Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 445 AM EDT Tue Aug 09 2022 Day 1 Valid 12Z Tue Aug 09 2022 - 12Z Wed Aug 10 2022 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE MID-MISSISSIPPI VALLEY TO THE CENTRAL APPALACHIANS AS WELL AS FOR PORTIONS OF THE SOUTHWEST AND GREAT BASIN... ...Lower Mississippi Valley, Ohio Valley, and Central Appalachians... A front extending from the Red River of OK/TX through to New England is acting as a focus for convection along and south of the front this morning. It will be weak and very slow-moving today into tonight, sagging southward as the day goes on. Nonetheless, the front will act as a forcing mechanism for continued convection that will develop in the moist, unstable air mass south of it. This airmass is characterized by PWATs of 2-2.5 inches, which is 2 standard deviations above normal. Further, instability will be quite notable, generally in the 2,000 to 3,000 J/kg range. Finally, FFG is exceptionally low in this area, generally between 1 and 2 inches in 1 to 3 hours due to fairly persistent heavy rainfall since July 25. Portions of eastern KY have received 10-15"+ during this time frame which is ~600% of normal...Southern IL has fared similarly. This combination of ingredients is forecast to have the following evolution: The periodic areas of convection ongoing from OH through MO will maintain themselves or very slowly weaken through the early morning. Widely scattered slow-moving showers and thunderstorms will develop along and south of the front in the Slight Risk area as soon as early afternoon. The storms would develop strong updrafts/heavy rainfall rather quickly due to the high instability. Most storms should not be able to organize much, remaining mostly cellular, though they may form into a line right along the front along almost the entire length of the Ohio River by mid-to-late afternoon. Topographic effects may also increase rainfall amounts further east into eastern KY and WV. Enough low-level inflow is forecast that it's possible that random pockets of loose organization/training could occur. Slow storm movement and the possibility of random loose organization and cell mergers could lead to isolated spots of up to 3" an hour. Widely scattered to scattered flash flooding will be possible this afternoon, particularly over areas with saturated soils. Due to the front hanging up further north to start the day today, the Slight Risk was expanded northward with this forecast package to include more of southern IL, IN, OH, northern WV and the southwest corner of PA. ...Southwest/Great Basin... The monsoon continues in the Southwest as the combination of southwesterly flow ahead of a strong upper level low off the California coast and southeasterly upper level flow across NM and AZ rounding the base of an upper level high centered over UT and CO meet to form a corridor where heavier convection is likely to form Tuesday afternoon. Some moisture appears to be drifting up the Gulf of CA/Sea of Cortez around the periphery of Tropical Storm Howard. This corridor over AZ into NV will feature much greater instability than the surrounding area with CAPE values between 1,000 and 2,000 J/kg. The highest values of CAPE are expected over southern AZ. The available moisture and instability support the potential of local amounts in the 2" range, which could occur within an hour. The abnormally wet conditions this region has been seeing the past few weeks means the soils in this area are abnormally moist. The largest issues are expected in slot canyons, dry washes, burn scars, and urban centers. Precipitable water anomalies are as much as 4 standard deviations above normal through northwest NV and southeast OR, making this area very unusually prone to the potential for flash flooding. With continued southerly flow ahead of an upper level low that will be moving into the northern CA coast today, the Slight Risk was expanded through northern NV into southeastern OR in coordination with the BOI/Boise, ID forecast office. Wegman/Roth Day 2 Valid 12Z Wed Aug 10 2022 - 12Z Thu Aug 11 2022 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE MID-ATLANTIC THROUGH THE MID-TENNESSEE VALLEY, PORTIONS OF THE SOUTHWEST AND GREAT BASIN, AND THE CENTRAL GULF COAST... ...Mid-Atlantic through the Lower Mississippi Valley... Confidence is increasing on the frontal position generally near the OH River and interior sections of the Mid-Atlantic to southern New England Wednesday. Continued elevated moisture (generally 2 to 2.5 sigma above normal) and ample instability (1000 to 2000 J/kg MUCAPE), the surface front and height falls aloft as troughing amplifies over the Northeast should allow an organized risk of convection, particularly over the TN Valley/Cumberland Plateau through the central Appalachians and to the Mid-Atlantic. Wherever cell training or mergers occur, hourly rain totals to 2.5" would be possible. The Slight Risk has been expanded eastward to the Jersey Shore south through the Delmarva in coordination with the LWX/Sterling, VA and PHI/Mount Holly, NJ forecast offices. FFGs are low (generally around 1.5"/3hr) in some areas due to recent wetness/ground saturation/urbanization. A Marginal Risk extends from the lower MS Valley through southern New England where there is an isolated flash flood risk. ...Southwest and Great Basin... The monsoon plume continues to drift east over the Southwest/Great Basin as the upper level high center drifts east over CO on Wednesday. A corridor of continued high moisture and instability (perhaps more unstable Wednesday than today - it depends on cloud cover) over NW AZ, SE NV, and SW UT including portions of the slot canyon area of southern UT continues to support a Slight Risk. The elevated moisture plume continues north over NE NV and much of western Idaho (1.25" PW in the Snake River Valley is 3 to 4 sigma above normal) where there is consensus for locally heavy terrain based activity. A low level boundary -- likely a thermal trough -- is expected to shift modestly across the region acting as the focus for heavy rainfall. Changes with this forecast package include expanding the Slight Risk northward into southern ID in coordination with the BOI/Boise, ID and PIH/Pocatello, ID forecast offices. The Slight Risk has also been expanded southwestward along the Mogollon Rim into New Mexico as this topography continues to be a focus for convection. ...In and near Southeast LA... A low-level disturbance ahead of an inverted trough aloft drifts from the eastern Gulf of Mexico into the central Gulf coast, bringing a southerly wind surge (10-15 kts) at 850 hPa in its wake (roughly double the mean wind due to the northerly flow aloft) with precipitable water values of 2-2.25". As the flow is off the warm Gulf, CAPE values of 2000+ J/kg are forecast. This all should contribute to rainfall efficiency with occasional loose, short training bands and quasi-stationary/merging cells that could bring hourly rain totals towards 3" -- well above what most metropolitan areas can handle. Portions of Southeast LA and southwest MS have been anomalously wet over the past week. No changes were made to the Slight Risk for this area with this forecast package. Wegman/Roth Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt