Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 23 2023 08:30:12 FOUS30 KWBC 230830 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 429 AM EDT Wed Aug 23 2023 Day 1 Valid 12Z Wed Aug 23 2023 - 12Z Thu Aug 24 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST AND THE GREAT LAKES/UPPER OHIO VALLEY... ....Texas Big Bend Region into the Southwest... The mid to upper level shortwave associated with the remnants of tropical cyclone Harold will continue to move around the southwestern periphery of the central U.S. high, lifting north from northwestern Mexico into the southwestern U.S. today.=20 Showers and storms fed by an axis of deep southeasterly flow and moisture will likely be ongoing across portions of the Big Bend and West Texas this morning, producing locally heavy amounts and isolated to scattered flash flooding concerns. As the system continues farther north, its associated moisture and the more typical monsoonal moisture are expected to increase shower and thunderstorm coverage and a greater threat for isolated flash flooding across portions of the Southwest from southern New Mexico to southern Utah. A Slight Risk was maintained from West Texas into southern New Mexico. PWs at or above 1.75 inches (1 to 2 standard deviations above normal) in addition to enhanced forcing east of the center are expected to support heavy rain and flash flooding translating northwest across this area today.=20 ....Great Lakes to the upper Ohio Valley... Ongoing elevated convection over the northern Great Lakes is expected to continue and may produce some locally heavy amounts as it drops southeast into the eastern Great Lakes and upper Ohio Valley early in the period. However, the greater threat for heavy rain and flash flooding is forecast to develop late today into the overnight hours as the low level jet intensifies and storms ignite once again east of a slow-moving warm front. Storms that develop will have ample moisture at their disposal, with models showing PWs of 1.75-2.25 inches pooling on the nose of 40+ kt westerly inflow. While there are still plenty of differences in the details, at least some of the guidance, including the 00Z HRRR and ARWs, indicated a period of northwest to southeast training storms along the warm front from southeast Michigan to northeast Ohio and northwest Pennsylvania before a more organized system develops and moves across the region. While uncertainty remains, felt there was enough of a growing consensus to upgrade a portion of the area to a Slight Risk. The Slight Risk generally highlights the area where the HREF shows higher probabilities for rainfall amounts exceeding 2 inches. The lower resolution NAM, GFS, and ECMWF also indicate heavy amounts developing across this region as well. Pereira Day 2 Valid 12Z Thu Aug 24 2023 - 12Z Fri Aug 25 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF SOUTHERN UTAH.. ....Southwestern U.S.... The mid-level remnants of Harold and monsoonal moisture spreading north along of the west side of the central U.S. ridge will support increasing shower and thunderstorm coverage across the Four Corners states and southern Nevada on Thursday, with PW anomalies of 1 to 3 standard deviations above normal forecast within much of the highlighted Marginal Risk area. Given the complex terrain and the center of some of the greater PW anomalies (2 to 3 standard deviations above normal), a Slight Risk was maintained across portions of southern Utah. ....Lower Great Lakes and upper Ohio Valley to the Northeast... A series of mid level shortwaves will continue to move across the top of the central U.S. ridge, moving east-southeastward across the Great Lakes -- supporting additional rounds of showers and storms. With little confidence in the details, some models suggest another period of northwest to southeast training storms from the southern Great Lakes into the Ohio Valley ahead of wave moving from Lower Michigan to the eastern Great Lakes Thursday night. A strong low level jet will once again supply ample moisture, raising the potential for heavy amounts. There is also some model indication that warm advection precipitation, with locally heavy amounts, may spread farther east into portions of the Northeast by early Friday. While upgrades may introduced in future updates, given the uncertainty did not have the confidence for anything greater than a Marginal Risk at this point.=20=20=20 Pereira Day 3 Valid 12Z Fri Aug 25 2023 - 12Z Sat Aug 26 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS THE CENTRAL HIGH PLAINS/ROCKIES.. ....Southwest to the central High Plains.... In addition to the more typical monsoonal showers and thunderstorms, the mid-level remnants of tropical cyclone Harold will continue to spark activity as it moves northeastward along the western periphery of the central U.S. ridge. Organized heavy rainfall is expected to become increasingly likely as the system moves east across southern Wyoming and northern Colorado late Friday into early Saturday. Moisture associated with Harold, along with strengthening low level easterly flow will support deepening moisture over northeastern Colorado, southeastern Wyoming, and the Nebraska Panhandle. The general model consensus shows PW anomalies of 2 to 4 standard deviations above normal covering the region on Friday. Overall the 00Z guidance was in generally good agreement, indicating 1-2 inches, with locally higher amounts, centered over this region. Expect the heavier totals to center along the upslope regions of northeastern Colorado and southeastern Wyoming, along and near the Foothills and Front Range.=20 ....Mid Atlantic... Deep moisture accompanying a cold front dropping south through the region is expected to produce scattered storms with locally heavy downpours possible. Training storms may help elevate the threat for localized heavy amounts. Not much confidence in the details, therefore subsequent adjustments are likely forthcoming in future outlooks. However for now, a Marginal Risk was added where guidance generally indicates PWs increasing to around 2 inches and the NAM, GFS, and Canadian Region show at least some potential for locally heavy amounts. Heavy amounts that do occur may pose isolated flooding concerns, especially over urbanized areas. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!95smLB74z8YludSJq1OGbA0HRWH_0YjZcFcE3vflGuXp= 5l2Um67J7s9CulWkTdd7NDeaU7s9CJb5D6nG9bsriJHjOSw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!95smLB74z8YludSJq1OGbA0HRWH_0YjZcFcE3vflGuXp= 5l2Um67J7s9CulWkTdd7NDeaU7s9CJb5D6nG9bsr0iyZqYU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!95smLB74z8YludSJq1OGbA0HRWH_0YjZcFcE3vflGuXp= 5l2Um67J7s9CulWkTdd7NDeaU7s9CJb5D6nG9bsr2HpZ48w$=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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