(Photo Credit: XIT Museum in Dalhart, Texas)
While certainly a center of severe weather activity during the summer months, the southwestern United States is not commonly thought of as a geographic area prone to strong and impactful winter storms. This is undoubtedly true at the end of March when temperatures average well above freezing throughout a majority of the region. However, a three day event at the end of March in 1957 brought portions of southwestern Kansas and the Texas/Oklahoma panhandle one of the most devastating blizzards the country has ever seen.
After a relatively cold start to the month, historical data from 1957 in Dodge City, KS reveals an abnormally warm mid-March with a maximum high temperature of 79°F observed on March 10th. While temperatures remained in the 70’s for another week, they fell quickly at the end of the month and by March 23rd the high temperature barely reached the freezing mark, 32°F exactly. A surface analysis chart for the night before shows the presence of a low pressure system situated over western Colorado, initially bringing heavy rain to the panhandle region (pictured below).
(Photo Credit: NOAA Central Library)
As the system strengthened and moved southeast, an influx of cold air from the north simultaneously dropped temperatures and initiated a transition from rain to snow overnight into the day on March 23rd. By that evening, the system had deepened 10 mb. from its pressure just 24 hours earlier to a pressure of 986 mb. and quickly moved southeast into northern Texas (for more information on pressure systems and how they work click here!). For nearly two straight days the area was engulfed by heavy snow and high winds that resulted in near whiteout conditions.
In total, this event brought a multitude of locations upwards of 15 inches of accumulating snow in just a 48 hour period. Wind gusts near 60 mph created huge snow drifts, some of which measured up to 30 feet. One of the most dangerous winter storms to ever impact the area, this blizzard claimed 11 lives, a substantial amount of livestock and caused $6 million of damage (in 1957 dollars). The historic 1957 winter storm impacted all who lived in the Texas panhandle region at the time and as stories from those who survived are passed down through generations, it shall not soon be forgotten.
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©2020 Weather Forecaster Dennis Weaver
Let us spin the clock back a century… On March 28th, 1920, one of the largest tornado outbreaks in the modern era affected a large swath of states in the Deep South and Midwest in what became known as the Palm Sunday tornado outbreak of 1920. This outbreak saw the spawn of 37 confirmed total tornadoes of which 16 of them were rated F3 or higher (based on the traditional Fujita scale for tornado strength). All in all, 380 lives were lost during the event. Of course, this was long before any weather forecasting technology or public awareness for severe weather came about as the current-day National Weather Service (then known as the U.S. Weather Bureau) didn’t implement the watch/warning system until 1953. 1920s forecasting guidelines and techniques were still being crafted and given the accepted regulations at the time, were rather crude and vague which did not convey as much information as the public.
The surface analysis shared above depicts an environment conducive for such intense convective activity to occur. A mature lee cyclone developed east of the Rockies and traversed over the central Plains. A trailing cold front sagged southward over the southern Plains while a warm front moved poleward across the mid-south and well into the Midwest states. The result is a setup which provided ample moisture fetch across many of the southern and Midwestern states and, coupled with decently warm temperatures and favorable winds, set the stage for a potentially dangerous outbreak. Also, let’s recall that during the 1920s, the science of meteorology was nowhere near the levels of today so parameters such as storm helicity, jet stream dynamics, and instability were not known. Forecasters could not convey such information so they relied on basic weather observations. Severe thunderstorms were numerous across many states including Michigan, Indiana, and Illinois were multiple tornadoes touched down and caused considerable damage to farms and houses in rural areas to buildings and established business in the more concentrated suburbs and cities. Storm motion was rapid; most storms had forward motion exceeding 50 mph which is presumed to be due to strong steering flows aloft.
Since writing this piece, this event has risen to relevance in recent times as the latest severe weather outbreak that affected states such as Missouri, Iowa, and Illinois. March 28th, 2020 had similar features observed on the surface analyses and upper-air charts such that the environmental setup favored organized development of surface-based supercells. The Storm Prediction Center issued a 15% risk of significant tornadoes across portions of central Illinois even…
Since the devastating outbreak in 1920, weather forecasting, modeling, and verification has gone to great lengths to provide the useful information needed by the general public to prepare and be weather-aware for significant severe weather events. But whether it is 1920 or 2020, the mission of forecasters remains the same: Protecting life and property.
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© 2020 Meteorologist Brian Matilla
The phrase “Hurricane Hunters” refers to aircrews that fly into hurricanes to gather meteorological data. The U.S Air Force Reserve 53rd Renaissance Squadron and NOAA’s Hurricane Hunters undertake these missions. The idea of flying aircraft into hurricanes started off with a bet. Pilots were being instructed to fly away from an impending hurricane headed towards the Galveston,Texas area. After hearing this, British pilots challenged and disagreed with the policy. In response, Colonel Joseph Duckworth flew an AT-6 trainer into the 1943 “Surprise Hurricane”. This was an unapproved mission and Duckworth, accompanied by Navigator Colonel Ralph O’Hair, became the first two hurricane hunters. The nickname Hurricane Hunters was coined in 1946 and has become the calling card for the airmen ever since.
The goal of these missions is to provide meteorologists with data to aid them in making accurate forecasts. They also aid researchers studying hurricanes. The NOAA uses two Lockheed Martin WP-30D Orion aircraft to man these missions. They are named Kermit and Miss Piggy respectively. The planes go through the eyewall and various levels of the storm taking in data from each level until reaching the calm eye of the storm. This process is repeated over the 8 to 10 hour mission. As they fly through the storm, they drop GPS radiosondes which monitor pressure, humidity, temperature, and wind direction and speed. This gives valuable data on the structure of the hurricane and its intensity. The Doppler Radar attached at the plane’s tail and other radar systems give meteorologists a 3-D look into the storm. In research operations, they are equally as valuable. They have been used in storms approaching America and Europe in studies of El Nino, atmospheric aerosols, and large convective storms.
NOAA Gulfstream IV-SP (G-IV) “Gonzo” is another facet of Hurricane Hunters, and this plane primarily focuses on the upper and lower portions of the atmosphere. It has a cruising altitude of 45,000 ft. They use the same concept of dropping radiosondes to collect data. This gives data on the direction of the storm and what direction the atmosphere may steer it in. They also are deployed for winter storms and the study of atmospheric rivers. These two aircraft are manned at NOAA’s Aircraft Operations Center in Lakeland, FL. During the time period where hurricanes are not prevalent, planes are flown for other meteorological research purposes.
Photo Creds: NOAA Hurricane Hunters
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©2020 Weather Forecaster Dakari Anderson
A combination of the day-night band and high resolution infrared imagery from the Suomi NPP satellite shows the historic blizzard near peak intensity as it moves over the New York through Boston Metropolitan areas at 1:45 am EST on January 27, 2015. (NOAA/NASA)
In addition, to the birthdays of Wolfgang Amadeus Mozart, Lewis Carroll, and famous ballet dancer Mikhail Baryshnikov, January 27th is also known for some historic snow storms. The 1981-2010 climate normals show a generally zonal range of temperatures from 62 degrees F near southern New Mexico/ Baja California to 8 F near NW Minnesota on this day. At this point, the northeastern U.S. has already reached their peak coldest day of the season- with Syracuse, NY the last major city at January 25th. Precipitation is centered around northern and central California and northern Washington- a key season for their rainfall.
1776- George Washington reported three feet of snow near Mount Vernon, and Thomas Jefferson about 3 feet at Monticello during the “Washington and Jefferson Snowstorm”
1922- 28” of snow fell from Jan 27-29 across the Washington D.C. area, the highest snowfall since record keeping began in 1885. The roof of the Knickerbocker Theatre collapsed the next day during a comedy show due to significant snow accumulation killing 98 people and injuring 130. (NWS Aberdeen, SD)
1966- In the midst of a 5-day lake effect storm, Oswego NY was buried under 102 inches of snow (David Ludlum)
1967- 23 inches of snow fell in 29 hours near Chicago, causing the city to become paralyzed in the days following (NWS Chicago)
1987- 1.9” of snow fell in Florence, SC for a 2-day total snowfall of 3.9” (NWS Wilmington, NC)
1988- A push of cold arctic air caused Hollywood, FL to report a record low of 39 degrees (National Weather Summary)
1989- Two-thirds of Alaska fell below normal for January 1989, with Fairbanks at its 8th consecutive day below -40, and Tanana at a low temperature of -76 F and a high of -68 F for the day. Wind chills were near 100 below zero (The Weather Channel)
1990- A series of cold fronts brought over 60 inches of snow to the Cascade Mountains of Washington State between the 23rd and the 27th (Storm Data)
2005- Month-to-date snowfall at Boston’s International Airport totaled 43.1 inches and making it the snowiest January on record
2015- Winter Storm Juno reached peak intensity of 975 mb in the early morning hours of January 27, dumping up to 3 feet of snow over parts of New England, winds up to 60 mph, and major flooding along the Massachusetts coast (The Weather Channel)
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©2020 Meteorologist Sharon Sullivan
The city of Washington D.C. comes to a halt during the deadly 1922 Knickerbocker blizzard with snow drifts as high as 16 feet ; (Left) a man walks through deep snow drifts near the Smithsonian Institution.
If one were to bring up the year 1994 in the Southeast, most everyone will immediately recall the incredible ice storm that occurred that year. During a 4-day span in February 1994, parts of Tennessee, Louisiana, Arkansas, Alabama and Mississippi experienced the worst ice storm of the decade, if not the century. Mississippi was hit the worst, but the people in the surrounding states were also affected heavily and would remember this event for decades to come.
On February 9, 1994, an unusually deep trough with an unusually large cold front was perturbating through the Southeast with the deepest part of the trough extending as far down as southern Texas. Ahead of the front, temperatures were in the lower 70’s Fahrenheit across the delta and into Alabama; however, behind the front, temperatures had plummeted to the mid 30’s in Memphis and even to single digits further behind the frontal boundary. For Memphis, there was a recorded temperature drop of almost 40°F overnight as the front passed through. Moderate rain showers began to develop behind the front as moisture from the Gulf of Mexico rushed in, causing a changeover to wintry precipitation by evening as temperatures behind the front continued to drop.
By 6:00 AM CST on February 10th, the front had pushed as far south as the panhandle of Florida. Meanwhile, freezing rain and sleet continued to fall in the areas behind it with accumulations of up to one inch of pure ice by morning. By this point, the wind became southwesterly across much of the Southeast, causing moisture to accrue over the cold polar air mass. The front began to slow as it progressed through southern Alabama and the panhandle of Florida, which allowed a deep area of low-pressure out of the Gulf of Mexico to collide with the already cool, moist air mass behind the front. This caused even more precipitation to occur over the following day, February 11th, which added another 1-2 inches of freezing rain over Memphis and northwest Mississippi that had already had an inch of frozen precipitation from the days prior. Unfortunately, for many across the area, power lines and trees were heavily affected by the weight of the ice, causing the trees to fall and break, and the power lines to sag, and even worse, topple over. Many in the area were without power for days and cellphones had no service. Over the event, a total of 3-6 inches of ice had accumulated in Mississippi and Tennessee, with the higher amounts accrued in the Mississippi Delta. One power line worker reported ice on a power line that was measured to be one foot in diameter!
On February 12th, wind flow remained southwesterly, but temperatures began to warm up across the area to begin a rather slow thaw. An area of high pressure was approaching and passed through the area on February 13th, which led to clear conditions and an eventual complete melting of all the frozen precipitation.
It is estimated that a total of 741,000 people were without water during the event, and 4,700 miles of power lines were down causing 750,000 people without power across Mississippi, Louisiana, Tennessee, Alabama, and Arkansas. Most people had power restored within a week, but some were without power for an entire month. Overall, this event caused over $3 billion in damage across the Southeast.
This event is why most people who live in this region take winter weather so seriously. This system over-performed compared to what was forecasted to occur; however, the sophistication of forecasting and forecasting instruments have substantially increased since then so if this event were to happen again, it would be relayed ahead of time so residents can prepare appropriately.
©2020 Meteorologist Ashley Lennard
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On September 28th, 2019 the college town of Davis, California was taken by surprise when a tornado warning was issued by the Sacramento National Weather Service. Situated just outside of Sacramento, the area is not one typically known for tornadoes, especially not in the fall month of September. The California valley only averages about 10 tornadoes a year, the majority of those occurring during the spring, and more so towards the northern apex of the valley in Butte, Calaveras, and Amador counties thanks to the Buttes and Coastal Mountain range there that help drive orographic lift. Although tornadoes in California are not unheard of, they are certainly few, and even fewer in the fall! So what exactly occurred with this wild weather?
Around 4:30pm, 10 miles north of Davis, California in the city of Woodland, California a large storm cell began to take form and inundated the city with precipitation. The storm rapidly developed into a severe thunderstorm and left streets flooded with rain and hail. Cloud to ground lightning was reported as the storm continued to grow and swell throughout the late afternoon.
At approximately 5:30pm, the cell had drifted southwest towards the town of Davis, home of the University of California, Davis, passing over highway 113 as it lumbered on. A large hail shaft was evident in the storm, dropping one half to one inch sized hail as it crossed the highway bridge between the two cities, bringing traffic to a near stop as hail pummeled the thoroughfare. Banks of hail formed, taking on the appearance of snow drifts as the hail and graupel continued to fall. From this inundation, cars would later be found to be spun-out, having lost control on the icy roads.
Come 6pm, the storm had strengthened immensely, and reached just short of Davis. A large anvil was evident, stretching and creeping along the skyline. Thunder and lightning activity intensified with increased cloud-to-cloud strikes occurring and precipitation continued to dominate the center of the storm. Nearby temperatures dropped with the Sacramento International Airport reporting a record-breaking low of 45 degrees Fahrenheit.
At 6:37pm, the storm had gone tornado warned as radar-indicated rotation. Multiple gustnadoes were reported from 6:15 until approximately 6:40pm, all precursors to the tornado that touched down at 6:41pm. The touchdown occurred in North Davis along county road 29 in agricultural fields where the twister tossed tumbleweeds and brush into the air, but did no serious harm. With wind speeds up to 74 miles per hour, the tornado was assessed to be an EF-0 and dissipated at about 6:55pm.
The twister, although short-lived and weak, was none-the-less an exciting introduction to Davis for many of the incoming and returning students to the university who had just begun their first week of the fall quarter. Although not quite a rarity, a California tornado is certainly a surprise to all when they come about. The September 29th tornado took many by surprise that day, but is forever etched into the memories of those who witnessed and experienced the exciting atmosphere that day.
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© 2019 Weather Forecaster Alexis Clouser
Reflecting on Rainfall in Hurricane Hugo: How Storm Speed Affects Rainfall Rates (Credit: NWS Weather Prediction Center)
Thirty years ago on September 22, 1989, Hurricane Hugo made landfall in the United State on Sullivans Island, SC as a category 4 hurricane causing impacts through both North Carolina and South Carolina. Although this storm was a major hurricane on the Saffir-Simpson scale, the amount of rainfall it brought to these areas was significantly less compared to more recent storms that also impacted these areas such as Hurricane Florence in 2018, which made landfall in North Carolina as a category 1. What could have caused this categorically “weaker” storm to have a more significant rainfall-related flooding impact than a major hurricane?
On September 10, 1989, a tropical depression that was soon to become Hugo developed southeast of the Cape Verde Islands. By the next day, this storm had become Tropical Storm Hugo before rapidly intensifying and reaching hurricane strength by September 13. Hugo made its first landfall between Guadeloupe and Montserrat as a category 5 hurricane before making another landfall in St. Croix less than a day later. Hugo then made subsequent landfalls that same day in Puerto Rico. On September 22, Hugo made landfall once again at Sullivan’s Island, South Carolina as a category 4 hurricane with sustained maximum winds of 140mph and a minimum central pressure of 934millibars. The storm weakened rapidly as it moved inland, however, it was still a category 1 by the time it impacted Charlotte, North Carolina the same day as its South Carolina landfall. By September 23, Hugo had weakened to a remnant low and had then dissipated south of Greenland by September 25, 1989.
On August 31, 2018, Tropical Depression Six formed south of Santiago in Cape Verde traveling on a west-northwest trajectory. By September 1st, this storm officially became a named storm as Tropical Storm Florence before undergoing an unexpected rapid intensification September 4th and 5th. At this point, Florence was a category 4 with estimated maximum sustained winds of 130mph and a central pressure of 950millibars. By September 12th, Florence had rapidly weakened due to increasing wind shear, strengthened once again to a category 4, reaching its peak strength of 150mph maximum sustained winds and a central pressure of 937millibars, before dropping back below major hurricane status. On September 14, 2018, Florence made landfall south of Wilmington, North Carolina as a category 1 hurricane with maximum sustained wind speeds of 90 mph and a central pressure of 956millibars. Florence slowly made its way inland, weakening to a tropical depression by September 16th and later dissipating over Massachusetts by September 18th.
Although both of these storms impacted many areas, including areas outside of the United States, the rainfall-related impacts focused on in this article will be on North Carolina and South Carolina since these were areas greatly impacted by both Florence and Hugo. Hurricane Hugo brought anywhere from 1-10 inches of rain to North Carolina and South Carolina with a maximum rainfall measurement of 10.28 inches in Edisto Island, South Carolina, southwest of Sullivan’s Island where Hugo had made landfall. The majority of heavy rainfall was relatively localized in the southeastern coastal region of South Carolina with many of these areas seeing between 3-7 inches of rain. While Hugo was a major hurricane and was incredibly destructive, it produced significantly less rainfall than the categorically weaker Hurricane Florence. The maximum measured rainfall was 35.93 inches in Elizabethtown, North Carolina. Surrounding areas in central North Carolina to southeastern coastal areas of North Carolina and northeastern coastal areas of South Carolina saw between 10-35 inches of rain during Florence’s time in these areas. Although Florence was a “weak” category 1 hurricane in terms of wind speed and minimum central pressure, the flooding impacts brought on by this intense rainfall were particularly significant. So, how could a category 1 hurricane produce significantly more rainfall in an area and have similarly destructive impacts as a major category 4 storm? The primary key was the speed at which the storms were moving. Hurricane Hugo quickly made its way from its landfall in South Carolina through inland North Carolina within a day while Florence moved at a much slower pace. This slow speed allowed for Florence to continue to produce large amounts of rain in the same areas, leading to significant flooding.
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©2019 Meteorologist Stephanie Edwards
Photo: Lauren Long https://www.syracuse.com/news/2014/07/deadly_tornado_smithfield_new_york_weather_madison_county.html
July 8th, 2014 was a typical summer day for Central New York State; hot, humid, with a slight risk of thunderstorms in the air as a cold front was expected to cross over the region. But that typical summer day quickly turned into an event that would change the perception of severe weather events for many people and local meteorologists for years to come.
A series of severe storms were expected to occur across the Northeast that afternoon with threats for widespread damaging winds, a couple of tornadoes, and isolated cases of large hail possible. A public severe weather outlook had mentioned that severe thunderstorms were expected over parts of the north-central Appalachians that afternoon, and a slight risk issued by the NOAA SPC in previous outlooks was later upgraded to a moderate risk that included much of central and western NY, as well as the Southern Tier. Below is one of the SPC mesoscale discussions issued for that day:
All the ingredients for severe weather and tornado development were present, and several meteorologists were aware of but caught off guard by the turn of events that would later unfold.
Credit: NOAA SPC
Base reflectivity from 13Z 20140708 to 05Z 20140709
Nearly five years ago this July, as the line of storms made its way across the region, an EF-2 tornado with winds up to 135 mph touched down at around 7:02 PM EDT in Smithfield, Madison county NY. This tornado killed four residents of the community, destroyed several homes and property, and is known to this day as one of the deadliest tornadoes ever to occur in New York State.
Several severe thunderstorm warnings were issued throughout the area. Madison county was included in the warning as the storms swept through. Neighboring counties had tornado warnings issued for storms showing signs of radar indicated rotation. This tornadic event however was unusual in the fact that it “spun-up” from the ground up and was not formed in the usual way out of a strong, rotating thunderstorm called a supercell. While several forecasters were already occupied on a separate tornado warned storm approaching Onondaga county, the storm over in Madison county that was merely severe thunderstorm warned dropped the unexpected and deadly tornado over Smithfield where it disappeared before anyone could be warned about it.
This tornado was hard for meteorologists to see, because it had occurred along the Smithfield ridge sitting at 1400 feet high where the National Weather Service radar took approximately five minutes to make a complete scan of the area. With the radar beam at the Binghamton airport sitting at an angle of 0.5 degrees and scanning several thousands of feet into the air, this made anything occurring below the height of the ridge difficult to see if at all, and when the tornado finally showed up on radar it was already happening, and within minutes it was too late. This line of storms produced five tornadoes total in New York State all in one day; four in central NY and one in Warren county.
Below is an image from the SVRGIS page from the NOAA SPC United States severe report database:
This graphic represents the tornado paths recorded from 1950 to 2017 in the U.S.
What many people often forget is that the typical areas that get the most frequent cases of tornadic events are not the ONLY places that get can and do get tornadoes that can cause loss of property and life. When tornadoes happen in the Northeast their lifespan is often shorter, and blocked by trees and other topographic barriers making it more difficult to seek shelter from. However, it’s important to recognize this event not only as an anomaly but as a tragic event that should remind everyone to take every warning and watch seriously no matter where they are and what they’re doing. More awareness needs to be made that tornadoes can and DO happen outside of typical areas like tornado alley in the great plains. A quote by New York State governor Andrew Cuomo strikes the feeling of this reality all too well as he once stated, “We don’t get tornadoes in New York, right? Anyone will tell you that. Well, we do now.”
Forecasting for rare events like these remains a difficult issue for meteorologists to this day, and goes to show how the strength and speed of nature’s greatest forces can overwhelm our best efforts of prediction even in places outside of Tornado Alley.
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@2019 Weather Forecaster Christine Gregory
On this past 4th of July, the temperature in Alaska’s largest city, Anchorage, soared higher than at any other point on record as record-crushing heat sprawled across the “Last Frontier” state.
The observed high temperature at the Ted Stevens Anchorage International Airport climbed to a mind-boggling 90 degrees crushing its previous all-time record high of 85 degrees set 50 years prior on June 14th, 1969. This high temperature also crushed the daily record high for Anchorage on July 4th which was 77 degrees set back in 1999.
And just to put this into perspective, the average high in Anchorage on July 4th is 65 degrees. Anchorage was so warm, that it tied a few other places across the United States that also topped out at 90 degrees on the 4th. This includes West Palm Beach, FL, Memphis, TN, and Rockford, IL.
The source of this record-setting heat is due to the placement of the jet stream. Typically, the jet stream is located south of the state which keeps all the “scorching heat” away from Alaska. That wasn’t the case on this Independence Day. A rise in the jet stream has allowed an expansive dome of high pressure to form over Alaska. Underneath this high pressure system, sinking air has suppressed rain chances which has led to plenty of sunshine and record heat.
Anchorage wasn’t the only one posting record heat on America's birthday. Below is a graphic created by the National Weather Service in Anchorage, AK showing other places around southern Alaska that broke all-time records on Thursday:
King Salmon: Observed high: 89°. Previous record: 88° (June 27th, 1953).
Kenai: Observed high: 89°. Previous record: 87° (June 26th, 1953).
Gulkana: Observed high: 88°. Previous record: 86° (1956).
This dome of high pressure and heat wave is forecast to peak this weekend and last through the middle of the month. The Climate Prediction Center’s outlook (below) calls for the abnormal warm trend to stretch through the next 6-10 days. High temperatures for Anchorage are set to top out in 80s for the next several days before the it is set to subside late next week.
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©2019 Meteorologist Joey Marino
On May 20th, 2019, NOAA's Storm Prediction Center (SPC) released their severe weather outlook. This outlook called for a high risk of a severe weather outbreak. The last time that the SPC went high-risk was on May 18th, 2017. The SPC only chooses to forecast for a high-risk day when they are confident in the chances of numerous long tracked tornadoes or a long-lived thunderstorm system such as a derecho. The high-risk forecast occurs when they see the four main ingredients, shear, lift, instability, and moisture in excess and if they could cause these storms to last.
Looking back on forecasts for May 20th, they showed a low-pressure system that would be position in the upper part of the Great Plains. This low-pressure system had a central pressure less than 1000 millibars. Systems with pressure less than 1000 millibars can help to initiate storm development. Often with a low-pressure system, fronts start to form, where they are connected to the low-pressure system. Fronts can also provide a way for the storms start, after all, they are a boundary where two different types of air masses meet.
Looking at another ingredient for severe weather on this day, moisture, the lower portion of the Great Plains has a special type of boundary called a dryline. The dryline is a boundary that divides the warm, moist air from the warm, dry air. As a result, the dew points on either side can not only provide a measure of the moisture in the air, but the dryline can also provide another lifting mechanism for parcels of air to rise. The dew point values in the area highlighted by the Storm Prediction Center were into the upper 60’s to lower 70’s, showing that there was plenty of moisture for convection to occur. These high dew points also added more instability into the atmosphere as well. So in order for severe weather to happen, one more ingredient was needed: shear.
Analyzing the soundings from the NWS offices located in this area, the winds were moving from a slight southeast direction the surface to turning more towards the southwest as the weather balloon moved higher up into the atmosphere. Not only that, the speed of the wind increased as the parcel of air rose.
So all four ingredients were present, now why was May 20th not as devastating when looking at the tornadic potential? These storms, produced baseball-sized hail, numerous flooding incidents, and a couple of tornadoes, kept merging together. As a supercell started to show signs of becoming tornadic, it would merge with another cell. After this pattern occurs, instead of discrete supercells, there would be a quasi-linear convective system. These systems can produce rapid spin-up tornadoes and hail, but these types of severe weather are often limited due to the fact that they are in a line and connected to one another. The linear nature of the storms on May 20th caused heavy downpours, leading to multiple flash flood warnings throughout the high-risk area.
While any type of severe weather is damaging, the high-risk area did not see long-lasting tornadoes, instead, the high-risk area got hail and rain, leading to a major flooding event. These areas did experience high wind gusts along with a few brief spin-up tornadoes. These areas did not have to experience the results of a long track tornado as the SPC predicted.
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Sources: NOAA SPC, https://blog.nssl.noaa.gov/ewp/2019/05/21/nucaps-data-from-yesterdays-may-20th-event/
©2019 Weather Forecaster Shannon Sullivan