Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Sun Jul 30 2023 09:36:00 FOUS30 KWBC 300819 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 418 AM EDT Sun Jul 30 2023 Day 1 Valid 12Z Sun Jul 30 2023 - 12Z Mon Jul 31 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST, THE NORTHERN PLAINS TO CENTRAL PLAINS/LOWER MISSOURI VALLEY, AND SOUTHERN APPALACHIANS/SOUTHEAST... ....Southwest... An inverted upper trough will continue to lift northeast from northern Mexico into the southwestern U.S., pushing the subtropical ridge farther to the east and allowing deeper moisture to advect across Arizona through the Four Corners region and into the central Rockies. The general consensus of the guidance continues to show PWs increasing to over 1.5-2 standard deviations above normal across southern California and along the southern Colorado Basin into the Four Corners region today. This moisture along with decreasing heights aloft is likely to support more widespread afternoon and evening thunderstorms and an increased threat for locally heavy amounts and isolated flash flooding. ....Northern to central Plains and lower Missouri Valley... Mid-level shortwaves moving off of the top of the ridge may pose a localized heavy rainfall threat as they interact with a plume of deeper moisture fed by southeasterly low level flow. While there are differences in the details, the overnight CAM guidance is highlighting two areas of storm development later today. One over southern Montana and central Wyoming, with storms spreading east into western South Dakota and Nebraska. The highest probabilities from the HREF for heavy amounts (greater than 2 inches) currently centers northeastern Wyoming into southwestern South Dakota. This may create isolated flash flooding concerns across the region before these storms move into the less flood prone Sandhills of central Nebraska. Storms that continue or develop farther downstream may also produce locally heavy rain and pose an isolated threat for flash flooding across portions of the lower Missouri Valley as well. Guidance is also showing storms developing farther to the north across northern Montana and southern Canada before moving into western North Dakota, with some of the CAMs showing localized heavy amounts across portions of northeastern Montana into western North Dakota. ....Southern Appalachians/Southeast... Plenty of moisture (PWs around 1.75 inches or higher) is expected to remain along and south of a frontal boundary as it continues to dip south through the Carolinas today. This moisture interacting with energy associated with overnight convection over the Ohio and Tennessee valley may produce some locally heavy amounts as it moves across the southern Appalachians and into the Southeast later today. This deep moisture will extend south to the Gulf Coast and across the Florida peninsula, bolstering the potential for heavy downpours and localized runoff concerns as the more typical sea breeze convection develops during the afternoon. Pereira Day 2 Valid 12Z Mon Jul 31 2023 - 12Z Tue Aug 01 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE SOUTHWEST TO THE CENTRAL HIGH PLAINS, THE MID MISSOURI TO THE MID MISSISSIPPI VALLEYS, AND OVER PORTIONS OF SOUTHERN GEORGIA AND FLORIDA... ....Southwest to the central High Plains... A favorable monsoon pattern will continue to support storms across portions of the Southwest, especially for Arizona and interior southern California, while expanding farther north and west across the Great Basin and the central Rockies. Deepening moisture and energy moving along the western periphery of an eastward-shifting upper high is likely to support the increasing coverage of showers and thunderstorms across southern Nevada, as well as much of Utah, Colorado, and Wyoming. PWs of 1-2 standard deviations above normal are expected across much of this area. Storms that develop across the region will have the potential to produce intense rainfall rates and raise flash flooding concerns, especially across urbanized and areas of complex terrain. ....Mid Missouri to the Mid Mississippi valley... Mid-level energy embedded within northwesterly flow interacting with a deep moisture pool associated with a low level front may produce rounds of convection with locally heavy amounts. The 00Z guidance continued to show a fair amount of spread with regard to the magnitude of amounts, but those that did show heavy amounts generally agreed on an axis extending from southwestern Iowa through Missouri. ....Southeast Georgia and northern to central Florida... Deep moisture (PWs around 2 inches) associated with a stalled frontal boundary is likely to enhance shower and thunderstorm coverage and the potential for heavy rainfall amounts across the area. Pereira Day 3 Valid 12Z Tue Aug 01 2023 - 12Z Wed Aug 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL ROCKIES... ....Southwest to the northern High Plains... While drier air is expected to begin limiting the potential for heavy rainfall and flash flooding concerns across southern Arizona and southern California, deep monsoon moisture and shortwave energy will continue to support the threat farther northeast through the Great Basin and into the central Rockies and High Plains. By Tuesday, some of the greater PWs anomalies (2+ standard deviations) are forecast to move into eastern Wyoming and Colorado. This includes recent burn scars and areas with relatively low flash flood guidance values. A Slight Risk was added to highlight the greater flash flood threat for this area. Additional upgrades may be needed, especially if confidence in heavy amounts across other vulnerable areas increases. ....Mid Missouri to the lower Ohio Valley... The pattern from Day 2 into Day 3 remains much of the same, with a similar threat for southeastward propagating storms, producing heavy amounts along much the same axis. Deeper moisture (PWs reaching above 2 inches) bolstered by strong westerly inflow into the low level front is expected to increase the threat for heavy amounts this period. Upgrades above a Marginal Risk may be forthcoming, especially if the models start to show a stronger signal for repeating heavy amounts across the same area. 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