Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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557 FXUS63 KLSX 301416 CCA AFDLSX Area Forecast Discussion...CORRECTED National Weather Service Saint Louis MO 916 AM CDT Sun Jun 30 2024 .KEY MESSAGES... - Cooler temperatures (5-15 degrees below normal) will be accompanied by a noticeable drop in dewpoints to result in a couple of very pleasant summer days today and Monday. Temperatures could near record lows Monday morning in some locations. - Warm, moist conditions return Tuesday into Wednesday. The pattern becomes active once again late Tuesday onward with a large share of dry time between rounds of showers and thunderstorms. && .SHORT TERM... (Through Late Monday Afternoon) Issued at 344 AM CDT Sun Jun 30 2024 Regional analysis shows a cold front at the southern fringes of the forecast area with northerly surface flow that encompasses much of the CWA. Temperatures and dewpoints continue to drop behind the front as skies clear from north to south. As of 08z, temperatures were 70s with dewpoints in the upper 50s to low-60s. The reprieve from uncomfortably warm, moist air is thanks in large part to a strong surface high (1026 hPa) that will take control through Monday as it builds in from the northern Plains. Today`s northerly surface flow will turn east to southeast Monday, which draws even drier air into the region to start the work week. Though there are no impacts to mention, it remains notable that the surface high is anomalously strong, peaking (1028 hPa) at or near NAEFS maximum climatological percentiles (99.5+). Dewpoints that drop into the 40s and 50s tonight into Monday morning, where co-located with light winds, will allow temperatures to become quite cool. Forecast lows (upper 50s) around the STL metro could come close to record lows for July 1st (56 degrees in 1937). It will be a stretch with the center of the surface high slide to our north. However, roughly 25% of ECMWF ensemble members are within 2 degrees of a record low. The GFS ensemble is a bit more aggressive with roughly 40% of the members at or below a record low. Records at Quincy provide a little more breathing room (52 in 1988), while Columbia isn`t going to come close at all (45 in 1892). Additionally, NBM probabilities show a small percentage (5-10%) of members bottoming temperatures out just below 50 degrees east of I-55 and north of I-70 in Illinois. In short, Monday morning could be jacket weather for northeast sections of the forecast area. Southwesterly mid/upper flow aloft draw moisture northward into the area Monday morning into early afternoon. Some guidance brings low chances (around 10 percent) for showers as far east as central Missouri. However, dewpoint depressions of up to 30 degrees reside near or below 850mb. This favors dry conditions, so I held PoPs west of the CWA through the period. Maples && .LONG TERM... (Monday Night through Saturday) Issued at 344 AM CDT Sun Jun 30 2024 There is expected to be an eastward shift in the overall ridging pattern late Monday into early Tuesday. Meanwhile, a broad upper trough progresses west to east along the Canadian/U.S. border. While south to southwesterly flow advects warm, moist air northward out of the western Gulf and into the Plains, the orientation of the stacked eastern ridge will make this process more gradual Tuesday into Wednesday. Mid-level 850mb temperatures of around 20C encroach on the Mississippi River Tuesday morning as warmer air holds over the Plains at least temporarily. Temperatures do begin to warm with 90s favored along and west of the Mississippi River, combining with dewpoints in the low-70s for heat index values to reach near 100 Tuesday afternoon. Though warmer (mid to upper 80s) than Monday, dewpoints in the 50s over interior section of Illinois should make it more comfortable. Late Tuesday into early Wednesday begins to get a little interesting as a cold front drops into Iowa and extends southwestward into northwest Missouri. The strong pressure gradient that lines up between the ridge to the east and the approaching trough results in intensifying jet dynamics as a shortwave rotates around the southern edge of the upper trough. Copious moisture continues to feed northward around the western periphery of the ridge as a strong low level jet of 40-50 knots develops Tuesday night, extending from the south-central Plains into the Great Lakes. There are some differences in the position of the front between individual guidance. The general theme is for convective initiation to occur over Iowa Tuesday evening into Tuesday night. The front then sinks southward into the CWA sometime late Tuesday night into Wednesday morning before stalling. The setup looks to support severe thunderstorms to the north, but the initial placement and timing of the surface convergence will be key in how far south the severe threat reaches. This will continue to be monitored over the next couple of days to fine tune the spatial distribution of the greatest threats with the potential for all hazards to be in play. This boundary then has a play on the area from mid to late week, including the Fourth of July holiday. Global guidance shows the boundary stalling, becoming west-east oriented as multiple shortwaves rotate around the parent upper low over southern Canada. This supports thunderstorm potential along the front to some magnitude during the day Wednesday. The strongest of the shortwaves moves overhead late Wednesday night into Thursday morning, providing ample upper ascent over the surface boundary, all within a region of deep layer moisture. PWAT values of 2+ inches look to be 2-3 standard deviations above normal, which points to heavy rainfall the night of the 3rd and heading into the 4th. Additional thunderstorms are possible late in the week. Fortunately, global guidance shows an amplified trough pushing the front south of the area heading into next week, but not before yet another round of thunderstorms late Thursday into Friday as another cold front approaches the region. Ahead of the front, scattered, diurnally driven thunderstorms could impact the Fourth of July. Given the distance in time, finer details will become more clear to nail down timing as the holiday approaches. Maples && .AVIATION... (For the 12z TAFs through 12z Monday Morning) Issued at 610 AM CDT Sun Jun 30 2024 VFR conditions are anticipated throughout the TAF period. Light northerly surface flow will gradually turn out of the east tonight into early Monday. Maples && .CLIMATE... Issued at 344 AM CDT Sun Jun 30 2024 Record Lows 7/1 St. Louis 56 in 1937 Columbia 45 in 1892 Quincy 52 in 1988 && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX