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
285 FXUS63 KLSX 071948 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 248 PM CDT Tue May 7 2024 .KEY MESSAGES... - Isolated to widely scattered thunderstorms will dot parts of east-central and southeast Missouri and southwest Illinois into the early evening. A few of these storms could be strong to severe with large hail the dominant threat. - Scattered to numerous severe thunderstorms are expected on Wednesday, primarily from late morning through the afternoon into early evening. The overall set-up is quite volatile supportive of large hail, damaging winds, and a few strong tornadoes. && .SHORT TERM... (Through Late Wednesday Night) Issued at 236 PM CDT Tue May 7 2024 Surface features are rather nebulous this afternoon with largely veered southwesterly surface winds across the area and very weak convergence within the flow across southwest IL into southeast MO. The air mass in this area is currently quite unstable with SBCAPE of 3000+ J/KG and deep layer shear on the order of 70 kts. If we can get some sustained deep storms, which is more challenging with the weak convergence/forcing, then supercells would be the favored mode. Given the veered low-level flow, the hodographs are rather straight favoring large hail as the main threat. The convergence zone is expected to shift east of the area by early evening with an ill-defined cold front settling southeastward across the area as well and the cumulative boundary becoming stationary across southern MO and southern IL later tonight. After the current storms, things look rather tranquil tonight with potential for some patchy fog. Wednesday continues to be a day of big severe weather potential but there is uncertainty on how it will all play out. The overall set-up hasn`t changed much with a surface low moving through west-central MO in the morning, and lifting through northeast MO and into west- central IL by evening. Accompanying the movement of the low, the aforementioned boundary to the south will move northward as a warm front with a trailing cold front accompanying the surface low. Undiluted, the warm sector will become rather expansive and very unstable by afternoon with potential for MLCAPE of 2500-3000+ J/KG. There is also very little CIN forecast within the warm sector due to heating, thus convection could evolve not only along the boundaries but seemingly throughout the open warm sector. The possibility of morning storms however leads to uncertainty in how convective evolution will occur. At least a few of the CAMS including the 12Z HRRR develop robust convection across western MO rather early in the day, growing upscale into a severe QLCS and racing this eastward across the southern half of MO and southern IL during the morning and early afternoon. The best I can tell this early development seems to be in response to a weak vort max in the southwesterly flow and convergence well ahead of the cold front/surface low. While I certaintly can discount this scenario, it seems a more plausible scenario is that convection will develop from late morning into the early afternoon throughout central/southwest MO and points east. Given the lack of cap, this might result in waves of storms. The aforementioned strong instability and deep layer shear of 60+ knots would favor supercells, however as mentioned in the SPC SWODY2 outlook, the convective modes could be messy with a mix of supercells and organized bowing segments. The environmental set-up is quite volatile with the aforementioned CAPE/SHEAR space, low LCL heights, steep mid-level lapse rates and anticipated convective modes favoring all severe weather hzards. Forecast hodographs across eastern MO and southwest IL in the afternoon depict impressive clockwise curvature in the lowest few kilometers resulting in large low- level SRH, also supportive of strong tornadoes. By early evening, all the activity should have moved east of the CWA and into the Ohio Valley region. Glass && .LONG TERM... (Thursday through Next Tuesday) Issued at 236 PM CDT Tue May 7 2024 An elongated positively-tilted longwave trof evolves from SE Canada through the SW CONUS by early Thursday, anchored by upper lows in the Great Basin and lower Great Lakes. The Great Lakes upper low/trof continue to deepen across the eastern U.S. through Friday, largely in response to upper ridging building from the Pacific into the Pacific NW, while leaving a cut-off low in the vicinity of the far southwestern U.S. While there are differences in details with the eastern upper trof structure and depth amongst the deterministic models/ensembles and LREF, there is a dominant signal for the presence of a trof and persistence of NW flow aloft Friday through the weekend. This pattern suports near to below normal temperatures beginning immediately post-frontal on Thursday and continuing through Saturday as a series of shortwaves in the NW flow bring weak reinforcing cold fronts through the area. Accompanying the shortwaves could be periodic precipitation chances, especially on Thursday, however there is no strong signal for high pops at any point in time. Heights aloft are on the rise by late Sunday as the eastern trof progresses into the western Atlantic and by early next week the flow aloft across the region is relatively weak by mid-May standards. In response we should see temperatures moderate and return to above normal. The aforementioned temperature trends are fairly well depicted in the NBM temperature guidance and the IQR generally ranges from 5-8 degrees in the Thursday-Tuesday time frame. Glass && .AVIATION... (For the 18z TAFs through 18z Wednesday Afternoon) Issued at 1224 PM CDT Tue May 7 2024 VFR flight conditions are expected to dominate most of the forecast period. Diurnal instability driven cumulus clouds will persist across east-central MO into southwest IL during the afternoon impacting KSTL/KSUS/KCPS. There is also a threat of a strong isolated thunderstorm in this region through 23-00z, however the anticipated coverage/probability of a TAF impact is low and thus it has not been mentioned. Overnight there could be some MVFR vsbys from radiational fog at KSUS/KCPS from around 09-13z. The next significant round of showers and thunderstorms will largely be centered on Wednesday afternoon, however I have a mention as early as 16z at KCOU/KJEF but there is a good amount of uncertainty in whether they will develop this early in the day. Glass && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX