Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 22 2024 00:56:04 FOUS30 KWBC 220055 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 855 PM EDT Wed Aug 21 2024 Day 1 Valid 01Z Thu Aug 22 2024 - 12Z Thu Aug 22 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF ARIZONA AND SOUTHWESTERN UTAH... Robust convection persists this evening across portions of northern and eastern AZ, and a flash flood threat is likely to continue. The activity over southeastern AZ will likely fade first, but the convection over central to northern AZ should have more=20 longevity into the overnight hours. A weak MCV tracking north into the area and continued southerly moisture transport should allow=20 for this maintenance of convection, with at least some flash flood risk expecting to persist. By later tonight into early Thursday=20 convection may move into southern UT as moisture advects into the=20 area. Thus while the convective focus thus far has been over AZ,=20 southwestern UT may see an uptick in flash flood risk later=20 tonight into early Thursday. A Marginal risk covers a small portion of central SD. A couple slow moving cells have been producing areas of heavy rainfall this=20 evening...and expect the activity over western SD to grow upscale=20 a bit as it moves into this portion of central SD over the next few hours. The combination of slow moving cells and potential cell=20 mergers suggest at least a localized flash flood risk may exist. Another small Marginal extends across portions of southwest NE=20 into northwest KS. Expect convection currently moving across=20 western NE to grow upscale and push southeastward across this=20 Marginal risk area. The expected progressive nature should limit=20 the flash flood risk, however some brief backbuilding/cell merging could lead to an isolated risk. Chenard Day 2 Valid 12Z Thu Aug 22 2024 - 12Z Fri Aug 23 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE GREAT BASIN AND FOUR CORNERS AREA IN THE WESTERN U.S... ....2030z Update... Very little adjustments needed to the inherited outlook based on the 12z guidance. The Slight Risk area generally encompasses localities across the Southwest where 12z HREF 40-km neighborhood exceedance probabilities for 2" (most skillful metric in the region) are 20% or greater. Although the updated outlook looks very similar to the prior, there is higher confidence in higher-end Slight (25%+ probabilities) from northwest AZ into southwest UT with the trends in the 12z model data. Would like to see another model cycle of continued trends in this regard to consider a targeted Moderate Risk upgrade, which would be particularly helpful to get the word out to the more isolated, sensitive slot canyon recreation areas. Churchill ....Previous Discussion... An increasingly amplified pattern out west will take shape as the digging upper trough and closed-low positioned off the Pacific coast will begin interacting with the anomalous moisture feed advecting northward around the western flank of the ridge to the southeast. PWATs will remain settled between +2 to +3 deviations above normal across much of the Southwestern U.S with advancement deeper into the Inter Mountain West leading to an enhanced convective regime extending from Northern AZ through UT and Western CO. Areal average MUCAPE across the Southwest will range from 500-1000 J/kg with a maximum of 1500-2000 J/kg situated within the core of the IVT pulse bisecting portions of Southern UT down through Northern AZ. This area in question will be the primary region of focus for heavy rainfall and relevant flash flooding, especially given the ongoing nature from the D1 time frame, as well as the complexity of the terrain with numerous slot canyons and remnant burn scars still situated over the area. Ensemble QPF footprint between 1-1.5" on average is a testament to the increasingly bullish outlook for heavy rainfall as these signals are usually reserved for the better flash flood setups in recent past. The back end of the 00z HREF for reference has a sharp increase in the 1"/hr rainfall rate probabilities between 18-00z in the period with a blossoming convective pattern likely extending through the following evening when correlating to global deterministic. The pattern will evolve with convective lingering late into the period with even some nocturnal storm prospects further inland into UT and Western CO. The previous SLGT risk forecast was maintained with the higher-end SLGT potential located within that zone between Flagstaff up through Zion National Park, including the Grand Canyon. The SLGT extends further inland through the interior of UT into Western CO with a secondary maxima being depicted within the western slopes of the Rockies, east of Grand Junction. Elsewhere: The setup across Maine and Florida will lend to non- zero chances for flash flooding, but the overall signal is still just below the 5% risk threshold to necessitate a MRGL risk. The previous MRGL across Maine was removed due to timing of the heaviest convection occurring mainly within the D1 period with an overall decay in the heavy rainfall prospects during the front half of the period. Across Florida, the setup is conducive for locally heavy rainfall thanks to a stalled front and pooling moisture near the boundary. Right now, the threat is borderline with the area of interest mainly over the Northeast FL coastal plain between Jacksonville to Daytona. Area FFGs are very high in this area leading to the primary threat being almost pure urban flash flooding with less concern for flooding outside those local zones. With some time remaining leading up to the period, will continue to assess the signal to see if a MRGL addition is prudent. Kleebauer Day 3 Valid 12Z Fri Aug 23 2024 - 12Z Sat Aug 24 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE FOUR CORNERS REGION AND INTERMOUNTAIN WEST... ....2030z Update... Only a minor expansion of the inherited Marginal Risk was considered necessary based on the new 12z model data. The global models tend to have a dry bias at this range, so used the CMC-reg QPF footprint as a basis for the expansion. Churchill ....Previous Discussion... ....Western U.S... The anomalous moisture feed that allowed for the active periods prior will vacate to the northeast with a skinnier axis of elevated PWATs generally focused from the AZ/NM state line up through the Inter Mountain West of UT/CO. This positions the the Four Corners area up through the Wasatch as the the main targets for heavy convection during the afternoon and evening Friday before the setup dwindles the back end of the period. A secondary focus across the terrain in Southeast AZ over into the NM Bootheel will also be an area that could see some scattered heavy rain signals with some support from global deterministic and ensemble members. This is a period in flux with local totals still reaching between 1-2" in the heaviest rain cores, but just not as widespread comparatively to those periods prior. It's a fairly fluid setup, so it will be a period to monitor for targeted upgrades pending how everything evolves. For now, a MRGL risk was sufficient in the aforementioned areas with only minor adjustments made from the previous forecast. Elsewhere: Will also be keeping an eye on a convective cluster being forecast off the South Carolina coast likely evolving from a weak disturbance crossing over the Gulf Stream. As of now, convective impacts are generally offshore of the Carolinas, but they are close enough with a fairly vigorous QPF footprint to take note as any further westward expansion of the convective potential could offer a targeted risk area along the confines coastal SC. Will continue to monitor in the coming days. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7SRMpVcGTJwqIigrR5SeW9cnEMillFLKR3_C-z2stJfj= v49QUXAQXr3DD-e-McERpQWLNuwY1uTb_nLWDdWjAMFeM1E$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7SRMpVcGTJwqIigrR5SeW9cnEMillFLKR3_C-z2stJfj= v49QUXAQXr3DD-e-McERpQWLNuwY1uTb_nLWDdWjMF2wcs8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7SRMpVcGTJwqIigrR5SeW9cnEMillFLKR3_C-z2stJfj= v49QUXAQXr3DD-e-McERpQWLNuwY1uTb_nLWDdWjrcnN0Vg$=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) .