Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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077 FXUS63 KLSX 182356 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 656 PM CDT Wed Sep 18 2024 .KEY MESSAGES... - While rain chances are increasing at times this weekend into early next week (up to around 50%), confidence remains low (less than 10%) in rainfall capable of relieving widespread drought conditions. - Confidence remains high (90%) that above normal temperatures will continues through end of the week, and while cooler weather is expected to end the weekend and start next week, exactly how cool remains uncertain. && .SHORT TERM... (Through Late Thursday Night) Issued at 218 PM CDT Wed Sep 18 2024 A tightly-wound shortwave can be seen in current water vapor imagery moving into the Northern Plains as a Rex Block slowly drifts eastward over the eastern CONUS. Between these two features, the CWA is beneath weak northwesterly flow aloft, with high clouds from the shortwave moving into the region. An expansive area of high pressure at the surface coving much of the eastern half of the country continues to keep our weather calm. With ample sunshine and weak southerly surface flow, temperatures are so far tracking similarly today as yesterday, with highs in the upper 80s to around 90 degrees expected this afternoon. Tonight, temperatures will fall to around 60 degrees for lows, and while the dry air in place over the area should help temperatures cool quickly, upper-level clouds will at least initially inhibit radiational cooling to some degree. If clouds are able to clear soon enough and temperatures can cool more than forecast, another round of fog, mainly in valleys, is possibly across southeastern Missouri and southwestern Illinois. The upper-level pattern mostly holds in place tomorrow, with the shortwave beginning to make its way into Canada as a trough slowly drifts eastward over the Intermountain West. Given the rather stagnant upper-level pattern over the CWA, mostly a repeat of today can be expected, with dry and warm conditions present. This stretch of relatively calm weather comes to an end late Thursday night/early Friday morning, as a weak shortwave passes north of the CWA and a weak low-level jet veers into the region, combining to spawn scattered showers and a couple of thunderstorms. Coverage, placement, and timing of this convection is uncertain at this point given the spread among CAMs, though the general timing is it could enter the CWA as early as midnight and exit as late as 8am Friday. Elmore && .LONG TERM... (Friday through Next Wednesday) Issued at 218 PM CDT Wed Sep 18 2024 At the start of the weekend on Friday, guidance still differs on the phasing of an upper-level trough and possible cutoff over the western CONUS. Solutions vary from the trough becoming cutoff to remaining intact with the main flow, and handle differently the speed and amplitude of the trough/cutoff when it begins to progress eastward. In general, the crest of an upper-level ridge ahead of the western trough will be poking into the Middle Mississippi Valley Friday into Saturday, keeping the thermal ridge in place over the region characterized by 850 mb temperatures around 20 degrees C per ensemble means. This climatologically favors surface temperatures around 90 degrees. Thanks to mostly clear skies on Friday, temperatures will reach the low 90s, but increasing cloud cover on Saturday from the approaching trough/cutoff leads to uncertainty in how warm temperatures will be able to get then. On the warmer end, temperatures may be close to if not similar to those on Friday, and on the cooler end, values could be roughly 5 degrees cooler compared to Friday. As for rain chances, deterministic guidance shows subtle disturbances rounding the crest of the ridge through the Middle Mississippi Valley on Saturday ahead of the main trough/cutoff. Moisture return over the CWA is still expected to be meager ahead of this feature, leading to low chances (20-40%) for rain mainly along and west of the Mississippi River. Guidance begins to diverge notably Sunday into early next week, with varying solutions in the evolution of the trough/cutoff as it enters the Midwest. The general consensus is that this trough/cutoff will be the first in a series of troughs or shortwaves to move through the Midwest, leading to an unsettled pattern as we end the weekend and start next week. The spread in the handling of the upper-level pattern has lead to a broad smattering of low rain chances (roughly 30%) Sunday through Wednesday. In reality, rain chances will likely be confined to much smaller time frames during this stretch. Moisture return is expected to remain meager, with any rainfall that occurs not amounting to much - ensemble medians are around 0.5" for total accumulation Saturday-Wednesday. The spread in the evolution of the upper-level pattern also leads to a wide spread in temperatures among guidance through the first part of next week. At times, the difference between the 25th and 75th percentile reaches 10 degrees during this stretch. While this degree of spread makes it difficult to pin down scenarios at this lead time, the general pattern and ensemble means support cooling temperatures to somewhere around climatological normals, with values varying from about 5 degrees above normal to 5 degrees below normal. Elmore && .AVIATION... (For the 00z TAFs through 00z Thursday Evening) Issued at 633 PM CDT Wed Sep 18 2024 Surface high pressure will maintain control through the period. High clouds will stream overhead as surface flow goes light and variable tonight. There is some concern for fog development should high clouds clear out, but conditions are more favorable over southeast Missouri. Withheld any mention in the TAFs with VFR favored. Maples && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX