Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 12 2024 00:49:48 FOUS30 KWBC 120049 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 849 PM EDT Sun Aug 11 2024 Day 1 Valid 01Z Mon Aug 12 2024 - 12Z Mon Aug 12 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR CENTRAL UTAH=20 THROUGH NORTHWESTERN ARIZONA, PORTIONS OF THE OZARKS AND CENTRAL=20 PLAINS, AND FOR THE COASTAL CAROLINAS... ....Central Utah through Northwestern Arizona... Upper ridge center located across NM into TX supports WSW flow=20 across NV/UT/AZ where ongoing late afternoon convection persists.=20 Maintained the Slight Risk across central/southern UT into northern AZ where the most concentrated activity presently resides (see MPD #864), though available instability will be waning in most places=20 over the next few hours. Broader Marginal Risk outline covers much=20 of the rest of the Southwest though the coverage is fairly=20 isolated. Still, presence of 1"+ precipitable water values west of=20 ~109W support this risk area, including Southern California in the=20 very near term.=20 ....Portions of the Ozarks and Central Plains... Tough forecast this evening/overnight as the next complex will=20 develop later this evening as the leading edge of a 30-50kt LLJ=20 will nose into the Four State area (MO/KS/AR/OK) coincident with a shortwave bisecting the region as it ejects out of KS. The setup=20 is driven by a strong focus of elevated instability with MUCAPE=20 between 1000-2000 J/kg across the above region within an axis of +1 to +2 deviation PWATs positioned across the Central Plains to the=20 western half of the Mid-Mississippi Valley. There quite a bit of=20 uncertainty on the exact placement of the highest QPF axis (or=20 axes, perhaps) with guidance jockeying on how the mid-level=20 evolution will come to fruition which plays a significant role in=20 the timing of initiation of convection, as well as the magnitude of convection that does develop over the area. Highest probabilities=20 for heavy precip are located over Southeast KS into southern MO but also over northeastern OK, owing to the bifurcation of the CAMs.=20 Latest 18Z RRFS showed less separation between potential convective paths of the ones that were separated vs some recent HRRR runs=20 which were a bit more coherent. Regardless, the forecast SLGT was=20 maintained and nudged north toward the KC area given the threat=20 likely lying within urbanized areas (as FFG values were around=20 2-2.5"/hr). ....Coastal Carolinas... Trimmed the Slight Risk outline to cover the ongoing convection=20 that will slowly diminish over the next couple of hours. Flash=20 flooding has been observed recently but this should diminish now=20 that it is well past sunset.=20 Fracasso/Kleebauer Day 2 Valid 12Z Mon Aug 12 2024 - 12Z Tue Aug 13 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OZARKS... ....20Z Update Summary... A Slight Risk of excessive rainfall was introduced across Southwestern MO and neighboring corners of AR/KS/OK for the ongoing convective pattern forecast to wander from D1 to D2. The MRGL risks across the remainder of the Central Plains to areas out west remains, as well as the MRGL across the coastal Carolinas. More on each region below.... Kleebauer ....Ozarks... A complex of thunderstorms at the end of the D1 period will continue through the morning tomorrow with locally heavy rainfall expected to traverse the Four State area with sights on the Ozarks of MO into Northern AR. Lingering elevated probabilities for >3" across the Southwest corner of MO with some deterministic output pinpointing 2-4" within the time frame of 12-18z before settling as the LLJ weakens and the convection decreases in intensity. A SLGT risk was issued to ensure some overlap from the previous period SLGT located over the area as the expected thunderstorm development will initiate late tonight and carry through the morning hours, similar to what transpired over OK today. The threat remains on the lower end of the SLGT risk threshold, but for messaging purposes and the prospects of heavy precip lingering through the first 6 hours of the period, wanted to make sure there was no gap in the enhanced flash flood prospects, especially with forecast rates up to 1-2"/hr expected with upwind backbuilding possible given the latest soundings off various CAMs. ....Southwest and Central Plains... A broad ridge positioned over the Southwest to South-Central U.S will allow for a migration of elevated PWATs and quick moving shortwaves to the east with focal points for convection located within the interior west, eventually spilling into the plains. MRGL risk was carried for a large area encompassing much of the Central Rockies and Four Corners region with an extension across the Central Plains into the Mississippi Valley as multiple thunderstorm complexes will likely spawn within a zone of enhanced large scale ascent and modest instability. Several clusters of QPF maxima between 1-2" based within the ensemble bias corrected mean QPF and the HREF mean output all point to a relatively active period where isolated to potentially scattered flash flood signatures could occur during the forecast period. The best chances will occur in the afternoon and evening across the west with evening and early morning convective patterns likely to be the driver for the areas east of 100W longitude. This is a traditionally classic summer climatological pattern with a broad coverage of Monsoonal moisture west of the Continental Divide while MCS's and more organized convective clusters drive the threats downstream. There is no one signal that is more robust than the others with the burn scar areas and complex terrain out west the drivers for flash flood potential while backbuilding convection within areas of more organized clusters will be the driver for areas in the Plains and Mississippi Valley. A broad MRGL was maintained to promote the overall pattern with only a few areas worthy of high-end MRGL risk consideration (Mid-Mississippi Valley and Colorado Front Range). Kleebauer ....Coastal Carolinas... 20Z Update: Only minor adjustments were made to align with the overall QPF output based in the latest ensemble data sets. The highest threat for heavy rainfall is likely within the Southeast NC area, including Wilmington proper thanks to elevated neighborhood probabilities >50% located over the area, coinciding with one of the more flash flood prone areas thanks to all the recent rainfall. General continuity was chosen thanks to the above variables. Kleebauer ...Previous Discussion.. Some lingering storms are possible along the coast Monday afternoon as the front over the area dissipates. With much reduced storm coverage, the Marginal remains over the area for the potential for isolated instances of flash flooding. This would likely be the result of sea breeze interactions. Wegman Day 3 Valid 12Z Tue Aug 13 2024 - 12Z Wed Aug 14 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL PLAINS, MISSOURI VALLEY, AND OZARKS... ....20Z Update Summary... A Slight Risk of excessive rainfall was added across portions of the Central U.S with a general continuity west of the Plains. The MRGL risk across the Carolinas was expanded a bit to the west to match trends in the heavier QPF signals further inland of the coast. ....Central U.S... An increasing consensus within the global deterministic suite, as well as relative ensemble output is pinning an enhanced heavy rainfall threat across much of Missouri and neighboring areas in KS and AR. A robust mid-level vorticity maxima will eject eastward around the northern periphery of a broad ridge located south of 30N in the Plains and Southwestern U.S. leading to strong large scale forcing into the center of the country in combination with deep layer moisture presence and primed antecedent conditions from previous periods. Ensemble bias corrected QPF has increased substantially within the past few cycles indicating a wide swath of 2-3" totals depicted over much of the Missouri Valley with extensions peeking into far East Kansas and the Ozarks of MO/AR. The prospects for a large complex have increased and there was enough of a signal to introduce a SLGT risk across the above areas. Kleebauer ....Carolinas... 20Z Update: The MRGL risk was extended further inland to note a bit of a broader convective signal within Central NC and SC as of the latest ensemble based forecasts. The prospects for extensive flash flooding is lower than previous periods with the QPF footprint generally weaker in magnitude comparatively. This was still enough of a signal to warrant a maintenance and expansion to cover a bit more of the interior Carolinas. Kleebauer ...Previous Discussion.. A bit stronger moisture push off the Atlantic may result in more widespread showers and storms a bit further inland into North Carolina. With little to organize the convection however, the flash flooding threat remains isolated. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5PPyM-ENpxy4DPFou2esWFhwJ1A1chDRaeeowPofKW3H= 7DK6ks3kEKNRJK_3VoAwY5cD-GsZJeqwZCx8uhB6kzCcnaA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5PPyM-ENpxy4DPFou2esWFhwJ1A1chDRaeeowPofKW3H= 7DK6ks3kEKNRJK_3VoAwY5cD-GsZJeqwZCx8uhB6df-nvJE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5PPyM-ENpxy4DPFou2esWFhwJ1A1chDRaeeowPofKW3H= 7DK6ks3kEKNRJK_3VoAwY5cD-GsZJeqwZCx8uhB6K97P6a8$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .