Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 21 2023 20:20:39 FOUS30 KWBC 212020 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 419 PM EDT Mon Aug 21 2023 Day 1 Valid 16Z Mon Aug 21 2023 - 12Z Tue Aug 22 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE WEST... ....16Z Update... Minor changes to the southwest flank of the MRGL as precip is ending across southern CA near Los Angeles with little expected precip forecast from now through tonight. There's still a marginal axis of instability located east of Los Angeles, so opted to maintain at least a MRGL for portions of southeast CA. Expanded the NE edge of the MRGL as well over MT given a tongue of higher PWATs and attendant instability axis shown on hi-res guidance with afternoon convection forecast from most CAMs. Monitoring the position of the convective development tonight across the upper Midwest over northern WI/Duluth as 12z HREF consensus has shifted the main precip axis closer to the northern edge of the MRGL in place. Given some 00z CAMs were on the southern envelope of convective positioning later this evening, will maintain the previous forecast, but trends are more for the northern extent of the previous forecast. The rest of the forecast remains on track. Kleebauer ....Previous Forecast... ....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.=20 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.=20 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 IS IN EFFECT FOR PORTIONS OF THE WEST AND SOUTH TEXAS... ....2030z Update... Main changes occurring across the western CONUS. SLGT located over the NW US was expanded to the Canadian border in conjunction with a higher QPF axis extending from the ID/OR border up through northwest zones in MT. Elevated low-mid level moisture remains entrenched across the area from the anomalous setup in previous days creating an environment favorable for heavier rainfall with marginal instability present. Second SLGT located over UT has been extended to the northern border of AZ up into SW WY as convective coverage is once again the main driver of the change. Both 12z HREF mean and global ensembles generate an elongated axis of heavier precip as we continue to see mid-level flow draw elevated deep layer moisture northward over the higher terrain, especially the Wasatch. Across South TX, have nudged the SLGT a bit up the Rio Grande, but remaining south of Del Rio to account for a small shift in QPF maximum on the northern extent of PTC Nine moving over the region. Confidence has increased for the bulk of the precip moving to the north of the lower Rio Grande, but anomalous low-mid level moisture flux with the tropical disturbance and a favorable region of instability on the southern half of the circulation, the threat for locally heavy rain and flood concerns remains a threat through the course of Tuesday. Lastly, given latest CAMs and northern shift of the blended HREF mean QPF across MI, opted to expand the northern periphery of the previous MRGL. Kleebauer ....Previous Discussion... ....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 EXCESSIVE RAINFALL IS IN EFFECT FOR THE SOUTHWEST/CENTRAL ROCKIES AND THE GREAT LAKES/UPPER OHIO VALLEY... ....2030z Update... No discernible changes to the D3 ERO. Will be monitoring for the trends across Lower MI down through western NY, PA, and NE OH as the ECMWF continues to harp on a more robust convective signature that would certainly lend credence to greater flood concerns. Out west, expecting another round of general convection extending from UT, south into far west TX thanks to an elevated moisture field stemming from tropical remnants moving out south TX and Mexico. Kleebauer ....Previous Discussion... ....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 Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7OjqKJ9JzAG5G29XQgmD2UH1htVJTGbTb8ZgHVZQGzty= g2dbrJ7x1PdhyiKHdIowU-M5iCz8Q20B2SugeEUZfTjwmJE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7OjqKJ9JzAG5G29XQgmD2UH1htVJTGbTb8ZgHVZQGzty= g2dbrJ7x1PdhyiKHdIowU-M5iCz8Q20B2SugeEUZkEZinqk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7OjqKJ9JzAG5G29XQgmD2UH1htVJTGbTb8ZgHVZQGzty= g2dbrJ7x1PdhyiKHdIowU-M5iCz8Q20B2SugeEUZMa0yHL4$=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. 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