Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jul 16 2023 20:22:22 FOUS30 KWBC 162022 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 421 PM EDT Sun Jul 16 2023 Day 1 Valid 16Z 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... 16Z Update... With heavy precipitation still ongoing and the potential for redevelopment farther to the west later in the day, did not make any significant changes to the previous outlook areas across the Northeast. Given the heavy rains that occurred earlier and the potential for additional heavy amounts associated with a mid-level vort lifting northeast across the region, the Moderate Risk area was extended farther south to include the northern Delmarva.=20 A Slight Risk was added to an area extending from North Texas to southern Louisiana. Deep moisture pooling along a boundary ahead of the convective complex now moving along the Red River is forecast to fuel slow-moving storms ahead of the system.=20 Consensus of the HREF guidance shows these storms along with the convective complex contributing to an axis of heavy rainfall extending across the region. HREF neighborhood probabilities indicate that locally heavy amounts of 3 inches or more are likely within the Slight Risk area. Pereira ....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. ....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 Valid 12Z Mon Jul 17 2023 - 12Z Tue Jul 18 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR THE MID-MISSOURI RIVER VALLEY, OHIO VALLEY TO THE EASTERN GREAT LAKES, AS WELL AS MUCH OF THE FLORIDA PENINSULA, SOUTHERN LOUSIANA, AND SOUTHEAST ARIZONA/SOUTHWEST NEW MEXICO... 2030Z Update... Made only minor updates based on the 12Z guidance, with no significant changes from the previous forecast. Pereira Previous Discussion... No major changes were made to the various Marginal Risk areas across the country on Monday. A secondary front pushing south and east across the Midwest will provide the focus for widely scattered showers and storms over the Ohio Valley today. Since the main plume of moisture responsible for the heavy rain Sunday in the Northeast will be well off the coast by Monday, this secondary front will have much less access to atmospheric moisture. This will dramatically cut down on the storm coverage and intensity, which in turn will limit the flash flooding threat as well. Nonetheless, isolated training/backbuilding storms can't be ruled out, with the strongest signal from southeast OH into western PA. The Marginal Risk was maintained across this area. The southern end of the same front will be between forcings, and therefore the signal for heavy rain has diminished, thus the biggest change from inherited was to drop much of MO and IL from the Marginal Risk area. Meanwhile further northwest across the Mid-Missouri River Valley, a developing LLJ will begin pushing the front east as a warm front Monday night. This interface will be the focal point for scattered storms, primarily over SD and northeastern NE. Isolated flash flooding can't be ruled out. This same LLJ will be much more influential and impactful further south and east on Day 3/Tuesday. The FL Peninsula will remain on the southern end of the impressive moisture plume now entirely off the rest of the East Coast. Thus, scattered but largely disorganized convection may still produce flash flooding since any storms will have ample moisture to draw from. The greatest threat will be along the Treasure Coast. A weaker signal persists over southeastern LA, but with possible flash flooding from Day 1/Sunday, opted to leave the Marginal as a low-end threat for Monday. This is the lowest confidence of the various Marginal Risk areas. Finally, weak monsoonal flow may produce a renewed round of afternoon showers and thunderstorms over southeastern AZ and southwestern New Mexico. This area was hit by convection the past couple days, so an isolated flash flooding threat can't be ruled out. Wegman Day 3 Valid 12Z Tue Jul 18 2023 - 12Z Wed Jul 19 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR THE LOWER MISSOURI VALLEY TO THE LOWER OHIO VALLEY AS WELL AS FOR PORTIONS OF THE NORTHEAST... 2030Z Update... No significant changes from the previous forecast. Models continue to show a strong signal for heavy rains developing across portions of the lower Missouri, mid Mississippi, and lower Ohio valleys this period. However, there remained enough location uncertainty to hold off on an upgrade to a Moderate Risk for now. Pereira Previous Discussion... ....Lower Missouri Valley to the Lower Ohio Valley... An MCS at the nose of strengthening LLJ is expected to develop over eastern MO and IL Tuesday evening, which will intensify as it slowly moves east across IL and IN Tuesday night. The LLJ will strengthen to 50 kts in the mid-levels Tuesday night as it advects plentiful Gulf moisture northeastward into a warm front draped across the area. This front will be the primary source of lift for strong and slow-moving thunderstorms capable of over 2 inch per hour rainfall rates in the strongest storms. While the area has been about average for rainfall over the past 2 weeks based on AHPS data, the potential for very heavy rain would likely overcome these antecedent conditions should the heavy rain persist for a long enough period of time. Since the front will be slow moving, this will support training thunderstorms which will move southeastward with time. The most persistent areas of rain may see rainfall totals exceed 5 inches for storm total rainfall from this event. There is considerable uncertainty as to where the MCS is likely to develop. The areas mentioned above are most likely to see the greatest rainfall, but the spread covers from central MO east to southwestern OH. Assuming the signal remains high for localized heavy rain, and with better consistency among the guidance with future forecast cycles, a Moderate Risk upgrade is possible. For now the high-end Slight Risk encompasses from St. Louis east across southern IL and southwestern IN. ....Northeast... The front pushing east across the Mid-Atlantic will have considerable upper level support from a deepening negatively tilted longwave trough. However, moisture will be limited as the primary moisture plume will be well off the coast. Thus, while storms will have the potential to grow strong with the strong forcing, the relative lack of moisture will limit strength to some extent, but moreso coverage. Thus, the Slight Risk is primarily driven by highly favorable antecedent conditions that will be ongoing after today's rainfall event. Tuesday's storms will be less strong and with less coverage, but since FFGs are likely to be low, widely scattered flooding is still possible. The Slight Risk was expanded southward to include all of the Hudson Valley and into northern NJ with this update, following the axis of heaviest rainfall. ....South Florida... Typical sea breeze convection will be ongoing with the stationary tropical air mass persisting. The Marginal Risk was nudged northward to include more of central FL with this update. ....Southeast AZ/Southwest NM... Monsoonal flow is likely to result in another round of afternoon showers and thunderstorms over the same area as previous days. Isolated flash flooding in sensitive areas can't be ruled out. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-a-v-xrezNv6XyFdrOJ7gpTvjzrwKAPO7Hny2RQLvZX3= rYW-4CBJsz_BGma4NQMnxXgjY-EFKUXwUrdI9Jg4ANa929M$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-a-v-xrezNv6XyFdrOJ7gpTvjzrwKAPO7Hny2RQLvZX3= rYW-4CBJsz_BGma4NQMnxXgjY-EFKUXwUrdI9Jg4K_6XzsQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-a-v-xrezNv6XyFdrOJ7gpTvjzrwKAPO7Hny2RQLvZX3= rYW-4CBJsz_BGma4NQMnxXgjY-EFKUXwUrdI9Jg4pWuW-sA$=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) .