Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Apr 28 2023 08:35:38 FOUS30 KWBC 280835 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 434 AM EDT Fri Apr 28 2023 Day 1 Valid 12Z Fri Apr 28 2023 - 12Z Sat Apr 29 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE WASHINGTON D.C. AND BALTIMORE URBAN CORRIDOR... ....Mid-Atlantic... An area of surface low pressure over the Ohio Valley this morning will transfer its energy to a developing coastal low near the VA/NC border by this evening, then the new low will crawl up the coast to the Delmarva by early Saturday morning. North of the lows, a strong southeasterly 850 mb LLJ is developing, which will strengthen to between 40-50 kt. This LLJ will draw prodigious amounts of Atlantic moisture into the Mid-Atlantic. With the slow-moving low holding the LLJ in roughly the same place for 12-18 hours, this will promote widespread moderate rain remaining nearly continuous throughout the day today. Fortunately, conditions and soil moisture have been drier than normal prior to today, which will certainly help mitigate potential flooding. However, HREF EAS probabilities are over 50 percent for 2 inches of rain in the DC-Baltimore corridor, and similar values for over 3 inches of rain in the less accurate 40 km neighborhood probabilities. Thus, it's the long duration of the rain that will aggravate flooding concerns. The urban corridor also has lower FFGs than the surrounding area, with typical poor drainage spots, which increases the chances of more widely scattered flash flooding. Trends in the rainfall totals have been increasing now for a few days, with the official WPC rainfall forecast now showing widespread 2 inch forecast amounts from northern VA through northern MD, with embedded 3 inch totals likely. In coordination with the LWX/Sterling, VA forecast office, a Slight Risk area upgrade was introduced for the DC and Baltimore metro areas with this forecast update. For the surrounding Marginal Risk area, similar rainfall totals are forecast from the Chesapeake through the Allegheny Front of western PA through the eastern WV panhandle. FFGs are higher outside of the metros, which have otherwise also experience drier than normal conditions recently. Thus, it's expected that any flooding concerns in the surrounding Marginal Risk area will be more isolated. ....Southern Plains... A vigorous positively tilted trough will dig southward across New Mexico and west Texas this afternoon and tonight. Ahead of this forcing, a bit of southerly return flow will bring Gulf moisture with PWATs to 1.25 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. The latest CAMs guidance is in good agreement that individual supercell thunderstorms will develop this afternoon, which will rapidly congeal into a squall line, which will then quickly move to the Texas Gulf Coast, all the way south to near Brownsville. With this pattern of very intense, but rather quick moving thunderstorms, flash flooding will be isolated, since no one area is likely to see heavy rain for more than an hour or two. During that short time it's raining, it's likely to be heavy, with embedded hail likely included in many instances. There's some concern, particularly along the I-35 corridor from San Antonio through Austin that antecedent wet conditions for the past 7-10 days that there's a higher chance of flash flooding, but because of the small areal coverage and rather quick expected movement of the storms to the southeast, for now will maintain the Marginal Risk. This may need to be upgraded with today's midday update if guidance either slows down the movement of the storms or the convection is stronger than currently forecast. No large changes were made to the inherited Marginal Risk area, as it is situated well for where the developing line of thunderstorms is expected to form. Wegman Day 2 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... 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. No significant changes were made with this morning's forecast update. Multiple rounds of storms are expected, most of which will be overnight Saturday night across the Slight Risk area. While each storm is likely to be fairly quickly moving, repeated storms over this area which just saw thunderstorms move through yesterday are likely to result in isolated to widely scattered flash flooding, especially in urbanized and poor drainage areas, such as Tallahassee, Jacksonville, and Valdosta. ....Southern New England, New York City Metro Area, Long Island, and the Hudson Valley... The same front that is likely to cause flooding over the Mid-Atlantic today will continue drifting northward into New York and Southern New England Saturday. The front will be significantly weaker on Saturday than today, though 30-40 kt southerly winds will bring 1 inch PWAT atmospheric moisture into the area. The forcing in the upper levels are likely to remain well west of the area, and the separation of the moisture and forcing should help to mitigate any heavy rainfall. This remains a lower end Marginal risk, and decreasing rainfall totals the past few forecast cycles may have this Marginal canceled with future updates. Wegman Day 3 Valid 12Z Sun Apr 30 2023 - 12Z Mon May 01 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF SOUTHERN MAINE AND EASTERN NEW HAMPSHIRE... ....New England and the Mid-Atlantic... In coordination with GYX/Gray, ME forecast office, a Slight Risk upgrade was introduced with this morning forecast update. An area of low pressure over the Southeast that helps force the convection there Saturday night will race up the East Coast and then curve westward over New York State towards a second low over the Great Lakes Sunday night. Once the low turns westward, the warm front ahead of the low, in tandem with vigorous upper level forcing from a negatively tilted longwave trough tracking northeastward through the Mid-Atlantic will invigorate the southeasterly flow off the Atlantic to a large degree. The flow will increase to as strong as 60 kt at 850 mb, with the LLJ advecting PWATs to 1.25 inches into the Slight Risk area. Add in significant topography in the White Mountains of NH and southern ME, and it's likely there will be 3 inches or more of rain, much of which falling after midnight Sunday. This large amount of rain falling in an area with steep topography will result in rapid increases in local small creeks and streams. Thus, the possibility of flash flooding in this area is more than just isolated. Somewhat lesser amounts of rain are expected both north of this region in central Maine, as the front that stalls out and causes the locally heavier rain over southern Maine doesn't get that far north, and further west there will be lower availability of moisture, despite similar upper level forcing closer to the center of the longwave trough. Further, across the northern Mid-Atlantic, especially where a bit heavier rain is expected over northeastern PA and much of NY, it's been relatively dry, so the 1-2 inches of rain, locally heavier, will fall over a longer period of time over an area that can better handle those amounts of rain. The Marginal Risk area was expanded southward across the Delmarva and northern VA with this update, largely to encompass the same areas expected to pick up multiple inches of rain today, and are also likely to see an additional inch or so of rain again on Sunday. Given much of the heaviest rain in the Slight Risk area is expected around the Day 3/Day 4 transition around 12Z Monday, any increased speed of the front may nudge the heaviest rainfall rates and chances of flooding into the Day 4/Monday ERO period. ....Florida... The trailing end of the front and the low over the Southeast will be over the middle of Florida from the western Panhandle through the northern Peninsula on Sunday. With the parent low tracking northeast early in the day, showers and thunderstorms associated with the trailing cold front will develop in the morning and persist through peak heating, likely diminishing in intensity, coverage, and duration by the time the storms reach southern Florida after sunset Sunday evening. With storm motion parallel to the front (northeastward) and Corfidi vectors at 10 kt or less over the Marginal Risk area in FL, along with instability to 1500 J/kg Sunday morning ahead of the storms, the likelihood for training storms is significant. Since the storms will have plenty of moisture to work with, expect heavy rain rates to 2 inches/hour associated with the storms. Depending on how the storms ongoing Saturday night evolve, an upgrade over the northern portion of the area is possible if the storms from Saturday night are slow to clear the Day 2/Saturday Slight Risk area. Thus, unlike in New England, the timing for the main event in this area will be first thing Sunday morning, with a likely early cancellation of the Marginal Risk area by Sunday afternoon. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7J_ABi_dMcCwdJSIJzzn_n7Dm21kl53GO0WuNeVXvF-H= KzVMcTXkqHbD9RYkrEtvUlsBXtmy9EJDL-AOUJIXzv4Of_s$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7J_ABi_dMcCwdJSIJzzn_n7Dm21kl53GO0WuNeVXvF-H= KzVMcTXkqHbD9RYkrEtvUlsBXtmy9EJDL-AOUJIX-sMvl3Y$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7J_ABi_dMcCwdJSIJzzn_n7Dm21kl53GO0WuNeVXvF-H= KzVMcTXkqHbD9RYkrEtvUlsBXtmy9EJDL-AOUJIXuyE1SW0$=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) .