Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Apr 03 2024 08:00:55 FOUS30 KWBC 030800 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 AM EDT Wed Apr 3 2024 Day 1 Valid 12Z Wed Apr 03 2024 - 12Z Thu Apr 04 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE CENTRAL MID-ATLANTIC UP THROUGH THE NORTHERN MID-ATLANTIC COAST... Current UA analysis depicts a broad upper-low circulation over the central Midwest with an occluded SLP positioned over Lake=20 Michigan. Broad cyclonic flow encompasses a good chunk of the CONUS east of the Mississippi with strong mid and upper dynamics driving a multi-area heavy rain threat this evening with one batch over=20 the Southeastern US, and the other located over Pennsylvania. This=20 pattern will evolve with the northern cluster of rain slowly=20 propagating to the northeast and the southern cluster exiting to=20 the east over the Carolinas into southern GA. By morning, a strong=20 upper-level jet core approaching 160-170kts will round the base of the mean trough with increasing large scale ascent focused over=20 the Mid-Atlantic generating another round of convection in-of the=20 southern Appalachians working up into the central Mid-Atlantic=20 Piedmont. A warm front will slowly lift north out of VA into at=20 least southern MD and the adjacent Delmarva with increasing=20 buoyancy along and east of the I-95 corridor up to Baltimore. To=20 the north, despite not being within the warm sector, very strong=20 mid-level ascent from the approaching trough will help overcome any surface based capping prompting a rapid development of elevated=20 convection over west-central VA up into south-central PA. This=20 convection will move northeastward thanks to the mean flow, but the anomalous 2-2.5 deviations above normal PWAT environment will=20 favor locally heavy rainfall within any organized updrafts capable=20 of dropping 0.5-1"/hr rates from late-morning through the afternoon hrs before pivoting to the northeast.=20 Recent trends from 00z HREF blended mean have bumped forecast=20 totals to 1-1.5" with locally as high as 2.5" based on the=20 probability matched mean forecast. This is due to the favorable=20 dynamics and increased convective nature of the precip, regardless=20 if in the warm sector or not. Recent FFG intervals have come down=20 considerably thanks to the previous 24-36 hrs worth of rainfall=20 ahead of the current period. The urban corridor from NoVA up=20 through the suburbs of NYC are a pedestrian 0.5-1"/hr FFG interval with both the 3-hr and 6-hr intervals only sitting at 1-1.5", at=20 best. The antecedent soil moisture will only exacerbate the threat=20 for runoff with small creeks and streams already towards the top of their banks. Larger river beds have responded from the recent=20 rains as well, so the threat for flash flooding and river flooding=20 in tandem will generate a heightened risk anywhere within the=20 aforementioned areas.=20 HREF EAS probability of at least 1" of total rainfall is between 30-60% over the central Mid-Atlantic with 70-90% over Northern NJ through the Lower Hudson, NYC, and Long Island. The 2" EAS is 20-40% across the Lower Hudson, east to Long Island as well which is significant enough for a risk upgrade due to the usual conservative nature of EAS probabilistic guidance. The urban corridor near NYC is the highest risk for seeing totals reach close to 2" due to the multi-round threat from the morning and later in the evening as a surface low spawns over the Mid-Atlantic and intensifies to the northeast with SLP location off the Northern NJ coast by evening. A strong southeasterly, onshore flow will create some surge potential along the coastal plain that will be accompanied by the heavy rainfall ongoing within the NW side of the surface cyclone. Onshore flow and heavier precip will make it as far north as Eastern MA, but colder temps and falling heights from the evolving upper pattern will allow for areas inland of the coast to slowly shift over to winter weather, reducing the threat for flooding as you move west of a line from Boston down to Providence.=20 Given the above variables and the very low FFG intervals in place heading into the period, have upgraded a large chunk of the previous MRGL risk to a Slight Risk with the primary focus over the urban corridors from NoVA up through DC/Baltimore, Philadelphia,=20 central and Northern NJ, NYC, Long Island, Lower Hudson Valley, and southern CT. The SLGT also extends up through the Lower Susquehanna Valley into central and eastern PA due to the low FFG intervals also in place in those regions with a general 1-2" of additional rainfall forecast from 12-00z this period. This correlates well with the current Flood Watches in place by the respective WFO's.=20 Kleebauer Day 2 Valid 12Z Thu Apr 04 2024 - 12Z Fri Apr 05 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Kleebauer Day 3 Valid 12Z Fri Apr 05 2024 - 12Z Sat Apr 06 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-o5-B3fk7snMeay-AMSmqZqGLFf-kUivyP0JUdt32no2= THmBp0tcWJT1Tz7UuYmF8CF_v_t1AmF886LAtautsW27PJQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-o5-B3fk7snMeay-AMSmqZqGLFf-kUivyP0JUdt32no2= THmBp0tcWJT1Tz7UuYmF8CF_v_t1AmF886LAtautKJKZeMw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-o5-B3fk7snMeay-AMSmqZqGLFf-kUivyP0JUdt32no2= THmBp0tcWJT1Tz7UuYmF8CF_v_t1AmF886LAtaut5Fc5lS8$=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) .