Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jul 15 2023 19:33:36 FOUS30 KWBC 151933 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 332 PM EDT Sat Jul 15 2023 Day 1 Valid 16Z Sat Jul 15 2023 - 12Z Sun Jul 16 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHEAST NEW MEXICO, THE TEXAS PANHANDLE, CENTRAL APPALACHIANS, FLORIDA GULF COAST, THE UPPER SUSQUEHANNA RIVER BASIN, AND NORTHERN NEW JERSEY... 16Z Update: ....Ohio Valley and Southern Appalachians... Shortwave energy moving across the Ohio Valley will continue to provide the necessary lift/forcing to maintain the area of showers and thunderstorms tracking through Kentucky, southern Indiana and southern Ohio this morning. Broad southwesterly flow ahead of the approaching front will bring higher moisture into the area and despite extensive cloud cover, building instability will favor localized 1-2" hourly totals across far northern KY, southern Ohio this afternoon. Later this afternoon into the evening hours, the 12Z hi-res guidance continues to show widespread thunderstorms developing across the Southern Appalachians, particularly western NC. 12Z HREF neighborhood probs show a strong signal for hourly totals of 1-2" through this evening and across western NC, 24-hr neighborhood probs for 3" peak at 45 percent and for 5" are 15 percent, so the Slight Risk was expanded some based on this. ....Florida Gulf Coast... An extremely moist airmass (PWs 2-2.5") with a lingering MCV in the region will provide the focus for enhanced rainfall and intense rain rates through the period. The 12Z HREF continues to suggest hourly totals between 1-2" are likely at times today with a near 25 percent chance of 24-hr totals exceeding 5". Based on this and some wetter antecedent conditions in the area, a Slight Risk was introduced for scattered instances of flash flooding. Taylor ....Eastern Half of the Country... A digging upper level low starting out the day today over northern Manitoba will dig southward, reaching the US/Canadian border in MN late tonight. An upper level wave rotating around that low and its associated cold front will sweep eastward across the Midwest today. The convection ongoing from MI southwest to IL is occurring partially because of that upper level forcing. It is expected to sweep eastward along with its forcing across the Midwest and into the Mid-Atlantic tonight. PWATs ahead of this convection will advect northward out of the Southeast, rising to between 1.5 and 1.75 inches broadly across the Northeast. This will provide plenty of moisture and instability for the storms as they cross the Midwest. Fortunately, as has been the case in recent days, the guidance remains consistent that the storms will be quite progressive, so no one area is expected to see excessive amounts of rain. Thus, the large Marginal Risk area across almost the entire half of the country east of the Mississippi remains in place. After sunset this evening, the storms will be into the Mid-Atlantic, but weakening due to the loss of solar heating. However, the upper level flow will favor more northward movement of the storms with a slowing eastward progression. This will result in a larger area of light to moderate rainfall to develop into PA and NY, resulting in a longer-duration but lighter intensity of rain. This too will favor generally low chances of flash flooding. The two Slight Risks generally over the Virginias and again over PA/NY are largely in place owing to the much lower FFGs in those areas, as the areal rainfall totals will be similar to areas outside the Slight Risk areas. Across the Southeast, the MCS starting out the day over MS will devolve into a broken line of widely scattered storms, which with plenty of available atmospheric moisture will support continued convective development both ahead of and behind that line of storms, as the real air mass change largely stays north from the TN Valley north. As there's little indication that the storms will be able to organize today, largely due to lack of forcing, the Marginal Risk area remains across the Southeast. Somewhat heavier rains are possible across the FL Panhandle today, where sea breeze convection may merge with advancing storms from the north, resulting in locally heavier rainfall totals. As FFGs are also quite high here, especially over the eastern Panhandle, which guidance is in good agreement will see the bulk of the heaviest rainfall today, a high-end Marginal remains in place, which may need to be upgraded to a localized Slight with future forecast updates. ....Southern High Plains... Storms ongoing as of the time of this writing over much of the Slight Risk area from northeastern NM into northwest TX are helping prime the soils and lower FFGs to increase the flash flooding risk when an MCS develops this afternoon into this evening across this region. A complex of storms will develop over northeast NM, then turn eastward as it moves over the southern TX Panhandle, following the upper Red River basin on the TX side of the river overnight tonight. The guidance has shifted south and west with the axis of heaviest rain associated with this MCS, which is in keeping with typical MCS behavior where the south and western sides of the MCS are more likely to get held up and move more slowly than the rest of the complex of storms. The Slight Risk area was shifted south and west, removing all of OK and much of the northeastern TX Panhandle from the Slight with this update. Wegman Day 2 Valid 12Z Sun Jul 16 2023 - 12Z Mon Jul 17 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF NEW ENGLAND, EASTERN NEW YORK, NORTHERN NEW JERSEY, AND NORTHEASTERN PENNSYLVANIA... ....Northeast... The environmental ingredients are coming together for localized extreme rainfall resulting in potentially significant flash flooding across portions of the Northeast Sunday. Ahead of an approaching shortwave trough, the low level flow will become more southerly later today into tonight. This will allow for the plume of anomalously high moisture to spread northward across the entire region. Precipitable water values, currently 1-1.5", will rise toward 2" and even 2.25+" by Sunday morning, especially along the coastal areas. These values will be near 2.5-3 standard deviations above normal for this time of year and looking at the sounding profiles, warm cloud depths will exceed 4.5 km from near Baltimore to Boston along the I-95 corridor. The 12Z HREF shows fairly high probabilities for 3-5" totals across portions of northern New Jersey, northeast Pennsylvania through southern New York and New England while the various hi-res CAMs have localized streaks of higher amounts. Overall there was a southward jog in the best setup for training thunderstorms and the QPF fields adjusted southward as well though there remains some spatial uncertainty in where the training bands do set up. Some of the 12Z guidance has trended faster, with the highest QPF/training bands further east across southern New England, while other models like the NAM, Nam3km are further west across northern New Jersey. Regardless, the combination of the very high moisture and forcing will make for very impressive rain rates (hourly totals up to 2" possible) and some extreme rainfall totals are possible in localized spots over a short time period, particularly across the urbanized I-95 corridor from northern NJ through southern NH where the high urbanization combined with the very wet antecedent conditions due to recent heavy rainfall could result in more significant flash flooding. With this in mind, the Moderate Risk was adjusted southward across New Jersey, New York (including NYC metro), as well as eastward across Massachusetts.=20 Taylor ---previous discussion--- A potent cold front that starts out the day near the 80 W meridian will push east through the Mid-Atlantic and into western New England by early Monday morning. The front will be supported by a potent upper level low over the Great Lakes, which will swing a potent upper level disturbance eastward into the Northeast. Meanwhile, a highly anomalous moisture plume with PWATs exceeding 2 inches, or up to 3 sigma above normal, will advect northward into New England ahead of this front. Ample moisture and forcing will set the stage for widespread moderate rain, with embedded heavier convective elements likely. CAMs guidance is in okay agreement that steady rain well ahead of the front will be ongoing right at the start of the period Sunday morning across western portions of the Moderate risk area in PA/NY/NJ. As the morning goes on, convection across southern areas of the Moderate Risk area will increase in coverage with daytime heating, though convective elements are likely to persist into northern New England with steadier rain in between the convective elements that far north. This will be due to the influx of additional Atlantic moisture into New England. Nearly all of the Moderate Risk area is characterized by very sensitive soils and low FFGs due to numerous recent previous days with heavy rainfall and flooding. The addition of multiple hours of steady rain, which will be heavy at times, is likely to initiate new flash flooding and worsen ongoing river flooding. During the afternoon, the first round of rain will move into eastern New England, but a secondary line of heavy showers and thunderstorms is expected to develop over NY and NJ and follow the first line eastward behind it. While this second round should be progressive enough to keep the duration of heavy rain rather short, the brief time frame in between rounds of rain may cause additional flooding to result from this second line. After sunset, the rain will continue slowly pushing eastward to coastal New England and Maine, while the second line largely dissipates with the loss of solar heating. Much of the event should be over by around midnight except across eastern ME. The Moderate Risk area was expanded in several regions. First, it was expanded south and west into far northeastern PA and much of northern NJ, owing to the flood sensitivity resulting from the training thunderstorms over the area a few hours ago. While there's added uncertainty as to how far south and west more widespread storms capable of heavy rainfall rates will get, the very wet soils in the area may make up for that uncertainty resulting in scattered flash flooding into the Catskills and eastern Poconos. The Moderate was also expanded northward to include more of northern VT given the increased signal in the guidance, along with storms that caused flash flooding yesterday evening as well. Finally, the Moderate was also expanded to include all of coastal CT and much of RI due to increased signal in the guidance there as well. There remains significant uncertainty as to how much rain will fall over the NYC area and Long Island, as southerly onshore flow off the Atlantic may hinder instability and prevent stronger storms from developing until they're further inland. Thus, NYC and Long Island remain in a high-end Slight Risk for now. The Moderate was shrunk a row of counties westward away from the MA, NH and ME coastline for similar reasons where proximity to the ocean may hinder stronger storms and thus, higher rainfall totals from being realized. The surrounding Slight Risk area remains largely the same. Elsewhere the Marginal Risk area was shrunk from the west to remove western NY, PA, MD, and VA with better agreement that the rainfall will be largely east of those regions by the start of the period. ....Gulf Coast... No significant changes expected along the Gulf Coast as the trailing end of the stalled front helps focus additional convection over southern MS, AL, GA, the FL Panhandle and eastern LA. Isolated flash flooding is possible where any storms may locally organize, though there is little confidence on exactly where that will happen. ....Lower Missouri River Valley... No significant changes on the potential for an MCS to drop locally heavy rains from KS through western IN Sunday afternoon. With a few urban centers in the area, there is local potential for more than isolated flash flooding, particularly near Kansas City, but confidence is too low at this time for any localized upgrades, though they remain possible. 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!5GgsG3OQDQ58nKahUXZE5iDyFDkF0lGdPGd36ViEqE7Q= 2xAGGswoW0xB7qokaXbEPcAe-DiTHxFrOd2xyWhDQO7QrS8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5GgsG3OQDQ58nKahUXZE5iDyFDkF0lGdPGd36ViEqE7Q= 2xAGGswoW0xB7qokaXbEPcAe-DiTHxFrOd2xyWhD6-ma73k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5GgsG3OQDQ58nKahUXZE5iDyFDkF0lGdPGd36ViEqE7Q= 2xAGGswoW0xB7qokaXbEPcAe-DiTHxFrOd2xyWhD1IvaBBE$=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) .