Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 10 2023 08:36:29 FOUS30 KWBC 100836 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 435 AM EDT Sun Sep 10 2023 Day 1 Valid 12Z Sun Sep 10 2023 - 12Z Mon Sep 11 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE CENTRAL PLAINS AS WELL AS MUCH OF THE MID-ATLANTIC AND NEW ENGLAND... ....Mid-Atlantic into New England... A very slowly drifting positively tilted upper level trough will continue east through the day today. Meanwhile a cutoff low over the Tennessee Valley early this morning will quickly open up into a trough and track northeastward into the Northeast through tonight. This pattern will continue to advect tropical moisture straight up the East Coast and into New England. The combination of large-scale lift in the atmosphere from the longwave trough over the Midwest and localized added forcing from the cutoff low turned shortwave trough will result in a renewed round of showers and thunderstorms for much of the East Coast and Appalachians from NC northward. Recent rains from the past several days have primed the soils in many regions, from the central Appalachians of the Virginias through New England. Thus, yet another round of potentially heavy rain and training storms will only restart/worsen preexisting flash flooding all up and down the Slight Risk area. The Slight Risk area was expanded southwestward into portions of southwestern VA and southern WV in coordination with RLX/Charleston, WV and RNK/Blacksburg, VA forecast offices. This area has been hard hit for multiple days with flash flooding from repeating thunderstorms, and while overall coverage of storms should be lesser today than previous days, the aforementioned antecedent soil conditions will support lesser amounts of rain resulting in flash flooding. The axis of heaviest rainfall has shifted a bit to the east further north into the Jersey shore, NYC and Long Island, and southern New England, so the Slight Risk area was also expanded eastward to include those areas. Expect higher amounts of instability from onshore flow off the warm waters of the Atlantic into NJ, NY, and southern New England, which will support stronger storms which will also repeat/train in the unidirectional southerly flow. Finally, the Slight Risk area was also expanded to include nearly all of northern New England except portions of the eastern Maine coast. Thunderstorms tracking northeastward ahead of the longwave trough will move over the same areas that were hit with storms in previous days. While the storms may weaken as they leave the coast into northern Maine, antecedent soil conditions favor lower FFGs in far northern Maine, which could result in flash flooding even that far north, especially into tonight. ....Central Plains... The Slight Risk area is largely unchanged from inherited. Guidance has come into better agreement that western KS will be the focus for the strongest storms as a strong cold front collides with a LLJ tracking northward out of TX. Forcing will be lesser into TX as the focus for convection is more the dry line with less upper level support, so the Marginal Risk down along the Caprock remains in place. The Slight risk area was trimmed out of Colorado as the strongest and most persistent storms look to form east of there. Further, the Marginal Risk was trimmed out of Iowa entirely with this update. Any storms that form along the front will advect eastward across southern Iowa, gradually weakening with time. This should favor perhaps a prolonged period of light to moderate rain. Antecedent conditions across much of the Plains into the Midwest have been bone dry, with soil moisture levels at less than 5% of both climatology and overall. Since the storms that get into Iowa will both be weaker and unlikely to train/repeat, think the large majority of any rainfall seen in Iowa today will be beneficial for the area, and flash flooding appears highly unlikely. Further, since much of Iowa is agricultural, even the bone dry soils should be less repulsive of moisture, especially since it won't be very heavy. Wegman Day 2 Valid 12Z Mon Sep 11 2023 - 12Z Tue Sep 12 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST, MIDWEST, AND SOUTHERN PLAINS... ....Northeast... A cold front sweeping across the region from far western NY and PA early in the day Monday will clear eastern Maine by Tuesday morning. The front will clear the anomalous atmospheric moisture from the Northeast. While the upper level trough over the Midwest will dig southward through the day, it will make relatively little eastward progress. Thus, in the upper levels, the unidirectional southwesterly flow will persist, but with little embedded forcing. As the moisture pushes off the coast, any resultant thunderstorms during the day Monday will progressively shift offshore. This will limit the potential for additional flash flooding. Any storms that form during the afternoon are progged by most of the guidance to remain widely scattered and in a line that gradually tracks east. This should limit the potential for flash flooding. ....Midwest... Strong forcing from a digging longwave trough over MN/WI will approach lower MI on Monday. The northeasternmost extension of the southern Plains LLJ will move into lower MI in advance of a cold front associated with the upper level trough. The LLJ will advect some moisture into MI, which will be lifted by the approach of the trough. The guidance shows an extensive area of light to moderate rain with embedded convective elements will move over Lower MI. Isolated flash flooding is possible, particularly in any urban areas and other low FFG portions of lower MI as a result. A small portion of northwestern Lower MI near Traverse City was hit hard by recent rainfall, and the additional rain expected Monday may allow for additional flash flooding, so the Marginal Risk was expanded to include this area. ....Southern Plains... The convergence of a strengthening LLJ Monday night with a southward advancing cold front moving down the CO Front Range and into NM and portions of northwest TX will cause scattered thunderstorms to develop over the area. Much of this area has been dry as noted by NASA Sport soil moisture imagery, which will greatly limit any flash flooding except in areas with training/repeating thunderstorms. This is most likely across north central TX south of Wichita Falls. No significant changes were made to the Marginal Risk area. Wegman Day 3 Valid 12Z Tue Sep 12 2023 - 12Z Wed Sep 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN NEW MEXICO... ....Four Corners Region... In coordination with ABQ/Albuquerque, NM forecast office, a Slight Risk upgrade was introduced with this update. Southeasterly low level flow will advect Gulf moisture northwestward out of TX and into NM on Tuesday. Meanwhile a passing longwave trough will move eastward across the Plains, with a weak jet streak developing along the base of the trough. This will contribute broad-scale lift over NM. Further, monsoonal moisture from the Gulf of California will track northeastward into the desert Southwest. The clash of these air masses as well as a stalling out cold front over the Plains will result in widespread showers and thunderstorms across much of the Four Corners region. Further west over UT/AZ, fast storm motions and somewhat drier antecedent conditions should keep any flash flooding isolated. Meanwhile NM is both a bit wetter, and expected to pick up additional rainfall on Day 2/Monday, such that by Tuesday, the rainfall resulting from the aforementioned forcings should result in more widely scattered flash flooding. Flooding is most likely over any burn scars, slot canyons, and in the Sangre de Cristo Mountains of north-central NM. ....Northeast... Moisture recovery along the coast ahead of another strong cold front at the leading edge of energetic forcing associated with an advancing longwave trough/upper low over the Great Lakes will result in another round of showers and thunderstorms over much of the Mid-Atlantic and Northeast Tuesday. Despite the moisture recovery, it will not reach levels currently present over the area, and with some recovery time from Monday, the chances of flash flooding in the Northeast will be a little bit lower than today and previous days. Thus, the Marginal Risk inherited was largely unchanged. Any slowing of the overall pattern or more atmospheric moisture available to developing convection could result in a targeted Slight Risk upgrade over portions of the area with future updates. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_mgUGAyf51x2lD4WAvibh2n0cWH5hiOUCHOyZFTtyav3= fZiPep5c_bNeZ7WkGmc1WglB2JNRwaRqx50TwqbeSSNCjTM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_mgUGAyf51x2lD4WAvibh2n0cWH5hiOUCHOyZFTtyav3= fZiPep5c_bNeZ7WkGmc1WglB2JNRwaRqx50TwqbeagK2is4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_mgUGAyf51x2lD4WAvibh2n0cWH5hiOUCHOyZFTtyav3= fZiPep5c_bNeZ7WkGmc1WglB2JNRwaRqx50TwqbeGUMVRps$=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) .