Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Wed Aug 21 2024 08:50:00 FOUS30 KWBC 210752 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 352 AM EDT Wed Aug 21 2024 Day 1 Valid 12Z Wed Aug 21 2024 - 12Z Thu Aug 22 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF ARIZONA AND SOUTHWESTERN UTAH... ....Southwest... A relatively strong mid-level ridge will be broken down on the western flank as a series of mid-level shortwaves begin ejecting off the Pacific with a more amplified, closed-low pattern evolving towards the back end of the forecast period off the Pacific Northwest. The weakening force of the pattern will allow for a more north-south alignment of streaming mid-level energy to ride out of Sonora and traverse the Desert Southwest to the Great Basin. In tandem, a pretty formidable moisture flux will begin advecting into the region, highlighted by a well-forecast IVT pulse (500-700 kg/ms) pivoting through much of AZ with roots from the tropical Eastern Pacific. This will allow for PWATs to surge closer to +2 to +3 standard deviations, a factor that typically enhances the convective pattern across the West and has a history of more widespread thunderstorms with heavy rainfall. Latest 00z HREF is privy to the enhancement with the blended mean QPF signal depicting a more robust convective threat with local maxima bordering upwards of 2" in spots confined to the terrain. The greatest threat(s) will be located across Southern to Southeast AZ where the initial surge will be taking place, intersecting the terrain situated near the border. The second areas will be within the Mogollon Rim and Grand Canyon areas of AZ into the Southwestern portion of UT. Those areas will see their best potential the second half of the period as the surge of elevated moisture reaches the region and continues the convective threat into the evening. HREF probability fields for >1" of precip are high (60-90%) across much of the above areas with >2" markers a respectable 25-40% within the neighborhood probability field. These are textbook signatures for elevated convective flash flood threats leading to a general maintenance of the SLGT risk from previous forecast with only minor modifications to the risk area. Interior areas across UT and CO will also see an uptick in convective coverage, but the deeper moisture plume will still be making progress around the northern periphery of the ridge, so the threat isn't expected to be as robust as further southwest. The environment is favorable for some isolated impacts, especially within any stronger mid-level perturbations that rotate overhead allowing for better organized cell structures in their presence. The MRGL risk was modified only to outline the trends within the ensemble QPF where locally heavy rainfall could produce some flash flood prospects. ....New York and Northern New England... Upper low across Quebec will slowly migrate south into Upstate NY, eventually drifting eastward through VT/NH during the second half of the period. Large scale ascent pattern under the low will generate periods of showers with embedded thunderstorms as the disturbance pivots through the region. Some of the signals across the Green/White Mountains of New England are sufficient for potential locally heavy rain cores that become tied to the terrain thanks to a slower mean storm motion. The lack of surface instability will limit the threat to a more elevated convective regime which typically doesn't allow for pronounced rainfall rates. The antecedent moist soils still lingering after what transpired the past week and the slow mean storm motions are still enough to sway to the side of a MRGL risk which was added the previous update. There was little deviation in the setup from that point, so decided to maintain continuity and keep the MRGL risk over portions of NY state and Northern New England. Kleebauer 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... 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 --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .