Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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046 FXUS63 KLSX 012300 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 600 PM CDT Mon Jul 1 2024 .KEY MESSAGES... - An active weather pattern will begin late Tuesday with opportunities for showers and thunderstorms including the threat of strong to severe thunderstorms and locally heavy rainfall with isolated flash flooding. - The initial threat of strong to severe thunderstorms Tuesday evening is greatest across northeastern, central MO and west- central IL. - Hot and humid conditions are expected across portions of the area Tuesday through Thursday. Heat index values could exceed 100 F in central MO Tuesday afternoon. && .SHORT TERM... (Through Late Tuesday Night) Issued at 339 PM CDT Mon Jul 1 2024 A very slowly decaying MCS has persisted across western MO and is now tracking more progressively eastward toward central MO this afternoon; however, the MCS should continue decaying and eventually dissipate by the time it reaches the CWA due to the combination of decreasing instability and increasingly pronounced dry low-level easterly flow with eastward extent. Therefore, only sprinkles and light rain are anticipated to reach central MO. Otherwise, clouds associated with the MCS and weak CAA has kept temperatures 5 to 15 F below average. Aside from MCS remnants this afternoon, dry conditions will prevail through most of Tuesday across the region under the crest of an upper-level ridge shifting eastward tonight into Tuesday. With much greater insolation and onset of low-level WAA by Tuesday, significantly warmer temperatures are anticipated with high temperatures returning to above average, in the upper 80s to mid-90s F. A warm front will also be lifting northeastward into central and northeastern MO on Tuesday but should act more as a dewpoint discontinuity than temperature difference. Most model guidance indicate that dewpoints on the warm (west) side of the front will be around 70 to the mid-70s F, with afternoon heat index values rising above 100 F. However, confidence is low that values will rise to Heat Advisory criteria since there is still some variability on how high dewpoints and heat index values will get with slight differences in front position. Thunderstorms are forecast to develop in the vicinity of a cold front Tuesday afternoon across northwestern MO and IA ahead of an upper-level trough, growing upscale into an MCS reaching northeastern MO/west-central IL. Model guidance generally indicate 2500 to 3500 J/kg of MLCAPE and 30 to 40 of deep-layer wind shear available as thunderstorms initially arrive, sufficient for at least a few strong to severe thunderstorms with a primary hazard of damaging winds, but marginally severe hail and an isolated tornado are possible. Although the threat of severe thunderstorms will gradually decrease through the evening, the strongest thunderstorms may tend to become focused toward the southwest with time since instability will decrease with eastward extent and low-level convergence will be maximized where the LLJ interacts with the southwest side of any surging outflow. If this evolution can take place and encourage periods of training thunderstorms, locally heavy rainfall and isolated flash flooding will also become potential hazards across northeastern MO into central MO with PWATS above 2" and deep warm cloud depths. Antecedent soil conditions are dry but the latest HREF LPMM of 24-hour QPF is 3 to 7" in those areas, with the disclaimer that it is heavily skewed toward a small membership with high rainfall amounts. These factors preclude higher confidence in flash flooding. Pfahler && .LONG TERM... (Wednesday through Next Monday) Issued at 339 PM CDT Mon Jul 1 2024 An active weather pattern will persist through Wednesday and Thursday as the cold front moves into the CWA and wavers beneath a series of shortwave troughs/perturbations within upper-level west- southwesterly flow. Thunderstorms are not anticipated continuously through this entire period, but there are several timeframes that are more favorable for showers and thunderstorms. The first is Wednesday afternoon and evening along and ahead of the cold front that will be near the I-44 (MO) and I-70 (IL) corridors during that time. Some of these thunderstorms could become severe since MLCAPE of 2000 to 3500 J/kg will coincide with deep-layer shear of 20 to 30 kt. The primary severe thunderstorm hazards will be damaging winds and large hail. It is unclear exactly how widespread thunderstorms will become with weak upper-level forcing but upscale growth of thunderstorms into an MCS is possible. The next timeframe is Wednesday night as the front retreats back north and is overrun by a LLJ. The expectation is that these thunderstorms will be largely evaluated, but high, anomalous PW and potential for some training of thunderstorms in the event an MCS forms, leads to at least some threat of locally heavy rainfall and isolated flash flooding. The location is not yet clear with differences in front position, although the threat of flash flood would increase if any heavy rainfall falls in areas that receive heavy rainfall Tuesday night. On Thursday (Fourth of July), model guidance is in agreement that a more potent upper-level shortwave trough will traverse the Central Plains into Midwest, forcing the front back through the CWA late in the day as a cold front after it lifts north of the CWA, providing another opportunity for showers and thunderstorms. The exact timing of showers and thunderstorms on Thursday is uncertain with the warm front possibly remaining in or near the CWA, but the chance of showers and thunderstorms increases during the evening as large- scale ascent ahead of the trough increases and the cold front arrives. Temperatures Wednesday through Thursday will be largely dictated by which side of the front a given location is: to the north of the front temperatures are forecast to be near average and along and south of the front temperatures are anticipated to be above average. With humid conditions also present, afternoon heat index values could reach above 100 F in some locations south of I-70. However, exact values and whether or not they reach dangerous thresholds will be influenced by thunderstorms. After Thursday, global model guidance remain in relative agreement that upper-level flow over the Mid-Mississippi River Valley will become broadly cyclone as longwave troughing develops over the north- central CONUS. Embedded shortwave troughs within this flow will provide additional opportunities of showers and thunderstorms at times with a series of frontal passages. However, there is spread in ensemble model guidance on the when showers and thunderstorms will occur as well as coverage and rainfall amounts, due to differences in moisture return quality. Temperatures are also favored to be generally cooler and near average in this pattern. Pfahler && .AVIATION... (For the 00z TAFs through 00z Tuesday Evening) Issued at 555 PM CDT Mon Jul 1 2024 Dry VFR conditions will persist through this TAF period, with winds veering out of the south overnight and strengthening Tuesday. Showers and thunderstorms will approach KUIN and the central Missouri terminals after 00z on Tuesday evening, outside of this current forecast window. There is some question as to how widespread they will be once they reach those terminals, if they manage to reach them at all. MRB && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX