Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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470 FXUS63 KLSX 112319 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 619 PM CDT Tue Jun 11 2024 .KEY MESSAGES... - Temperatures will warm well above normal over the course of the next week. The warmest conditions are expected Thursday, Sunday and Monday with heat index values reaching 100-105 in some locations. - There is a slight risk for severe thunderstorms Thursday afternoon into Thursday evening. The primary threats will be large hail and damaging winds. && .SHORT TERM... (Through Late Wednesday Night) Issued at 212 PM CDT Tue Jun 11 2024 A broad view of the CONUS shows ridging in the 700-250mb layer over parts of the southwest and south-central U.S. resulting in northwesterly flow over the mid-Mississippi Valley. Mid to high clouds have occasionally streamed overhead with limited impact on sensible conditions. Surface high pressure has moved far enough southeast for southerly flow to draw slightly warmer temperatures into the Midwest, along with a modest increase in moisture. Surface observations at 19z show dewpoints have climbed well into the 50s to near 60 over central Missouri with 40s hanging on over interior sections of Illinois. Temperatures generally ranged from the mid-70s to near 80 degrees. As we go through the remainder of the period, we will be watching a an train of disturbances track west to east over the northern section of the country. The disturbances are expected to trickle southward with each consecutive pass. The parent upper low continues to track across southern Canada with a cold front that trails southwestward into the Upper Mississippi Valley. Modest upper ridging over the northern Lower 48 will transition to zonal flow Wednesday. Shortwaves embedded within the zonal pattern will introduce potential for showers and thunderstorms over the Upper Midwest, progressively reinforcing the surface cold front southward. The first of these passes over the Wisconsin/Illinois border, which is far enough to keep most precipitation out of the area. While CAMs show an isolated shower or two along the front at times, soundings depict a capped environment, limited moisture, and no upper support to reasonably suggest any activity will survive this far south. The dry forecast has been maintained. Southwesterly flow will usher 850mb temperatures into the mid- teens to near 20C Wednesday, along with a subtle increase in moisture. Dewpoints in the low to mid-60s will accompany temperatures well into the 80s and low-90s, which is only the leading edge of the heat to come Thursday into early next week. Maples && .LONG TERM... (Thursday through Next Tuesday) Issued at 212 PM CDT Tue Jun 11 2024 Yesterday`s guidance proves the NAM was the northern outlier with most now showing the cold front in in the vicinity of the Iowa/Missouri border Thursday afternoon. The front sinks southward Thursday evening into Thursday night, bringing the greatest potential for showers and thunderstorms, not without a few caveats to consider. Deterministic guidance shows a rather impressive west/east oriented corridor of SBCAPE reaching 3000-4000 J/kg with the commonly excited NAM unsurprisingly over 5000 J/kg in some places. The 75th percentile of LREF more reasonably projecting between 2500-3000 J/kg with averaging pulling numbers down slightly. Low/mid-level moisture pools along and ahead of the front with PWAT values of 2 to 2.25 inches falling within in the top 5th percentile of climatological means. Moisture and instability do not seem to be in question, but surface to mid-level flow nearly parallels the boundary, lessening potential magnitude of convergence underneath a strong mid-level capping inversion. Bulk shear of 30-35 knots and mid-level lapse rates of 7-8C should be sufficient enough to aid in severe potential over northeast Missouri and west-central Illinois. However, shear drops below 30 knots near I-70. Additionally, there is no indication of a definitive low level jet feature. While there is certainly potential, there is some uncertainty as to how soon convective initiation occurs and just how far south convection tracks before instability wanes through the evening. While the cap may be an initial inhibitor, what does break through could lead to large hail, provided updrafts are robust and deep enough to overcome melting in the very warm surface and mid-level layers. Parallel flow should quickly congeal any individual cells into a linear segment that drifts south with time. Soundings do show dry air aloft that will also exhibit wind potential, but how long this survive and how far south will be the question with greatest potential likely remaining north of I-70. Thursday night`s cold front is expected to stall over southern sections of the area Friday. Where the front stalls could keep conditions uncomfortable Friday into Saturday as dewpoints in the 70s pool along and south of the front. This is accompanied by light and variable surface flow, limiting any breeze that could otherwise provide some relief for those outdoors. LREF graphical output shows the greatest spread in temperature quartiles (5-9 degrees) south of I-70, signaling the front likely stalls somewhere over southern Missouri into southern Illinois. North of the boundary could be a different story. Though still warm, dewpoints range from upper 50s in the far north to the mid-60s along I-70. This isn`t a dry airmass, per say, but it looks to be relatively better than what is draped through southern sections of the CWA. Surface high pressure over the Upper Midwest moves into the Great Lakes as upper height rises build from west to east Saturday. Mid- level warm air advection kicks southwest to northeast Saturday afternoon with surface flow turning out of the south later in the day. This pushes 20-23C air northward with a resurgence of heat late in the weekend into early next week. Some guidance suggests a few showers and thunderstorms along the front as a weakening upper shortwave trough traverses the central Plains into the Upper Midwest. Much of the lift passes to the northwest, though, keeping most of this potential over northwest Missouri into Iowa. Though NBM quartiles are tightly clustered through the next several days, I think the magnitude of warmth (upper 90s) is a bit aggressive. Not only would this approach record highs in some cases, the ample amount of low level moisture (dewpoints in the 70s) and 180-200 degree surface wind direction does not typically result in near triple digit heat. Low to mid-90s is more reasonable. Additionally, the GFS MOS is much lower with upper 80s to low-90s and ECM MOS 90th percentile in the mid-90s. It`ll still be warm regardless, but should limit the spread of triple heat index values Sunday and Monday. It`s safe to say that a majority of the area will be in the low to mid-90s with few upper 90s around urban areas. The forecast becomes a little murky beyond Monday. Global guidance shows a system over the Gulf of Mexico that could either eject westward into Texas or northward along the Mississippi Valley as a cold front drops southward out of Iowa. This will all depend on the evolution of the southeastern U.S. ridge, which plays out quite differently between long range deterministic guidance. Maples && .AVIATION... (For the 00z TAFs through 00z Wednesday Evening) Issued at 546 PM CDT Tue Jun 11 2024 Dry and VFR conditions are expected through the period with generally light winds. Britt && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX