


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
45
46
47
356 FXUS63 KLSX 140310 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1010 PM CDT Sun Jul 13 2025 .KEY MESSAGES... - The threat for locally heavy rainfall and flooding continues into early this evening. A few strong storms could produce isolated damaging winds as well. - Typical Summer weather continues this week. Temperatures will remain within a few degrees of seasonal normals with an almost daily chance for afternoon and early evening thunderstorms. && .SHORT TERM... (Through Late Monday Afternoon) Issued at 152 PM CDT Sun Jul 13 2025 A long wave mid/upper level trough extends from the Upper Midwest southwest into the southern Plains this afternoon . This trough, along with a quasi-stationary front extending from near Moberly eastward into central Illinois is producing a broad area of showers and and thunderstorms this afternoon. The threat for locally heavy rain continues for the rest of the afternoon into the early evening as P-wat values are in excess of 2 inches and the movement of some of these storms is relatively slow. There`s also an isolated severe threat with the storms for the rest of the afternoon into early evening. RAP is forecasting MLCAPE between 1500-2000 J/Kg across southwest and south central Illinois through sunset with 0-6km shear between 20-30kts. This is enough for marginally severe multi-cell clusters with an isolated damaging wind threat. Soundings are not particularly supportive of large hail showing deep moisture and only 200-500 J/Kg of hail CAPE, However, if a storm can start rotating, the enhancement to the updraft could produce some isolated 1 inch hail as well. Thunderstorms should diminish with loss of daytime heating this evening. Attention turns to fog potential tonight. There should be plenty of ground and low level moisture available after this afternoon and evening`s rain. Much of the high cloud cover is expected to clear out tonight, and light winds should promote fog development. There may be more low stratus than fog if the sky doesn`t clear as much as expected, and this is more likely across parts of south central and southwest Illinois. Elsewhere, areas of fog are likely, with patchy dense fog possible. Not sure how widespread the truly dense fog (visibility 1/4 mile or less) will be, but a Dense Fog Advisory may be needed before sunrise on Monday morning for parts of the area. Monday`s weather looks relatively quiet compared to today`s with a lingering chance for afternoon thunderstorms, mainly across the eastern Ozarks and far southwest Illinois. Afternoon MLCAPE values once rise once again into the 1500-2000 J/Kg range, but here is far less deep-layer shear at only around 5kts, so severe storms are not expected at this time. Carney && .LONG TERM... (Monday Night through Next Sunday) Issued at 152 PM CDT Sun Jul 13 2025 The flow aloft will become quasi-zonal over the Mid Mississippi Valley for much of the work- week. Global deterministic guidance shows a few vorticity maxima moving through the region in the zonal flow during this period. The most prominent of these is the remnants of the trough which is now extending southwest from the Midwest into the southeast Plains. This vortmax is forecast to lift northeast from Oklahoma/western Arkansas through Missouri and Illinois on Tuesday. The resulting mid-level dynamical lift will probably give us the best chance for widespread rain across the forecast area for Tuesday through next weekend. Mean to 75th percentile P-wat remain relatively high in the 1.8-2.0 inch range, so locally heavy rainfall will continue to be a possibility as this vortmax moves northeast across the area. Mean 500mb heights bottom out Tuesday night and then begin rising Wednesday as the upper ridge over the Southeast U.S. spreads westward into the Lower and Mid Mississippi Valley. As mentioned above, there are subtle indications of weak short waves moving across the Mid Mississippi Valley during the week in the deterministic GFS and ECMWF, though these are little more than brief shifts in the flow from slightly west-southwest to slightly west- northwest. An actual synoptic scale short wave trough moves across the Upper Midwest/southern Canada late in the week which pushes a weak cold front into southern Iowa or northern Missouri by Thursday night/Friday according to the deterministic guidance. If the front actually makes it that far south it will be a focusing mechanism for convection on Friday, most likely across northern Missouri into west central Illinois. It`s questionable though how far south the front will actually move due to the ridge building from the Southeast U.S. The deterministic GFS and ECMWF hint at another short wave moving across the Mid Mississippi Valley on Saturday or Sunday, but there are timing and strength differences between the two. The wave barely registers on global ensembles which only show a general shift from weak zonal flow to weak west-northwest flow so the precipitation forecast for the weekend is fairly low confidence. Temperatures through the period should continue to be within a few degrees of normal, with highs largely controlled by afternoon convection. Carney && .AVIATION... (For the 06z TAFs through 06z Monday Night) Issued at 1004 PM CDT Sun Jul 13 2025 The primary concern during the 06Z TAF period will be the potential for dense fog and low ceilings during the first 6-10 hours of the period. While a few weak showers continue to linger across the area, most of this activity has drifted south of local terminals and skies are gradually clearing. As this continues, light winds and residual humidity will likely lead to a combination of low clouds and fog, with LIFR ceilings and at least patchy dense fog likely. The highest probability of 1/2 mile visibilities or less exist at COU/JEF/UIN where clouds are clearing more quickly, but some of this is possible at St. Louis area terminals as well. However, metro area terminals may be more likely to see LIFR ceilings rather than persistent dense fog. Visibilities are expected to improve by mid morning, and ceilings will gradually lift. While there will be improvement in ceiling heights by midday, ceilings may be slow to reach VFR as stratus gradually transitions to scattered cumulus during the afternoon. Otherwise, a few showers will be possible mainly at St. Louis area terminals in the afternoon, but probabilities remain too low for TAF inclusion at this time. BRC && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX