Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Aug 18 2024 19:56:15 FOUS30 KWBC 181955 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 355 PM EDT Sun Aug 18 2024 Day 1 Valid 16Z Sun Aug 18 2024 - 12Z Mon Aug 19 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID ATLANTIC AND THE SOUTHWEST US... 16Z Excessive Rainfall Discussion Update... Introduced a Slight Risk area over portions of southern Utah and far northern Arizona given on-going hydrological concern from yesterday's rainfall and the signals shown by the convective allowing models for additional storms capable of producing heavy rainfall. Of particular note was the concern over the Paria River drainage being extremely saturated and the fact that convection had been able to persist through the convective-min of the morning. On the other side of the coin, there was concern how the early morning showers could limit the threat farther south in Arizona. Maintained the Slight Risk area in the eastern US with only minor modification to account for a weak meso low moving northward along the New Jersey coastline and to account for localized but very impactful heavy rainfall over parts of the southern Connecticut coast with the thinking any additional rainfall could make areas prone to flooding even more susceptible today. Few changes were needed elsewhere. Bann ....Mid Atlantic and Northeast... An approaching cold front will bring another day of scattered to numerous showers and thunderstorms to portions of the Mid Atlantic and Northeast today into tonight. Aloft, the broad upper trough will slowly advance eastward into the eastern Great Lakes and central Appalachians. Ahead of it, southwesterly flow will allow for warm and moist air to continue lifting into the region and will be characterized by dewpoints well into the 70s and PWs above 1.5" with locally higher values exceeding 1.75", which is around 2-2.5 standard deviations above normal. With peak heating, instability of between 1500-2000 J/kg is forecast to develop. The 00Z guidance continues to key on the urban corridor from near Washington, DC to near NYC metro for the greatest QPF and probabilities for intense rain rates today into tonight. Several clusters of thunderstorms are likely to develop (some this morning) and with the mean flow out of the southwest near parallel to the storm motions, some enhanced training or backbuilding will be possible, particularly over eastern PA, northern New Jersey, and into southern New York. Here, the HREF neighborhood probabilities are high for 3" totals (above 60 percent) and show a slight signal for 5" (15-20 percent). Rain rates of 2-3"/hr will be possible given the PW environment. Given the proximity and likelihood for this to occur over the more urbanized corridor, scattered instances of flash flooding will be possible, some of which could be locally significant. Elsewhere in the region outside of the Slight Risk, thunderstorm activity is expected to be more isolated in nature, but given the environmental ingredients could still result in localized flash flooding. This includes much of PA and NY states where there is still a decent signal in the HREF probabilities for 2-3" isolated totals. Another area of concern is further south from eastern VA into the Piedmont and coastal regions of the Carolinas where the combination of locally heavy rainfall over more sensitive/saturated ground conditions could lead to additional flash flooding concerns. ....Southwest and Intermountain West... Impressive and deep monsoonal moisture will continue to stream around the western periphery of the monsoon ridge and combine with daytime instability to produce scattered to widespread showers and thunderstorms from AZ as far north as WY and ID. Convection will be capable of producing hourly totals up to 0.75" at times, particularly across portions of northern AZ and southern Utah where guidance continues to show potential for locally higher rainfall amounts. ....South Florida... Another day of scattered, slow-moving thunderstorms are expected across the region where rain rates may approach 2-3"/hr at times. This is due to the lingering surface trough in the vicinity focusing the higher PWs. The consensus among the 00Z guidance is that this should largely stay south of the major metro areas, but will be close enough to continue the Marginal Risk for portions of South Florida and the upper Keys. ....Ozarks Region... Shortwave energy rounding the top of the upper ridge will quickly drop through the Ozarks region late in the period (this evening/overnight). With a modest low level jet forming in response and a decent moisture influx, a narrow strip of locally heavy rainfall is likely to develop in the region. Given the northwest flow aloft and storm motions, some backbuilding is possible. While confidence in these scenarios isn't very high, some of the outlook area has seen recent heavy rainfall and the potential for a narrow/localized area of 1-3" is possible based on the 00Z hi-res guidance and HREF neighborhood probabilities. This could lead to a few instances of flash flooding, mainly in the 06Z-12Z period early Monday morning. Taylor Day 2 Valid 12Z Mon Aug 19 2024 - 12Z Tue Aug 20 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHEAST U.S. AND SOUTHWEST INTO THE INTERMOUNTAIN WEST... 2030Z Update... Overall...potential for excessive rainfall will start diminishing on Day 2 but the threat will still linger over portions of the Southwest US/Intermountain Region given monsoon moisture still circulating around the edge of a sprawling upper ridge. Even brief downpours over burn-scars may result in flash flooding. In=20 addition...the potential for excessive rainfall lingers in the east but the risk is more from embedded higher amounts within a broader shield of lighter rainfall rates rather than from prolonged=20 downpouirs. The previous outlook had this pretty much in hand and=20 only minor adjustments were needed. Bann Previous Excessive Rainfall Discussion ....Northeast... The upper trough will make a final pass across the Mid Atlantic to Northeast on Monday with the cold front expected to move across the coastal areas during the late morning into the evening hours. With peak heating and sufficient destabilization, vigorous convective elements are expected to develop again, mainly from the coastal Mid Atlantic through much of eastern New York and portions of New England. Storm motions should be fairly progressive, but a few storms could repeat/train due to merging/colliding cells and storm totals could approach 1-2" in spots leading to flash flooding. ....Southwest/Intermountain West... By Monday, the bulk of the anomalous moisture is expected to have lifted northward through the Four Corners region and into the Intermountain West/Rockies. With another shortwave trough embedded in the flow moving through, this should be sufficient to produce another day of isolated to scattered thunderstorms, most likely over portions of northern Utah, Colorado, and into western WY, far eastern Idaho. Flash flooding will be possible, particularly for the most sensitive locations like areas near recent burn scars. Taylor Day 3 Valid 12Z Tue Aug 20 2024 - 12Z Wed Aug 21 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST U.S... 2030Z Update... Models still point to the westward expansion of the upper level ridge over the southern US on Tuesday...limiting the extent and coverage of late day and evening thunderstorms. Any of the storms which form would pose an isolated flash flooding threat. Bann Previous Excessive Rainfall Discussion The upper ridge over the Southern U.S. is expected to build/grow and expand westward into more of the Southwest U.S. on Tuesday. This will likely limit the extent and coverage of monsoon thunderstorm activity across the region, but with PWs still elevated across Arizona and the expectation for at least a modest amount of instability, isolated thunderstorms are possible through the late morning into evening hours. If they do develop, these storms would pose an isolated flash flood risk for the typical vulnerable locations like areas near burn scars, dry washes, and slot canyons. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-VIaoE0XKskiznr7H0R_ViCvA6Lxs5ehIxHYQ-OVrymU= qvTo4EoqL6TCYIjttyKhDCq86kkQeCa44qn3-8D89ymcRKs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-VIaoE0XKskiznr7H0R_ViCvA6Lxs5ehIxHYQ-OVrymU= qvTo4EoqL6TCYIjttyKhDCq86kkQeCa44qn3-8D8i4l4keY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-VIaoE0XKskiznr7H0R_ViCvA6Lxs5ehIxHYQ-OVrymU= qvTo4EoqL6TCYIjttyKhDCq86kkQeCa44qn3-8D8ES9CFV8$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .