NWS-Jackson MS Report - Severe Thunderstorms / Tornados - January 21-23 1999 [Central Atlantic Storm Investigators]

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Severe Thunderstorms / Tornados
January 21-23 1999

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The following detailed report is from CASI Member Tim Armstrong of NWS - Jackson/MS:

Storm Summary of the 1/21 - 1/22/99 Mississippi Severe Weather Outbreak

Tim Armstrong NWSFO JAN

The MRF and NOGAPS did an excellent job forecasting a potential severe weather outbreak across the south even five days out. By Monday, it was becoming clear that a major spring-like storm could form in the southern plains and move across the mid-south with possible severe weather.

Thursday January 21st actually started seeing severe weather in the Jackson CWA (County Warning Area) around 2:30 PM local time. I reported to work for my noon to 8 PM shift. The synoptic pattern at the time included a rapidly developing surface and upper level system to our west with a strong flow of Gulf moisture from the south. Temperatures were in the mid 70s to near 80 over much of central and southern Mississippi with dewpoints at or above 60 in many locations. In the upper levels, a strong west/west-southwest flow was setting up with 500 mb temperatures hovering around -15 C. Thunderstorms began forming in the early afternoon and several cells began displaying severe weather signatures in our northeastern Louisiana parishes around 2:30 PM. The first warnings were issued for Morehouse and West Carroll parishes in northeastern Louisiana.

As the evening progressed, storms ignited all throughout eastern Texas, Louisiana, Arkansas, and extreme western Mississippi adjacent to the river. Tornado watches were in place over most of Louisiana and Arkansas from the start. Sunflower & Bolivar counties, MS which border the Mississippi River, were placed under tornado warnings time and time again that evening as rotating cells continued to redevelop over that county. The Storm Prediction Center had requested special 18Z soundings to be taken from JAN and surrounding stations -- and the data helped immensely with their forecasts. CAPE values were 1500 to 2500 across the region with LIs from -4 to -7. Low level helicities were also increasing with values over 300 m^2/s^2 already.

The activity seemed to die out around 11:30 PM local time as surface temperatures continued to fall due to loss of heating from the sun. The main upper level energy had yet to round the base of the trough in Texas to keep things going. Things became "fairly" quiet over JAN's CWA during the late evening of Thursday and early morning Friday. I was able to go home around midnight and sleep for a few hours then...

...When I reported back to work at 7 AM Friday, tornado watches were already being posted for the area and storms were firing even at this early hour! Winds across the region, even outside of the thunderstorms, were gusting over 40 mph in places. The 7 AM temperature across most of southern and central Mississippi was in the low 70s with dewpoints well into the 60s. Almost as soon as I sat down and logged into the ASOS, warnings were being issued just west of Vicksburg along the Mississippi river again. The upper level wind energy was now crossing the region today and winds at jet stream level were well over 100 mph. The winds at the surface which had been from the south and southwest were now beginning to back to the southeast as a surface low intensified to our west. This only intensified the local helicities further. Total turning due to the wind as indicated by Doppler Radar shear profiles was nearly 130 degrees through the depth of the storms! Inflow at the base of the storms was now from 160 degrees and cirrus-level outflow was from 290 degrees producing a tremendous amount of turning motion in the updrafts.

As the upper level trough approached, mid level temperatures began slowly dropping which only increased surface-based instability. Sfc dewpoints actually made it into the upper 60s in some locations which is enough to easily produce thunderstorms even in a summer airmass. Lapse rates throughout the mid levels nearly became dry-adiabatic (8.4 degrees/km at LCH) meaning any moist convection would strengthen into an explosive updraft.

The character of the storms was also different Friday. On Thursday, the radar operators were only grudgingly issuing tornado warnings because the rotational signatures were rather weak and shallow. Although many of those warnings Thursday did verify with tornadoes actually spotted on the ground...today's storms were much different. Mesocyclones 30,000 feet deep and with gate-to-gate shears over 100 kts were being observed, three at a time, crossing the river into Mississippi. Well defined inflow notches and wear echo regions were also observed on radar. One senior forecaster said it was the most classic case of tornadoes on radar he'd seen in many years in Mississippi.

Our WDSS radar display (this is different than the standard PUP radar interface) ranks mesocyclones based on their intensity -- and showed one as a rank 13 meso. Tornado warnings have been issued with as little as a rank 5 just to put that in perspective! As the line crossed Jackson, a strong mesocyclone was observed to the south-southwest of the radar site at very close range. The radar operator remarked that you could almost see what appeared to be a ground-based debris cloud circling the circulation on the lowest scan elevation. Luckily, tornado warnings had already been out for the Jackson area for over 20 minutes.

I was working the radio both Thursday and Friday and was basically glued to my chair in front of the radio console. We operate five weather radio transmitters across Mississippi covering portions of four states. The Ackerman transmitter which covers 20 counties in north-central MS was playing 8 seperate tornado warnings at one point. The storms continued moving northeastward at 45 mph as the line drifted east at less than 20 mph. Nearly every county in our CWA was under at least one warning during the event.

By the time it had ended Friday night, the Jackson office had issued a total of 175 severe weather warnings: 91 tornado warnings, 73 severe thunderstorms warnings, and 11 flash flood warnings. I personally issued all but 18 of those warnings on weather radio. Storm surveys are planned for next week and it will probably take all week to visit just the most heavily damaged sites. This is likely going to go down in history as one of the most widespread early-season tornado outbreaks in history for this part of the country.

On a lighter note, the 00Z weather balloon release just happened to concur with the passage of the storms over the weather service office. The official log sheet for that release now reads "BALLOON SUCKED UP AND SPIT OUT BY +TSRA" The balloon made it up to about 600 mb, and was then coated with ice so thick it promptly fell back to the ground. Just shows what happens when a weather balloon becomes an artificial hailstone inside a thunderstorm.