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
548 FXUS63 KLSX 041135 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 635 AM CDT Tue Jun 4 2024 .KEY MESSAGES... - Isolated severe thunderstorms are possible again this afternoon into evening across the region. Primary hazards in the strongest thunderstorms include damaging winds (up to 60mph) and large hail (up to 1 inch). - Our greatest chance at seeing thunderstorms will be tonight along a cold front. If we see any severe weather from these storms, the primary hazard will be damaging winds (up to 60 mph). - Largely dry conditions and near average temperatures will follow the cold front. A low chance of thunderstorms (up to 20%) returns this weekend. && .SHORT TERM... (Through Late Wednesday Afternoon) Issued at 338 AM CDT Tue Jun 4 2024 A mid-level shortwave trough and surface MCV are located over eastern Kansas/western Missouri as of 07z and slowly moving east- northeast. The MCV has only been able to produce generally short- lived showers along a wing of weak moisture convergence and low- level warm air advection. With no SBCAPE and limited MUCAPE (up to 500 J/kg), it`s unlikely that this feature will materialize into anything more than it currently is, and confidence in this feature drastically impacting this afternoon`s forecast is low. Meanwhile, ongoing convection in eastern Oklahoma is expected to spark additional northeastward moving convection along an outflow boundary early this morning. A weak (20 to 30kt) LLJ along with warm air and moisture advection will keep convection alive as it reaches south- central Missouri late this morning. We have confidence that limited surface based instability (500 - 1000 J/kg) and weak bulk shear (up to 20 kts) will keep these thunderstorms below severe criteria. However, confidence begins to wane regarding how they will impact afternoon convection. This afternoon`s near storm environment will be similar to that of yesterday`s: high SBCAPE (2000 to 3000 J/kg) with low bulk shear (20 to 25 kts). Although a mid-level trough will crawl through the region, forcing for ascent will be weak, leaving the heavy lifting to warm air advection to spark disorganized, pulse convection. Thunderstorm coverage will increase from isolated to scattered through the afternoon as instability increases. However, the coverage of severe thunderstorms specifically will remain isolated at best. Deterministic soundings show long, skinny CAPE this afternoon, translating to slow building updrafts with a lack of substantial hail CAPE. Largely saturated profiles, low shear values, and weak low-level lapse rates indicate that downdraft potential and momentum transfer will be minimal as well. These factors are expected to keep thunderstorms largely sub-severe this afternoon. If a thunderstorm is able to become severe damaging winds of up to 60 mph and large hail of up to 1 inch will be the primary hazards. Thunderstorms may have a greater potential at becoming severe if this morning`s convection establishes outflow boundaries in the area. The finale of our early week thunderstorm chances will come late this evening along a cold front. Upper level ascent will maximize during the frontal passage and MUCAPE values are expected to still range from 1500 to 2000 J/kg. Thunderstorms are thus expected along the front as it enters the forecast area from the northwest. Low bulk shear (20 to 30kts) and 0 to 3km shear (10 to 20 kts) in addition to diminishing daytime instability and mid-level forcing decrease confidence in the front`s severe threat. If any segment does become severe, damaging winds of up to 60 mph will be the primary hazard. The best potential of this happening would be in northeast Missouri and west-central Illinois, as the line is expected to weaken and break with time due to the aforementioned factors. The threat for flash flooding has been previously discussed with this frontal passage, but the progressive, southwest to northeast oriented, weakening line of showers and thunderstorms should preclude this in most locations. There`s no substantial signal for backbuilding or training, either, further lowering the potential for flooding. The latest LPMM reflects this, with very localized spots of 2 to 3 inches of rain, decreased from days past. Again, northeast Missouri and west-central Illinois have the greatest chance at seeing flash flooding given the strength of the line as it passes through those areas. Of course, any area with poor drainage and/or areas that have been impacted by recent heavy rain there is still a have the potential for very localized flash flooding. The cold front is expected to clear the area by Wednesday morning with all precipitation exiting shortly after. High temperatures Wednesday will land in the low to mid-80s despite the frontal passage, but it will be notably less humid as moisture is scoured from the region. A dry secondary cold front will bring cooler air to the region Wednesday night, dropping lows into the low to mid-60s. Jaja && .LONG TERM... (Wednesday Night through Monday) Issued at 338 AM CDT Tue Jun 4 2024 A deep mid-level closed low will spin over the Great Lakes region late this week, keeping the region in northwest flow aloft. There is signaling in deterministic guidance of two shortwaves passing Saturday and Sunday, but given the expected flow pattern not being favorable for thunderstorms and the differences in the troughs among models, confidence is low in what these features will bring us this weekend. The NBM interquartile range reflects this as well, with 9+ degree differences between the 25th and 75th percentiles beginning on Saturday. What we are confident in is a cooler, seasonable weekend ahead. Jaja && .AVIATION... (For the 12z TAFs through 12z Wednesday Morning) Issued at 621 AM CDT Tue Jun 4 2024 Guidance overnight has struggled to keep up with ongoing conditions, but consensus is that IFR to MVFR ceilings will slowly lift and scatter this morning, becoming VFR by noon to 1pm. Coverage and timing will vary some depending on location, so amendments will likely be needed this morning. Isolated to scattered showers and thunderstorms will push through the forecast area today. The first taste of these thunderstorms are in central and south-central Missouri as of 1130z. These are expected to push northeast today, increasing in coverage later this afternoon into this evening. During this timeframe isolated severe thunderstorms capable of 1 inch hail and 50 to 55kt gusts. Any thunderstorm that directly impacts a terminal today will be capable of producing brief reduced flight conditions. Mention of VCTS has been maintained into this evening due to low confidence in direct impacts to any terminal. A cold front will arrive tonight bringing a greater chance of thunderstorms to each terminal and potentially MVFR ceilings. This front will be progressive, and any flight restrictions caused by it are expected to be brief. Widespread VFR conditions and northwesterly winds will follow the front. Jaja && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX