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Just days after another deadly tornado outbreak wreaked havoc on the southeastern United States, another severe thunderstorm threat is in the forecast moving into Palm Sunday. This time, the risks for Dixie Alley are slightly lower, but the bullseye for significant severe weather is now focused over portions of the mid-Atlantic on Sunday afternoon (28 March 2021). In fact, the categorial convective outlook from the Storm Prediction Center (SPC) hints at a day 1 enhanced risk (level 3 of 5) over the mid-South at 20Z 27 March 2021, while another enhanced risk is centered over southeastern Virginia with an expansive slight risk (level 2 of 5) extending from Maryland to North Georgia for Palm Sunday (Figure 1). According to the SPC, the forecasted severe weather event will primarily come in the form of damaging thunderstorm winds or wind gusts of 50 knots (58 mph) or higher, with locally higher wind gusts to near hurricane force possible. There is also a lower, but still decent, chance of tornadoes and hail as the synoptic feature moves northeastward into southeastern Canada.

Figure 1: NOAA/NWS Storm Prediction Center (SPC) categorial outlooks issued on 27 March 2021 for a) Day 1, 20Z 27 March 2021 to 12Z 28 March 2021, and b) Day 2, 12Z 28 March 2021 to 12Z 29 March 2021. Source: NOAA/NWS SPC Convective Outlooks.

 

As of 21Z 27 March 2021, the center of the low pressure system that will be propelling the severe threat is situated over eastern Missouri and has a mean sea level pressure of 1007 mb (Figure 2). Weaker, broader areas of low pressure are situated along the cold frontal boundary to the north and to the southwest of the main area of circulation. Given the orientation of the jet stream beginning at 18Z 27 March 2021, the cyclone is located within the right jet entrance region of the jet streak situated over the Great Lakes and New England. To the south of the primary low pressure center is a weaker jet streak that encompasses the cyclone in its left jet exit region. The animation in Figure 3 shows the irrotational wind vectors in the upper troposphere, which simply describes the component of the wind without any rotation or directional changes associated with it. In the upper troposphere, regions of upper level divergence occur in locations where the irrotational wind vectors are pointed away from, and, to no surprise, includes Missouri. When a column of air is underneath an area where air diverges in the upper troposphere, the air density aloft will decrease, and causes upward vertical motions from lower levels of the atmosphere, which acts to either strengthen a surface low or weaken a surface high. Through the GFS (Global Forecast System) run at this time, the broader areas of low pressure begin to intensify as a result of these strong upward vertical motions as it moves into southeastern Canada on Sunday afternoon.

Figure 2: Surface analysis at 21Z 27 March 2021 with selected station plots. Source: National Weather Service Weather Prediction Center (WPC) Surface Analysis Archive

 

Figure 3: Loop of GFS forecast data over the Contiguous United States from 18Z 27 March 2021 to 06Z 29 March 2021 using six-hour intervals. Precipitable water (PW) is shaded using bottom left scale; 250-hPa jet velocity (in m/s) is shaded using bottom right scale. 300-200 hPa potential vorticity (PV) denoted in black contours; 600-400 hPa vertical velocity greater than zero denoted in red contours. Arrows denote irrotational wind component. Source: Alicia Bentley Real Time GFS Analyses and Forecast Maps.

 

If you look closely at how the jet stream is oriented in the previous figure, you may have noticed that there is also an upper level trough that dives into the Great Lakes as the low pressure center intensifies. Just like diverging upper level winds, troughs have their own ways of strengthening low pressure systems, as long as the center of circulation is to the east of the trough axis in the Northern Hemisphere. In a trough, the spinning motion, or vorticity, around its base is cyclonic (counterclockwise) due to the curvature of the trough, while the motion around a ridge is anticyclonic (clockwise). Granted that our cyclone is in the Northern Hemisphere, any parcel of air that is bounded by a trough to the west and a ridge to the east will have positive vorticity (cyclonic motion) advected into its vicinity, and will cause the air above to rise. Figure 4 confirms this process, as vertical ascent is greatest to the east of the trough, right where the low pressure system is forecasted to move. In terms of the severe threat for the mid-Atlantic on 28 March 2021, the contours for vertical ascent also account for the growth of strong thunderstorms that have smaller-scale updrafts.

Please note that this post is a forecast for 27-28 March 2021, and conditions can change. In the event of inclement weather, ensure that you have multiple ways to receive weather warnings and have a plan of action before the storm arrives.

Figure 4: Loop of GFS forecast data over the Contiguous United States from 18Z 27 March 2021 to 06Z 29 March 2021 using six-hour intervals. Cyclonic relative vorticity is shaded using bottom scale. 500-hPa geopotential height denoted in black contours; 500-hPa temperature denoted in red dashed contours; vertical velocity greater than zero denoted in blue contours. Source: Alicia Bentley Real Time GFS Analyses and Forecast Maps.

This week had a significant weather event in the southeast region. Over the span of a couple of days, a low-pressure system, shown by the counterclockwise flow, strengthened, and led to a lot of precipitation coming across Alabama and Mississippi before moving north towards Kentucky and Tennessee. As seen in Figure 1, the precipitation the night of March 17 was severe and even resulted in a few tornadoes and as the low-pressure system continued moving, the precipitation lightened slightly but was still very present in the 12Z surface map.

Figure 1: Surface map showing pressure contours and precipitation type on 2021- 03-17- 21Z (right) and on 2021-03-18-12Z (left) to show how the low pressure system moves and strengthens as it moves towards the eastern ridge.

 

The surface analysis map below, shown as figure 2, shows the cold front that came across and the warm sector ahead of it. The warm front present at 21 Z Wed March 17 goes away nearing 12Z Thur March 18 but there is still a slight east to west wind and a turning of wind by the South Carolina border. The air north of the warm front, it is cooler and has lower dew points. In the pocket of the warm front, there is moisture advection as the warm moist air is pulled from the Gulf of Mexico which strengthened this low pressure system. Looking at the surface analysis at 12Z there is possibly a weak wedge event but it did not do anything to stop the severe weather present.

Figure 2: Surface analysis map showing the different temperatures, dew points and wind speed across the United States. The different fronts can also be seen, speficially the cold front moving across the south eastern U.S. form 21Z Wed Mar 17 to 12Z Thur Mar 18

 

Analyzing the upper levels, there is not a present jet streak occurring in the 300 mb level but there is convection and strong wind speeds northeast of the low-pressure system. The trough ridge flows at this level are what contribute more to the dynamics that strengthened this low pressure system. The 500 mb did show a maximum vorticity over the low shown by the deeper red colors in figure 3 but no positive vorticity advection at the surface since the low is not in the optimal location at 500 mb between the trough and the ridge. The 500 mb height anomaly shows the low pressure system moving and how it lines up with the vorticity map. This low is positioned between a somewhat western trough and an eastern ridge so other upper level dynamics present are ageostrophic divergence and moisture advection from the gulf.

Figure 3: (Left) 500 mb map showing relative vorticity
(Right) 500 mb height anomaly map showing the extreme low pressure system in the south east.

A potent severe weather event devastated parts of Mississippi and Alabama on St. Patrick’s Day leaving widespread tornado damage in its wake. The severe weather wasn’t over however as the Carolinas had to deal with their own bout of severe weather including tornadoes on the 18th. After the 18th, the following 24-36 hours brought about strong subsidence on the backside of the trough feature that brought about all of the active weather and a drier, quieter weather pattern ensued. This blog post will take a look at what was forecasted to transpire in the 48 hours after the tornado outbreak across the Gulf coast and what dynamically was present to allow for the active weather pattern to continue for another day and what led to conditions drastically improving across the South

During the day Friday, March 18th the upper-level low was still present over the Southeast (over eastern KY) with a weaker jet streak present at 250mb over Eastern GA into the Carolinas (See figure below from 18z Friday the 18th). This implied that upper-level divergence was ongoing and based on mass continuity this evacuation of air in the upper levels causes upward vertical motions as air from the mid/lower levels must rise and replace the air at the upper levels. This resulting loss of air at the surface will keep the surface cyclone strong through the short term. The jet streak is positioned in such a way that the left-jet exit region resides over the Carolinas. This is a region

where upper-level ageostrophic divergence is maximized which will translate downward to the surface and be one of the causes for strong convection to develop during the afternoon hours. The jet streak at 250mb looked to influence the mid-tropospheric trough at 500mb in such a way that it remained amplified and with a negative tilt as it progressed eastward. This orientation of the trough will serve two important purposes for future convective development: Cold air advection on the backside of the cyclone which will tighten the temperature gradient and increase low-level wind shear, and increased vorticity in the mid-levels due to the curvature of the trough. The figure below is forecasted 500mb vorticity at 18z Friday the 18th and shows relatively strong positive vorticity advection occurring over the Southeast. Positive vorticity advection will help to

increase upward vertical motions as well as shear in the mid-levels. This feature along with the 250mb jet are working in tandem to increase overall upward vertical motions and shear which eventually translates down to the lower levels.

Looking at the lower levels, specifically at 850mb shows how southerly flow off of the GOMEX was forecasted to tighten the temperature gradient and contribute to low-level wind shear. The figure below shows tight thickness contours with strong southerly flow over the Southeast. Thickness lines that are packed tightly indicates a strengthening thermal wind. Thermal wind is what will not only increase warm air

advection, but also increase low-level wind shear. The strong southerly flow of air near the surface via the thermal wind can be classified as a low-level jet. This is a feature that is a result of upper-level divergence due to the present jet streak translating down to the lower levels in the form of faster air converging near the surface and the strong temperature gradient only enhances this feature even more. Winds are noticeably veering with height which means a clockwise rotation of the wind barbs as you observe each level of the troposphere which not only is an indication of warm air advection but also directional shear which is an important factor in the development of severe weather. All of the different features present in the various levels of the troposphere will likely translate to rigorous convective development at the surface and another round of severe weather for the Southeast.

Beyond the 18th and over the next 24-48 hours there will be a significant change in the weather pattern across the Southeast. As the trough feature begins to move off the coast a much drier and quieter environment will take shape. The backside of troughs is where you can find areas of subsidence which means sinking air and calm conditions. The overall air flow becomes more northerly and cooler, drier air filters into

the region as a result. As opposed to upper-level divergence like with the jet streak, upper-level convergence will be occurring now which will cause downward vertical motions little to no surface convection. An anticyclone is beginning to form on the backside of the trough which will help to maintain the flow of cool, dry air from the north into our region, drying out the troposphere. At the mid-levels, the forecast shows

negative vorticity advection due to geostrophic flow advecting smaller values of vorticity towards larger values of vorticity, however, there does appear to be some vorticity signatures sticking around our area. This could hint at some possible residual instability and perhaps some enhanced cloud cover or light precipitation over the next 24-48 hours. However, as the ridge begins to build into the region and the anticyclone (high pressure) strengthens, an overall calm and quiet weather pattern appears to follow the active weather pattern that proceeded it.

During the weekend of March 12-13, 2021, significant weather was forecast to occur across much of the western and central United States, including a severe weather outbreak in the Plains states and a major snowstorm in the Rocky Mountains. These events were associated with a low pressure system over eastern Colorado, the development of which was caused by several synoptic factors.

A good place to analyze synoptic-scale weather systems is the jet stream level (250 or 300 mb). This is because convergence and divergence of the ageostrophic wind at this level is directly related to vertical motion in the mid-troposphere: if divergence is occurring in the upper levels of the atmosphere, air from below will rise to replace it, causing upward vertical motion in the mid-troposphere. This causes an area of low pressure to form or strengthen at the surface and is generally associated with clouds and precipitation. Figure 1 shows the forecast 300 mb wind and height for 0000 UTC 14 March 2021.

Figure 1: GFS 300 mb height and wind (12Z 11 March 2021, valid 00z 14 March 2021)

 

The map shows an upper-level trough centered over the Four Corners region and an upper-level ridge located over the eastern half of the country. Ageostrophic divergence occurs in regions with an upstream trough and downstream ridge, such as much of the central United States. In addition, a jet streak is shown on the eastern side of the trough base, centered over west Texas and New Mexico. Eastern Colorado is in the left exit region of this jet streak, a region associated with ageostrophic divergence. Since both the jet streak and trough/ridge positions are contributing to upper-level divergence over eastern Colorado, this is an especially favorable area for upward vertical motion.

Another way to forecast the development of synoptic-scale storms is to use the QG omega equation. This equation has a lot of complicated math, but in general it relates upward or downward vertical motion in the mid-troposphere to two variables: the change in vorticity advection with height, and temperature advection. According to the equation, positive vorticity advection increasing with height and warm advection in the low-mid troposphere are associated with upward vertical motion. Both of these were forecast to occur over northern and eastern Colorado, and the GFS model indicated significant upward vertical velocities at 700 mb in this region as a result (Fig. 2).

Figure 2: GFS 700 mb height, vertical velocity (18Z 11 March 2021, valid 00z 14 March 2021)

 

The end result of all this is the formation of a low pressure cyclone over eastern Colorado. While severe weather may be possible over the Texas and Oklahoma panhandle region, perhaps more interesting is the potential for a major snowstorm in parts of Wyoming and Colorado. The forecast maps below (figure 3a) produced by the Cheyenne, WY National Weather Service office illustrates this well: forecast totals in some cities are up to three feet. The Low End Amount graphic (figure 3b), which shows the snow amount with a 90% chance of being exceeded, is equally interesting. As the map shows, forecast confidence is quite high in very large snow accumulations, with a 90% chance of at least 16 inches of snow in the city of Cheyenne. Similar amounts were forecast for the mountains of Colorado and the Denver metropolitan area: both the NWS offices in Cheyenne and Boulder used rather strong wording in their forecast bulletins, reflecting the potential for a historic event.

Figure 3: National Weather Service Cheyenne, WY forecast maps for a) total snow accumulation and b) low end amount (90% confidence that this amount of snow falls).

March 10th – 11th were exciting days for weather enthusiasts. To start out, California had significant precipitation. As shown in figure 1, the low-pressure system that brought precipitation to southern California was somewhat in the left jet exit region of the jet streak. This often indicates divergence aloft, causing rising motion, and ultimately an increased chance to strengthen a low-pressure system. Meanwhile the Midwest, a storm system was developing.

Fig. 1: A wind mat at 250 mb with sea level pressure contours in black, 1000-500 mb thickness in red and blue dashed lines, and the wind speed shaded based on the color bar (source: http://www.atmos.albany.edu/student/abentley/realtime/standard.php?domain=northamer&variable=mslp_jet and Satellite and Radar Blog for March 11th.)

 

This was not the only dynamic mechanism causing the in system in California to exist though. Relative vorticity was in the area of the cyclone (the yellows and reds seen in the top of figure 2), as well as rising motion (the blue lines in the top of figure 2). The relative vorticity helped generate and strengthen the storm in the Midwest as seen in figure 2 as well by helping increase vertical motion. Vertical motion allows clouds to form more easily by allowing warm, moist air to rise, cool and condense. There was positive moisture advection, or moisture moving into a region, in the Midwest where the Midwestern system developed. This is shown in the bottom half of figure 2.

Fig. 2: (top panel) This is a map of 500 mb relative vorticity (shaded using the scale listed),wind barbs in knots, 500 mb geopotential height contours in black, and ascent in blue taken at 12 UTC on 2021 March 11 (source: http://www.atmos.albany.edu/student/abentley/realtime/standard.php?domain=northamer&variable=rel_vort), and (lower panel) is a map of 850 mb height dew points where the green shaded regions are where dew point is equal to temperature. This means an air parcel is saturated and holding the maximum amount of moisture for the conditions (source: https://weather.cod.edu/analysis/).

 

The observations in figure 3 give an idea of what happened. If southern California is observed, the low-pressure system sweeps across the region. In the mid-west, it is possible to follow the development of the system along a stationary front. It is eventually strengthened to look more like a Norwegian Cyclone between the 06 UTC and 12 UTC maps on 2021 March 11. This can be identified by the occluded front north of Wisconsin and Michigan. The positive moisture advection and position of the jet are given to further evidence for the development of the low. It did end up strengthening.

Fig. 3: This is a progression of surface observations from the Storm Prediction Center. From top left to bottom right moving across the rows, the data shown is from 18 UTC 2021 March 10, 00 UTC 2021 March 11, 06 UTC 2021 March 11, and 12 UTC 2021 March 11 (source: https://www.wpc.ncep.noaa.gov/html/sfc-zoom.php).

Today, scattered convection was evident across the Pacific Northwest, especially around Washington state. The surface analysis from this morning revealed a low-pressure system off of the coast of British Columbia, bringing WNW flow from the Pacific onshore. This provided a source of moisture for the area to help provide a base for convective activity as surface level temperature and dewpoints were close together signaling locally high relative humidity, but what was going to provide the lifting mechanism necessary for initiation of convective activity? Looking to the upper levels (specifically @250mb), there is evidence of a trough feature over the Pacific Northwest and in particular, the right side of the trough is situated over the area with a noticeable jet streak present based on the analysis. While not a particularly strong or amplified trough feature, this implies upper level ageostrophic divergence and mass evacuation which will induce upward vertical motions filtering down to the lower levels. The cyclone and subsequent onshore flow is evident as you go down to the 500mb and 700mb levels as well. The 700mb analysis gives you an idea of low-level moisture advection which was present with the onshore flow.

00z 11/19 and 12z 11/19 250mb upper air maps (AOS-UW-Madison) respectively
Isobars (black contours), Temperature (red dashed contours), Wind Speed (wind barbs), Stronger Jet Stream winds (fill pattern)

 

Now that the synoptic stage is set, we’re going to look at some sounding data to analyze the state of the atmosphere near the surface and see if there are more signs of convective initiation. Today’s 12z sounding (on the left) from KUIL (Quillayute,WA) was observed and from this we can see a very saturated low-level profile at least up to 800mb meaning there is definitely precipitation occurring over the area. Modest lapse rates (5.5-7.5 C/km) were shown with this sounding, implying modest parcel buoyancy to aid in the overall lift and upward vertical motions to induce convective activity. While this sounding depicts rain as the form of precipitation, there can be a changeover to snow depending on where you are in the region. The sounding on the right is from KOTX (Spokane, WA) also taken today at 12z depicts a similar sounding in terms of saturation and likely precipitation, however the temperature profile is close to the freezing temperature and there was in fact snow falling in the area. One reason for this is the effect of orographic lifting over the Cascade Mountains that forces air up from the surface into higher elevations where it condenses and in this case precipitates snow. The other thing that suggests snowfall is the wet-bulb temperature (thin blue line in between temp and dwpt profiles). Wet-Bulb temperature is the temperature a parcel of air will be at when it is cooled to saturation. When the wet-bulb temperature is at freezing or lower, this can imply snowfall even when the actual temperature is above freezing.

 

The image below shows radar imagery from this morning over Washington state, and you’ll notice that in addition to the onshore precipitation there seems to be an absence of precipitation to the east of the onshore flow but then all of sudden starts up again around and just east of Seattle. This area where the precipitation has started up again is known as the Puget Sound Convergence Zone (PSCZ).

 

The PSCZ is an area in which WNW winds converge and initiate lift and convection after diverging around the Olympic Mountains to the west. The abundance of moist WNW flow due to the low-pressure system talked about earlier have converged and were able to induce upward vertical motions, working in tandem with the upper level ageostrophic divergence from the upper trough, producing shows and thunderstorms near and east of Seattle this morning.

Forecasting rain totals in Washington state, particularly around Seattle, can be a challenge due to the topography of the state. On 19 November 2020, while the skies were clear across most of the country, the west side of Washington state instead experienced rain (Fig. 1). The Cascade mountain range, which runs through vertically through the middle of the state, confine most of the showers induced by the moist Pacific air to the western edge of the state. However, the exact location of these showers comes down to an even smaller orographic (aka mountain induced) effect.

Figure 1: Radar Reflectivity at 1351 UTC (5:51 am PST) on 19 November 2020. Source: UW-Madison Department of Atmospheric and Oceanic Sciences

 

The Olympic mountains, which are smaller than the cascade mountain range in both size and height, are located on the Olympic Peninsula directly west of the city of Seattle. The mountains have differing effects depending on the direction of the wind, resulting in either the Puget Sound convergence zone or the rain shadow effect. When the wind is flowing from the southwest towards the Olympic mountains, it is largely forced upward, cooling and condensing the moist pacific air and causing it to rain on that side of the mountain. As a result, the air that flows down the mountain contains less moisture than the air that was not forced up the mountain, causing a rain shadow (Fig. 2).

Figure 2: Puget Sound Convergence Zone and rain shadow caused by the Olympic Mountains in Washington state. Source: The Seattle Times

 

When the wind is flowing from the Northwest, it flows around the Olympic Mountains, and converges on the other side. This area of convergence forces the moist air upwards, resulting in a thin line of showers such as the one that can be seen over Puget Sound at 5:51 am PST (Fig. 1). Due to both of these effects, an accurate wind forecast is essential to estimate both the location and total amount of rain that falls. While the initial observations at 1400 UTC (6:00 am PST) on 19 November 2020 show winds with a westerly flow near the Washington coast (Fig. 3a), the wind shifts to become southwesterly six hours later (Fig. 3b).

Figure 3: Wind barbs and pressure contours at a.) 1400 UTC (9:00am PST) and b.) 2000 UTC (3:00pm PST), and the c.) 1-day observed precipitation for 19 November 2020. The red circle highlights the rain totals on the windward side the Olympic mountains, while the pink circle highlights the rain totals due to the convergence zone. Source: NOAA/NWS/SPC

 

As a result, the majority of the rainfall totals in the Seattle area for 19 November 2020 are due to the convergence zone present just north of Seattle earlier in the day and on the windward side of the Olympic mountains from the wind shift later in the day (Fig. 3c).

Today I want to discuss and explain the concept of thermally direct circulations. How do they work and how does the circulation impact the development of fronts? Let’s try and answer these questions.

A thermally direct circulation is a circulation that can exist along a frontal boundary. A typical directly thermal circulation is initiated by solar heating just ahead of a cold front. When air is heated, the air rises. Rising air is useful; after all, rising air is needed to generate lift and convection. The rising air creates a region of lower air pressure near the surface and a higher pressure at the top of the circulation. That air must go somewhere, so a second piece must exist in order to generate and complete the circulation. Often, that second piece is sinking air behind the cold front. Behind the front, there is often clouds and precipitation which cools and compresses the atmosphere. This compression creates a lower pressure aloft and a higher pressure at the surface from the sinking motion in the atmosphere.

So, we have rising motion ahead of the front, and sinking motion behind the front. In order for the circulation to become complete, there must be a complete motion. The rising air ahead of the front will want to move towards the region of lower pressure aloft behind the front.. And the sinking air will want to move towards the region of surface low pressure ahead of the front. (figure 1).

Thermally direct circulations can aid in the development of cold fronts. A key to the strengthening of a cold front is an increase in the temperature difference ahead and behind a front. This increasing of the temperature gradient is a key to frontogenesis. Additionally, the motion near the surface aids in the movement of the low level cold air mass into the warmer air mass ahead of the front.

One caveat, thermally direct circulations is only one way for fronts to develop or increase in strength. Other factors can either aid or inhibit frontal development. But thermally direct circulations can give us insight into one of the processes that are behind the development and strengthening of fronts.

Figure 1. An example of a thermally direct circulation. Here, the purple colors represent rising air and the grey colors represent sinking air. A. Expansion of the atmosphere, in this case, induced by heating of the air near the surface, generates a rising motion in the atmosphere. B. The rising air then moves towards the region of sinking air behind the front. C. Behind the front, the rain has cooled, and clouds have limited solar heating of the air. This cooling causes a contraction of the air, which is then filled by the air moving in via B. D. The sinking air reaches the ground and moves towards the region of lower pressure that has been created by the rising air from A. (source https://www.tropicaltidbits.com GFS Model Run).

 

References

Markowski, P and Richardson, Y., Mesoscale Meteorology in Midlatitudes. 2016, Wiley Blackwell.

Tropical Tidbits webpage. https://www.tropicaltidbits.com/

The seemingly endless 2020 Atlantic Hurricane season continues as the latest named storm, Hurricane Eta, has just become a Category 4 storm. While Eta continues to churn in the Caribbean Sea, the focus of this blog post will be on Hurricane Zeta, which made landfall in Louisiana last Wednesday, October 28th. As seen with many other tropical cyclones this year, Zeta rapidly intensified into a Category 2 hurricane with sustained winds of 110 mph just before it made landfall. After making landfall, Zeta moved northeastward to the mid-Atlantic region at speeds upwards of 30 mph (Figure 1). Due to the fast movement of this storm, the risk for inland flooding was lower compared to slower moving storms such as Hurricane Sally, but the threat for tropical storm force winds were prevalent all the way from Louisiana to Delaware. As Zeta raced inland, it entered a less favorable synoptic environment for tropical cyclone strengthening which caused the storm to weaken. In addition to this, Zeta merged with a mid-latitude system that was present over northern Texas. Due to this, Zeta began to exhibit more extratropical characteristics, such as frontal boundaries (Figure 2).

Figure 1: This loop displays a portion of the 5 day outlook for Hurricane Zeta as it moves across the United States. (Source is under “Figure 1”)

 

Figure 2: This image displays surface fronts and sea level pressure over the United States at 18z Thursday, Oct 29th. (Source is under “Figure 2”)

 

Shortly after landfall, Zeta was downgraded from a category 2 hurricane to a tropical storm. A quick glance at the synoptic level can explain why Zeta weakened as it moved over land.  The upper level wind shear produced by the jet stream over the southeastern United States contributed to an unfavorable environment for tropical cyclone development (Figure 3). In addition to this, Zeta moved from the warm, moist Gulf air to more cool, dry air over land. Tropical cyclones are warm core systems that need warm, moist air as fuel, but as dry air is entrained in the cyclone, it begins to fall apart. Due to these factors, Zeta lost its classical symmetrical tropical cyclone look as it became more asymmetrical due to frontal boundaries being associated with the system. (Figure 4) This is typical of cyclones as they transition from tropical to extratropical over mid-latitudes.

Figure 3: This image displays winds at the 250 mb level. The darker, more red and purple colors represent faster wind while lighter blue shading represents slow wind. The image is from 18z on Thursday, Oct 29th. (Source is under “Figure 3”)

 

Figure 4: The image on the left of figure 4 depicts a tropical cyclone with a classic symmetrical shape and a warm core. The image on the right depicts an extratropical cyclone with frontal boundaries and a cold core. (Source is under “Figure 4”)

The mesoscale environment becomes much more interesting regarding Zeta as it weakens and merges with the extratropical cyclone over Texas as it now begins to exhibit frontal characteristics. Frontal boundaries are associated with features such as large horizontal contrasts in temperature or moisture and maximums in cyclonic vorticity. The following figures in the panel below display each characteristic respectively. There is a stark contrast in temperature, as you move eastward from Louisiana to Georgia. The temperatures are colder west of Alabama and warmer east of the state, and this is indicative of a cold front. Looking at the mid-Atlantic to northeastern United States, the temperature is warmer from Kentucky/West Virginia southward and cooler to the north of these states, indicative of a warm front. (Figure 5). This frontal setup involving a warm sector is a classic characteristic of an extratropical cyclone. In addition, regions of maximum cyclonic vorticity at the 850 mb level are displayed along the cold and warm frontal boundaries. Regions of maximum cyclonic vorticity are also observed around the center of Tropical Storm Zeta due to its counterclockwise rotation (Figure 5). Now that frontal boundaries have been identified, it can be determined if frontogenesis is occurring along them.

Figure 5: In this panel of images, the chart on the left displays 850 mb level temperature at 18z Thursday Oct 29th. The blue shading is cooler temperatures whereas red is warmer. The image on the left is 850 mb cyclonic vorticity at 18z Oct 29th. The darker reds/oranges indicate where cyclonic vorticity is at a maximum. (Source is under “Figure 5”)

 

The frontogenesis equation involves 4 unique terms that can be analyzed to determine if frontogenesis or frontolysis is occurring at the frontal boundary with respect to that term. (Figure 6 Source: Dr. Zachary Handlos EAS 4813). If the term being analyzed has an overall positive sign, that means the front is strengthening, therefore the term is frontogenetic. If the term has an overall negative sign, that means the front is weakening, therefore the term is frontolytic. I analyzed 2 terms, shearing and diabatic, with respect to the warm front region and area of frontogenesis displayed by the purple contours (Figure 7). To analyze the shearing term, wind direction and temperature must be investigated. Since the winds go from a more westerly component to a more easterly component in the y prime direction, that portion of the term is negative, and since the temperature seems to increase as you move eastward, parallel to the front in the x prime direction, that portion of the term is positive (Figure 8). Overall, the sign of the shearing term would be negative, making it frontolytic. The diabatic term is more ambiguous when preforming sign analysis. Due to the fact there is cloud cover and precipitation on both sides of the front, indicating diabatic cooling, the sign of the term is negative, but due to the negative on the outside of the term, overall it becomes positive (Figure 9). Since the overall sign of the diabatic term is positive, it is frontogenetic. Due to the shearing term being negative and the diabatic term being positive, it is difficult to say whether the front is experiencing frontogenesis or frontolysis. Due to the strong cyclonic vorticity and temperature gradient associated with the front along with the positive diabatic term, the front can be said to be experiencing mild frontogenesis. Zeta’s characteristics changed drastically from tropical to extratropical as it moved from the Gulf as a hurricane and merged with an already existing mid-latitude cyclone.

Figure 6:: This equation displays the frontogenesis equation. Term A is the shearing term, Term B is the confluence term, Term C is the tilting term, and Term D is the diabatic term. (Source: Dr. Zachary Handlos EAS 4813

 

Figure 7: This map displays temperature advection (blue and red shading), temperature contours (blue and red contours), and frontogenesis contours (purple contours) at 12z Thursday Oct 29th. The area of strongest frontogenesis is near the warm front associated with the extratropical cyclone. Areas of frontogenesis are also seen around Zeta as it transitions to extratropical. (Source is under “Figure 7”)

 

Figure 8 & 9: Figure 8 (left) is a map displaying temperature contours and wind barbs at 12z Thursday Oct 29th. These contours and wind barbs were used in the sign analysis of the shearing term. Figure 9 (right) is a map displaying 6-hour averaged precipitation rate (green/yellow/red fill), mean sea level pressure (black contours), and geopotential height (red and blue contours). This map was used in the sign analysis of the diabatic term as areas where precipitation was occurring lead to cloud cover and cooling. (Source is under “Figure 8” and “Figure 9”)

 

Hurricane Zeta made landfall in Louisiana on Wednesday, October 28th with winds around 110mph and then downgraded to a Tropical Storm on Thursday, October 28th over Alabama. As Zeta continued its path northeast, the strong winds left significant damage in its wake as shown in Figure 1 below.

Figure 1: As Hurricane Zeta moved through New Orleans on Wednesday, October 28th, strong winds caused tornado-like damage blowing down a tree limb onto a powerline.

 

Early on Thursday, October 29th, Tropical Storm Zeta passed through Atlanta. Figure 2 below shows some of the damage that was found on Georgia Tech’s campus hours after the storm had passed through.

Figure 2: Photo taken of downed powerlines and limbs on Fowler Street NW on Georgia Tech’s Campus on Thursday, October 29th at 10:00AM.

 

Tropical Storm Zeta moved along its path relatively quickly due to the position of the jet stream and a midlatitude cyclone to the northwest. We can see in Figure 3 the predicted path that shows Zeta moving back over the ocean in the mid-Atlantic around Thursday evening.

Figure 3: The Hurricane Zeta advisory for Wednesday, October 28th shows the predicted path of Zeta.

 

Although Zeta will be moving back over the ocean, it will not be able to strengthen any because the ocean waters are not warm enough to fuel the storm. In Figure 4 below, we can see the sea surface temperatures where Zeta is going to go over are around 18 degrees Celsius which is much too cold.

Figure 4: Sea surface temperatures over the mid-Atlantic in degrees Celsius with the warmer temperatures represented by the warmer colors.

 

When we look at what else could result from Zeta, we can look for any areas of frontogenesis. Frontogenesis is the process of tightening horizontal temperature gradient to create and strengthen cold and warm fronts. By using sign analysis on the four terms of the frontogenesis equation, we can hypothesize whether or not it will occur. Positive terms contribute to frontogenesis, and negative terms contribute to frontolysis, the weakening of horizontal temperature gradients. The four terms are shearing, diabetic heating, tilting, and confluence. Sign analysis on the first two terms resulted in a negative shearing and a positive diabetic heating. For the tilting term, we can use Figures 5 and 6 below to arrive at an overall negative tilting term.

Figure 5: A vertical cross section showing pressure and cyclonic potential vorticity. As pressure decreases, the dark green potential temperature values increase.

 

Figure 6: A vertical cross section showing pressure and omega (upward vertical motion). Going from point B to point A, omega becomes more negative which indicates upward vertical motion. This combined with the analysis from Figure 5 results in a negative tilting term.

 

For the confluence term, we can analyze Figure 7 below resulting in a positive confluence term.

Figure 7: A 500mb Geopotential Height, Cyclonic Vorticity, and wind map with the adjusted x and y prime axes. The y prime axis points toward lower potential temperatures. In the positive y prime direction, the wind barbs change from strong winds going northeast to slightly weaker and more easterly winds. This signifies an overall positive confluence term.

 

Altogether, we have negative shearing, positive diabetic heating, negative tilting, and positive confluence with the positive terms slightly outweighing the negative ones. Thus, we could possibly have some frontogenesis resulting in a front.

Figure 8: This map shows fronts and weather types valid through Friday, October 30th. The red arrow points to the low-pressure system of Zeta.

 

In Figure 8 above, we can see the cold front that is predicted for Friday resulting from Zeta’s low-pressure system. Zeta is forecasted to continue degenerating over the mid-Atlantic.

Sources:

https://www.foxnews.com/us/south-damage-hurricane-zeta

www.tropicaltidbits.com

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