Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Apr 28 2023 00:58:32 FOUS30 KWBC 280058 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 857 PM EDT Thu Apr 27 2023 Day 1 Valid 01Z Fri Apr 28 2023 - 12Z Fri Apr 28 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHERN FLORIDA AND SOUTHEAST GEORGIA OVERNIGHT... ....North Florida Into Southeast Georgia... Activity which had been along the Gulf coast earlier today continued to move eastward and was mainly focused over the northern Florida peninsula northward into southeast Georgia as of early evening. Isolated rainfall amounts of 2 to 4 inches embedded within a broader area of rainfall amounts generally will continue to be observed as the whole area continues to propagate eastward. Maintained the Slight Risk ahead of the surface boundary where there was better moisture and instability...and expanded a bit northward and eastward in Georgia based on latest trends seen in satellite and radar imagery. Given the steady forward propagation...the threat for flash flooding should be diminishing during the late evening once the activity moves off-shore. Given the northeast to southwest orientation of the heavier rainfall axis on radar...realigned the Marginal Risk area a bit. Areas along the Gulf coast behind this activity were removed from the outlook now that drier air was overspreading the area.=20 ....Ohio/Tennessee Valleys and Adjacent Mid Atlantic... The 18Z HREF continued to show a low-end probability of rainfall rates of an inch in a hour over portion of the Ohio and Tennessee Valleys overnight...with a signal for some rates to increase again late tonight along the along or near the Blue Ridge. The Marginal Risk area was adjusted eastward a bit once again over western portions of the Carolinas into Virginia to account for the probability of 1 inch per 3 hours increasing to 70 pct or greater...with the assumption that locally higher amounts in shorter time spans are possible...by 00Z. Much of the area has been very dry so no more than a Marginal appears warranted.=20 Farther west...the large scale circulation will continue to support locally enhanced rainfall rates...and no more than minor tweaks were needed here for this outlook. Bann Day 2 Valid 12Z Fri Apr 28 2023 - 12Z Sat Apr 29 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE MID ATLANTIC REGION AND TEXAS... ....Mid-Atlantic... 21Z update... Consensus continues to show 1 to 3 inches falling from the northern Mid-Atlantic coast to central Pennsylvania, with the highest QPF likely focused near the coastline. No changes were made to the Marginal Risk area as it currently highlights the region most likely to have an elevated threat for excessive rainfall and isolated areas of flooding. Campbell As the shortwave trough that caused the heavier rains over the Ohio and Tennessee Valleys Thursday lifts northeast, it will be able to take advantage of the warm frontal/tropical moisture from the Gulf being pumped northwestward into the Mid-Atlantic on a 40 kt southeasterly jet. PWATs will increase to over 1.5 inches into southeastern VA, and over 1 inch across much of the rest of the mid-Atlantic by Friday afternoon. This moisture will include instability to 500 J/kg which will also be advected north up the coast. Light to moderate rain will be widespread across all of the mid-Atlantic on Friday, but the instability will contribute to locally heavier embedded elements. Soils are very dry over the area, but the long-duration rain with heavier rain embedded will likely result in isolated flash flooding, especially in urbanized and poor drainage areas. The feed of Atlantic moisture will be slow to diminish and move northeast, so it will take until well into Friday night for the plume of rain to make it to the New York City area. ....Southern Plains... 21Z update... The latest guidance has trended the QPF a little heavier and further north into parts of Oklahoma; with the GFS suggesting as much as 3 inches across southeast Oklahoma. Given this uptick among the consensus, the Marginal Risk area was adjusted to reflect this trend. Campbell A vigorous positively tilted trough will dig southward across New Mexico and west Texas Friday afternoon and overnight. Ahead of this forcing, a bit of southerly return flow will bring Gulf moisture with PWATs to 1.5 inches north across eastern Texas. Further, there will be extreme instability advected and developing across central Texas, as MUCAPE values exceed 3,000 J/kg, with values diminishing to closer to 1,000 J/kg as you go north to the TX/OK border. Much cooler and drier air racing south on 20-30 kt winds will provide additional forcing along the front for strong to severe storms to develop across northern and eastern Texas Friday afternoon. Since the flow behind the front will be N to NNW, it will be moving only slowly eastward, which in turn will mean any storms tied to the frontal forcing will also be moving slowly. Given the extreme instability, moisture, and a slow-moving front, the Marginal Risk remains in place, with a small expansion to the Red River. There's some uncertainty as to how the storms will organize, but given the heavy rainfall that fell today, it's possible a small Slight may be needed with future updates when there's more certainty as to how the storm evolution will develop. Wegman Day 3 Valid 12Z Sat Apr 29 2023 - 12Z Sun Apr 30 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR THE FLORIDA PANHANDLE INTO SOUTHERN GEORGIA... ....Southeastern US... 21Z update... Recent heavy rainfall across the Gulf Coast and the Southeast will have lowered local FFG across the region. The latest guidance is depicting another round of 1 to 3+ inches for much of the same area. The inherited Slight Risk area remains in good order, minor west-east expansion was made with this update. The northern bounds of the Marginal Risk area across central South Carolina was readjusted to reflect recent QPF trends as well. Campbell A developing low over the Mississippi Valley and a pair of upper level shortwaves moving northeast across the Southeast will provide the forcing as highly anomalous Gulf moisture to 3 sigma above normal with PWATs approaching 2 inches, and a 50-60 kt LLJ by late Saturday night over the FL Panhandle will all combine over the Slight Risk area to result in multiple rounds of thunderstorms each capable of rainfall rates exceeding 2 inches/hour, which will move northeast out of the Gulf and into southeastern GA and SC. The first round of storms will be with a weaker front moving across the area Saturday afternoon, then the second will follow it with the stronger surface front late Saturday night. Instability will increase through this time as greater instability is advected northward with the Gulf moisture on the strengthening LLJ. The Slight Risk area was coordinated with the TAE/Tallahassee, FL, JAX/Jacksonville, FL, and MOB/Mobile, AL forecast offices. Depending on how much flooding is seen today/Thursday, today's round of storms may prime the soils for the greater rainfall event expected on Saturday. Each individual storm should be fast moving, which should help mitigate the flash flooding potential a bit. ....New York City Metro Area and Southern New England... 21Z update... no changes were made for this area. Campbell Remnant rainfall from the front that moved up the Mid-Atlantic Friday and Friday night will continue slowly moving northward into the Marginal Risk area Saturday morning. A steady southeasterly flow of rain will continue for much of the day, with low amounts of instability that may locally increase rainfall rates. Expect the best chances for isolated flash flooding in urbanized and poor-drainage areas. As the front causing the rainfall will be weakening throughout the day as it slowly lifts northeastward across New England, this poses an overall low flash flooding risk. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-wPxgYX2U3LzjWsNhtMJruozEdb5KXCid7rP5dhVK-bW= 1GZaLGq6FanqllJrww79GdbnzoIfKTBRS7vPysAeZWsf7FE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-wPxgYX2U3LzjWsNhtMJruozEdb5KXCid7rP5dhVK-bW= 1GZaLGq6FanqllJrww79GdbnzoIfKTBRS7vPysAeXX23FwE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-wPxgYX2U3LzjWsNhtMJruozEdb5KXCid7rP5dhVK-bW= 1GZaLGq6FanqllJrww79GdbnzoIfKTBRS7vPysAe09o5e3Y$=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) .