Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Mon Aug 21 2023 08:41:00 FOUS30 KWBC 210829 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 428 AM EDT Mon Aug 21 2023 Day 1 Valid 12Z Mon Aug 21 2023 - 12Z Tue Aug 22 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS OVER PORTIONS OF THE WEST... ....Western U.S.... The low to mid level circulation associated with the remnant's of Hilary is forecast to track through the northern Intermountain West today. Strong southerly winds along the eastern flank of the circulation will support an area of enhanced convergence, with deep moisture moving northeastward from eastern Oregon and southwestern Idaho. Guidance continues to show highly anomalous PWs (+4 to 6 sigma) from the northern Intermountain West through the northern Rockies. Deep warm cloud depths are expected to support areas of very efficient rainfall, with locally heavy amounts likely. The overnight guidance continues to highlight portions of northeastern Oregon into Idaho as an area where heavy rainfall is more likely. Orographic effects are expected to help to produce some 2-3 inch amounts in the Wallowa Mountains. Farther to the south across California and Nevada, PWs will be on the decline, however ample moisture will remain to support additional showers and storms. While additional widespread heavy amounts are not expected, given the antecedent conditions, even locally heavy amounts may cause additional flash flooding concerns. Across central Arizona into Utah, enhanced southerly flow sandwiched between an upper low/trough moving into California and the broad high over the central U.S. will support an axis of deeper moisture, which along with daytime heating, is expected to fuel shower and thunderstorm development, with locally heavy amounts possible later today. ....South Texas... A trough of low pressure moving across the Gulf of Mexico is expected to move inland across South Texas near the end of the period. NHC continues to highlight environmental conditions favorable for development, noting a tropical depression or storm is likely to form as it nears the coast. Regardless of development, heavy rainfall will become increasingly likely Tuesday morning as PWs increase to 2-2.5 inches across the region Tuesday morning. ....Upper Midwest... A strengthening low level jet is expected to support elevated storms developing north of an east-west oriented warm front positioned over the region. PWs are forecast to increase to 1.75+ inches as the jet amplifies. While widespread heavy amounts are not expected, guidance does show some signal for training/redeveloping convection, which may produce a ribbon of locally heavy amounts. Pereira Day 2 Valid 12Z Tue Aug 22 2023 - 12Z Wed Aug 23 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE WEST & SOUTH TEXAS... ....Northwest/Northern Intermountain West/Northern Rockies... Energy associated with the remnants of an upper low moving into California on Monday is expected to lift across the region, tracking ahead of a compact upper low dropping southeast into the Pacific Northwest on Tuesday. This along with lingering moisture is expected to support additional showers and thunderstorms, extending the flash flooding threat across the region for another day. ....Northern Arizona to western Wyoming... An axis of deeper moisture is expected to remain, with increasing mid level flow ahead of the remnant low moving into the Great Basin helping to draw some deeper moisture north across Utah into western Wyoming. This moisture is likely to support additional rounds of showers and storms, and may create more flash flooding concerns, especially for areas where storms develop subsequent days across southern into central Utah. ....South Texas... It is likely that a tropical disturbance or a more organized system will move across South Texas early Tuesday before continuing along the Rio Grande into north-central Mexico later in the period. Regardless of development, PWs of 2 to 2.5 inches and the potential for training bands, are expected to raise the threat for heavy rainfall amounts across South Texas. Limiting factors for flash flooding will be the anticipated progressive nature of the system and the antecedent dry soil conditions. ....Upper Great Lakes... A slow-moving warm front and a nocturnal low level jet are expected to support another round of elevated storms late Tuesday into early Wednesday. Storms that develop are expected to focus a little farther east than the previous night, centering from northeastern Wisconsin and the U.P. into lower Michigan. As with the previous storms, models do not suggest widespread heavy amounts, however training/redeveloping storms may produce an axis of locally heavy amounts. Pereira Day 3 Valid 12Z Wed Aug 23 2023 - 12Z Thu Aug 24 2023 ....A MARGINAL RISK OF THE SOUTHWEST/CENTRAL ROCKIES AND THE GREAT LAKES/UPPER OHIO VALLEY... ....Portions of New Mexico and Arizona northward to northern Utah... A mid to upper level shortwave associated with the tropical disturbance emanating from the Gulf of Mexico is forecast to move around the southwestern periphery of the central U.S. high, lifting north from northern Mexico into the Southwest U.S. late Wednesday into early Thursday. This along with more traditional monsoonal moisture are expected to produce showers and storms, and potentially locally heavy amounts and isolated flash flooding across the region. ....Great Lakes to the upper Ohio Valley... A warm front and the threat for overnight convection is expected to shift farther east, with areas from southeast lower Michigan to western New York and Pennsylvania most likely impacted. Models are far from agreement, with some, particularly the ECMWF, much more emphatic on the threat for heavy amounts. While confidence is limited, solutions such as the ECMWF cannot not be discounted given the general consensus of the models that shows the inclusion of deeper moisture and stronger upper forcing with this round of storms. Pereira $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .