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The morning of Thursday, February 25, 2021 was relatively quiet across much of the continental United States, with one of the few areas of precipitation over the Pacific Northwest. This occurred due to a low-pressure system moving into western North America from the northern Pacific. The WPC surface analysis map from 12Z that morning shows the cyclone center over the Alaska panhandle with cold, warm, and occluded fronts extending to the south:

Figure 1. WPC surface analysis, 12Z 25 February 2021.

 

One way to analyze storm systems like this is water vapor imagery, which measures brightness temperature at certain water vapor-specific wavelengths that are unaffected by other components of the atmosphere. Higher brightness temperatures (yellow-orange colors) indicate dry air, while lower brightness temperatures (blue-white-green colors) indicate moist air or clouds. On the map, the low-pressure system moving into North America is easily identified as the area of counterclockwise spin moving into the Alaska panhandle, with a large area of clouds to its east. The cold front can be seen as the sharp boundary extending south from the cyclone center, separating those clouds from much drier air just to the west.

Figure 2: GOES-17 Mid-level Water Vapor, 0330-1045Z 25 February 2021.

 

Another feature of note is the stream of lower brightness temperatures, indicating higher water vapor content, extending from the central Pacific near Hawaii northeastward across the ocean into the low-pressure system. This is similar to an atmospheric river, although it may not meet the specific criteria to qualify as one. Either way, it shows that the cyclone moving into North America had plenty of moisture to work with, resulting in significant precipitation over this region. This precipitation can be identified using radar reflectivity:

Figure 3: NEXRAD Reflectivity, 1103Z 25 February 2021.

 

Radar reflectivity is determined by the size and concentration of the raindrops or snowflakes. Although the product does not directly distinguish between rain and snow, the appearance of the reflectivity map can often be used to make a pretty good guess at precipitation type. Higher reflectivity values with sharper edges usually indicate rain, while snow has a much smoother appearance with lower reflectivity values. On the map above, most of the precipitation west of the Cascades in western Washington is probably rain, while snow is likely falling in eastern portions of the state. This can be verified with surface observations, which showed rain in most locations west of the mountains with temperatures well above freezing (high 30s-low 40s). In contrast, many sites in and east of the mountains reported snow with temperatures in the high 20s to low 30s.

Figure 4: Surface Station Plots (University of Wisconsin), 14Z 25 February 2021.

 

Ultimately, the storm dropped upwards of a foot of snow in many mountain areas, with several lower elevation areas also seeing at least a few inches. The storm continued to move eastward over the course of the next day, with winter storm warnings and advisories issued as far southeast as Wyoming and Utah.

An active day was in store for parts of the Southeast US on Thursday, February 18th. A digging trough brought very frigid air south that not only kept a lot of country that was still dealing with the aftermath of a devasting winter storm earlier that week iced over, but also helped to set up a strong temperature gradient as warm, moist air advected into Florida and Southern Georgia. Airmass RGB is a type of enhanced

satellite imagery that helps to identify different kinds of air masses based on temperature and moisture. Greens indicate warm, moist air while blues indicate cold, dry air. The U-shaped area of orange and red indicates upper tropospheric dry air which can help to identify troughs and the jet stream. Cleary based on this image there is a boundary being set up with warmer and more moist air in the southeast with colder and drier air behind it in the central and southern plains. A surface analysis at 12z helps to confirm the established air masses and where the frontal boundary is positioned. Florida

And parts of Southern Georgia are in the warm, moist air mass and are therefore in in the warm sector of the frontal boundary (bounded by the warm front to the north and the cold front to the west) with temperatures and dewpoints in the 70’s while further west you can see that these numbers plummet into the 20’s and teens.

Airmass RGB was able to give a hint at where the location of the trough might be, but in order to see this more clearly here is a picture of water vapor imagery over the same area. Water vapor imagery shows you the brightness temperatures of surfaces and cloud tops, but at a specific wavelength that targets water vapor. This

allows you to clearly see where moist air (brighter) and dry air (darker) is. This helps to further identify where a trough feature might be due to the dry upper-level air being brought down to the lower latitudes and there appears to be a similar U-shaped region as was seen with the Airmass RGB imagery. Clearly the southeast is under the influence of a warmer, moist airmass due to the very bright white colors shown over the area. A 300mb analysis map from the Storm Prediction Center at 12z confirms the location of the trough in the same general area as what was being shown on satellite.

The position of the trough will play a big role in developing convection in the southeast during the day. Strong upper-level wind associated with the trough are shown with the fill pattern with 80-100kt winds over Florida and Georgia.

Now that the warm/cold airmasses and the location of the trough have been confirmed, let’s see where any precipitation is occurring. This is IR imagery which

shows brightness temperatures of surfaces and cloud tops, like with water vapor, except that these measurements are based on outgoing longwave radiation emitted from these surfaces which means that the brighter the feature, the colder the surface and the darker the feature the warmer the surface. This is helpful with identifying convection because bright objects indicate high (cold) cloud tops which is a sign of developing convection. There appears to be a SW-NE line of convection stretching from the Gulf of Mexico up towards the Mid-Atlantic and Northeast which is likely associated with the cold front shown on the 12z surface analysis shown earlier. NEXRAD radar reflectivity

From ~13z that morning confirms the presence of convection across the southeast. A more cellular and well-defined line of showers and thunderstorms are ongoing across FL-GA-SC while the more blue-dominant and smoother reflectivity across TX-AR-LA indicates likely snow, especially with below freezing temperatures present over much of that region. A small area of potential ground clutter pops up over central Florida. This is an issue that comes up sometimes when a radar beam scatters and deflects off of the ground or other objects besides hydrometers, so there likely is little to no precipitation actually occurring over that area. Based on where the warm, moist airmass is with ongoing convection and the position of the trough feature, I wanted to take a look at a 12z sounding from a station in the warm sector. This 12z sounding is from Tallahassee, FL (TLH). From the surface up until ~700mb the red temperature profile and green

dewpoint profile are nearly touching which indicates a very moist environment which makes sense based on Airmass RGB and Water Vapor imagery. The wind profile to the right shows some strengthening and turning of the winds with height which hints at wind shear being a factor in helping to build and strengthen any thunderstorms that develop in the area. The positioning of the trough based on Airmass RGB, Water Vapor, and 300mb analysis show strong SW upper-level winds over the region which will help to turn the warm southerly surface winds with height and aid in building more convection later on in the day.

Figure 1: MODIS aboard the Aqua satellite captures a combined visible-infrared imagery (with corrected reflectance, resolution of 250m) of the Great Lakes region on February 20, 2021. Source: NASA Earth Observatory.

 

After an abnormally warm start to winter, a recent cold air outbreak affecting a large swath of the interior United States has contributed to a noticeable spike in ice coverage across most of the Great Lakes. The image above was acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite on February 20, 2021. This false-color image uses a combination of shortwave infrared, near-infrared, and visible bands that help distinguish ice coverage from snow, liquid water and clouds. Sea ice is denoted in the pale blue regions within the major bodies of water (dark blue to black), and is in its highest concentrations in western Lake Superior, Green Bay (northwest of Lake Michigan), and underneath the cloud cover over Lake Erie.

Figure 2: 1973-2020 average ice concentration compared to 2021 for the Great Lakes, updated February 20, 2021. Source: Great Lakes Surface Environmental Analysis (GLSEA).

 

According to measurements taken by the Great Lakes Environmental Research Laboratory (GLERL), only 2.81% of the Great Lakes were ice-covered to usher in 2021, and increased to 10.65% by February 1, 2021. In an average year, between 20 and 40 percent of these bodies of water would contain ice by February 1 before reaching its respective peak in ice coverage by late February. By February 20, 2021, total ice coverage across the Great Lakes jumped to 44.71%, which brought ice concentrations to near-average totals for this time of year. Compared to the 2019-2020 winter season, a similar lack of cold air throughout the season kept the maximum ice coverage under 20% the entire season, which was below the average ice concentration for the Great Lakes.

Figure 3: Loop of mean sea level pressure (MSLP) over the Great Lakes from 00Z 13 February 2021 to 00Z 19 February 2021 using a Rapid Refresh Reanalysis product. Higher values of MSLP (high pressure systems) are denoted in brighter colors, whereas lower values (low pressure systems) are denoted in cooler colors.

 

What’s helping to finally bring this cold air over the Great Lakes? Beginning on February 6, 2021, a series of high pressure systems would originate over central Canada and the northern Great Plains, with the most potent of these systems affecting all of the Great Lakes beginning on February 13, 2021. In a high pressure system, also known as an anticyclone, the air around the system flows in a clockwise pattern in the Northern Hemisphere. If an anticyclone is located to the west of a region, as was the case throughout the majority of the week, cold air will enter the region from the north, and will continue to be pumped into the region until the upper level dynamics either weakens the high pressure system or moves it out of the area. At its peak, the latest high pressure system over the northern Great Plains reached a mean sea level pressure of 1046 millibars on February 14, 2021, which corresponded to low temperatures ranging from -25 degrees to +10 degrees Fahrenheit along the shores of the Great Lakes, with lower wind chill values. As of February 20, 2021, this high pressure system will begin to move eastward, which will allow for warmer temperatures to enter the region through the early portions of next week, although not warm enough for ice formation to decline just yet.

The Weather Prediction Center is anticipating a messy 24-hours from 02Z 14 February through 02Z 15 February 2021. The map indicates a large swath of the Southeast receiving rain, mixed precipitation, freezing rain, and/or snow, as shown in Figure 1. The travel impacts will likely be vast, though this is dependent upon the type of precipitation that falls.

Figure 1: Weather Prediction Center national forecast chart, valid 02Z 14 February through 02Z 15 February 2021. 

 

Visible satellite imagery shows thick clouds blanketing much of the United States, as shown in Figure 2. Across the country, there is a general lack of convective activity, evidenced by the flat and uniform appearance of the cloud cover. There is some minor convective activity bubbling up from the Gulf of Mexico and making landfall over the Mississippi/Alabama coast. This may provide some moisture to the winter storm system. However, the main story with this system lies with the atmospheric profile, which determines what precipitation will fall as rain, freezing rain, sleet, or snow.

Figure 2: GOES-16 visible satellite imagery, valid 12:16Z through 16:26Z 14 February 2021.

 

This temperature difference can be deciphered in part by examining radar imagery, illustrated in Figure 3. Off the east coast, the precipitation is falling as rain. The scattering of the radar rays by raindrops makes this precipitation appear splotchy. This can be contrasted with the precipitation that’s falling as snow over the Great Plains, as this has a smoother appearance. The trouble with using radar to forecast an event such as this winter storm is that it’s very difficult to tell areas of freezing rain and sleet on radar imagery. This is where the atmospheric temperature profile needs to be considered.

Figure 3: Radar imagery, valid 17:11Z 14 February 2021.

 

At the intersection of the snow/sleet/freezing rain line sits Nashville, Tennessee. A forecast skew-T sounding for 04Z 15 February shows a very small warm nose preventing the precipitation from falling as snow. The frozen precipitation will fall through the atmosphere until it reaches approximately 840mb. At this point, it will melt, as the atmospheric temperature is above freezing. Around 870 mb, the precipitation will freeze again, as the surroundings are below freezing down to the surface. The size and altitude of the warm nose determines whether the precipitation falls as either freezing rain or sleet. As a result, the impacts of this winter storm are extremely difficult to forecast.

Figure 4: Forecast skew-T sounding for Nashville, Tennessee, valid 04Z 15 February 2021.

 

Washington and other areas of the Northwest started receiving snowfall on Thursday, February 11th in the evening and continued to receive snowfall into the subsequent weekend. The highest amounts the area was predicted by the local NWS branch was around one foot around in the Cascades of Pierce and Lewis Counties. Closer to the coast, the area received a mixture of snow, sleet, and freezing rain, as shown by the radar taken from accuweather at 17Z on Friday February 12th. As of Saturday, February 13th at 18Z, a majority of the state of Washington still had light to moderate snowfall. The majority of Washington and Oregon as of 18Z on Saturday February 13th in either a winter storm warning or a winter weather advisory.

Figure 1: Accuweather Radar screengrab taken at 17Z on Friday, February 12th. This radar shows snow in parts of Washington and Northern Oregon and a combination of snow and ice on the coast.

 

There was a low-pressure system off of the coast of the northwest that caused this snowfall upon its arrival to the states. As of 15Z on Friday February 12th the mean sea level pressure of center of the low was 1007 millibars, the ‘L’ seen directly off of the Oregon coast on the WPC surface level analysis map. As displayed on the water vapor image taken at 15Z on Friday February 12th. the frontal boundary caused by the low has very high and cold cloud tops, the frontal boundary was also followed by a region of dry air, typical of a cold front like this. There was another low that followed the first that was responsible for bringing the second round of snow to the Washington and Northern Oregon coast. At the center, this low has a mean sea level pressure of 990 millibars as of 15Z on Friday, February 12th, as shown by the same WPC surface level analysis.

Figure 2: GOES 17 water vapor image taken at 15:50:32Z. Shows frontal boundary in addition to an area of cyclonic motion associated with the second low off of the Canadian coast.

 

Figure 3: WPC Surface Analysis taken at 15Z on Friday, February 12th. This surface map shows both the 1007 millibar low off of the Oregon coast and the 990 millibar low off of the Canadian coast.

 

This second low the area saw was formed in exact accordance with the Norwegian Cyclone model with an occluded front sourcing from the center, a cold front branching off to the west of the occluded front and a warm front branching to the east of the occluded front. This is indicative of an extratropical cyclone being in the mature stage meaning the low is at its maximum strength and that the cyclone will begin to slow down in movement, which is what happened. The result of these lows was Washington and Oregon receiving snow that ranged from traces up to 18 inches according to the National Weather Service. The snow continued to fall and caused snow as far East as Wyoming and Colorado.

 

 

 

Figure 1: Visible imagery from GOES-16 on 1416 UTC 24 November 2019 (https://www.star.nesdis.noaa.gov/GOES/index.php)

 

A curious cloud formation over the southeastern United States appeared on satellite imagery on the morning of 24 November 2019. Visible imagery taken at 1416 UTC on 24 November 2019 showed a well developed extratropical cyclone over the northeast and further south a line of clouds over eastern Tennessee that suddenly began to spread to the southeast once in Georgia (Fig. 1). An infrared cloud-top imagery loop showed low level clouds that initially (As of 0636 UTC 23 November 2019) stretched in a band from the northeast to southwest, but over time the southern part of the band extended southeast while the northern portion remained stationary and did not propagate eastward at all (Fig. 2). The answer to the cause of this cloud development lies in the height of the clouds, shown on IR imagery to be low in altitude.

Figure 2: Cloud-top IR Imagery from GOES-16 from 0636 to 1421 UTC 24 November 2019 (https://www.star.nesdis.noaa.gov/GOES/index.php)

 

Extending from the northeast to southeast are the Appalachian Mountains, which start in northern Georgia and end in southern Maine (Fig. 3). These mountains are low in height relative to the Rocky Mountains, but in the case of these clouds, they were just high enough. As the band of low-level clouds propagated eastward, the Appalachian Mountains blocked the majority of that movement due to their height. Some of that moist air is simply trapped by the mountains. Any air that is orographically forced upward over the mountains, upon reaching the other side of the mountain, has lost the majority of its moisture on the windward side and is warming, decreasing the relative humidity even further. Thus, there were no clouds on the leeward side. Further south, past the southern tip of the Appalachian Mountains, the clouds can continue on their march southeastward unperturbed and unbothered by the terrain.

Figure 3: Topographic map of the United States (https://jan.ucc.nau.edu/~alew/maps/basemaps.html)

Pre-Thanksgiving travel got a whole lot harder this week in the western region of the US as what is known as a “bomb cyclone” had begun to impact the region on Tuesday, 26 November 2019. A bomb cyclone is caused by bombogenesis, which occurs when an extratropical cyclone rapidly intensifies as a result of the pressure in the center of the storm dropping at least 24 mb in 24 hours. In the case of this most recent cyclone, the storm dropped from 1002 mb to 975 mb in only 12 hours. A mid-latitude storm with a pressure of 975 mb indicates a very strong storm, in this case capable of producing hurricane force winds and winter storm warnings across the west coast. The rapid organization and development of the storm can be seen in Fig. 1.

Figure 1: Airmass RGB satellite imagery from 1730 UTC 26 November 2019 to 0130 UTC 27 November 2019. Source: NOAA

 

The rapid intensification of the storm is due to its location in relation to a strong jet. As can be seen in Fig. 2a, the area of low pressure shifted from the center of the jet to the northern portion of the jet exit region. Strong upper level divergence of air in this portion of the jet leads to rising air at the surface, which in turn lowers the pressure. Between 1200 UTC 26 November 2019 and 0000 UTC 27 November 2019, the pressure at the center of the storm dropped 27 mb as a result of this effect. Over this 12 hour period, the pressure, as seen in Fig. 2b, went from 1 standard deviation below the mean sea level pressure to 5 standard deviations below the mean sea level pressure. Coupled with the other mid-latitude cyclone currently over the central US, travel across the US for the Thanksgiving holiday is not going to be easy this year.

 

Figure 2a (left): Mean Sea Level Pressure (MSLP) in black, jet stream shaded in blue/pink. Source: http://www.atmos.albany.edu/student/abentley

Figure 2b (right): Mean Sea Level Pressure (MSLP) in black, standard MSLP anomaly shaded. Source: http://www.atmos.albany.edu/student/abentley

 

Sources:
http://www.atmos.albany.edu/student/abentley
https://www.wpc.ncep.noaa.gov/archives/web_pages/sfc/sfc_archive.php
https://www.washingtonpost.com/weather/2019/11/26/bomb-cyclone-could-break-records-it-slams-into-west-coast-bringing-mph-winds-blizzard-conditions/
https://www.weather.gov/

Flash flood warnings were issued by the National Weather Service across portions of Arizona, Utah, and California due to the rainfall associated with a low pressure system moving across the region. This can be seen in Figure 1. The system formed originally as a tropical invest region off the west coast of Mexico, then moved northeastward over the Gulf of California. Over the Gulf of California, the system was able to continue to grow as the warm, moist air above the water contributed to convection. This can be seen in Figure 2. This figure depicts the moisture transport associated with the system. As shown, the moisture is being advected from over the Gulf of California northward to the Mexico and Arizona border.

Figure 1. National Weather Service issued map of warnings and advisories. Issued 19 November 2019 at 2006 UTC. Depicts majority of Arizona under a flash flood warning. Over 13 million people were under a flash flood warning at the time this was taken.

 

Figure 2. SPC Mesoanalysis Page produced moisture transport plot. Taken 2000 UTC 19 November 2019. Depicts the transport of precipitable water (fill) northward. Moisture transport vectors are plotted. https://www.spc.noaa.gov/exper/mesoanalysis/new/viewsector.php?sector=12#

 

Figure 3. Visible Satellite Image taken 1956 UTC 19 November 2019. Depicts Southwestern United States, including the system over Arizona bringing rainfall to the region. http://www.aos.wisc.edu/weather/wx_obs/GOES17_sw.html

 

The system is expected to propagate northeastward, bringing rain to more of the southcentral United States, and even snowfall at higher elevations. The current cloud structure can be seen in Figure 3. The system is currently located over the border of Mexico and Arizona. The flash flood warning will remain in place through Wednesday evening as the system moves northward, bringing continuous rainfall to the rest of Arizona. In regions that are as dry as the southwest, it does not take a lot of rainfall to induce flooding. Though there is some potential for hail formation and strong winds, the main threat will be the flooding.

Amongst preparations of turkey, cranberry sauce, and stuffing for Thanksgiving, West Coast residents in California and Oregon are also preparing for a potentially record-breaking winter cyclone. A powerful low pressure system, which is rapidly intensifying, is expected to make landfall near the border of Oregon and California during the afternoon of 26 November. With anticipated wind gusts up to 100 mph, this low-pressure system is expected to cause wind speeds comparable to a low-end Category 1 hurricane. Additionally, this system is predicted to cause up to 4 feet of snow in the Sierra Nevada and wave heights up to 37 feet along the coast.

Currently, the system’s central pressure values are predicted to drop 40 mb in 24 hours, which effortlessly surpasses the “bomb cyclone” requirements of a 24 mb decrease in pressure over 24 hours. In Fig. 1, a GIF of water vapor satellite imagery shows the variations of brightness temperature to the east and west of the low pressure system. To gain further insight into this pattern, it is helpful to use the Air Mass RGB satellite product. As shown in Fig. 2, dry upper level air with a cold air mass to the south, as well as high, thick clouds, contribute to the spiraling system. Fig. 3 shows a GFS model run of MSLP and 10m wind speed knots, which depicts the counter-clockwise wind barb orientations, which reflects the low pressure environment. Furthermore, if the central pressure decrease of 40 mb proves to be true, this could cause the region’s lowest pressure readings on record. The previous record was set in 2010, with a central pressure reading of 978 mb.

Figure 1: To view a GIF of water vapor satellite imagery of the low pressure system just off the western coast of the United States, please go to https://twitter.com/NWSBayArea/status/1199414866475270144 )

 

Figure 2: This Air Mass RGB satellite imagery shows the interactions of the dry upper level air (red), cold air mass (blue), and high, thick clouds (white) which spiral counter-clockwise at the bomb cyclone off the coast of California and Oregon.

 

Figure 3: This map from Tropical Tidbits shows a GFS model run of MSLP and 10m wind speeds. Here, the central pressure of the low system is predicted to be 971 mb.

Figure 1: True-Color RGB Visible imagery from GOES-16 (East) from 1726 UTC to 1931 UTC 31 October 2019 (https://weather.cod.edu/satrad/)

 

On 31 October 2019, a cold front moved through the eastern United States in association with an extratropical cyclone located over the Great Lakes region. Early in the afternoon (1726 UTC), a squall line feature formed due to the passage of the front and aided by daytime heating, which is visible in Fig. 1. The most interesting features of the visible satellite imagery, however, are the clearly distinct air masses associated with the front. When cold fronts move into a region, they bring cold, dry air. This air is more dense than the moist warm air from the tropics ahead of the front, and as such the cold air moves below the warm, forcing the warm air up (this is what causes the precipitation, and the added boost of instability from daytime heating can lead to severe weather) (Fig. 2).

 

Figure 2: Conceptual model of a cold front (http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/cf_xsect.html)

 

Extratropical cyclones in the Northern Hemisphere lead to cold air moving from the northwest, and in states such as Georgia, prior warm, moist air is from the Gulf of Mexico. This leads to the difference in wind directions in front of and behind the front – behind experiences northwesterly wind, and ahead experiences south-southwesterly. This is visible on the true-color visible satellite imagery for this event, as the cumulonimbus clouds ahead of the cold front move north-northeast, and behind the front, lower level clouds within the cold air mass are moving southeast (Fig. 1).

 

Figure 3: Composite Radar Reflectivity from 1725 UTC to 1935 UTC 31 October 2019 (https://weather.cod.edu/satrad/)

 

Radar imagery from the same time period also shows the different types of precipitation associated with the frontal passage (Fig. 3). The squall line ahead of the front represents convective precipitation, notable on radar imagery as tight regions of intense precipitation. Closer to and behind the front, within the cold air mass, is weak stratiform precipitation from the low level clouds that remain.