Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Tue Aug 20 2024 07:38:00 FOUS30 KWBC 200752 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 352 AM EDT Tue Aug 20 2024 Day 1 Valid 12Z Tue Aug 20 2024 - 12Z Wed Aug 21 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST U.S... Current upper air analysis across the CONUS depicts a sprawling mid-level ridge positioned over much of the Southern Plains into the Southwest. The ridge itself will maintain its orientation with some modest strengthening heading through the afternoon with the center of the ridge positioned across the Southern Rockies. Enough diurnal destabilization and moisture trapped under the ridge will offer a threat of widely scattered convection across the terrain in AZ, extending into Southern CO and Northern NM. The overall QPF footprint is fairly lackluster in the areal average, but a few pockets of heavier precip can be found within the CAMs indicating a low-end potential for heavy rain and potential flash flooding. The primary concerns will remain tied to the complex terrain and adjacent towns that could be impacted by rapid onset flooding. Slot canyons and remnant burn scars located across the Southwest will be under the threat as well, one of the main reasons for continuity from the previous forecast, as well as an expansion eastward into Southern CO and Northern NM where multiple burn scars are located. This remains the primary area of interest for the period. A small area across east-central SD will also experience a threat of locally heavy rainfall as remnants of a complex of thunderstorms migrates eastward along the leading edge of favorable mid-level ascent as a shortwave ejects out of the High Plains. Favorable instability axis across the Northern Plains favors points further west with the storm complex likely to enter a less favorable convective regime to maintain a strong enough intensity to induce flooding prospects. LLJ will also be on the decline leaving behind a relatively mundane setup for prolonged convective enhancement that would be necessary for flash flood prospects. Considering the above variables, and the addition of drier soil moisture located within the forecasted zone of impact, a nil risk area was maintained, however the threat is non-zero (<5%). Kleebauer Day 2 Valid 12Z Wed Aug 21 2024 - 12Z Thu Aug 22 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN ARIZONA... A more robust moisture advection pattern will ensue across the Southwest U.S with some Pacific moisture being pulled northward thanks to the combination of the Southern ridge shifting eastward and weakening, along with a shortwave trough quickly moving inland through CA leading to a pathway for elevated moisture and embedded shortwaves pivoting out of Sonora and nearby Baja to move northward. Ensemble QPF is much greater compared to the D1 time frame thanks to the poleward advancement of anomalous moisture (+2 to +3 deviation PWATs based off the latest NAEFS) and subsequent instability increase. The ascent pattern across the Great Basin is the driver for a better signal further north in-of AZ and UT thanks to diffluent flow out ahead of the shortwave. Ensemble QPF has increased since the last series of runs with a more pronounced convective depiction signaling some totals between 1-1.5" now introduced in the means. The back end of the 00z HREF is already showing the increased convective pattern at the beginning of the diurnal period with the remainder of the evening still to go. Ensemble bias corrected QPF is running between 0.75-1.25" for an areal average over a good portion of AZ with the focal points situated over the terrain in Southern AZ and across the Mogollon Rim and Grand Canyon areas of Central and Northern AZ. The prospects are certainly favorable for more localized flash flood concerns just based off the anomalous PWAT advection anticipated. After collaboration with offices out west, a targeted SLGT risk was added across Northern AZ encompassing places like Flagstaff to the Grand Canyon, representative of the strongest heavy precip signals expected during the D2 period. Additional upgrades to a SLGT risk are also possible within the next series of updates, especially within Southern AZ and Southern UT pending the run-to-run consistency within guidance. Kleebauer Day 3 Valid 12Z Thu Aug 22 2024 - 12Z Fri Aug 23 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST U.S... A continuation of the elevated moisture pattern across the Southwest will transpire in D3 with an axis of scattered to widespread convection likely across much of AZ through the Inter Mountain West. Small mid-level perturbations are still forecast to rotate northward around the western flank of the ridge positioned across the Southern Plains. A digging upper low off the PAC Northwest coast will allow the flow to become even more meridional across the West allowing for the more anomalous PWAT advection to occur further in the interior leading to the best heavy rain axis to likely situate over Northern AZ through much of UT and Western CO. The growing consensus on the specifics of where the heaviest precip will focus will likely correlate to an upgrade at some point in the future, but timing of the mid and upper pattern still has some points to hash out before going into more detail. As of now, the MRGL from the previous forecast was maintained but expanded on all sides as the 00z ensemble QPF footprint expands with agreement from the recent ML output on the precip placement. Across Florida, a stalled frontal boundary will become the focal for scattered convection on Thursday afternoon with the areal QPF average relatively elevated, but still not depicting a substantial enough signal to warrant a risk addition to the area. This will be a period to monitor for perhaps a targeted risk along either coast, pending sea breeze pattern evolution where heavy rain could affect the the urban corridors either side of the Peninsula, or an area inland like Orlando proper. Kleebauer --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .