Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Apr 28 2023 20:07:04 FOUS30 KWBC 282007 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 406 PM EDT Fri Apr 28 2023 Day 1 Valid 16Z 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... 16Z update... There has been a growing signal for the potential for heavy rains to reach or exceed FFG across portions of Ohio and West Virginia this afternoon and evening. While some of the hi-res guidance is showing fairly progressive cells averaging around 0.75 in/hr there are a few that may pulse up to 1-1.25 inches/hr over some of the areas of higher terrain. The Marginal Risk was expanded westward to eastern Ohio and most of West Virginia. The Slight Risk area continues to highlight where the urban corridor may be impacted by heavy rain and flash flooding. There was a small shift eastward on where the higher QPF may align therefore the Slight Risk was trimmed on the northwest side and bumped eastward to encompass the Annapolis area. Campbell 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... 16Z update... The latest guidance continues to suggest an increase in the potential for heavy rainfall across eastern Oklahoma and further north/east across western Arkansas. Given this trend and forecast amounts close or above local FFG, the northern bounds of the Marginal Risk was expanded. Campbell 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... 21Z update... The latest guidance continues to show 2 axes of QPF across the Southeast, the heavier and more dominant one along the Gulf Coast and into Georgia (within the Slight Risk area) and another one spanning from Mississippi to far southwest North Carolina. Consensus with the northern band has areal averages of 1 to 3+ inches and the local FFG are as low as 1/1.5 in and 0.25 in along the North/South Carolina border in the mountains. Given the potential to reach or exceed FFGs and the soil sensitivity, the Marginal Risk area was extended northward to cover this part of the country. Additionally, the Slight Risk had a minor tweak eastward across eastern Georgia to reflect the latest trends. 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. 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... 21Z update... With QPF trends showing decreased amounts across parts of Southeast New York the Marginal Risk area was trimmed over this part of the region. Still expecting the heavier rainfall to be over coastal areas. Campbell 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 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-8VO5EJ-VAtTEPqb0P6MU__Sn5Ivra4FHZFkRCRzQZ3w= 6qyw645rn0UcMY30VNCDQOyfmBpB0MmjJ7mLyuBjiEHUOcg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-8VO5EJ-VAtTEPqb0P6MU__Sn5Ivra4FHZFkRCRzQZ3w= 6qyw645rn0UcMY30VNCDQOyfmBpB0MmjJ7mLyuBjPwSDMr4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-8VO5EJ-VAtTEPqb0P6MU__Sn5Ivra4FHZFkRCRzQZ3w= 6qyw645rn0UcMY30VNCDQOyfmBpB0MmjJ7mLyuBjcNUZxDA$=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) .