Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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614 FXUS63 KLSX 292328 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 628 PM CDT Tue Oct 29 2024 .KEY MESSAGES... - Well-above normal temperatures and gusty winds will persist through Wednesday. Highs in the 80s will be accompanied by southerly gusts of up to 30-40 mph. - A cold front will bring widespread showers and thunderstorms to the region Wednesday night into early Thursday morning. A few of these storms may be strong to severe with wind gusts to 60 mph and a low chance for an isolated tornado (mainly in central MO). - A cooler and more active pattern could bring additional chances for beneficial rain from late weekend into next week. && .SHORT TERM... (Through Late Wednesday Night) Issued at 240 PM CDT Tue Oct 29 2024 The morning began incredibly warm with temperatures in the mid-60s to low-70s. This was already about 5 degrees above the normal highs (upper 50s/mid-60s) for today. Mainly clear skies have allowed for deeper mixing, leaning into the warmer side of the guidance as temperatures sit in the low-80s just prior to 20z. This environment also supports stronger winds sustained at 15-25 mph and gusts that have reached 30-40 mph. Consequently, relative humidity has ranged from the mid-30s to mid-40s (percent). It is conceivable that RH values could drop a little more as we continue to heat into the mid-80s, continuing the concern for elevated fire danger through the next few hours. Strong surface ridging over the east coast and the developing surface low at the front range of the Colorado Rockies will extend the warmth through Wednesday. The primary difference will be the slight increase in moisture with dewpoint subtly climbing through the 50s with some locations over central/southeast MO and southwest IL approaching 60 degrees. Minimum RH values jump by approximately 10 percent, which should be enough to mitigate another day of elevated fire danger. That being said, southerly winds remain strong with gusts not much weaker than today. Caution should still be practiced with the persistent dry conditions we`ve experienced in recent weeks. Attention turns to the approaching cold front that will return temperatures to normal (60s) and bring our first, near-certain (70- 100 percent) shot at measurable precipitation in weeks. Latest guidance has trended slightly downward in QPF amounts between Wednesday night and early Thursday. GFS/ECM ensembles show much of the area in the 0.25-0.50 inch range. This isn`t too surprising given how the upper trough and bulk of the vorticity remains north of the CWA, pulling the surface low through Iowa into western Wisconsin. Much of the precipitation potential over the mid- Mississippi Valley will rely on the anomalous moisture (PWATs 1.5- 1.75 inches/3-4 standard deviations above normal), strong surface convergence along the front and limited instability (MLCAPE 200-600 J/kg) to maintain remnant showers and thunderstorms into the diurnal lull of the nighttime period. Provided the convective nature, while QPF average will generally fall in the 0.25-0.50 range, locally higher amounts are possible under the stronger cells. There are some minor timing differences between deterministic guidance with NAM/RAP showing a later arrival than most other guidance. The run-to-run ensembles also point to this trend with the last 3 update cycles of the 6-hourly precipitation amounts delaying precipitation accumulation past 06z Wednesday night/Thursday morning. Fortunately, we may keep severe weather isolated and limited to the western sections of the CWA. Shear is certainly not the limiting factor with speeds at 45-50+ knots. MLCAPE values are marginal and highest (200-600 J/kg) over central Missouri. Values trail off quickly through the night as the front approaches the Mississippi River in the same manner than mid-level lapse rates erode from 7-8C to our west, to essentially nothing as the front closes in on central Missouri. Ongoing thunderstorms, especially any that show bowing, may produce isolated damaging wind gusts near 60 mph and a low chance for an isolated tornado over the far western CWA. This threat is expected to quickly diminish to the east through early Thursday morning. Maples && .LONG TERM... (Thursday through Next Tuesday) Issued at 240 PM CDT Tue Oct 29 2024 Thursday will see improving conditions as the cold front progresses to the southeast. Aside from a remnant shower over far southeast Missouri and southwest Illinois late Thursday morning, precipitation chances will diminish by afternoon. Much cooler, near-normal air then surges into the region with an 850mb airmass that fall from the mid-teens to the single digits (C). Halloween events will feel cool in comparison to previous days with temperatures falling through the 60s and into the 50s at most locations. The coolest conditions will be found over central/northeast Missouri and west-central Illinois, where we cool into the 40s for the tail end of trick-or-treating (8pm/9pm Thursday evening). Quasi-zonal upper level flow briefly sets up over the central sections of the CONUS Friday as surface high pressure extends from the north-central U.S. southward through the mid-Mississippi Valley. The surface high continues to build south into the Great Lakes Region Friday as mid/upper level ridging amplifies over the east third of the country. This keeps conditions dry and more like fall with highs in the low to mid-60s. The next system comes ashore over the Pacific Northwest late Friday into Saturday, when deterministic guidance diverges on more specific details in the synoptic pattern. The differences are related to the orientation of the strong surface ridge over the eastern U.S. and eastward extent of the western trough. This pattern eventually introduces additional shortwaves in the upper level flow, which eject out ahead of the deepening western trough. NBM guidance spreads low chances (15-40 percent) from west to east through the day Saturday, but this a product of the diverging solutions. Realistically, if the surface high holds on long enough, Saturday may largely end up dry. All global ensemble members show beneficial rain arriving as soon as Saturday evening with accumulating rainfall continuing into next week. One thing to note is that not as many members are producing some of the higher numbers (3-4 inches) as far east as they were yesterday. Suffice to say, it is only one forecast cycle and it still to far out to rely on specifics. The outlook remains positive with much-needed rainfall favored for late weekend into next week. Maples && .AVIATION... (For the 00z TAFs through 00z Wednesday Evening) Issued at 628 PM CDT Tue Oct 29 2024 VFR flight conditions expected through the forecast area. Otherwise, the main issue continues to be the gusty south winds. Will see winds diminish slightly at onset of forecast period, then winds will pickup by 15z-16z Wednesday with gusts up to 30 to 35kts at times. As for any LLWS tonight, with decent mixing from the strong winds aloft, confidence is low for any LLWS. Thus did not add mention to TAFs at this time. Byrd && .CLIMATE... Issued at 241 PM CDT Sat Oct 26 2024 Daily High Temperature Records: |---Oct 29---| KSTL 86 F (2016) KCOU 86 F (1937) KUIN 85 F (1937) Daily MAX Low Temperature Records: |---Oct 29---|---Oct 30---| KSTL 69 F (2004) 64 F (1876) KCOU 67 F (2004) 65 F (1950) KUIN 67 F (1946) 66 F (1927) && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX