Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jul 16 2023 08:24:45 FOUS30 KWBC 160824 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 423 AM EDT Sun Jul 16 2023 Day 1 Valid 12Z Sun Jul 16 2023 - 12Z Mon Jul 17 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF NEW ENGLAND, DOWNSTATE NEW YORK, NEW JERSEY, THE PHILADELPHIA AREA OF PENNSYLVANIA, AND NORTHERN DELAWARE... ....Northeast... A potent cold front and attendant negatively-tilted upper level shortwave has caused a large area of rainfall to develop over the Mid-Atlantic from the Delmarva through New York State. The showers and thunderstorms are moving over a large flood-sensitive area from northeastern PA through much of New England, which is causing renewed flash flooding over western portions of the Moderate Risk area. As the individual thunderstorm cells track northeastward, new thunderstorms are expected to develop around midday from the Eastern Shore and the Delmarva northeast across much of the Moderate Risk area. This will further worsen any ongoing flash flooding, as well as cause additional flash flooding to develop. Besides the highly favorable antecedent conditions, a plume of anomalously moisture-rich air is being advected northward ahead of the cold front. PWATs may exceed 2.25 inches along the immediate Jersey coast northeastward through coastal New England, which in some areas is over 3 sigma above normal for mid-July. Since July is the warmest month of the year climatologically, and warm air can hold the most moisture, an anomaly 3 sigma above normal is highly unusual. The showers and thunderstorms that have both already formed and will continue to develop across the Northeast today into tonight will have access to that level of moisture, making it highly likely that the strongest storms will have the potential to produce prodigious rainfall rates. Already this morning the storms that formed over northeastern PA are locally producing rainfall rates to 2 inches per hour. Add in diurnal instability late this morning through this afternoon and the storms will have even greater rainfall producing potential. Thus, the Moderate Risk is looking on track, and is a higher-end Moderate Risk from NYC and northern NJ northeastward through central NH. The I-84 corridor from CT through MA is particularly vulnerable, as numerous showers and storms have developed in this region out ahead of the main line of rain now crossing into NJ, with additional storms likely to develop later this afternoon. While these areas mentioned have a somewhat higher risk of flash flooding today, the signal was not consistent that the storms producing the heaviest rain will be widespread enough anywhere in the Moderate Risk area that High Risk-level flash flooding is anticipated. Rather, much of the guidance has "streaks" of higher rainfall totals in the 4-6 inch range corresponding with semi-stationary lines of training thunderstorms, which will be interspersed with areas with much less rain in between storms. This characterizes better potential for scattered instances of flash flooding with localized significant/considerable flash flooding, and not widespread flash flooding with numerous instances of significant flash flooding that would characterize a High Risk flash flooding day. In coordination with many of the forecast offices, the Moderate Risk was maintained with this update. A few notable changes were made to the area of the Moderate Risk, however. The biggest change was to expand the Moderate southward quite a bit to now include the Philadelphia area, much of NJ, and the northern tip of Delaware, including Wilmington. Both guidance and present radar trends suggest the flash flooding potential in this area has increased markedly with heavy rain likely to move over the aforementioned urbanized areas more prone to flash flooding both early this morning and redeveloping late this morning through this afternoon in this area. This change was coordinated with the PHI/Mt. Holly, NJ forecast office. The Moderate Risk area was maintained across NYC and the surrounding metro, and expanded a bit to include more of the eastern suburbs on Long Island, including much of Nassau County, as urbanization supports a higher flash flooding potential. The eastward shift in the axis of heaviest rainfall now puts NYC squarely in the cross-hairs of the potential for training thunderstorms. As mentioned the highest threat continues from northeastern CT through central MA and into NH, as highly favorable antecedent conditions combine with likely training thunderstorms embedded within a broad area of moderate rainfall for much of the day today. Should the rainfall over-perform the forecast, this is the most likely area to potentially need a High Risk upgrade later today, but this will be highly contingent on more widespread high rainfall totals, as opposed to individual storms delivering quick hits of heavy rain over the area. The Moderate Risk was also expanded a little up into southwestern ME as this area is also sensitive from previous rainfall events and the storms should have no problem maintaining themselves as they push north. For a bit of good news, the slight eastward shift allowed for a small trimming of the western peripheries of the Slight and Marginal risk areas over central PA and upstate NY. Much of the rain these areas are expected to see is occurring presently and should be over for the day around sunrise and the start of this Day 1 period. However, the caveat is that additional afternoon storms are likely to develop over eastern NY and far eastern PA, which may form a bit further west. ....Gulf Coast and Southeast... The aforementioned eastward shift in the axis of heaviest rainfall has effectively pushed most of the forecasted rain to the immediate coast or off the coast of the Carolinas. Thus, the previous Marginal Risk in this area was dropped with no flash flooding anticipated. The trailing end of the front will extend into FL and west along the immediate Gulf Coast. A few of these areas had localized heavy rain in previous days, which is likely to be more widespread today as compared to previous days, however, given the high FFGs along the immediate Gulf coast, think any flash flooding will be very localized. Some return flow will allow for a few showers and storms to develop along the Red River forming the TX/OK border, but they are not expected to be organized enough to pose any more than an isolated flash flooding threat, so the Marginal Risk for this region was maintained with minimal changes. ....Central Plains through the Chicagoland Area... Widely scattered showers and thunderstorms will form this evening through the overnight hours from KS/NE east through northwestern IN. The storms should be fast-moving and actually have a somewhat limited supply of moisture, so heavy rainfall totals are not expected. The inherited Marginal Risk area was largely left the same. Wegman Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5fXZk2-m2JrNwthrnaxkzDIJnlKqWBpwxFOrDlYMEnem= JPisfk_i7jEYZNAlZpaI5dq6Cj97xywKQ4uMAPZqihw9bmQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5fXZk2-m2JrNwthrnaxkzDIJnlKqWBpwxFOrDlYMEnem= JPisfk_i7jEYZNAlZpaI5dq6Cj97xywKQ4uMAPZqmK7JLXw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5fXZk2-m2JrNwthrnaxkzDIJnlKqWBpwxFOrDlYMEnem= JPisfk_i7jEYZNAlZpaI5dq6Cj97xywKQ4uMAPZqT65sEws$=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. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .