Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Apr 28 2023 08:27:40 FOUS30 KWBC 280827 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 426 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 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!-ytRvjxpI8IT06IyNQuPULTDuc_CLs2TcZIedKHcUi5D= UsvpgckvWEhHIfdMXmqcQQn2uEjPVpAXS_wm6dyC2_i1sGA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-ytRvjxpI8IT06IyNQuPULTDuc_CLs2TcZIedKHcUi5D= UsvpgckvWEhHIfdMXmqcQQn2uEjPVpAXS_wm6dyC3cfwFJo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-ytRvjxpI8IT06IyNQuPULTDuc_CLs2TcZIedKHcUi5D= UsvpgckvWEhHIfdMXmqcQQn2uEjPVpAXS_wm6dyCgBfUNhI$=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) .