Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Mar 20 2024 20:29:14 FOUS30 KWBC 202028 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 428 PM EDT Wed Mar 20 2024 Day 1 Valid 01Z Thu Mar 21 2024 - 12Z Thu Mar 21 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent.=20 Otto Day 2 Valid 12Z Thu Mar 21 2024 - 12Z Fri Mar 22 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF=20 THE RED RIVER VALLEY TO THE WESTERN GULF COAST... A 500 mb southern stream trough is forecast to track eastward from the southern High Plains Thursday morning to the west-central Gulf Coast by Friday morning. The advancing of this feature will allow=20 a northward returning warm front in the western Gulf to lift into=20 southeastern Texas during they day on Thursday. A weak low along=20 the front is forecast to hover near or just south of the Gulf Coast into Thursday evening, but the low becomes less defined in the guidance after 00Z Friday. The 12Z hires consensus of simulated reflectivity shows convection near the front/low and=20 along the middle/upper Texas coast around 12Z Thursday, possibly=20 expanding into southwestern Louisiana within peak precipitable=20 water values of 1.5 to 1.8 inches. From the convection near the=20 coast, an organized convective cluster is expected to track ESE=20 near/north of a baroclinic zone well south of the central Gulf=20 Coast into Friday morning. Therefore, the greatest threat for an=20 organized excessive rainfall threat near the coast will occur from=20 about 12Z through ~21Z. The best instability (1000 to 2000+ J/kg) is still forecast to be=20 confined to within ~100 miles of the Coastal Plain but sufficient instability will exist farther north into the Red River Valley as well to support high rainfall rates of at least 1-2 in/hr. A small window of time will occur Thursday evening ahead of the main=20 eastward advancing shortwave from West Texas with weak instability and organized thunderstorms which will have the potential for=20 isolated 2-4 inch totals from storm scale interactions. As the=20 upper low continues to advance east Thursday night, there will be=20 a loss of instability across northern locations (drier air=20 advecting in and the loss of daytime heating) with a remaining=20 focus for heavy rain farther south over eastern/southern Texas,=20 which is forecast to transition into a largely progressive rainfall event by 12Z Friday. Otto Day 3 Valid 12Z Fri Mar 22 2024 - 12Z Sat Mar 23 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AND SOUTHERN MID-ATLANTIC, AS WELL AS FOR COASTAL NORTH CALIFORNIA AND FAR SOUTHWESTERN OREGON... ....Southern Florida... Convection may be ongoing to the immediate west of the Florida=20 Peninsula Friday within a moist environment (PWAT values locally=20 exceeding 2 inches), a carryover from expected Gulf of Mexico convection from Thursday night with an expected movement toward=20 the east. A second round of heavy rain is anticipated ahead an advancing 500 mb trough tracking eastward along the Gulf Coast. Strong divergence/diffluence aloft ahead of the upper trough and 850 mb winds from the south are forecast by the 12Z GFS to climb=20 into the 30-50 kt range from the south, likely supporting locally=20 high rainfall rates impacting the southern half of the Florida=20 Peninsula. Through the period, there is a decent consensus from the 12Z models for 3 to 6 inches over southern Florida. Granted, much=20 of the real estate for this region is swamp but the urban corridor=20 from Tampa Bay to Marco Island and near Melbourne down to the Miami metro and Florida Keys will see some threat for excessive=20 rainfall. As the event draws nearer, the possibility for an upgrade to Slight Risk remains for a portion of the region but confidence=20 on placement is not high enough at this point in time. ....Southeast into southern Mid-Atlantic... Rainfall will expand eastward across the Gulf Coast states and eventually into the Southeast/Mid-Atlantic region region ahead of a potent mid-level trough. Increasing onshore flow and moisture=20 transport wrapping into the region from the south and southeast.=20 Precipitable water values over 1 inch for inland locations and=20 nearing 1.5 inches for the coast can be expected. 850 mb winds in=20 excess of 50 kt are expected ahead of a developing surface low to=20 track across the Southeast. Despite the anomalous moisture for the=20 Southeast (+1 to +2 standardized PW anomalies), poor mid-level=20 lapses will limit instability to fairly weak values (up to ~500=20 J/kg and perhaps near 1000 J/kg near the coast), which should limit rainfall rates overall. In addition, the anomalous moisture plume=20 will begin to advance eastward/northward up the East Coast toward=20 the end of the period. A Marginal Risk, focused where instability=20 will be best, was left in place from continuity but with some=20 trimming on the northwestern/western edges. ....Coastal northern California and far southwest Oregon... The previous Marginal Risk was removed with this update for the=20 West Coast given a perceived lower threat for excessive rainfall.=20 The latest model consensus shows a quick-hitting plume of moisture ahead of a cold front (peak IVT values near 500 kg/m/s) reaching=20 the OR/CA border around 12Z Friday with limited/negligible=20 instability but short term potential for rainfall rates exceeding=20 0.5 in/hr, followed by the axis of higher IVT sliding southward=20 down the CA coast through 00Z Saturday. As the cold core of a 500=20 mb low nears the Oregon and northern California coast Friday=20 evening, instability will increase with steepening 700-500 mb lapse rates which could yield rainfall rates above 0.5 in/hr but=20 these values should be highly localized. Otto Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7WJRr0juCa8bFzKyljgB6uxvlWTOI6lUhUyrNBLLUmdB= kGA3nMuRdw-XBTY9M7kSOE1f9GCAmwLYDnkqcoqQ_dRg0hc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7WJRr0juCa8bFzKyljgB6uxvlWTOI6lUhUyrNBLLUmdB= kGA3nMuRdw-XBTY9M7kSOE1f9GCAmwLYDnkqcoqQh3HDZAI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7WJRr0juCa8bFzKyljgB6uxvlWTOI6lUhUyrNBLLUmdB= kGA3nMuRdw-XBTY9M7kSOE1f9GCAmwLYDnkqcoqQOXO-t6U$=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) .