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
208 FXUS63 KLSX 301712 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1212 PM CDT Tue Apr 30 2024 .KEY MESSAGES... - Widespread dense fog across southwestern IL, southeastern MO, and portions of the Ozarks will persist this morning, before dissipating an hour or two after sunrise. - A few strong to severe thunderstorms are possible across central, northeastern MO and west-central IL late this evening (10 pm to 1 am) with damaging winds and marginally severe hail the main hazards. - Opportunities for showers and thunderstorms will persist at times through the rest of the work week and through the upcoming weekend, but more of that time will be dry than wet. The greatest chance is late Thursday into Friday at 60 to 80 percent. && .SHORT TERM... (Through Late Wednesday Afternoon) Issued at 402 AM CDT Tue Apr 30 2024 Dense fog with visibilities of 1/4 mi or less has become increasingly widespread across southwestern IL, southeastern MO, and portions of the Ozarks as residual BL moisture continues to radiationally cool in the presence of a nearly stationary front with light/calm winds. A Dense Fog Advisory remains in effect for these areas through mid-morning. To the west, any dense fog is expected to more patchy/localized and confined to river valleys. Fog will dissipate within an hour or two of sunrise as diurnal BL mixing and heating commences. Plentiful insolation and low-level southerly WAA will support warmer, above average high temperatures today than Monday and in the mid-70s to mid-80s F along with dry conditions beneath slight rising heights within quasi-zonal upper-level flow. An upper-level shortwave trough ejecting over the Central Plains into Upper Midwest and associated cold front is progged to promote thunderstorm development across western IA, northwestern MO, and eastern KS this afternoon and evening. These thunderstorms will organize into one or more east-southeastward-moving broken lines, which CAMs indicate will move into central, northeastern MO and west- central IL during the 03 to 05z timeframe. Conditions will be more supportive of these thunderstorms being severe earlier and to the west and northwest of the CWA. Exactly how quickly thunderstorm intensity wanes as they enter the CWA is uncertain but thunderstorms will be shifting southeast of stronger deep-layer wind shear, instability/ moisture, and large-scale ascent. Therefore, the window appears brief for any strong to severe thunderstorms with hazards of damaging winds and marginally severe hail in central, northeastern MO and west-central IL. Subsequent thunderstorm dissipation could also be gradual overnight as they continue east-southward, in conjunction with a SPECIFICS FOR KSTL: slow weakening of a southwesterly LLJ ahead of the approaching cold front. CWA-wide dry conditions are anticipated to return by sunrise Wednesday. The cold front will stall across the CWA somewhere in the vicinity of the I-70 corridor as low-level WAA and moisture transport continues to the south and over the front. With daytime insolation, short-term model guidance advertise 1500 to 2000 J/kg of MLCAPE being generated by the afternoon along and south of the front. Mid-level heights will again be rising and frontal convergence is not anticipated to be particularly strong, but little convective inhibition may be sufficient for isolated to scattered showers and thunderstorms during the afternoon and evening. Deep- layer wind shear will be modest, decreasing confidence in any organized strong to severe thunderstorms at this point. Along and south of the front, high temperatures in the low to upper 80s F are forecast and, north of the front, high temperatures in the mid to upper 70s F are anticipated within a relatively cooler airmass. Pfahler && .LONG TERM... (Wednesday Night through Monday) Issued at 402 AM CDT Tue Apr 30 2024 An increase in showers and thunderstorms is forecast Wednesday night as a southerly LLJ develops and increases low-level WAA over and north of a northward retreat of the front as a warm front, especially across central and northeastern MO in closer proximity to the core of the LLJ and further away from the most vigorous mid- level height rises associated with upper-level ridging across the eastern CONUS. Global model guidance is in agreement that another upper-level shortwave trough will eject across the Central Plains into Upper Midwest on Thursday with another trailing cold front reaching the CWA. Along and ahead of this cold front, there is a signal in model guidance for one or more multicell clusters of thunderstorms to develop between late morning and afternoon and traverse the CWA during sometime during the Thursday evening into early Friday timeframe, with timing differences among model guidance. Stronger mid-level flow and deep- layer shear will remain to the west and northwest of the CWA, but the threat of isolated strong to severe thunderstorms will still need to be monitored depending on whether or not thunderstorms are in the CWA within or shortly after peak diurnal instability. Friday through the upcoming weekend, the large-scale wave pattern over the central CONUS will remain generally similar with ejecting upper-level shortwave troughs over the Central Plains into Upper Midwest, permitting additional opportunities for showers and thunderstorms at times as associated fronts also oscillate across the Mid-Mississippi River Valley. Following late Thursday/early Friday`s frontal passage, temperatures will cool to near or slightly above average, persisting through the weekend. Global model guidance diverge early next week with respect to whether a more noteworthy upper-level pattern change takes place or not, but a relatively active pattern of showers and thunderstorms and seasonably warm temperatures appears on track in either case. Pfahler && .AVIATION... (For the 18z TAFs through 18z Wednesday Afternoon) Issued at 1207 PM CDT Tue Apr 30 2024 All eyes are on a cold front diving south through the Plains toward the lower Missouri and middle Mississippi River Valley this afteroon and evening. Thunderstorms are expected to develop along the front and be quite strong or severe to the northwest of our area before weakening as they reach northeast Missouri and west central Illinois. Nevertheless, thunderstorms are likely now at KUIN and a TEMPO has been added for late this evening that includes a wind gust to northwest. Confidence in precipitation decreases as the night goes on and the farther south the front sinks into the area. Have continued vicinity wording at all TAF sites as the front moves through. Southerly wind ahead of the front will swing around to the northwest in it`s wake. CVKING && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX