Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 12 2023 16:33:06 FOUS30 KWBC 121633 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1232 PM EDT Sat Aug 12 2023 Day 1 Valid 16Z Sat Aug 12 2023 - 12Z Sun Aug 13 2023 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FROM THE CATSKILLS OF NEW YORK THROUGH CONNECTICUT...EASTERN OHIO INTO WESTERN PENNSYLVANIA AS WELL AS FROM SOUTH-CENTRAL COLORADO EAST THROUGH SOUTHERN KANSAS AND SOUTHWESTERN MISSOURI... ....16Z update... ....Northeastern Ohio/Western Pennsylvania... Given heavy rain over northeastern Ohio into western Pennsylvania over the past 24 hours (1 to 3+ inches through 12Z), FFG values have lowered to roughly 1 inch (or less) per hour and 1-2 inches in 3 hours for the region. Another round of thunderstorms is expected this afternoon ahead of a progressive cold front to move through Ohio and Pennsylvania from the northwest, with increasing moisture and instability with daytime heating, rainfall rates of 1-2 in/hr appear likely with areas of short term training...despite an overall tendency for storms to track toward the south and east with the mean flow. ....Kansas/Missouri... The previous Slight Risk over southeastern Colorado/southwestern Kansas was extended eastward into southwestern Missouri to account for expected heavy rain overnight, likely continuing through 12Z Sunday for Missouri/Arkansas. Increasing 850 mb winds between 03-06Z above 30 kt will likely coincide with expanding convection, out ahead of an expected MCS advancing eastward trom the High Plains, supporting potential for repeating and training. 12Z HREF probabilities for spotty 2-4 inch totals across southeastern Kansas into southwestern Missouri were 40-60 percent, overlapping with some lower FFG across the region. ....Remaining Areas of the Lower 48... Adjustments for the remaining Marginal Risks across the lower 48 were minor, with 12Z HREF guidance supporting a couple of tweaks to the existing areas. Otto ....previous discussion follows... ....South-Central Colorado east through the Oklahoma Panhandle and Southwest Kansas... An MCS is expected to develop across the Slight Risk area this afternoon through this evening. This is along a stalled out front draped west to east from Colorado all the way through the Northeast. Convection is likely to develop all along the front at various times throughout the day. Additional rainfall will develop with the MCS across the Slight Risk area. Easterly mid-level flow will advect Gulf moisture up into the southeast Colorado High Plains, meanwhile, MUCAPE values will spike above 3,000 J/kg over much of KS. A vort max will track northeastward out of the 4 Corners region and into the Panhandles by late this afternoon and this evening. The combination of all of these ingredients will lead to shower and thunderstorm development as early as 19Z across CO. The convection will quickly merge into an MCS by 23Z as the storms approach the CO/KS/OK junction area. As the storms move into the moisture and easterly low-level flow, the developing MCS may allow for cell mergers and backbuilding to locally enhance rainfall amounts over the area. While this area has been relatively dry in recent days per NASA Sport imagery, expect the storms to have ample moisture to produce rainfall rates as high as 2 inches per hour at times. Each individual cell should be moving along at a fast enough speed to keep the time any one area is seeing heavy rain low, but the aforementioned cell mergers may lengthen that time locally. Thus, the Slight Risk was hoisted as the MCS propagates eastward, generally following the KS/OK border. By evening, the storms should be fast moving enough that any cell mergers and/or backbuilding will be lesser. Thus, much of the rest of southern KS into southwest MO was kept at a Marginal risk for now. ....Catskills, Lower Hudson Valley into Connecticut... A fast moving shortwave trough will cross northern NY through New England this afternoon into tonight. North of the Slight Risk area, the shortwave should be moving plenty fast enough to keep associated shower and thunderstorm activity also moving at a rapid pace. Thus, despite very favorable antecedent conditions in the form of wet soils across northern NY and northern New England, the Marginal was kept in place. In the Slight risk area, leftover energy on the southern periphery of the shortwave will linger back across the region. With increasing moisture advecting in from the south with PWATs approaching 1.75 inches by tonight. The influx of moisture and stalled out upper level energy will support backbuilding and training convection developing to the lee of the Catskills and tracking east across the Hudson Valley and into CT. The storms will be capable of 1 to 2 inch per hour rainfall rates, and somewhat favorable antecedent conditions could result in more widely scattered flash flooding across this area. With general agreement on this synoptic setup across the area, the Slight Risk was hoisted with this update. ....South Dakota and Vicinity... Decreasing trends in the guidance as far as rainfall totals resulted in the Marginal Risk being dropped with this update. Since the moisture will be cutoff by the storms further to the south across KS, this will greatly limit storm coverage across the Dakotas. ....Mogollon Rim... Renewed convection along the Mogollon Rim associated with the monsoon is expected to develop across northern AZ into far west-central NM. Storms in recent days have been successful at generating flash flooding in limited areas, and a repeat day as yesterday is likely once again as far as overall storm coverage is concerned. ....Central Sierra Nevada... No changes were made to the inherited Marginal Risk area. QPF values are underwhelming for 24-hour totals in this area, but the second straight day of convection in this area could support additional isolated flash flooding this afternoon and evening. Wegman Day 2 Valid 12Z Sun Aug 13 2023 - 12Z Mon Aug 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN EASTERN KANSAS, SOUTHERN AND CENTRAL MISSOURI, AND THE LOWER OHIO RIVER VALLEY... ....Eastern Kansas through the Ohio Valley... Forecasted rainfall across the region remains relatively unchanged. Convection starting out the day across KS/OK will slowly advance northeastward at the nose of a southwesterly LLJ. The LLJ will help raise PWATs as high as 2.25 inches across the Slight Risk area. The convection will wane during peak heating as the supporting LLJ weakens, though doesn't entirely dissipate. By Sunday night, the LLJ will quickly restrengthen. This will support training convection associated with an MCS that will track southeastward across MO. Pre-frontal convection forming ahead of the main line will result in total rainfall in the 2 to 4 inch range in localized spots. This area is particularly vulnerable to flash flooding given favorable antecedent soil conditions from St. Louis southward, which is the area that stands the greatest risk of flash flooding, both from antecedent conditions and likelihood of seeing the higher rainfall amounts. The Slight Risk area was expanded northward to include more of IL and IN with a little bit of a northward shift in where the rainfall is likely to develop Sunday night. This makes sense since the LLJ will be pushing the axis of greatest moisture northward into the warm front. The area from St. Louis southward remains in the highest likelihood for a future Moderate Risk upgrade, but with both inherent uncertainty as to where the axis of heaviest rain will set up between St. Louis and Cape Girardeau, and also no significant increase in forecasted rainfall, the upgrade was postponed with this update. ....Upper Midwest... A strengthening surface low will develop and track across the upper Midwest today. The heaviest rainfall is expected north and east of the low center across central MN and into northwestern WI. While localized rainfall totals exceeding 2 inches is likely, that rainfall should be both spread out over a longer duration, and this portion of MN and WI has been exceptionally dry recently, so much of the rainfall is likely to be beneficial to the area. Thus, the Marginal Risk is little changed. Should higher rainfall totals occur over urban areas such as the Twin Cities, localized Slight risk impacts are possible. These impacts are not expected to be widespread enough to warrant a Slight risk upgrade, but the area will continue to be monitored. ....Central Sierra Nevada... No changes were made to the Marginal Risk area in the Sierras with this update. The overall trend in rainfall in this area from day to day is decreasing, but since the impacts tend to increase with multiple days of rain, the Marginal Risk remains for this update. Wegman Day 3 Valid 12Z Mon Aug 14 2023 - 12Z Tue Aug 15 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE GREAT LAKES, AS WELL AS FOR THE CENTRAL AND SOUTHERN APPALACHIANS THROUGH SOUTHERN PENNSYLVANIA AND NEW JERSEY... ....Great Lakes Region... A low tracking from the Upper Midwest to western New York on Monday will produce locally heavy rainfall across the Slight Risk area as it tracks eastward. The signal for heavy rain in this area has been consistent for a few days now, though the details are still unclear. There has been a slow southward trend in the guidance, which makes sense as the best moisture and forcing trend southward. Thus, the inherited Slight Risk was moved southward. In coordination with APX/Gaylord, MI forecast office, the northern L.P. has been very dry in recent days, the soils are sandy, and the southward trend should result in less rainfall around the Mackinac Straits, with more rainfall into southern areas of the L.P., including metro Detroit. Some of the moisture will be advected northward from the Ohio Valley ahead of the low, and the plentiful forcing from the developing shortwave trough should effectively wring out that moisture in the form of heavy rain. Across northern portions of the Slight Risk area, the rain should be relatively light but long in duration, while southern areas will see much more intermittent rainfall, but with heavier rainfall rates. ....Appalachians to Southern New Jersey... An MCS tracking out of the Ohio and Tennessee Valleys will move northeastward across WV on Monday, as atmospheric moisture increases to 2.25 inches of PWAT. This will allow for very efficient rainfall rates, locally exceeding 2 inches per hour across central WV Monday afternoon. A 40 kt southwesterly LLJ will supply continued ample moisture to allow storms to backbuild and train across far southwestern VA and WV. The inherited Slight Risk is largely unchanged over this region. Further north and east, similarly anomalous moisture will track northeastward up the coast with the same LLJ. Expect a notable dry signature along the mountains of western VA, where the Slight risk area was trimmed. Think there will be an upslope component enhancing rainfall rates across WV, which will result in a dry downslope counterpart across much of VA. A warm front will be pushing north through the day and especially overnight Monday night. This front will help support lift of that moisture-laden air mass across southern PA and NJ. The eastward bend to the Appalachians over central PA should also help uplift storms across the area. Meanwhile the front will be supported by wet antecedent conditions from the Philadelphia area east to the Jersey Shore. There is inherent uncertainty as to where the warm front will be set up. It's possible heavy rain may make it as far north as the NYC area, so the Slight Risk may need to be shifted northward with future updates. ....Arizona/New Mexico Border Region... Strong easterly flow will advect Gulf moisture up the terrain and into AZ/NM on Monday. Shortwave disturbances tracking along the western periphery of a stubborn ridge over TX will provide additional support for afternoon and evening showers and thunderstorms across this area. There is considerable likelihood that portions of the Marginal Risk area may need to be upgraded to a Slight with future updates. Antecedent conditions will play a big role in this, as wetter soils will support more widespread flash flooding by Monday. There is still some uncertainty on both placement and rainfall amounts, so the Marginal Risk was left with this update. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-php9fCcY0taY4PxWR-3zQxA4Idn4PC564gcsQeCTvJi= aiVLKUflIFCYoBxkF0XFCjyBTeMB0bcKU9WJ88X5uM4urFA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-php9fCcY0taY4PxWR-3zQxA4Idn4PC564gcsQeCTvJi= aiVLKUflIFCYoBxkF0XFCjyBTeMB0bcKU9WJ88X5YPF9dfQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-php9fCcY0taY4PxWR-3zQxA4Idn4PC564gcsQeCTvJi= aiVLKUflIFCYoBxkF0XFCjyBTeMB0bcKU9WJ88X5qBaT9uE$=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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