Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
538 FXUS63 KLSX 221911 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 211 PM CDT Sun Sep 22 2024 .KEY MESSAGES... - A few strong to severe thunderstorms are possible this afternoon across southern Missouri and Illinois, damaging winds are the greatest threat. - After another round of showers and thunderstorms Monday a relative dry period will settle in for the work week. && .SHORT TERM... (Through Late Monday Night) Issued at 211 PM CDT Sun Sep 22 2024 The heavy cloud cover that has been in place from this morning`s rain has started to break up across southern Missouri and Illinois, increasing temperatures and instability in this area. While cloud cover is breaking up further north into the LSX forecast area and as far north as the I-70 corridor in some areas, it remains uncertain how much destabilization will be possible in these areas this afternoon. Where destabilization occurs, roughly 1000-2000 J/kg of MLCAPE will develop and thunderstorms will be able to form from a combination of an approaching cold front and a small mid-level disturbance. With roughly 25-30 kts of 0-6km shear a few storms will be able to become strong to severe, with damaging winds the main concern. Even if cloud cover doesn`t clear significantly over our forecast area, storms could track from a more favorable environment in southwest Missouri eastward into the southern forecast area. The location of the SPC Marginal Risk outlines the most favorable area for strong to severe thunderstorm development. The risk for thunderstorms will diminish with sunset as instability wanes and the cold front exits the forecast area. After a brief dry period overnight, another round of rain will move northeast into the forecast area, driven by the low-level jet and interacting with the lingering elevated front. This shot of rain will push through the area during the early morning to mid-morning hours. Precipitable water values near the climatological max (1.50-1.75") will boost rain rates despite the meager instability tomorrow. A 20-50% chance for showers and a few thunderstorms lingers through the day and into the evening hours as a mid-level shortwave approaches the forecast area ahead of an advancing mid-level trough and interacts with the persistent low-level jet. With the cold front expected to be south of the forecast area, instability will be at a premium. SREF guidance indicates only a 10% chance of MUCAPE values greater than 500 J/kg across the far southern CWA Monday afternoon. If the cold front were to stall within the southern CWA, instability would be higher and likely be able to generate a strong thunderstorm, but confidence in this is low. Given this, we have chosen not to message the SPC Day 2 Marginal at this time. Across the 24 hour period more beneficial rain is expected, with global ensemble guidance showing a 90% chance of at least 0.5" of rain and a 50-60% chance of 1.0" of rain south of the I-44 corridor in Missouri. Lesser amounts are expected to the north and east. Cold air advection behind today`s cold front will cool 850 mb temperatures to around normal (12-16 degrees C), and combined with rain and heavy cloud cover, will result in another cool day tomorrow with some portions of the forecast area struggling to exceed 70 degrees. Delia && .LONG TERM... (Tuesday through Next Sunday) Issued at 211 PM CDT Sun Sep 22 2024 The previously mentioned mid-level trough will approach the mid- Mississippi Valley Tuesday, though moisture will be limited behind Monday`s precipitation and significant rainfall is not expected. Through the remainder of the week the mid-level trough will linger over the lower-Mississippi River Valley, while a mid-level ridge will ooze over the trough to the north, generating a Rex block that will persist into the weekend. With the trough positioned across the lower-Mississippi River Valley, the best moisture return, lift, and precipitation chances will remain largely south of the forecast area. Some ensemble systems are indicating that a tropical system (currently a tropical disturbance over Nicaragua) may move northward and interact with the mid-level trough. This would bring a surge of moisture and lift into the lower to mid-Mississippi Valley, increasing precipitation chances for the weekend. This scenario would be our next best chance of precipitation in the long term, and obviously is highly contingent on the development and motion of the tropical disturbance. We will be monitoring this over the coming week. Under the more stagnant Rex block, 850 mb temperatures remain near normal, so fluctuations in daily high temperatures will be largely tied to how much cloud cover is present. Currently high temperatures are projected to remain near to just below normal, but increased sunshine would result in a bump of up to 5 degrees. Delia && .AVIATION... (For the 18z TAFs through 18z Monday Afternoon) Issued at 1159 AM CDT Sun Sep 22 2024 A mixed IFR to VFR ceiling persists over the area with gradual improvements expected at the terminals to high MVFR or even VFR flight conditions. Ceilings are expected to drop back down to MVFR or even IFR conditions as early as the evening hours as daytime mixes diminishes. Confidence is highest that conditions will degrade to MVFR with a round of rain that is expected tomorrow morning. Guidance points to IFR conditions building in overnight, largely tied to the previously mentioned rain, however confidence is not yet high enough to include a mention in the TAFs for the moment. Speaking of rain, showers are lingering across the area this afternoon but should gradually clear ahead of shower and thunderstorm chances mainly south of I-70 and the mid-Missouri and St. Louis metro terminals. The next best chance (50-60%) for rain will push into the area tomorrow morning from the southwest, impacting the mid- Missouri and St. Louis metro terminals. Southwesterly winds will become northwesterly to northerly over the afternoon and into early evening as a cold front slides through the forecast area. Wind speeds will stay largely below 10 kts. Delia && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX