Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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502 FXUS63 KLSX 231800 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 100 PM CDT Thu May 23 2024 .KEY MESSAGES... - Although there`s at least some chance for thunderstorms nearly every day, there are two primary time frames we`re watching for the best potential. The first is on Friday when a cold front moves through with the potential for damaging winds and possibly some hail. The uncertainty here revolves around whether there will be a round of storms in the morning or if the stronger activity waits until the afternoon. - After a dry Saturday, the best day of the weekend, the next trough brings the potential for two rounds of storms Saturday night and Sunday. There`s a potential for a greater severe weather threat on Sunday but there`s uncertainty on the timing of this trough and whether the strong instability will exist in our area or not. && .SHORT TERM... (Through Late Friday Afternoon) Issued at 246 AM CDT Thu May 23 2024 As surface high pressure is moving east this morning, low level moisture return has begun with some showers and thunderstorms along the leading edge of the better moisture. This activity will primarily be confined to the morning hours when the moist advection on the low level jet is most pronounced. As we get further into the day mid level shortwave ridging will promote subsidence and suppress convection despite increasing instability in the gradually moistening air mass. So after the morning showers it should actually be a pretty nice, dry day for most with temperatures near 80. The next shortwave trough is actually moving out into the Northern Plains today, sparking off a new round of thunderstorms over Nebraska this evening. As the trough moves northeast it will send a cold front eastward tonight into Friday. Similar to our last episode on Tuesday, the best forcing with the trough will be moving well to the north so the coverage of storms further south in our area is a little more in question. Instability will tend to be maximizing ahead of the front as moisture pools and temperatures aim for their warmest levels of this air mass, but convergence along the front will be weakening and lift from the shortwave trough stays well to the north. In addition, deep layer shear is only weak to moderate so it will be more difficult to get supercell structures from thunderstorms, with a tendency more toward multicell clusters with the primary severe weather hazard being damaging winds. While that is the general synoptic setup, there are more complications involved. The thunderstorms that get going over Nebraska tonight are forecast by most of the CAMs to form into a broader convective complex which pushes southeast well ahead of the front and moves clear across our area during the morning to early afternoon. If this is how it plays out, then our primary severe weather threat would be with this line, with the intensity perhaps decreasing as it moves in but reinvigorating as daytime heating adds instability ahead of it. This would also mean that during the afternoon most of the area will be behind the MCS affected by the cold pool which limits the potential for redevelopment as the actual front moves in later in the day. It`s worth noting that while many CAMs show this scenario, most of the global scale models don`t play out the morning MCS concept, instead focusing the threat along the cold front in the afternoon. While the CAMs have a general bias toward over-convecting and this could be causing the projected MCS to blow through too early, the global scale models have a tendency to underplay these mesoscale events and focus more on the synoptic scale forcing of the cold front. It`s hard to have much confidence in which way it will play out. The message we have the most confidence in is that there is a threat for strong to severe thunderstorms with primarily a threat for damaging winds on Friday, but the timing could be anywhere from mid morning to late afternoon. This also has an effect on the temperature forecast as a morning MCS would prevent strong heating. Kimble && .LONG TERM... (Friday Night through Wednesday) Issued at 246 AM CDT Thu May 23 2024 The cold front Friday night is strong enough to clear the area and push the moist, unstable air mass out of the way giving way to the driest day of the forecast on Saturday. It won`t last long, though, as surface high pressure already begins to push east of the forecast area Saturday afternoon and moisture begins surging back north in the plains ahead of the next trough. This next trough will be further south than the last several, placing us well within the better synoptic forcing. We`re expecting the development of a surface low with a warm front lifting north and a cold front swooping southeast in quick succession. The most widespread rainfall will be with the warm front Saturday night into Sunday morning. In this scenario the severe weather hazard would primarily be large hail with elevated thunderstorms if enough instability can develop. Whether another round develops in the warm sector ahead of the cold front on Sunday is much more in question. It`s this round that would have the greater overall severe weather threat if all the variables line up. The timing of the wave is the biggest factor here, as if the low moves by in the morning and swings the cold front through early, we may not develop enough surface based instability ahead of the front in our area. While there is still timing and track uncertainty on this wave, the trend continues to be for a rather quick passage of this trough which would have a tendency to place the better severe weather threat to the southeast of our area. Sunday`s trough initiates a pattern change across the continent. As it moves east it will deepen and establish a broader trough over the eastern US with a ridging building over the west. The resulting northwest flow over our region will promote drying and the potential for cooler temperatures as well. While dry and cool is the rule, it`s not absolute. Shortwave troughs dropping through the flow could bring brief periods of showers, but we won`t have the Gulf of Mexico moisture to work with to create a greater severe weather threat. Whether temperatures will be a few degrees below normal or a few degrees above will depend on whether we`re closer to the broader trough or the broader ridge. The general trend will be for initially cooler temperatures followed by a gradual warming. The greatest uncertainty is on Monday as most models indicate a secondary trough following Sunday`s wave, but there`s considerable variability on how strong it will be. The operational GFS actually portrays this as the stronger of the two troughs and is the most aggressive with continuing rain chances into Monday. If Sunday`s trough is significantly more dominant, then subsidence in its wake would promote a dry Memorial Day. Kimble && .AVIATION... (For the 18z TAFs through 18z Friday Afternoon) Issued at 1250 PM CDT Thu May 23 2024 Lingering high MVFR and low VFR ceilings will affect terminals over central Missouri and the St. Louis area terminals through this afternoon. Then mainly dry and VFR conditions are expected through tonight before a cold moves into the area on Friday. There will be an increasing chance for thunderstorms at UIN/COU/JEF by late morning at STL after 18Z. Winds will remain out of the southeast to south at 10 knots or less. Britt && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX