Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jan 09 2024 07:53:49 FOUS30 KWBC 090753 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 252 AM EST Tue Jan 09 2024 Day 1 Valid 12Z Tue Jan 09 2024 - 12Z Wed Jan 10 2024 ....A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE MID-ATLANTIC AND NORTHEAST, AS WELL AS A PORTIONS OF THE WESTERN CAROLINAS... ....Mid-Atlantic and Northeast... Amplified upper trough pattern across the central and eastern CONUS will continue as a prolific meridional flow regime with ample moisture advection and ascent will shift eastward within a broad area of difluence and intense 140kt jet coupling associated from the longwave trough and tail of a jet extension over the North Atlantic. Broad cyclonic flow around the upper trough over the central US will advect tropical air poleward with PWAT anomalies along the entire eastern seaboard reaching between 1-3 deviations above normal with IVT values of up to +8 sigma bisecting a large area from the Mid-Atlantic through the Northeast U.S. This type of pattern has historically produced flooding and flash flooding concerns just based off the premise of the anomalous moisture field and heavy rainfall within the population centers located between Raleigh up to Boston. The added concern of snow melt stemming from the significant winter storm from the previous 36 hours through the Mid-Atlantic and Northeast only exacerbates the risk as any additional QPE within the time frame enhances flood concerns within the locales that received snow, as well as downstream within river basins as the water dumps into the array of rivers and streams located within the Northeast. Current forecast of widespread 2-3" with locally as high as 4" presents a major problem for the megalopolis and adjacent areas, especially those with ample snow cover that will become part of the flooding equation. NBM probabilities for at least 2.5" through the period are over 90% for a very large area encompassing eastern PA through the Lower Hudson Valley into southwestern CT. This is the primary area of concern as the snow depth within this zone is a standard 6-12" with as much as 15" in portions of the Catskills and northeast PA. Much of the heaviest rain will occur this evening as the best ascent is realized with a strong PVA signature advecting northeast over the region. A line of heavy rain will extend from southern New England down through the southern Mid-Atlantic by that time with a growing consensus of 1-2" of rain likely in a 6 to 8 hour window between 22-06z Wednesday for the I-95 corridor from DC to north of NYC. Latest HREF probabilities have also increased in regards to total rainfall as 00z EAS probabilities for at least 2" are over 90% encompassing an area from southeast MA down the 95 corridor in CT, through the Lower Hudson valley, through Northern NJ, and eventually arching back into northeast PA. This is coincident with an area that received modest to significant snowfall from the previous event creating a targeted spot for the worst impacts possible. This is a higher-end Moderate Risk for those areas in question as snowmelt and ice jams from the snowfall along with the prolific rainfall will create a very tumultuous setup in regards to flooding concerns. Further south and west from the Blue Ridge up through central and eastern PA, rainfall will still be fairly significant with probabilities of 2" or more running between 40-60% within the lower elevations and over 70% within all of the higher terrain areas above 2500ft. This is likely due to the orographic ascent pattern caused from the prevailing southeast flow banking against the terrain to couple with the broad difluent pattern overhead. A growing consensus of impactful rainfall within the Blue Ridge of west-central VA was enough to warrant an extension of the Moderate to the terrain east of the Shenandoah. High-end Slight Risk is still anticipated for much of the southern Mid-Atlantic between Asheville up through Roanoke, especially within the terrain in those locations. 10-year ARI exceedance intervals were highest over the area east of the Shenandoah with some 100-year ARI interval probabilities showing up in the HREF fields, so that was a factor in limiting the extension further south, along with the higher FFG indices located to the east of Roanoke down to Danville. =20 =20 ....Southern Appalachians...=20 Signal for strong low-level convergence and orographic ascent within the confines of the southern Appalachians in northeastern GA up through the tri-state area of SC/NC/TN continues leading to a maintenance of the Moderate Risk across the aforementioned area. Amplified upper-level pattern upstream of the area will induce a strong low-level moisture advection regime stemming from backed 850mb flow out of the southeast. Guidance has ramped up the intensity of the anomalous flow pattern with boundary layer winds approaching 65kts between 850-700mb which correlates well with a +2-3 deviation PWAT anomaly and saturation to the tropopause as indicated by all major guidance beginning this morning and continuing through the early afternoon time frame. Latest ensemble probabilities for the area remain robust within both a spatial and numerical sense as the prospect for prolific rainfall exceeding 2.5" has not only maxed out in the probability fields, but also expands further east and northeast within the confines of the terrain. 00z HREF EAS probabilities for at least 3" of rainfall within the end of D1 into D2 are over 80% within the area encompassing Clayton, GA up through the terrain northwest of GSP into the mountains adjacent to Asheville in southwest NC. 5"/12-hr neighborhood probability is between 50-70% as well within the tri-state point of GA/SC/NC to the northwest of Greenville, a signature commonly seen in the higher impact events within the terrain focused areas east of the Mississippi. The expansion of the heavy rainfall footprint to include some of the population centers in the Piedmont allowed for a correlating expansion of the Moderate Risk further east and north, including the Greenville/Spartanburg metro area. This was done in coordination with the local WFO in Greenville/Spartanburg as the signatures for locally heavy rain and flash flooding concerns were very much at the forefront with the latest trends. Kleebauer Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4Hy0bfHSoX8Bo4CO5IEeedBuX24V3pe2SxIXPg7Ziu_f= SDo7fMP_zHqStWrHnASjB6OP27bk6dQW6TVWGOXkE09d8Ms$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4Hy0bfHSoX8Bo4CO5IEeedBuX24V3pe2SxIXPg7Ziu_f= SDo7fMP_zHqStWrHnASjB6OP27bk6dQW6TVWGOXkFP8UMOU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4Hy0bfHSoX8Bo4CO5IEeedBuX24V3pe2SxIXPg7Ziu_f= SDo7fMP_zHqStWrHnASjB6OP27bk6dQW6TVWGOXkb6swBTw$=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) .