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
966 FXUS63 KLSX 120353 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1053 PM CDT Thu Jul 11 2024 .KEY MESSAGES... - Isolated showers and thunderstorms are expected tonight through Saturday across portions of the area. Despite these chances (20- 30%), most will not see much, if any, showers or thunderstorms. - Temperatures will continue to rise to above climatological normal through early next week. By Sunday, heat index values could reach the 100s. && .SHORT TERM... (Through Late Friday Night) Issued at 213 PM CDT Thu Jul 11 2024 Current water vapor imagery shows a trough over the western Great Lakes region with a positively tilted axis extending across the CWA from northeast to southwest. Along this axis are weak disturbances that will provide support aloft for isolated to scattered afternoon convection mainly south of I-70. Any thunderstorms that do develop are expected to remain sub-severe as there is little forcing other than weakly aloft and daytime heating. Expect the strongest storms to have brief downpours. Winds are expected to remain light and variable as surface high pressure over the Upper-Mississippi River Vally expands southward. Light winds, a saturated near surface layer, and little cloud cover will create conditions favorable for fog. Patchy fog is likely mainly across northeast and east-central Missouri (including the St. Louis Metropolitan area) and western Illinois. It is possible that low-lying areas in these aforementioned locations could see periods of dense fog overnight. Apart from the potential for fog overnight in the eastern portions of the CWA, there is a 20% chance for showers and a brief thunderstorm across central Missouri along the nose of a low-level jet. The upper-ridge over the western CONUS will continue its eastward expansion overnight and as it does so will help push a weak warm front into the southern portions of the CWA by Friday morning. Daytime heating will allow for the atmosphere to become modestly unstable with SBCAPE values between 1000-2000 J/kg and in combination with the weak warm front, isolated showers and thunderstorms are possible through the day Friday primarily along and south of I-70. Otherwise, temperatures on Friday will be near normal. MMG/Elmore && .LONG TERM... (Saturday through Next Thursday) Issued at 213 PM CDT Thu Jul 11 2024 A weak region of positive vorticity advection will round the expanding upper-level ridge late Friday night into Saturday morning. As it does so, it will coincide with an area of increased moisture convergence and lingering instability from daytime heating across eastern and northeast Missouri and west-central Illinois. This will help focus very isolated (20% chance) showers and thunderstorms across the aforementioned locations late Friday night through mid-day Saturday. Most will not see much in the way of any showers or thunderstorms. After convection wanes early Saturday afternoon, allowing for daytime temperatures up warm-up into the upper- 80s through mid- 90s. An additional chance (20%) for showers and thunderstorms exists late Saturday through early Sunday across western Illinois as a vorticity maximum rounds the ridge and the low- level jet ramps up over northwest Missouri and southern Iowa. Overall thunderstorm coverage across our far northeast CWA currently looks very limited and unsupported given height rises aloft. The main forecast concern comes as a potentially substantial heat up gets underway Sunday through Tuesday. The upper-level ridging will continue its eastern expansion across the CONUS with an 850-mb ridge over the southeast CONUS. Current ensemble guidance is in consensus that 850-mb temperatures areawide will be between 22-24 C Sunday through Tuesday, supporting near surface temperatures in the high 90s and even push 100 degrees in some locations. There is also agreement (at least 70%) among ensemble guidance that dewpoints will reach at least the low 70s. This combination of hot temperatures and high dewpoints will allow to heat index values to reach well into the low 100s. Should these values be realized, heat headlines will need to be considered over the next few days for early next week. Confidence in these hot temperatures continues to increase as the atmospheric set up corresponds to the region`s favorable climatological for past heat events. The hot and humid conditions are not expected to last more than a few days as a shortwave associated with an upper-level low over the Hudson Bay traverses the Mid-Mississippi River Valley by Wednesday. This shortwave will help provide support aloft for the passage of a cold front sometime late Tuesday into Wednesday. The cold front is forecast to bring both precipitation and near normal temperatures to the area for the second half of next week. MMG/Elmore && .AVIATION... (For the 06z TAFs through 06z Friday Night) Issued at 1053 PM CDT Thu Jul 11 2024 Fog is expected to develop in northeast Missouri through south- central Illinois tonight, dropping flight conditions to MVFR through sunrise before quickly burning off. Despite incoming cloud cover from thunderstorms in western Missouri, left MVFR fog at KSUS and KCPS due to their lower elevation, quickness to fog in calm, clear conditions, and uncertainty in eastern extent of clouds. Decided to maintain dry TAFs at KCOU and KJEF despite convection as well since thunderstorms have been very slow moving and have dissipated before they`ve reached these terminals all evening. Otherwise, calm to light and variable winds will increase for most locations generally from the south Friday morning. There is a slight chance (20%) of convection through the day, but confidence in impacts to any terminals are too low to mention in the TAFs at this issuance. Jaja && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX