Month: April 2021

Fig. 1. WPC surface analysis maps valid at 21Z 7 April 2021 (left) and 06Z 8 April 2021 (right) showing frontal boundaries, high and low pressure centers (blue H’s, red L’s, respectively), sea level pressure (contours, hPa), and surface station plots over the eastern United States, and NEXRAD radar reflectivity mosaic from 15Z-23Z 7 April 2021 [1, 2].

 

Wednesday evening into Thursday morning, a double-low extratropical cyclone feature was centered over the central U.S. with cold frontal boundaries extending from southern Minnesota to eastern Texas. As indicated above in surface analysis maps, the double-low eventually merged into a single deeper low (coming in at 996 mb at 06Z 8 April 2021) and also forming with it a double cold frontal boundary extending to the south of the low pressure center now located in central Iowa. By 03Z 8 April 2021, the precipitation and convective activity associated with this extratropical cyclone and accompanying frontal boundaries encouraged the issuance of a handful of severe thunderstorm warnings across the storm chasing region of the central U.S. and a tornado warning in the vicinity of Shreveport, Louisiana. Radar imagery shows core and gap structures in the rainbands as well indicating horizontal wind shear present. Wind shear and forcing from frontal boundaries propagating through the region fired up some intense convective activity and precipitation observed in multiple cold frontal rainbands extending from the low pressure center to the Gulf Coast.

 

Fig. 2. “Natural” Color, Channel 13 Infrared, Day Cloud Phase RGB, Airmass RGB, and Upper Level Water Vapor GOES satellite imagery over the continental U.S. valid at approximately 2030 UTC 7 April 2021 [2, 3].

 

Satellite imagery shows a clear Comma-head structure to the north of tailing rainbands to the south with the center of the low circulating in the vicinity of the Iowa-Nebraska border. “Natural” Color imagery gives an indication of where frozen precipitation may be occurring in the “comma-head” region where colder air is usually located with extratropical cyclones as well as where intense convection is located (along the frontal boundary to the south) through cyan coloring of frozen cloud particles. In IR imagery, we observe intense cold brightness temperatures associated with the higher cloud tops and intense convection along the cold frontal boundaries. In the Day Cloud Phase RGB imagery, the yellowish colors indicated thick cloud structures with frozen particles at high altitudes associated with intense convection along the cold front front, while bluish colors indicated liquid droplets in clouds at low altitudes. The Airmass RGB imagery gives us an indication of where anomalously high potential vorticity (PV) is located via red-orange colors. Thus, it is an indication of where the jet stream is located (observed ridging over the rockies, slightly dipping south just upstream of the extratropical cyclone, and ridging again over the eastern U.S.) as well as potential intrusions of stratospheric air into the upper troposphere (observed just to the south-southeast of the low pressure center). Airmass RGB also gives an indication of the contrast in air masses at play with this extratropical cyclone via greens indicating warm moist air and blues indicating colder drier air. Upper Level Water Vapor imagery also shows the jet stream and a dry air intrusion being ingested into the extratropical cyclone at this time, indicating the likelihood of cyclogenesis (strengthening of the cyclone).

 

Fig. 3. Dual-pol WSR-88D imagery from Duluth, Minnesota from approximately 2230Z 7 April 2021 – 0030Z 8 April 2021. Top panel shows base reflectivity, the second panel shows differential reflectivity, the third panel shows base velocity, and the fourth panel shows correlation coefficient [2].

 

Radar imagery from within the comma head region on the northern edge of the extratropical cyclone shows some of your typical more stratiform precipitation for this area in base reflectivity above in Figure 3. The expected base velocity is also observed via radar data. Since Duluth, Minnesota is to the north of the center of cyclonic circulation, the flow field in the vicinity of the radar is expected and observed to be easterly and is shown above as green regions are motion towards the radar and red is motion away. There is also something else going on above called the “bright band” effect, which is where dual-pol radar indicates where the melting layer is. Base reflectivity seems to show a transition from liquid to frozen precipitation as well defined blobs of heavier precipitation give way to lesser intensity, smoother regions of indicated precipitation. Differential reflectivity subtly shows a band of noisy data in a similar region to where the precipitation type changes to the north and northeast of the radar site. Correlation coefficient on the other hand shows a much more distinct region of noisy data where it drops in the same band looking area. The band that is shown in the various radar data is indicative of the melting layer, which is where mixed precipitation is expected and the differential reflectivity and correlation coefficient readings become noisy due to the different types (and therefore shapes and sizes) of precipitation that are present at that level. When looking at surface temperatures and surface observations however, only liquid precipitation was observed and temperatures were above freezing. A quick look at a skew-t showed that temperatures were above freezing up to around 750 mb, giving any frozen precip at higher altitudes time to melt as it fell to the surface.

 

Fig. 4. Top left panel shows 250 mb wind speed (fill, kt), streamlines, and mean sea level pressure extrema (red L’s, blue H’s, mb). Top right panel shows 850 mb temperature (contours, °C), temperature advection (shaded, K/hr), frontogenesis (purple contours, K/100 km/3 hr), and wind barbs. Bottom left panel shows mixed layer CAPE (shaded, J/kg), 10m/850 mb/500 mb wind barbs (white, green, yellow barbs, kt), and mean sea level pressure (contours, hPa). The bottom right panel shows a vertical cross section of potential vorticity (shaded, 2 PVU dashed contour), wind out of page (purple contours, kt), and potential temperature (contours, K). All data show from 18Z 7 April 2021 over the continental U.S. [4].

 

Other analysis tools shown in Figure 4 give an idea of what could be expected from the extratropical cyclone overnight and the conditions contributing to the weather that occurred. The surface low is shown to be in between an upstream shortwave trough and downstream ridge in upper level flow, and is thus in a region of ageostrophic divergence which can contribute to upward vertical motions and strengthen the cyclone. The temperature advection and frontogenesis present in association with the cold front indicates that the front will likely strengthen and contribute as a forcing mechanism driving convection in the region. Also present in the region is some significant mixed layer CAPE and some decent vertical wind shear, which contributed to the intense convective activity present along the cold frontal boundaries throughout the Southeast. In an attempt to see if any upper level fronts were playing a role in the double cold frontal rainband feature observed in radar reflectivity, the cross section in Figure 4 was taken to see if higher PVU was intruding into the upper troposphere. The telltale dip in 2 PVU altitude near the jet stream is observed with the wind max being right where this slope is steepest, but there doesn’t seem to be a significant intrusion. Therefore, it seems that the double low that merged influenced the propagation of a double cold frontal boundary that provided forcing for the convective activity observed. This extratropical cyclone ended up producing 7 preliminary tornado reports, 59 preliminary wind reports, and 7 preliminary hail reports on 7 April 2021 across the central United States [5].

Sources:
[1]https://www.wpc.ncep.noaa.gov/html/sfc-zoom.php
[2]https://weather.cod.edu/satrad/
[3]https://www.tropicaltidbits.com/sat/satlooper.php?region=us&product=ir
[4]https://www.tropicaltidbits.com/analysis/models/?model=gfs&region=us&pkg=uv250&runtime=2021040718&fh=0
[5]https://www.spc.noaa.gov/climo/reports/210407_rpts.html

Figure 1: GOES-16 True Color RGB on 9 April 2021 from 1230 UTC to 2130 UTC, in 10 minute intervals. Source: University of Wisconsin-Madison Space Science and Engineering Center (SSEC).

 

For the first time since 1979, the La Soufriere volcano in the eastern Caribbean has erupted, sending large plumes of volcanic ash and dust eastward into the Atlantic Ocean. According to seismic research conducted by the University of the West Indies, this active stratovolcano in the island country of Saint Vincent and the Grenadines has only had five explosive eruptions since 1718. Although a series of smaller eruptions began in December 2020, the most recent explosive eruptions began on 9 April 2021, and continued through at least 13 April 2021.

 

Figure 2: GOES-16 Ash RGB on 10 April 2021 from 1045 UTC to 1415 UTC, in five minute intervals. Source: University of Wisconsin-Madison Space Science and Engineering Center (SSEC).

 

Through many recent advances in satellite detection, GOES-16 satellites are able to use multiple frequency bands through various forms of RGB (red, green, blue) imagery to detect key meteorological and geological features, including volcanoes. Since volcanoes are capable of producing ash and sulfur dioxide gas that can be hazardous to public health and aviation, the GOES-16 Ash RGB product can be useful for understanding where such plumes are headed. Figure 2 shows the Ash RGB loop on the morning of 10 April 2021, which features frequent explosive eruptions from La Soufriere. At higher altitudes, the brown color represents high, thick ice clouds, whereas the tans and dark yellows indicate ash mixed with sulfur dioxide. Likewise, pure ash is shown in red and magenta, and is often at lower altitudes than the volcanic plume as it settles onto the surface beneath.

 

Figure 3: Absorbing Aerosol Index (AAI) measured by GOME-2 onboard the MetOp (Meteorological Operational) satellites from 9-13 April 2021. Source: EUMETSAT Satellite Application Facility on Atmospheric Composition (AC SAF).

 

Through the frequent and dense volcanic emissions between 9-13 April 2021, high concentrations of ash and sulfur dioxide were present in several islands that neighbored Saint Vincent, to include Saint Lucia to its north, the Grenadines and Grenada to its south, and Barbados to its east. Figure 3 provides another vantage point of volcanic emissions by looking at the aerosol absorbing index (AAI) measured by the European-based EUMETSAT organization, which is a measure of the absorption of ultraviolet (UV) radiation by “active” particles in the UV spectral range. As consistent with the previous satellite products, the largest initial explosions occurred on 9-10 April 2021, and is indicated by greater values of AAI above, as volcanic clouds deflect solar radiation back into space or the upper levels of the atmosphere. Barbados, which is downstream of the plume on 10 April 2021, issued a severe volcanic ash warning and temporarily closed their international airport in response to the high concentrations of ash and sulfur dioxide that were transported eastward. In the sounding below (Fig. 4), westerly winds at higher altitudes confirmed the motion of the volcanic plume as it reached the upper troposphere, and continued to serve as the dominant motion of ash and sulfur dioxide through at least 13 April 2021.

 

Figure 4: Sounding from Grantley Adams International Airport (TBPB) in Barbados on 10 April 2021. Wind barbs (in knots) are shown in black on the right edge of the sounding.

At first glance, the weather across the United States appeared quiet on 14 April 2021. However, a blip over southern Texas and Louisiana caught my eye. A few convective storms seemingly appeared out of nowhere. Figure 1 shows visible satellite imagery of this event. The storms bubble up rapidly around 2200 UTC 14 April 2021, with the peak convective activity located just east of Houston, Texas. Storms suddenly taking shape like this is often a signature of the influences of diurnal heating. This lines up well with the timeline of these storms, as they took shape right around sunset. The intensity of the storms is evident by the overshooting cloud tops seen on this visible satellite imagery. The overshooting cloud tops are a result of rapid upward vertical motions that cause the clouds to extend into the lower stratosphere. A severe thunderstorm warning was issued by the National Weather Service in association with these storms, further verifying what is seen on visible satellite imagery.

 

Figure 1 Visible satellite imagery from GOES-East, valid beginning 2200 UTC 14 April 2021 (5pm local time). The cloud cover is shown in the white/light grey, depending upon the thickness of the cloud cover. Land can be seen through the clouds in dark grey.

 

Investigation of water vapor imagery can allow for analysis of the moisture available with this rapidly developing system. Figure 2 shows the GOES upper-level water vapor imagery beginning around 2200 UTC 14 April 2021 (5pm local time). Water vapor imagery can show locations of the jet stream and jet streaks, and we can deduce some synoptic-scale influences from this. The jet stream is located on water vapor imagery by locating regions of brightness gradient of the white coloration. In Figure 2, the jet stream is located south of Florida, with increasing intensity off the coast. The high level of moisture in the vicinity of the convective cells is evident by the green coloration over eastern Texas and southern Louisiana. This indicates that plenty of moisture was present to fuel the rainfall in association with these cells. The location of the jet stream is removed from the direct vicinity of the convective activity, but there might be some influences from upward vertical motions associated with the right jet entrance region as this jet streak moves toward the Gulf coast of Florida.

 

Figure 2 Upper-level water vapor imagery, valid beginning 2200 UTC 14 April (5 pm local time). Moist airmasses (with high water vapor concentration) appear in the shades of green. Dry airmasses (with low water vapor concentration) appear in shades of yellow. Land cover can be seen through the airmasses in blue.

 

The locations of the moist and dry air masses are verified by looking at Airmass RGB imagery, shown in Figure 3. On this satellite imagery, the warm, moist air mass can be seen by the green coloring. Dry air is shown in orange, and lower stratospheric air (indicating the location of the jet stream) is shown in dark orange. Cloud top temperatures are shown by the white regions. There is a warm, moist air regime over much of the Southeast. The developing convection is shown clearly by the intense white coloration of the cells popping up. There is a dry streak over the Gulf of Mexico, indicating a jet stream signature in the vicinity, moving towards Florida.

 

Figure 3 Airmass RGB imagery from GOES-16, valid beginning 2200 UTC 14 April 2021 (5pm local time). Warm, moist airmasses appear in the green coloration. Dry airmasses appear in the lighter orange coloration. Dry, highly stable air (advected from the stratosphere) appear in the dark orange/red coloration. Cold, dry (polar) airmasses appear in the dark blue/indigo coloration.

 

Finally, the information gathered from various satellite products can be verified by referring to radar imagery. Base reflectivity in Figure 4 shows the highest dBZ values located east of Houston. This corresponds to the areas of most intense precipitation. The regions with the highest dBZ values also raise concern for hail formation. By referring to other radar products (such as correlation coefficient imagery and differential reflectivity imagery, not shown here), hail does not appear to be forming with these storm cells. The cells show a clear core and gap structure as well, so there is concern for the formation of cold pools that may drive more local upward vertical motions.

 

Figure 4 Base reflectivity from KHGX (Galveston, Texas), valid beginning at 0000 UTC 15 April 2021 (7pm local time). The highest reflectivity values (in dBZ) are shown in the red/orange colors. These correspond to the regions experiencing the heaviest rainfall.

 

Though these storm cells rapidly appeared over the South, there was little impact from their formation. They ended up bringing rain to eastern Texas and southern Louisiana in a classic diurnal heating set up. This event brought a taste of what’s to come in this region over the next few months of late spring and summer!

The second full week of April 2021 was the first week in quite some time that no major severe weather outbreak was forecast for the southeastern United States. That said, though, interesting weather did occur in the US, including some significant precipitation over the Intermountain West region. This can be seen on a radar reflectivity image from the afternoon of Wednesday, April 14:

 

Figure 1: NEXRAD radar reflectivity over Utah and southwest Wyoming, 1945Z 14 April 2021 (source: College of DuPage)

 

Two distinct areas of precipitation can be seen affecting northern Utah: a stratiform region stretching from eastern Utah to southeast Idaho, and a convective line over western Utah. While radar reflectivity alone does not directly distinguish between rain and snow, snow tends to appear much smoother on maps of this type, while rain echoes are better defined with sharper edges. In addition, reflectivity values tend to be higher for rain than for snow. Based on these characteristics, some of the precipitation in the first stratiform region appears to be in the form of snow, especially in the northwest Utah/southern Idaho border areas. However, most locations only experienced rain, with snow limited to areas of higher terrain. This is likely due to the tendency for the radar beam to increase in elevation with distance from the radar. In addition, many radar sites in the western US are placed on top of hills or mountains in order to see over nearby terrain, enhancing this elevation effect. Precipitation is probably falling through the radar beam as snow, but melting before it reaches the surface. This is confirmed by a map of surface observations from 2200 UTC (1600 MDT) that afternoon, which shows temperatures well above freezing (40s-50s F) in the valley regions.

 

Figure 2: Surface observations, 22Z 14 April 2021. (source: UW-Madison AOS Department)

 

Significant snow did fall in the mountains, with some locations seeing well over a foot of accumulation. However, perhaps more interesting was the convective line that formed behind the earlier round of stratiform precipitation. At 2030 UTC, this line extended from the southern end of the Great Salt Lake southward through Salt Lake and Utah Counties as seen on the radar image below. This may not technically qualify as a squall line or mesoscale convective system (MCS) since the area covered was not large and the system was relatively weak. However, a shelf cloud and gusty winds over 40 mph were observed with much of the line, and a few stronger cells can be seen within it – especially the one over northwestern Utah County. That particular cell produced half-inch diameter hail in addition to frequent thunder and lightning.

 

Figure 3: NEXRAD base reflectivity from KMTX radar site, approximately 2030Z 14 April 2021. (source: College of DuPage)

 

Interestingly, surface temperatures were only in the mid 40s at the time. Although thunderstorms are unusual with temperatures this low, they are not unheard of if other atmospheric conditions are right. This can be seen from the 0000 UTC sounding from Salt Lake City later that evening: the surface temperature was only 44F, but mid-level temperatures were cold enough to result in some weak instability – around 250 J/kg of CAPE (Convective Available Potential Energy). While this is not a lot and is too low for any significant threat of severe weather, it was clearly enough to allow for the scattered thunderstorms observed Wednesday afternoon across northern Utah.

 

Figure 4: 00Z 15 April 2021 sounding from Salt Lake City. (Source: Storm Prediction Center)

The focus on April 8th, 2021 in the meteorological realm was an extratropical cyclone over the Midwest. There were four satellite images that were observed in class. These will be broken into two figures so the images will be larger and still comparable. The mid-level water vapor channel on GOES-16 shows dry air wrapping around and into the cyclone in a comma shape from North Dakota south and east all the way to Alabama and back north and west into Missouri at the center. The darker shades are dry air, and the whiter shades are moist air and higher cloud tops if they are very bright. Similarly, the airmass RGB has an orange rust color stand in for dry air where green air is moister. White is clouds, with brighter whites being higher cloud tops. The dry air intrusion at mid-level helped strengthen the cyclone by enhancing sinking of cold air and maintaining the updraft of warm air.

 

Fig. 1: (top panel) A mid-level water vapor satellite image where dark shades are dry air and light shades are moist air (source: https://whirlwind.aos.wisc.edu/~wxp/goes16/wv/goes16_namer.html) from 08 April 2021 at 1350 UTC, and (lower panel) is an airmass RGB satellite image from 08 April 2021 at 1400 UTC where rust is dry air, green is moist air, blue is colder air, and purple at the corners is limb effects (not accurate data). (source: https://weather.cod.edu/satrad/?parms=continental-conus-airmass-24-0-100-1&checked=map&colorbar=undefined)

 

The day cloud phase RGB at the top of Figure 2 is not useful at night (all the clouds turn red) but is useful for differentiating between types of clouds and snow during the day. The center of this cyclone is teal, meaning it has liquid water-based clouds. The northwest sector of the cyclone (the Dakotas and all of Minnesota) as well as the southeast sector of the cyclone (parts of Alabama, Georgia, and Florida) all have orange and yellow shades, which indicate higher cloud tops and convection. The bottom of Figure 2 shows the simple water vapor channel, where black represents dry air and light to white blues indicate intense convection and precipitation. Looking at Figure 2, as well as both the water vapor and airmass RGB channels, the heaviest precipitation looked to be over the panhandle of Florida. Figure 3 was radar reflectivity at the time, and it confirmed what the satellites displayed. Moisture being advected from the Gulf of Mexico likely helped the southern edge of the cyclone be more intense.

 

Fig 2: (top panel) A day cloud phase satellite image where teal clouds are liquid water drops, yellow and orange clouds are higher cloud tops, and red clouds are still under the terminator (source: https://weather.cod.edu/satrad/?parms=continental-conus-dcphase-24-0-100-1&checked=map&colorbar=undefined), and (lower panel) is a simple water vapor satellite image with black being dry air and bright blue whites being higher cloud tops with indication of convection. (source: https://weather.cod.edu/satrad/?parms=continental-conus-simplewv-24-0-100-1&checked=map&colorbar=undefined) Both are from 08 April 2021 at 1400 UTC.

 

Figure 3: A radar reflectivity image in dBZ from 08 April 2021 at 1400 UTC. The strongest precipitation is over the Florida panhandle as indicated on the side color bar. (source: https://weather.cod.edu/satrad/?parms=continental-conus-comp_radar-24-0-100-1&checked=map&colorbar=undefined)

 

Finally, looking ahead to 0000 UTC on 2021 April 09, there is a small risk for tornadoes as the rest of the southern part of the system makes its way across the Southeast. The sounding from Tallahassee (KTLH) on the left of Figure 4 shows an opportunity for a process known as forcing. Forcing is when an air parcel, if moved higher up into the atmosphere, remains cooler than its environment but keeps rising if something forces it upward into the atmosphere. The right part of Figure 4 shows convective available potential energy (CAPE) values high enough for tornadoes to form. This is coupled with directional and speed wind shear, which is when the wind changes direction and speed as it moves upward into the atmosphere, which could induce rotation. The Storm Prediction Center put out a 2% risk for tornadoes over parts of the Southeast today for this reason.

 

Figure 4: (left panel) A sounding taken on 08 April 2021 at 1200 UTC. The brown dotted line is the path an air parcel would take, the red line is the environmental temperature, and the green line is dew point temperature. Because the brown line is to the left of the red line, an air parcel would be cooler than the environment were it to rise, creating a situation where the parcel would have to be forced upward by a mechanism if it were to move because cool air tends to sink. (source: https://www.spc.noaa.gov/exper/soundings/21040812_OBS/). (right panel) A map showing convective available potential energy (CAPE) in J/kg (fill pattern) wind speed and direction at 10 m, 850 mb, and 500 mb, (white, green, and yellow, respectively) in kts.

On March 31 00Z, a broad area of low pressure can be seen centered over North Carolina and Virginia moving eastward (Figure 1). The low-pressure system, with cyclonic circulation, is producing an associated cold front that runs down the southeast, oriented southeast to northeast. Further northward, another low-pressure system can be observed just north of Maine producing an associated cold front that runs down the east coast appearing to almost connect with the front further south. The combination of these low-pressure systems and their associated fronts are producing precipitation and storms up and down the east coast. The pressure gradient does not appear to be very strong, indicating milder storms and weather.

Figure 1: Surface Analysis, April 1 00Z

 

The system in focus can be seen moving over the east coast of the US in the infrared image below, taken on March 31 23Z. The whitest shades of the image can be seen over Georgia and the Carolinas. These white shades indicate cold cloud temperatures, which usually correspond to higher clouds with associated convection and storms (Figure 2). On the radar image below, taken shortly after on April 1 01Z, highest reflectivity can be seen over Georgia and the Carolinas, corresponding to the whitest regions on the infrared image. These areas of high reflectivity indicate high levels of precipitation (Figure 3).

 

Figure 2: Infrared Satellite Image, March 31 23Z

 

Figure 3: Radar Image, April 1 01Z

 

In figure 4 below, showing 250 hPa windspeeds and 500-1000 hPa thickness, the low-pressure system centered over North Carolina appears to be in the right entrance region of the jet. This is an area of positive vorticity advection, which results in upward vertical motion and is favorable for the strengthening of extratropical cyclones. This would suggest our system should continue to strengthen as it moves off the east coast. Additionally, the lows can be observed to be centered in a region of an upstream trough and downstream ridge (Figure 5). This is an area with ageostrophic divergence, which results in upward vertical motion and is favorable for the intensification of the cyclonic system being discussed. The highest areas of divergence and upward vertical motion can be seen in figure 5 along the southeast coast.

 

Figure 4: 1000-500 hPa thickness, 250 hPa wind speed, April 1 00Z

 

Figure 5: 300-200 hPa PV, irro. Wind, 600-400 hPa ascent, April 1 00Z

 

This low-pressure system will continue to produce precipitation along the east coast throughout the day on April 1. The 12-hr. precipitation probability map shows moderate probability of rain along the east coast (Figure 6). This precipitation should not be associated with too much severe weather activity. Figure 7 shows that only low levels of CAPE can be observed over Florida and southeast Georgia. This indicates a lower probability for intense storms associated with this system.

 

Figure 6: 12 hr. probability of precipitation, April 1 00Z

 

Figure 7: 850 hPa heights, temp, CAPE, 1000-500 hPa shear, April 1 00Z

Wednesday March 31st brough another chance for severe weather across much of the Southeastern United States. While this week did not bring a 3rd consecutive SPC issued high risk area, the SPC did issue a slight risk spanning from southern Louisiana and Mississippi to North Carolina (Figure 1). The main reason for this convective activity was due to a cold front propagating eastward. This front was associated with a low-pressure system centered over the Mid-Atlantic. The convection associated with the front can easily be seen in the visible satellite imagery (Figure 2). White clouds associated with convective activity can be seen extending from Louisiana northward along the East coast, into Canada. The visible imagery does not clearly depict a low-pressure system with a classic counterclockwise flow, and the reason for this is due to a surface cyclone in Southeastern Canada with a cold front extending southward into the US. The interaction of convection with this system and the Mid-Atlantic system makes for a messier visible image.

Figure 1: SPC convective Day 1 outlook for the United States. This outlook was for 31 March 2021 13Z to 01 April 2021 13Z. Fill pattern displays outlook type. Image courtesy of the NOAA/NWS Storm Prediction Center.

 

Figure 2: This is a visible satellite image from GOES-16 at 18Z 31 March 2021. Image courtesy of University of Wisconsin-Madison.

 

The reason the Southeastern US had a slight risk for severe weather was due to a variety of synoptic and mesoscale factors. Water vapor satellite imagery can provide more insight about the position of the jet stream and associated jet streaks. On water vapor imagery, the jet stream can be identified by looking for contrasting bright and dark regions. Along this border is where the jet stream will be. Additionally, there are four indicators of jet streaks on water vapor imagery: 1) Strongest brightness temperature gradient, 2) Transverse banding, 3) Fastest region on animation, and 4) Intensifying ridging. Based on Figure 3A, transverse banding can be seen of the NE coast of the United States. Additionally, a strong brightness temperature gradient is in this region as well. As a result, a jet streak would be expected over the Great Lakes & NE United States into SE Canada. Looking at data from the SPC Mesoscale Analysis Archive from the same time as the water vapor image (1800 UTC), a jet steak is present in the exact region expected (Figure 3B). Much of the SE United States is in the region downstream of the trough, and this region is where mid-tropospheric upward vertical motions occur due to ageostrophic divergence aloft. Additionally, this region is in the right jet entrance region of the jet streak which is also associated with mid-tropospheric upward vertical motions (UVM). This UVM can be confirmed by the purple contours which indicate UVM on Figure 3B.

 

Figure 3A (left) is a water vapor satellite image from GOES 16 at 18Z 31 March 2021. Figure 3B (right) is a 300 mb height (contours, black, mb), wind speed (fill pattern), and divergence (contours, purple) from 31 March 2021 at 18Z. Images courtesy of the University of Wisconsin-Madison and the SPC Mesoscale Analysis Archive.

 

In addition, the mesoscale environment was favorable for convective activity as well. The former water vapor image shows ample moisture over the Southeast, indicated by the white and gray coloring. CIN was present across much of GA and SC around 14Z but dissipated around 18Z. CAPE values ranged anywhere from 250 – 1000 J/kg over the region of interest around 18Z (Figure 4A). In addition, 0-6 km shear values ranged anywhere from 40-60 knots over the region at 18Z (Figure 4B). Supercell composite parameter values were not that impressive at 18Z as the predominant value was .5 (Figure 4C). The moderate CAPE values along with the mild to moderate shear environment were the primary mesoscale factors that led to multicellular thunderstorm development rather than intense severe supercells. This type of convective activity can be confirmed with radar reflectivity data from March 31st. The data shows the most intense convection and precipitation ahead of the front, with lighter precipitation lagging behind (Figure 5).

 

Figure 4A (top left) is a plot of CIN (fill pattern, shaded at 25 and 100 J/kg) and CAPE (contours, red, J/kg) from 31 March 2021 at 18Z. Figure 4B (top right) is a plot of 0 -6 km wind shear (contours, blue, knots, every 10 knots) from 31 March 2021 at 18Z. Figure 4C (bottom) is a plot of Supercell Composite Parameter (contours, blue) from 31 March 2021 at 18Z.

 

Figure 5 is radar reflectivity data from 31 March 2021 at 21Z. The fill pattern (dBz) represents precipitation and its intensity.

 

Overall, the Southeast United States was situated in a mild area of instability in regard to the synoptic and mesoscale environments. The jet streak present over the Northeast was strong but positioned just a bit too far from the SE to maximize UVM in the region. Had the shear environment been more intense, a severe weather outbreak with supercells could have occurred, but the mild shear coupled with moderate mild to moderate CAPE was not enough.