Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Thu Mar 21 2024 08:18:00 FOUS30 KWBC 210900 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 500 AM EDT Thu Mar 21 2024 Day 1 Valid 12Z Thu Mar 21 2024 - 12Z Fri Mar 22 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE RED RIVER VALLEY TO THE WESTERN GULF COAST... No major changes to the previous thinking. A 500 mb southern stream trough is forecast to track eastward from the southern High Plains this morning to the west-central Gulf Coast by Friday morning. The eastward advance of this energy will facilitate development of a wave of low pressure near or just offshore of southeast TX early this morning and in conjunction with a northward returning warm front. A sufficient level of instability and forcing is expected to be in place for the development and expansion of convection this morning over areas of south Texas and especially over toward the middle TX coast and offshore. The 00Z HREF guidance supports the heaviest rains generally focusing offshore closer to the low center and the attendant frontal zone, but there may be sufficient rains along the coast or just inland for an isolated threat for runoff problems and flooding. Meanwhile, with the core of the ejecting mid-level trough and some amplification of the 700 mb low center toward the Red River Valley, there is expected to be multiple clusters/bands of convection that develop this afternoon and evening which will more broadly impact central/northern TX and into south-central OK. A more progressive evolution of the convective threat, and potentially a QLCS, may then reach across southeast TX and southwest LA Friday morning. For all of the convective areas across the southern Plains/western Gulf Coast area, there may be some spotty 2 to 4 inch rainfall totals. A Marginal Risk area remains intact at this time. Orrison Day 2 Valid 12Z Fri Mar 22 2024 - 12Z Sat Mar 23 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHEAST FLORIDA... ....Southern Florida... A well-organized area of convection, and likely a QLCS, is expected to be traversing the eastern Gulf of Mexico early Friday morning which is expected to impact much of southwest to southeast FL. The convection will be driven by a combination of factors including a very moist and unstable airmass pooling over the southeast Gulf of Mexico in close proximity to a warm front, and with strong low- level jet energy/forcing within a very divergent flow pattern aloft. The 00Z HREF guidance tends to be favor the potential for the heaviest rains staying southwest of the southern Florida Peninsula near the aforementioned front and in proximity the better instability, but this initial round of heavy rainfall may result in locally 2 to 3 inches of rain alone before it sweeps through. A second round of heavy rain is anticipated though ahead an approaching 500 mb trough tracking eastward along the Gulf Coast. Strong divergence aloft ahead of the upper trough and a strong southerly low-level jet of 40 to 50+ kts are forecast to refocus convection initially out over the central and eastern Gulf of Mexico which will then advance quickly east and impact south FL. Between the two rounds of heavy rainfall, as much as 3 to 6 inches of rain will be possible. The Marginal Risk area was tweaked to include all of south FL, and with concerns for the more sensitive urban corridor across southeast FL including areas from Fort Lauderdale down through Miami and Homestead, a Slight Risk was introduced here where there may be an urban flash flood threat. ....Southeast into the Southern Mid-Atlantic... As mentioned before, 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 transport wrapping into the region from the south and southeast. Precipitable water values over 1 inch for inland locations and nearing 1.5 inches for the coast can be expected. 850 mb winds in excess of 50 kt are expected ahead of a developing surface low to track across the Southeast. Despite the anomalous moisture for the Southeast (+1 to +2 standardized PW anomalies), poor mid-level lapses will limit instability to fairly weak values (up to ~500 J/kg and perhaps near 1000 J/kg near the coast), which should limit rainfall rates overall. In addition, the anomalous moisture plume will begin to advance eastward/northward up the East Coast toward the end of the period. A Marginal Risk remains in place and was generally tweaked to account for the 00Z HREF guidance. Orrison Day 3 Valid 12Z Sat Mar 23 2024 - 12Z Sun Mar 24 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE DELMARVA NORTHWARD INTO COASTAL AREAS OF SOUTHERN NEW ENGLAND... ....Mid-Atlantic into Southern New England... The latest multi-model consensus continues to support a southern stream area of low pressure and energy associated with it advancing northeastward up across the Southeast coastal plain and generally along the Mid-Atlantic coast on Saturday before then impacting southern New England by Saturday night. The advance of northern stream shortwave energy/troughing into the Northeast will gradually capture/deepen the low center as it advances north- northeast across the region. Very strong low-level forcing/warm air advection and moisture transport out ahead of the advancing low center should favor an axis of heavy rainfall across the Mid- Atlantic and southern New England coastal plain, with the heaviest rainfall amounts south and east of I-95. The guidance is quite impressive with the axis of frontogenesis across this region, which coupled with the level of low-level moisture convergence/jet energy ahead of the low may result in a southwest/northeast axis of rainfall rates that reach as high as 0.50" to 0.75"/hour, and despite what should be a general lack of instability. Locally as much as 2 to 4 inches of rain may impact areas of the Delmarva up toward Long Island and areas of southern New England where these rates tend to persist and this may result in areas of flooding as a result. The Slight Risk of excessive rainfall was expanded to cover these areas. ....Southeast Florida... Locally organized areas of convection may still be transiting areas of southeast FL early Saturday morning, as energy and moisture continues to focus across the region. A cold front will sweep across the region though early in the day, and this should allow for the convective threat to then come to an end. Given the heavy rainfall threat in the prior period, any additional rains Saturday morning may result in additional runoff concerns. A Marginal Risk area has been depicted for the southeast FL urban corridor as a result. ....Northwest California and Far Southwest Oregon... A Marginal Risk of excessive rainfall is highlighted for areas of northwest CA and southwest OR as additional Pacific moisture and energy arrives in association with a larger scale upper-level trough gradually crossing the West Coast. Additional rainfall amounts are expected to be as much as 1.5 to 2.5 inches, with potentially some spotty instances of 0.50"/hour rainfall rates. Given some of the rainfall expected here before this period, the antecedent conditions are expected to be rather wet, and the additional rains may pose some very isolated runoff concerns. Orrison = = = --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .