Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 22 2023 00:35:41 FOUS30 KWBC 220035 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 834 PM EDT Mon Aug 21 2023 Day 1 Valid 01Z Tue Aug 22 2023 - 12Z Tue Aug 22 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE WEST... ....Western U.S.... The vestiges of Hilary continue to move through the northern portions of the Intermountain West this evening. In its wake, deep layer southerly flow will continue to channel pockets of deeper moisture across much of the West, particularly Arizona into southern Utah, areas across central/northern California and further north into eastern Oregon, Idaho, and western Montana. This very anomalous moisture should support isolated to scattered showers and thunderstorms that will be capable of producing hourly rainfall totals between 0.5-1" at times, enhanced by orographic effects and some instability, especially across portions of Arizona and Utah. The ERO risk areas were trimmed and adjusted to fit the latest model trends and higher probabilities of seeing the most intense rain rates through 12Z. ....South Texas... The initial rain bands associated with Tropical Depression Nine will begin to move onshore the South Texas coast later this evening through tonight. Heavy rainfall will become increasingly likely with some stronger cores expected overnight which may produce hourly rainfall totals in excess of 1" at times. The anomalously high moisture (PWs 2"+) will contribute to high rainfall efficiency, though the relatively fast forward motion of the system combined with the very dry antecedent conditions should limit any flooding to mainly isolated instances over urban and low-lying vulnerable areas. ....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 and 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. Taylor 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!7HuPaD9VL7L_H8e5-d1amNOJ-CQ7NgrNvRCxFoj3iCPW= m_0tLEfoPRenQ84SA3pLsHmLd7VYjeZTr2g1IK8krUPSMs0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7HuPaD9VL7L_H8e5-d1amNOJ-CQ7NgrNvRCxFoj3iCPW= m_0tLEfoPRenQ84SA3pLsHmLd7VYjeZTr2g1IK8k6GNIHVc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7HuPaD9VL7L_H8e5-d1amNOJ-CQ7NgrNvRCxFoj3iCPW= m_0tLEfoPRenQ84SA3pLsHmLd7VYjeZTr2g1IK8kFDN2W2M$=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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