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
44
837 FXUS63 KLSX 121938 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 238 PM CDT Sun May 12 2024 .KEY MESSAGES... - Widespread showers and a few thunderstorms are expected tomorrow, with some breaks expected in the afternoon. A strong thunderstorm or two will be possible in the southern half of the Ozarks late in the afternoon and evening, with strong wind being the primary threat. - Lighter showers are likely to continue through Tuesday afternoon. - The next opportunity for rainfall will be Thursday and Friday. && .SHORT TERM... (Through Late Monday Night) Issued at 237 PM CDT Sun May 12 2024 Quiet weather continues this afternoon as surface high pressure remains in place, albeit temporarily. Meanwhile, temperatures have climbed into the low 80s as of 1 PM, thanks to a combination of mostly clear skies, warm southerly flow, and a passing upper level ridge. Later this afternoon and evening, patchy high level cloud cover will begin to spread into the area, the first hint of the next storm system approaching from the west. Otherwise though, weather for the remainder of the evening will remain uneventful. Changes are on the horizon, though, as a slow-moving low pressure system currently located across the Central Plains will move into the western Missouri. Ahead of this low, modest southerly low level flow will gradually advect Gulf moisture northward overnight tonight through early tomorrow morning, with PWAT values climbing to between 1.2 and 1.5 inches. This will set the stage for widespread precipitation between tomorrow morning and Tuesday afternoon. The first round of widespread rainfall is expected to move through the area between early morning and mid afternoon, as a subtle vorticity maximum pivots around the approaching trough. As this occurs, a wave of showers is expected to move from southwest to northeast, and almost all areas are expected to see at least some rainfall. Meanwhile, modest MUCAPE ranging from 100 to as much as 500 J/kg will support at least a few thunderstorms during this timeframe, although strong/severe storms are not anticipated with this initial round. Overall, rainfall rates are expected to be light to moderate outside of a few embedded convective cores, as forcing is not expected to be particularly strong. This initial wave of rain will also be progressive, and and training storms are not expected. As such, while a soaking rain can be expected for most areas, flooding is not a significant concern locally. By late afternoon, most of this rain is expected to exit the area to the northeast, with a relative lull in precipitation expected in many areas for a few hours. From here, attention turns to the possibility of redeveloping showers and thunderstorms during the mid to late afternoon, with a limited potential for a few stronger storms as well. As the first round of showers moves off to the northeast, varying levels of clearing may allow for some afternoon destabilization, particularly across central and southern Missouri. How much so will likely be limited by cloud cover and poor lapse rates, but between 1000 and 2000 J/kg of CAPE may be attainable. Meanwhile, mid level winds will increase across southern Missouri as the shortwave approaches from the west, with favorable jet dynamics underneath the left exit region of a modest 500 mb jet. With around 30-40kt of 0-6km shear to work with, it wouldn`t be out of the question to see a some more organized convection late Tuesday afternoon and evening capable of producing strong wind gusts across the southernmost parts of our area, or roughly the southern half of the Missouri Ozarks. This is highly conditional, but will be worth watching over the next 24 hours. Between Monday night and Tuesday afternoon, the low pressure system will continue its slow eastward march, and a cold front will sink into the area. Meanwhile, light showers along and behind the cold front will slowly spread into the area from the northwest, although there is some model disagreement regarding just how vigorous Tuesday`s shower activity will be. However, this activity will likely be less convective in nature save for perhaps parts of the southern Ozarks and southern Illinois, with lower rain rates and perhaps some drizzle mixed in later in the day as well. Once all is said and done late Tuesday, most areas will likely observe somewhere between .5 and 1.5 inches of rain, with forecast amounts dropping slightly from previous forecasts. Temperatures both days will also likely top out only in the low 70s, and perhaps not even out of the 60s in northern areas. BRC && .LONG TERM... (Tuesday through Next Sunday) Issued at 237 PM CDT Sun May 12 2024 A break in the rain is expected Wednesday as the cold front surges south and another transient upper ridge moves in overhead. However, this will not persist for long, as rain chances will increase yet again Thursday through Friday. Model guidance continues to suggest that a positively tilted upper trough will move through the area, providing just enough forcing to produce widespread but perhaps not particularly heavy precipitation. Moisture content will be sufficient to support this, but not anomalously high, and forcing from the upper trough and perhaps a diffuse surface cold front do not appear to be particularly noteworthy. As such, the potential for heavy rain appears to be low, but there is a consistent signal among a majority of ensemble members for at least some precipitation in most areas late in the work week. Meanwhile, seasonable, if not slightly below average temperatures are likely to persist through the end of the week. BRC && .AVIATION... (For the 18z TAFs through 18z Monday Afternoon) Issued at 1246 PM CDT Sun May 12 2024 VFR conditions are expected to prevail for the remainder of the day and well into the evening. Overnight, ceilings will gradually lower as precipitation arrives from the west. Ceilings will likely remain at VFR levels until early tomorrow morning, but will fall to at least MVFR, possibly IFR throughout the morning and through the end of the TAF period. Showers are also expected to impact all local terminals during this time frame, with at least some potential for a thunderstorm or two. Confidence in widespread precipitation is high, but low regarding the potential for thunderstorms. Visibility reductions to MVFR levels will be possible due to this precipitation, potentially lower within stronger convective cores. BRC && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX