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As with everything else being extra in 2020, the hurricane season has thrown yet another storm at the Gulf Coast. This time it placed our very own metro Atlanta under a tropical storm warning for early Thursday (29 October 2020) morning. And it certainly packed a punch as it moved through the region. Fig. 1 above shows some of the damage sustained in the metro Atlanta area with the main issues being downed trees and power lines. Although the metro area fared well in comparison, around 1 million Georgians were without power causing many counties to cancel school the following day. Zeta made landfall as a Category 2 storm over New Orleans, LA, strengthening more than the early forecasts expected (again this season). A great look at Zeta’s structure is shown in Fig. 2. The cross sections show the intensity of the v-component winds on the eastern side of the storm. These winds are stronger due to the strong cyclonic circulation and storm motion contribute to rapid wind speeds, which also contribute to the larger storm surge threat being where these winds are occurring. At one point in Boluxi, MS, which was right in this sweet spot of being just east of the eye at landfall, the forecast storm surge was 7’-11’. The cross sections to the far right also show a great look at the stability and sinking air in the eye of the storm, as the potential temperature contours bow down and have slightly less space between them indicating larger stability in the eye.

Fig. 3 shows the current forecast cone for now Tropical Storm Zeta as it moves through our region and its wind field, as well as the wind history, showing that the hurricane force winds sustained all the way into central Alabama. That, along with the narrow width of the forecast cone, indicate how quickly this storm is moving. At the time of that forecast issuance, Zeta was moving NNE at 39 mph. Contributing to this rapid motion are upper level flow and the setup of a high pressure region’s anticyclonic circulation off the East Coast and an extratropical cyclone’s cyclonic circulation just west of Zeta. Like a pitching machine, Hurricane Zeta was steered right between these two “cogs” and sent flying through the region. Fig. 4 shows the upper level jet contributing to Zeta’s path (Fig. 4a), as well as the height anomalies of the high region to the east and the cut off low feature to the west (Fig. 4b). Fig. 4c-d show a comparison of cyclonic vorticity between 500 mb and 700 mb, which shows the increasing vorticity with height of the cold core extratropical cyclone over Texas and Oklahoma (which can indicate upward vertical motions through the QG-Omega equation). These panels also show the symmetrical, compact region of extremely strong cyclonic vorticity associated with the rotation of Zeta (Zeta has a lot of zeta! – Zak).

While the majority of Georgia north of I-85 was under a tropical storm warning, with winds forecast in the 60-70 mph range and flash flood watches, we noticed a lack of hurricane related tornadoes throughout the region. That’s one distinct difference between when the remnants Delta rolled through and this time around with Zeta is the associated tornadoes (or lack thereof). One potential reason for this is the rapid speed that Zeta is moving through the region. Most of what could have potentially formed tornadoes was here and gone before having the chance to set up shop. Another potential reason is the lack of significant CAPE in the region. Fig. 5 shows the MLCAPE in the region as well as the surface to 6 km wind shear. While the wind shear is between 40-50 knots at 13Z 29 October 2020, there is only very modest MLCAPE in the region. The lack of CAPE, and presence of CIN there as well, could be another factor representing the lack of energy available to aid the formation of tornadoes. Overall, the CAPE doesn’t seem to be sufficient and the rapid speed at which Zeta is moving makes it difficult for it to produce many tornadoes.

Now, going back to the strengthening of Zeta to a Category 2 storm before landfall. We must begin to wonder why that rapid strengthening just before landfall has been a common theme this hurricane season (other than just talking it up to being 2020). Keeping in mind that hurricanes require low shear environments to strengthen due to the symmetrical nature of the storm’s structure, the presence of some modest vertical wind shear in Fig. 6a shows a potential dissuading variable to Zeta’s strengthening. Fig. 6b shows that water temperatures in the Gulf of Mexico just before Zeta’s landfall were definitely warm and able to provide warm moist air to invigorate the storm, but nothing that out there concerning the typical warmth of these waters. So why do the models keep underpredicting this season? Zak suggested in our discussion that maybe something along the lines of an anomalously warm loop current in the Gulf has been able to consistently provide warmer waters to give the storms a little bit more juice just before they make landfall, and the models just haven’t been able to capture it well this season. But that will definitely be something interesting and worth looking into in the future to improve our forecast models to provide more accurate information to the public regarding the dangerous conditions heading there way.

On October 5th, 2020, the 24th named storm of the 2020 Atlantic Hurricane season, Tropical Storm Delta, was named. It was named in the Caribbean as it approached the Yucatan Peninsula and the Gulf of Mexico. Within 24 hours of being a tropical storm, Delta rapidly intensified into a Category 4 hurricane with sustained winds of 145 mph. Thankfully the storm weakened into a Category 2 hurricane before making its first landfall near Cancun, Mexico around 6:30 EDT on October 7th. Due to frictional forces from the land below, Delta weakened further into a Category 1 hurricane. However, as it moved into the Gulf of Mexico, it rapidly intensified again into a Category 3 hurricane. Delta was now on track to make landfall almost exactly where Hurricane Laura did a month prior, and this would be a record 4th time in one year that Louisiana was hit by a named storm. On Friday October 9th, Delta made landfall near Creole, Louisiana. To fully understand why Delta intensified so rapidly, the synoptic scale environment, or large-scale atmosphere setting, must be looked at.

Figure 1: This loop shows the 5 day forecast and watches and warnings for Hurricane Delta. (Source Link)

 

When looking at the synoptic level setting regarding tropical cyclone development, there are 3 major things to consider: Sea surface temperature, moisture content, and upper level wind shear. Tropical cyclones are warm core systems that strengthen over very warm water.

A sea surface temperature of 27°C (80°F) or higher will be what is needed to fuel these cyclones. The sea surface temperature of the Caribbean and the Gulf of Mexico was around 30°C (Figure 2) which is more than enough to strengthen a tropical cyclone. In regard to moisture, tropical cyclones thrive in a moisture rich environment. The relative humidity in the atmosphere over the Gulf while Delta was moving though the region was 90% and higher (Figure 3). When looking at the wind shear environment, the key for tropical cyclone development and strengthening is little to no upper level wind shear. If it was present, it would not allow the storm to properly form. In the case of Delta, there was little shear over the waters it traveled, but some upper level shear was present over the Southeastern United States due to a jet streak just north of Mississippi, Alabama, and Georgia (Figure 4). Due to this synoptic setup, Hurricane Delta was able to rapidly intensify twice in its lifetime.

Figure 2: Global sea surface temperature at 07 Z October 7th. Warmer colors indicate warmer temperatures. (Source Link)

Figure 3: Mid to upper atmospheric moisture content measured at 18 Z October 7th. (Source Link)

Figure 4: Wind speed measured at the 200 mb level on 06 Z October 8th. Allows to see how jet is behaving. (Source Link)

 

As Hurricane Delta made landfall in Louisiana it was longer over the warm, moist, shear free environment it had been and began to weaken and fall apart. As tropical cyclones fall apart and weaken over the mid-latitudes, they can begin to show extratropical cyclone characteristics. This was exactly the case with Hurricane Delta due to dry air being entrained into the system over land. As Delta went more extratropical, it lost its classic symmetrical tropical cyclone look and became asymmetrical as it exhibited frontal boundaries. Now post-tropical cyclone Delta will be much more interesting to look at on the mesoscale level.

The cold front associated with post-tropical cyclone Delta (Figure 5) caused severe weather for much of metro Atlanta as it moved through Saturday evening into Sunday. Looking at a sounding diagram (Figure 6) from the region during this time, about 848 J/kg of CAPE were observed. This value matched the most-unstable CAPE which means the atmosphere as a whole was very unstable at this time. This high CAPE value along with a strong frontal system that provides a lifting mechanism can brew convective activity for severe weather. That is exactly what happened in the state of Georgia on Saturday/Sunday as the National Weather Service confirrmed 7 tornadoes in the state that day. In addition to this, there were 6 reports of hail and 36 reports of wind damage (Figure 7). Over the span of a week, Delta ranged from a category 4 hurricane bringing life threatening flooding, to a post-tropical system causing severe weather outbreaks in the Southeast.

Figure 5: Surface analysis map from the Weather Prediction Center of Atlanta at 15 Z on October 11th. (Source Link)

Figure 6: This is a sounding diagram of the metro Atlanta area at 18 Z October 11th. (Source Link)

Figure 7: This is a storm report of the state of Georgia from the severe weather over the weekend. (Source Link)

 

As of Thursday, October 8, 2020, Hurricane Delta was located in the Gulf of Mexico. The storm was classified as a Category 2 hurricane due to its maximum sustained wind speeds of 100 mph. The 5-day outlook published by the National Hurricane Center Thursday morning at 7 am CDT indicated the storm strengthening into a major hurricane by 1 am CDT Friday, then weakening back to a Category 2 storm before making landfall on the Louisiana coast Friday afternoon just after 1 pm CDT. Most of the Louisiana coastline was under a hurricane warning, while a tropical storm warning spread along the Texas coast to the far eastern portion of the Louisiana coast. After the storm was projected to make landfall, it was due to weaken into a tropical storm by early Saturday morning, and then further weaken to a tropical depression by Saturday afternoon. We will look at some of the atmospheric dynamics present that helped to determine the strength and track of the forecasted hurricane.

The following map shows the 300-200 hPa potential vorticity (shown in the gray contours), irrotational wind vectors (shown in the black vector arrows), the 600-400-hPa ascent (shown in the red contours), the 250 hPa jet (shown in the blue-pink fill pattern), and the precipitable water (shown in the green-orange-pink fill pattern). Let’s first look at the irrotational wind vectors. These illustrate the magnitude of the irrotational wind, or the wind without any vorticity taken into account. Irrotational wind also shows whether the air is converging or diverging in the upper troposphere. In the still image shown below, at 06 UTC on October 8, there is slight divergence over the center of Hurricane Delta. However, if we progress through the forecast period, the divergence greatly increases over the center of the storm as it approaches landfall. This indicates air evacuating the center of the storm, which corresponds to a lowering of the surface pressure and thus a strengthening of the hurricane. Then, right before the storm approaches the Louisiana coastline, the magnitude of the irrotational wind vectors decreases, indicating the storm weakening once again. The regions of vertical ascent, shown in the red contours, also indicate upward vertical motion. The strongest example of ascent associated with Hurricane Delta is shown later in the forecast period, after the storm has made landfall. There appears to be a frontal system associated with the remnants of the storm, enhanced in part by this upward vertical motion. The jet stream helps to show the direction of motion of the hurricane, especially following its landfall. The jet stream is oriented meridionally, stretching from the Rocky Mountains in Canada to the Atlantic Coast of the United States. As the forecast period progresses, the jet stream becomes even more horizontal. The remnants of the storm become ingested in the lowest reaches of the jet stream, and this helps to steer the storm off the Atlantic Coast. One final ingredient the storm needs to maintain its strength is moisture, and this map shows that there’s plenty of it. Over the Gulf of Mexico, the values of precipitable water are very large. This will feed the storm and allow it to bring high rain totals to the Louisiana coast. After the storm makes landfall, this moisture source will be cut off, so this will contribute to its continual weakening as it moves father from the Gulf of Mexico.

The following map shows 500 hPa geopotential height (shown in the black contours), temperature contours (shown in the red contours), ascent (shown in the blue contours), cyclonic relative vorticity (shown in the warm-colored fill pattern), and wind barbs. Let’s take a look at the temperature contours. There is a concentration of red contours around the center of Hurricane Delta, indicating that this is a warm core system. This gradient becomes slightly stronger while the hurricane is sitting over the Gulf of Mexico. At this point, the storm is absorbing the moisture from the warm ocean below. This will enhance the gradient of the temperature at the core of the storm. The cyclonic relative vorticity contours show the rotational nature of the hurricane. In the Northern Hemisphere, storms rotate counterclockwise due to the directional influence of the Coriolis force. The darker red shades of the fill at the time of the map indicates the intense rotation of Hurricane Delta. The cyclonic relative vorticity increases as the storm moves northward, and right before landfall, the intensity of the fill pattern decreases, indicating the storm’s weakening. After the storm makes landfall, there is some cyclonic relative vorticity that appears around the remnants. This frontal-like structure also has rotation associated with it. This front may also be enhanced by jet stream dynamics. Finally, the 500 hPa geopotential height contours indicate the general direction of the storm. Though they don’t illustrate the movement as clearly as the jet stream, the remnants of the hurricane will follow along the bottom of the mid-level trough sitting over the Great Lakes/Northeast region.

One important tool for any forecaster is the Skew-T diagram or plot. A Skew-T diagram (Figures 1 and 1A) shows the state of the atmosphere in a specific location at a specific time. Soundings are generated by the information received from a released weather balloon as it rises up in the atmosphere; these balloons are released around 00Z and 12Z Universal Time. The information is then relayed back to the National Weather Service local office and uploaded to create a map of the state of the atmosphere around the United States and southern Canada.

 

Figure 1. Sounding profile and analysis from Detroit, Michigan from October 1st, 2020 at 12Z. (source: spc.noaa.gov)

 

Figure 1A. Indexes of the Detroit sounding enlarged for easier reading. (source: spc.noaa.gov)

 

A lot of information can be gleaned from a Skew-T plot. In particular, Skew-T plots can tell if there is a risk of severe weather. Fortunately for us, computers can do the heavy lifting and we no longer have to do these calculations by hand. The index values are given on the bottom of the Skew-T plot in Figure 1. For severe weather, attention should be paid to the following (which are circled in red in Figures 1 and 1A), Convective Available Potential Energy (CAPE), the Lifted Index (LI), and the K index.

CAPE represents the amount of energy that exists in the atmosphere to generate thunderstorms. Values lower than 1000 J/kg are generally considered a low probability of severe weather, while values over 2500 J/kg indicate a high risk of severe weather (Markowski, 2016 p. 33). In the above example, the highest CAPE is 264, which suggest severe weather is unlikely. The Lifted Index is based on the location of the parcel when lifted from the surface and raised to 500 mb. The parcel temperature is then subtracted from the actual air temperature. If the parcel temperature is higher than the actual air temperature (ie. a negative number), the atmosphere is unstable. The greater the negative value, the greater the risk of severe weather. Severe weather isn’t likely with a LI > -2, and is considered very likely if the LI < -6. In our sounding, the LI is forecast to be -2, which would indicate that severe weather is possible, but not very likely (www.weathertap.com, 2020). Our final index, the K index, is also used to measure the probability of thunderstorms. If you had to calculate the K index, you would take the temperature at 850mb, add the dew-point temperature at 850mb, then subtract both the temperature at 500mb and the dew-point depression (the difference between the temperature and the dew point) at 700 mb (www.weather.gov, 2020 and www.weathertap.com, 2020). The Figure 2 shows the probabilities for thunderstorms and severe weather:

Figure 2. Severe weather and thunderstorm potential based on the Lifted Index and the K index values. (https://www.weathertap.com/guides/aviation/lifted-index-and-k-index-discussion.html)

 

By looking at the actual weather radar for 12Z, 16Z, and 20Z, rain and a few thunderstorms are seen in the Detroit, Michigan area (Figure 3), but by checking the storm prediction center archives, no severe weather was reported in that region for the day of the sounding (spc.noaa.gov, 2020) verifying that our analysis of the different severe weather indices was accurate.

Figure 3. Radar imagery near Detroit, MI on October 1st, 2020 at 12Z (left), 16Z (center), and 20Z (right). https://www.spc.noaa.gov/exper/mesoanalysis/new/archiveviewer.php?sector=16&parm=pmsl#

 

References

Markowski Paul and Richardson, Yvette. Mesoscale Meteorology in Midlatitudes, 2016, Wiley-Blackwell.

https://www.spc.noaa.gov/exper/archive/events/

https://www.weathertap.com/guides/aviation/lifted-index-and-k-index-discussion.html

https://www.weather.gov/lmk/indices

https://www.spc.noaa.gov/exper/soundings/

https://www.spc.noaa.gov/exper/mesoanalysis/new/archiveviewer.php?sector=16&parm=pmsl#

On 30 September 2020, weather in the continental United States was fairly uneventful. There was only one area that the Storm Prediction Center (SPC) had their eyes on – the state of Michigan. Throughout the day, convective thunderstorms rolled through the region, bringing occasional bouts of heavy rain due to a warm and cold front sweeping through the area within a few hours of each other (Fig. 1).

Figure 1: WPC surface analysis maps from 0000 UTC 30 September 2020 to 0000 UTC 1 October 2020. Isobars (in brown), frontal boundaries, and areas of high and low pressure can be seen on the map. Source: https://www.wpc.ncep.noaa.gov/archives/web_pages/sfc/sfc_archive_maps.php?arcdate=09/30/2020&selmap=2020093000&maptype=usfntsfc

 

Despite the convective nature of the storms, there was minimal surface-based CAPE present (Fig. 2). CAPE, or Convective Available Potential Energy, is a metric derived from atmospheric soundings to quantify the instability present in the atmosphere. Areas of less than 1000 J/kg of CAPE, seen throughout most of Michigan in Fig. 2, represent areas of weak instability.

Figure 2: SPC Mesoanalysis of surface based CAPE and CIN overlayed onto radar and wind barbs from 1300 UTC on 30 September 2020 to 0000 UTC on 1 October 2020. CAPE is in red contours while CIN is shaded in blue. Source: https://www.spc.noaa.gov/exper/mesoanalysis/new/archiveviewer.php?sector=16&parm=thea&underlay=1#

 

However, minimal CAPE does not always mean there is no convective activity present. In the atmospheric soundings in Detroit, Michigan between 1200 UTC on 30 September 2020 (Fig. 3 left panel) and 0000 UTC on 1 October 2020 (Fig. 3 right panel), the temperature path almost perfectly follows the moist adiabat. This means that any amount of lifting (such as a frontal passage) would cause an air parcel to rise, resulting in convective thunderstorm activity.

Figure 3: Atmospheric sounding for Detroit, Michigan at 1200 UTC on 30 September 2020 (left) and 0000 UTC on 1 October 2020 (right). Temperature is in red, while dewpoint is in green. Source: https://www.spc.noaa.gov/exper/soundings/20100100_OBS/

 

Both warm and cold frontal passages can cause precipitation by forcing warm, moist air upward, where it cools and condenses. The right panel of Fig. 4 shows how a warm front (such as the one passing through Michigan around 1500 UTC) could cause precipitation, while the left panel of Fig. 4 shows how a cold front (such as the one passing through Michigan around 1800 UTC) could cause precipitation.

Figure 4: Conceptual model of a cold frontal passage (left) and warm frontal passage (right). Source: https://laulima.hawaii.edu/access/content/group/dbd544e4-dcdd-4631-b8ad-3304985e1be2/book/chapter_6/fronts.htm

 

Another way to measure the potential for convective thunderstorm activity in this situation is to look at PVA, or positive vorticity advection. Areas of PVA, as can be seen ahead of thunderstorms in Fig. 5, represent areas of upper level divergence that typically leads to rising air at the surface. Just like in the case of a front, this rising air cools and condenses, resulting in precipitation.

Figure 5: Vorticity, positive vorticity advection (PVA), and negative vorticity advection (NVA) overlayed onto radar from 1300 UTC on 30 September 2020 to 0000 UTC on 1 October 2020. Vorticity is shaded, PVA is contoured in blue, and NVA is contoured in red. Source: https://www.spc.noaa.gov/exper/mesoanalysis/new/archiveviewer.php?sector=16&parm=thea&underlay=1#

 

Even though the frontal passage on the 30 September 2020 did not result in significant rainfall totals in Michigan, the atmospheric setup that resulted in convective activity in the region remained, bringing more rain to the area in the days that followed.

The forecast period for our weather discussion was rather quiet, so we decided to focus in on an area we were already interested in as it encompasses the current Weather Challenge forecast city: Grand Rapids, Michigan. Michigan has experienced multiple frontal passages over the past few days contributing to rainfall, pressure fluctuations, and temperature shifts in the region. The GFS 06Z forecast over the next couple days (12Z 1 October to 12Z 3 October) has the precipitation mostly moving out of the region aside from pockets remaining around coast of the Lake Michigan until another disturbance moves through Sunday. On the synoptic scale, we analyzed the jet configuration shown in Figure 1 over North America noting the strong northwesterly meridional flow at 300 mb extending from the Alaska-Canada border through Missouri and Illinois. The ridge-trough pattern in the jet is reflective of the geopotential height pattern across North America displayed in Figure 2, which bears strong resemblance to the Pacific-North American pattern, which is a low-frequency teleconnection pattern known to influence below average heights and temperatures in the eastern United States and above average heights and temperatures over the western United States. A persistent geopotential height dipole is present across the U.S. over the forecast period with a longwave ridge over the western U.S. and a long wave trough over the east, and associated above and below average heights, respectively.

The strong temperature gradient, as evidenced by the gradient in geopotential heights, is what leads to the strong winds in the jet flow aloft. The jet flow pattern set up aloft is advecting cold and dry polar continental air into the upper Midwest, contributing to the cold frontal events observed recently. This flow will persist over the next couple days but will become more disorganized and weaken, shown in Figure 1, as the forecast progresses and the temperature gradient weakens. The advection of the polar continental air into the upper Midwest leads to drier and cooler conditions, but also plays a role in the precipitation we observe and forecast as it moves through the Great Lakes region due to the lake effect.

Lake effect precipitation occurs when cold, dry air moves over a relatively warmer body of water. In the case of our region of interest centered around southwest Michigan, this body of water is Lake Michigan. Figure 3 shows lake surface temperature is forecast to remain around the mid-50 °F range, while the air being advected over Lake Michigan will be approaching the low 40’s°F and potentially enter the 30’s°F over the forecast period. When looking at the precipitable water in Figure 4, which is the amount of water you would find if you condensed all the water vapor in the overhead air column, it confirms that the air mass moving through the region is quite dry and will continue to be so through the forecast period. The westerly surface flow in the area will advect that cold, dry air over the surface of Lake Michigan, which will cause the air to pick up moisture and heat as the specific heat of water is much higher than that of air, causing the air to be warmer with higher moisture content, and therefore less dense, once it reaches the downwind shore.

The decrease in density will already influence the air to rise as it reaches the downwind shore, but other factors can influence upward vertical motion of this air at the surface as well as it reaches the opposite shore. For instance, the surrounding land around Lake Michigan is marginally higher in elevation which can assist some rising motion of the air. More telling, I think, is the role that friction plays on influencing some low-level convergence. Friction changes the speed, and therefore slightly affects the direction, of wind. Since friction opposes flow, the wind will slow and so the Coriolis force will decrease, which will give the wind a directional component toward lower pressure. The force of friction of water on air is much less than that of land. As is shown by Figure 5, there is a slight change in speed and direction as the winds come onto the eastern shore of Lake Michigan where the flow shifts slightly more westerly from a more northwesterly flow and drops from around 15 knots to around 5 knots in the vicinity of Grand Rapids. This speed and directional low-level convergence due to impacts of friction will help contribute to rising motion, which will influence condensation. So, the lake effect potentially plays role in the precipitation seen in the Figure 6 forecast for southwest Michigan and as indicated by the soundings taken on the upwind and downwind sides of Lake Michigan displayed in Figure 7.

Tropical cyclones form due to the warm, moist air over the ocean near the equator. The warm, moist air rises and creates a low pressure area near the surface. The pressure gradient force causes the surrounding air to move towards the lower pressure area. That air then warms and rises as well. As the warm air rises, it cools and the water vapor in the air forms clouds. This continues to happen until a whole system of clouds spins and grows, and an eye forms in the center, marking a calm, clear, area of low pressure.

Figure 1 below shows what a cyclone would look like if you could slice into it. The red arrows symbolize the warm, moist air rising from the ocean’s surface while the blue arrows symbolize the cooler air being pushed towards the surface. The spinning causes the clouds to form the bands that are shown around the eye.

(Figure 1)

On Thursday, September 17th, a low pressure system in the Gulf of Mexico evolved into a well-defined circulation that was called Tropical Cyclone Beta. Tropical cyclones have four different labels that are based on wind speed. From lowest to highest wind speed, they are tropical depression, tropical storm, hurricane, and major hurricane. At this point, Beta was labeled as a tropical depression. Figure 2 below is a visible satellite image taken on the 17th, and you can see the system of clouds that had formed into the circulation.

(Figure 2)

On Friday, September 18th, Tropical Depression Beta was upgraded to Tropical Storm Beta. It was expected to grow and intensify, but dry air coming in from the west over Northern Mexico and Texas caused dry air entrainment that significantly weakened the storm. Figure 3 below shows the dry air in the atmosphere on that Saturday.

(Figure 3)

(Figure 4)

Figure 4 above is a sounding from Corpus Christi, Texas on Saturday the 19th. The orange dashed lines represent the dry adiabatic lapse rate, and the red solid line is the environmental temperature. By noticing that the environmental temperature follows parallel to the dry adiabatic lapse rate near the bottom of the graph, we know that the atmosphere near the surface that day in Corpus Christi was very dry. We can see from Figure 3 that this was definitely the case.

As Beta slowly approached land, it caused lots of rain and storm surges across the coasts of Northern Mexico, Texas, and Louisiana. Figure 5 below shows a picture taken in Galveston, Texas on Monday, September 21st.

(Figure 5)

On Tuesday, September 22nd, Beta made landfall on the coast of Texas between Corpus Christi and Galveston. After making landfall, Beta continued to slow and weaken until denigrating into a post tropical cyclone remnant low.

Sources:

https://www.post-gazette.com/news/nation/2020/09/20/Tropical-Storm-Beta-forecast-Atlantic-hur ricane-season-Texas-Louisiana-Gulf-Coast-Hurricane-Teddy-Wilfred/stories/202009200193

https://spaceplace.nasa.gov/hurricanes/en/

https://www.nhc.noaa.gov/refresh/graphics_at2+shtml/205430.shtml?cone

https://www.nhc.noaa.gov/archive/2020/BETA.shtml?

Tropical Storm Beta formed in the Southwest Gulf of Mexico on September 17th and meandered up near the coast before finally making landfall in Southeast Texas. Being stuck in between a digging trough feature over the southern plains and a high-pressure system to the East kept Beta at a slow pace before moving inland. Once inland, however Beta began to weaken significantly and become more of an open circulation. This process opened the door for nearby stronger flow to pick up and steer the system to the NE which would lead to the transition of Beta into a post-tropical cyclone and eventual remnant low. As upper level winds accelerated the system eastward across the Southeast US, the system began showing frontal-like tendencies and would become more than just a remnant low.

As Beta continued to weaken as it moved inland, its once closed and cutoff circulation became open and elongated. This not only allowed for the ingesting of dry air to the NW into the system, further weakening it, this made Beta vulnerable to be pulled away by strong upper level flow which was evident by a digging trough feature over the southern plains. This would begin

to accelerate Beta to the northeast, bringing it across the rest of the southeast. As the system advanced over the Tennessee Valley it ran into a shortwave trough from the Northeast US. This helped to amplify the other trough feature that Beta was riding eastward as shown in the figure above with the jet streak signature from Arkansas to the Mid-Atlantic coast. This shortwave interacting with the large trough feature over the southern plains would provide the necessary energy to allow Beta to undergo some frontogenesis before passing over the southeast and moving off the coast.

In class we’ve learned how to identify frontal boundaries. The most obvious thing to look out for is temperature change, after all a frontal boundary marks the separation of two different air masses and the most common types of fronts (cold/warm) indicate where the boundary between a cold and warm airmass resides. However, there are other things that can indicate a frontal boundary such as moisture, wind direction/shift, and vorticity. Beta showed evidence of a frontal boundary as it made its way across the southeast. The middle figure shows precipitable water (moisture content of air above a fixed point) that shows moist air as the colder colors and dry air as the warmer colors. Beta is positioned over Tennessee and looks to be pulling moist air northward from the Gulf. A distinct boundary running from SW Alabama to NE Georgia separates moist and dry air and indicates where a potential cold front could be while another boundary across KY-VA indicates where a potential warm front could be. The figure on the left shows cyclonic vorticity which can be maximized at the boundaries of fronts. Two areas of maximized vorticity appear around the same areas of where the moist boundary lines hinted at a cold and warm frontal boundary. The slight packing of thickness contours in the figure on the right seem to coincide with the presence of the shortwave trough mentioned earlier. This small tightening of the temperature gradient sets up a weak baroclinic zone that will help to establish a more distinct boundary, particularly at the location of the potential warm front.

Another concept we learned in class is the frontogenesis equation and being able to perform sign analysis on the four terms (shearing, confluence, tilting, diabatic) in order to figure out whether frontogenesis (overall positive) or frontolysis (overall negative) is present. The

middle figure shows plotted frontogenesis (purple contours), temperature (red contours), and warm air advection (red fill pattern). The frontogenesis plotted here is a magnitude, so while it’s not clear which terms are positive and negative, it does at least indicate that these terms are present in some capacity and this plot appears to focus along where a potential warm front might be which could be why there is a local maxima of warm air advection. The first two terms (shearing, confluence) can be deduced from this figure. The wind barbs along with the temperature contours show that both of these terms are positive as you have both terms showing both a decrease in temperature in both the x and y directions (negative) as well as a shift from westerly winds to easterly (negative –> shearing) and southernly to northerly (negative –> confluence) resulting in both terms being positive. For the tilting term, the figure on the left shows omega values which can help determine the change in pressure over time in the y direction. Based on the figure this results in a positive change in omega, but with an increase in potential temperature with decreasing pressure, this results in an overall negative tilting term. The diabatic term is difficult to predict, but based on the figure on the right, the diabatic term is negative due to the slightly positive increase in temperature as you go across the front due to lack of cloud cover and overall small change in thickness. Overall, we have two positive and two negative frontogenesis terms, indicating that whatever frontogenesis that’s occurring is relatively weak. This is not surprising given the fact that typically remnant lows associated with posttropical cyclones produce weak frontal boundaries if they develop into anything at all outside of a weak low.

Tropical Cyclone Beta went from a tropical storm to a meandering low that was picked up by an upper level trough, amplified by a shortwave, and likely produced a weak cold front with an accompanying warm front that, while weak, was able to produce some convective activity across the southeast (see figures below).

On 10 August 2020, a strong “derecho” developed over Iowa as a result of a strengthening pre-existing MCS (Mesoscale Convective System) combined with an unstable atmosphere ripe for vigorous convective activity.  The derecho led to numerous severe wind reports primarily over eastern Iowa, northern Illinois and southern Wisconsin, with a handful of wind gust reports exceeding 100 mph (Fig. 1 top-panel).  Extensive straight-line wind damage was observed, including destruction of farm buildings and equipment (Fig. 1 bottom-panel), crops, trees and weak building structures.

Fig. 1: (top-panel) SPC filtered storm reports for 10 August 2020 (source: https://www.spc.noaa.gov/exper/archive/event.php?date=20200810), and (bottom-panel) heavily damaged grain belt in Marion, IA as a result of straight-line winds from the derecho event (source: https://www.weather.gov/dvn/summary_081020)

 

Fig. 2 shows a conceptual model of a derecho.  A derecho is an MCS (Mesoscale Convective System) in which the leading edge of the storm evolves into an intense “bow echo.”  Derechos initially form as a single or multicellular thunderstorm that develops within an environment with modest to high vertical wind shear, instability (i.e., sufficient CAPE, or Convective Available Potential Energy), moisture and some sort of lifting mechanism (typically a frontal boundary or mid-to-upper level “wave;” Fig. 2 panel 1).  As the thunderstorm complex matures, cold air sinking within the thunderstorm downdraft diverges at the surface and propagates radially outward.  The leading edge of the cold pool moving in the same direction as the mean wind forces warm, moist air downstream of the advancing cold air to lift, and it will create the “bowing” effect of precipitation on radar reflectivity imagery, as this portion of the cold pool propagates faster than the cold pool edges not aligned with the mean wind (Fig. 2 panel 2).  The lift ahead of the cold pool leads to further thunderstorm development, and, if the complex continues to strengthen, will develop a stronger downdraft and faster propagating cold pool (Fig. 2 panel 3).  The winds associated with the fast-moving cold pool can exceed hurricane force (i.e., greater than 74 mph, or 64 knots), and, within strong derechos, 100 mph (i.e., 86.9 knots)!  Furthermore, a “bookend vortex” can form on the northern edge of the bow echo, which gives the derecho a “rotation” appearance on radar.

Fig. 2: Conceptual model of a derecho (illustrations by Dennis Cain, and images from https://www.spc.noaa.gov/misc/AbtDerechos/derechofacts.htm#types).  See text for details.

 

At 1200 UTC 10 August 2020, remnants of an MCS over southeastern South Dakota moved eastward into Iowa over the next several hours.  During that timeframe, the synoptic-scale environment exhibited an upper-level trough over the Saskatchewan province associated with a strong surface extratropical cyclone over the region.  The cyclone was associated with a cold front extending south and then southwestward through northwest Wisconsin, southeast Minnesota, central Iowa and southeastern Nebraska moved slowly eastward during the day (Fig. 3).  This front, maintained within a deformation zone driven by anticyclonic flow over the northern U.S. Rockies and southeastern U.S., along with the upper tropospheric trough/ridge and jet streak dynamics, provided synoptic-scale lift to aid the MCS during the day.

Fig. 3: WPC surface map analyzed at 1200 UTC 10 August 2020.  Weather station data shown using traditional conventions and units, isobars (mb) are in solid brown, and frontal boundaries and high and low pressure centers shown using traditional conventions.  Source: https://www.wpc.ncep.noaa.gov/archives/web_pages/sfc/sfc_archive_maps.php?arcdate=08/10/2020&selmap=2020081012&maptype=namussfc

 

Along with the above, surface-based CAPE values in some portions of eastern Iowa and central Illinois exceeded 4000 J/kg, with modest directional shear in place due to the presence of an upper level trough positioned over the North Dakota/Minnesota/Saskatchewan border (not shown).  For example, the KDVN sounding at 1700 UTC 10 August 2020 (with the balloon launch occurring at a non-traditional time of day) shows observed CAPE values over 4500 J/kg (Fig. 4)!  These type of conditions were optimal for strengthening of the pre-existing MCS as well as its evolution into a derecho.

Fig. 4: KDVN sounding at 1700 UTC 10 August 2020 (source: https://www.spc.noaa.gov/exper/archive/event.php?date=20200810).  Note the large surface-based CAPE value as well as modest speed shear prior to derecho arrival.

 

As a result of the above (plus some other factors not focused on in this post), the derecho that evolved had a “textbook” appearance on radar, as shown in Fig. 5 below.

Fig. 5: Radar reflectivity factor (color scale based on conventional color scale used for radar reflectivity) .gif of the derecho event (from the Quad Cities, IA radar site) on 10 August 2020.  Note the bow echo that develops over time.  Note the bookend vortex on the northern edge of the bow echo.

Welcome to our course blog for Mesoscale Meteorology at Georgia Tech!  Throughout the semester, students and faculty of this course will post information about mesoscale meteorology forecasting and observations based on topics discussed within the class.