Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Thu Aug 22 2024 08:39:00 FOUS30 KWBC 220749 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 349 AM EDT Thu Aug 22 2024 Day 1 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... ....Western U.S... A continued amplified pattern across the west will evolve further with a retaining deep moisture fetch arcing through the Desert Southwest and further into the Inter Mountain territories within UT and CO. A general +2 to +3 deviation PWAT anomaly will bisect the region of Northern AZ through Central UT and Western CO by the afternoon hours today leading to an enhanced convective output during peak diurnal destabilization. A growing consensus amongst CAMs members for widespread thunderstorm development with stronger cores able to produce 1"/hr rates or better aligns well with the pattern and will induce a greater potential for flash flooding along the aforementioned areas. A strong HREF EAS signal for 1" of rainfall exists within the Southwest portion of UT, including some of the busier National and State Parks where slot canyon positioning and stronger flash flood prospects overall create a heightened prospect for impacts during the expected convective time frame. HREF neighborhood probabilities for >2" of rainfall are also very high (50-80%) within the local area of Southwest UT down into the Northern tier of AZ, a signal generally befitting for flash flood scenarios in recent history. The threat is not as robust further south into the Mogollon Rim as the deeper moisture advection regime will be located further north, but the environment will still be primed for scattered heavy rain signatures within that zone, so continuity on the southern end of the SLGT risk was maintained from previous forecast. Further north, the SLGT risk remains, but the threat is on the higher-end of potential for the areas near Flagstaff up through that area of Southwest UT thanks to the alignment of the best instability, deep layer moisture, and correlated IVT pulse traversing the area. Pending the evolution of hi-res guidance and radar trends leading into the afternoon, a targeted Moderate Risk is plausible, especially within the slot canyon corridor that has the highest flash flood susceptibility. The areas further north across UT and CO will see their fair share of convective impacts, especially late in the afternoon and evening when instability reaches peak and any mid-level perturbations stemming from convection to the Southwest advect northeastward allowing enhanced ascent locally as they maneuver overhead. Another area of interest will occur across the CO Front Range as combination of elevated moisture, instability, and a stronger shortwave ejecting out of the Central Rockies will ignite another round of heavy thunderstorms to the east of Denver, an area that has seen impacts as of late with regards to flash flooding. A secondary QPF max located across the area allowed for a pretty elevated signal within the 3hr FFG exceedance probabilities (40-60%), mainly after 00z when LLJ influence peaks the intensity of the forecasted convection. A MRGL risk is currently located over the area, but that corridor across Eastern CO, away from the divide is the focal point for that time frame. ....Florida... Two separate zones for flash flood potential will exist across FL this period. The first of which will be situated across the western coast of the Peninsula with the Clearwater/Tampa/Sarasota corridor as the primary focus. A quasi-stationary front will meander over the northern half of FL with smaller mid-level impulses wandering eastward off the Gulf, creating a multi-wave convective scenario during the entirety of the forecast. 00z sounding out of KTBW indicated a sufficiently deep warm cloud layer (>15.5k ft) with areal PWATs running over 2" with expected continuity during the period. Mean flow running parallel to the front will create an, "Express lane" for multiple mid-level shortwaves off the adjacent Gulf to bisect the area, each carrying their own convective conglomeration as they move ashore across the region. Rainfall rates of 2-3"/hr will be plausible in the scenario, impacting an already water logged area of FL that is still dealing with the aftermath of Debby. The lower FFG intervals within the urban corridors, and now surrounding areas with the slow receding waters create a higher likelihood of flash flooding to occur, especially with the multitude of convective waves anticipated during the forecast period. A MRGL risk was added to account for the threat. The second area of interest lies further northeast, closer to the Jacksonville metro and surrounding coastal portions of Northeast FL south of Jacksonville, as well as far Southeast GA where the recent CAMs output is the most aggressive with regards to heavy rain prospects. A strong HREF neighborhood probability for >5" (50-80%) exists within that zone between GA down to Jacksonville proper with a low-end probability for >8" also depicted within the prob fields. This all stems from the front location and expected instability axis to develop during diurnal heating leading to development along the boundary with slow storm motions anticipated. In coordination with the Jacksonville WFO, a MRGL risk was added for that area encompassing Southeastern GA down through Northeast FL with emphasis on the urban areas along the coastal plain. Kleebauer Day 2 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... 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. Current QPF signal is modest with areal averages closer to 0.25-0.5" with a maxima focused near the Four Corners over Southwestern CO (>1"). The secondary max is across the Wasatch through the terrain in Western CO where isolated totals >1" are plausible, but more sporadic in coverage compared to the Four Corners region. Overall, the setup is more benign in the grand scheme, especially considering the previous active periods. Kleebauer --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .