Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jul 14 2023 19:45:45 FOUS30 KWBC 141945 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 343 PM EDT Fri Jul 14 2023 Day 1 Valid 16Z Fri Jul 14 2023 - 12Z Sat Jul 15 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER HUDSON VALLEY THROUGH WESTERN AND CENTRAL MASSACHUSETTS...=20 ....16Z Outlook Update... The ongoing forecast is generally on track, although several changes have been made based on morning convective trends and 12Z model guidance.=20 ....Northeast... A very small northwestward adjustment to the Moderate Risk area was made, and a westward expansion of the Slight was made across northeastern Pennsylvania. Through the remainder of the day, models focus most convective development from northeastern PA through southern NY, with lower chances of storms farther southeast across Connecticut in the wake of morning convection in that area. The categorical risk adjustments are also consistent with model trends. Areas of 1-2 inch/hr rain rates and backbuilding/training could affect especially the Moderate Risk area and spread eastward from northeastern PA into the NYC Metro later this afternoon and early evening. ....Mid-Atlantic... Robust convective development across central/eastern North Carolina this morning has left behind an outflow boundary across that area, leaving some doubt regarding convective coverage in eastern Virginia this afternoon. Surface dewpoints are still in the 70-76F range in eastern Virginia though, and any sunshine in the wake of morning rainfall could destabilize the area and cause a few storms to develop. With the greater focus for convective development now evident across central/eastern NC per 12Z guidance and surface observations, the Slight Risk has been adjusted to account for better flash flood potential in NC today. It is worth noting that HREF QPF totals of 1-3 inches across southern/central North Carolina could materialize across areas that have received copious amounts of rainfall this morning. This area will be monitored for the possibility of a targeted Moderate Risk upgrade should a corridor of training convection materialize for a few hours this afternoon. ....Mid-Mississippi Valley to the Florida Panhandle... A persistent MCS continues to result in localized flash flood potential across coastal areas of the Florida Panhandle this morning. Latest CAMs/observations suggest that the ongoing MCS could backbuild westward toward southern Alabama, while additional deep convection develops to the north toward the MS/AL border area. Convective mergers and training associated with these features could result in locally heavy rainfall totals - perhaps exceeding 5 inches in a few spots. This area will also be monitored for the possibility of a targeted Moderate risk upgrade especially if one or two axes of extreme (2.5+ inch/hr) rain rates can persist for multiple hours. It is worth noting that models redevelop storms near the central Florida Panhandle between 06-12Z that could also produce locally heavy rainfall and rain rates exceeding 2.5 inches/hr, though that rain should occur across areas of fairly high FFG thresholds (3+ inch/hr), suggesting that multiple hours of heavier rates would be needed for any widespread flash flood potential. Another area of concern exists from the MS/OH River confluence eastward into western Kentucky. Recent CAMs suggest that localized training potential could materialize there as upstream MCS activity reaches the region after 06Z tonight. The timing and extent of convection is a bit uncertain, although the potential was high enough to expand the Marginal area eastward and include a Slight Risk for this update. Cook ....Previous Discussion... ....Northeast... Confidence continues to increase in the potential for even more flash flooding in the hard-hit areas of the lower Hudson Valley through the Berkshires and into central Massachusetts. Additional heavy rainfall occurred through this exact region overnight, and flash flooding is ongoing across portions of the lower Hudson Valley and southern Catskills as training and backbuilding storms develop and move across that area. These storms are expected to largely dissipate over the next few hours, but given their longevity so far, that remains to be seen. In fact, recent radar trends have been showing additional shower and thunderstorm formation across the Catskills and into the Capital Region. Around noon today, the second round of storms, for which the Moderate Risk is in effect, is expected to begin from northeastern PA through southeastern NY and northern NJ. Each individual storm cluster is likely to be moving at a reasonably fast clip. However, the combination of very favorable antecedent conditions (flash flooding is ongoing now in many of these areas!), and then this second round, which is actually likely to consist of several rounds of storms generally forming around the Catskills/Poconos then moving off to the north and east into New England. A cold front moving across New York and Pennsylvania will push east today as a warm front races north across New England, leaving much of New England in the warm sector, consisting of PWATs to 1.75 inches. An upper level shortwave across Ohio and the Virginias will rapidly lift northeastward today which will add to the atmospheric lift. With daytime heating, most unstable CAPE will rapidly increase locally to as high as 3,000 J/kg, and more broadly to 2,000 J/kg across the lower Hudson Valley and southern New England, which may allow for storms to begin even as early as late morning. Regardless, this will be plenty of fuel for storms to produce rainfall rates as high as 2 inches per hour. With training and backbuilding likely due to largely parallel mid-level flow to the cold front, local amounts could approach 6 inches in the hardest hit areas, which again are very likely to be the same areas that have much lower FFG values from rainfall both in the recent past and ongoing. The Moderate Risk was coordinated with all of the impacted forecast offices: BGM/Binghamton, NY, ALY/ Albany, NY, OKX/Upton, NY, BOX/Norton, MA, and GYX/Gray, ME. ....Mid-Atlantic... Multiple rounds of training showers and thunderstorms are ongoing across southern VA and northern NC. The training storms moving east along an east-west oriented boundary resulted in numerous flash flood warnings from far southern WV through southern VA overnight. A moisture plume consisting of PWATs approaching 2.25 inches will be drawn northward ahead of a cold front also moving across the Northeast. CAMs guidance suggests central NC will be the primary area that will see strong storms capable of 2+ inch/hour rainfall rates this afternoon. However, given the sensitivity of the soils across southwest VA, this area was included in the newly expanded Slight risk, despite somewhat lower signal for heavy rains later today. Storms will form in this highly favorable environment around midday today across central and western NC, likely including somewhat lower coverage into south central VA. As the storms move east into the evening, storm mergers are likely as the storms that form along the mountains in western NC catch up to the slower moving storms across central NC, resulting in a greater likelihood of flash flooding across NC, where fortunately FFGs are at least somewhat higher. Regardless, given the impressive amount of moisture these storms will have at their disposal, the heavy rainfall rates are likely to locally exceed FFGs. ....Mid-Mississippi Valley to the FL Panhandle... Yet another pair of MCSs are ongoing across the far northern TX Panhandle and northeastern OK this morning. The storms towards the Panhandle are expected to gradually weaken as they move into western OK, while the storms over northeastern OK are expected to grow upscale into a larger MCS across AR by midday, tracking into MS and TN during the afternoon. As second round of storms starting the day in NE will race southeastward into AR overnight and eventually weaken across northern MS early Saturday morning. These repeated rounds of storms, some of which are expected to move over hard-hit areas of eastern MS and far western AL, are expected to cause widely scattered flash flooding across eastern OK, AR and into MS. Thus, the Slight Risk area inherited was expanded well to the west to include central OK. Depending on how much agreement can occur between the CAMs regarding tonight's convection, further westward expansions into the Panhandles may be needed with a new round of showers and thunderstorms that may develop there late tonight. The Slight risk area extends down to the FL Panhandle for storms that are developing now and will persist through the morning. Close proximity to the very warm Gulf will supply the storms with an abundance of moisture, and the slow-moving storms could back build or have a second round move through from the inland MCS's weakening as they move towards the area. FFGs in this area are rather high, so the higher flash flooding threat is likely further inland near the MS/AL border due to yesterday's very heavy rains, but flash flooding is possible, particularly in urbanized areas around Pensacola. Wegman Day 2 Valid 12Z Sat Jul 15 2023 - 12Z Sun Jul 16 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR IN AND NEAR NORTHEAST NEW MEXICO, THE OKLAHOMA/TEXAS PANHANDLES, CENTRAL APPALACHIANS, NORTHERN PENNSYLVANIA, AND SOUTHERN NEW YORK STATE... ....2030Z Outlook Update... Mid-level troughing and the present of widespread moisture along and east of the Mississippi River will result in several areas of lines and clusters of thunderstorms. While many areas of east of the Mississippi will have a chance at occasionally heavy rainfall, a couple areas exhibited locally lower FFG thresholds (generally less than 1 inch/hr across NY/PA and around 1-1.5 inch/hr across the central Appalachians) to coincide with these rainfall chances. The peak thunderstorm potential across the central Appalachians will likely occur during the afternoon and early evening, and HREF probabilities of FFG exceedence were locally higher here compared to other areas of the country. A Slight Risk was introduced in this area based on low FFGs, the 12Z HREF, and other model guidance. Through the overnight hours, forcing for ascent associated with a mid-level wave traversing the Ohio Valley and moisture/weak instability resulted in a separate peak in precipitation potential across central Pennsylvania and southern New York between 06Z-12Z. The low FFGs (mentioned above) and overall synoptic setup yields enough confidence for flash flood potential to include a separate Slight Risk for this regime.=20 Additional information on the NM/OK/TX Slight Risk is included below. Cook ....Previous Discussion... ....Northeast NM/TX and OK Panhandles... Model agreement has deteriorated somewhat regarding the likelihood of an MCS forming near or over the Panhandles late tonight. Thus, the Slight Risk is a lower-confidence one. However, given storms that are ongoing now and the likelihood of another MCS forming at the end of Day 1, should a stronger one form Saturday night, as some of the CAMs suggest, then the potential it could cause flash flooding is certainly there and would fall in the Slight category as regards the coverage of flooding it could produce. Due to the uncertainty as to whether or not it will form, the Slight risk was maintained for this forecast cycle with few changes with continuity being the primary driver of the forecast. ....US from the Mississippi East... A weak and highly diffuse signal for general showers and thunderstorms continues across almost the entirety of the eastern half of the country east of the Mississippi River. Based on the CAMs that extend through the first half of the period for the day on Saturday, it's likely that one or two lines of storms will cross the country from west to east, but will be moving at a rapid pace with little thunderstorm activity between the lines of storms. This weather pattern favors a very large area of mostly light daily rainfall totals. However, given the abundance of moisture that will continue to be advected northward ahead of these storms, they could still easily grow strong enough to produce locally heavy rainfall totals to 2 inches. The areas most likely to see flash flooding are those with the most favorable antecedent conditions and low FFGs. Since there is little to no signal for repeating or training storms, it could not be pinpointed where any higher rainfall threat will be located on Saturday, and thus no upgraded areas were introduced with this forecast update. Should a better signal arise in portions of the Marginal Risk area, a localized Slight is quite possible given the abundant moisture streaming northward ahead of any lines of storms. Wegman Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9Mk8Uhb1wp-4FOPe_dMO_doAtgKp11tK-DdEqVx5ELAG= tt0jWtcDzYrAqaJNdGfcWkFT3mMWDPx7iQ8vhGoh8hk9Roo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9Mk8Uhb1wp-4FOPe_dMO_doAtgKp11tK-DdEqVx5ELAG= tt0jWtcDzYrAqaJNdGfcWkFT3mMWDPx7iQ8vhGohmPAmTI0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9Mk8Uhb1wp-4FOPe_dMO_doAtgKp11tK-DdEqVx5ELAG= tt0jWtcDzYrAqaJNdGfcWkFT3mMWDPx7iQ8vhGohKhNJoXc$=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) .