Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 21 2024 20:55:59 FOUS30 KWBC 212055 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 455 PM EDT Thu Mar 21 2024 Day 1 Valid 16Z 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... ....16Z Update... The forecast thinking remains unchanged regarding the isolated=20 excessive rainfall threat today across the Red River Valley and=20 Western Gulf Coast. Showers and thunderstorm activity continues to expand this morning in south Texas in the vicinity of a surface=20 low, warm front, and inverted trough analyzed between CRP and BEA. The ongoing convection has reinforced the warm front offshore,=20 which is expected to prevent more robust surface based instability from returning inland as reflected in the 12Z CAM guidance.=20 However, can not rule out isolated runoff issues close to the=20 coastline as the surface low and warm front migrate east today.=20 Otherwise, a pair of vorticity maxima and weak surface dryline will lead to thunderstorms later this afternoon across the=20 Southern Plains. Localized rainfall totals of 2-4" remain possible=20 over portions of the Southern Plains this afternoon where storm=20 motions are reduced close to the ejecting closed low. Asherman ....Previous Discussion... A 500 mb southern stream trough is forecast to track eastward from the southern High Plains this morning to the west-central Gulf=20 Coast by Friday morning. The eastward advance of this energy will=20 facilitate development of a wave of low pressure near or just=20 offshore of southeast TX early this morning and in conjunction with a northward returning warm front. A sufficient level of=20 instability and forcing is expected to be in place for the=20 development and expansion of convection this morning over areas of=20 south Texas and especially over toward the middle TX coast and=20 offshore. The 00Z HREF guidance supports the heaviest rains=20 generally focusing offshore closer to the low center and the=20 attendant frontal zone, but there may be sufficient rains along the coast or just inland for an isolated threat for runoff problems=20 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=20 convection that develop this afternoon and evening which will more broadly impact central/northern TX and into south-central OK. A=20 more progressive evolution of the convective threat, and=20 potentially a QLCS, may then reach across southeast TX and=20 southwest LA Friday morning. For all of the convective areas across the southern Plains/western Gulf Coast area, there may be=20 some spotty 2 to 4 inch rainfall totals. A Marginal Risk area=20 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 SOUTH FLORIDA... ....Southern Florida... Multiple rounds of organized deep convection currently located over the central Gulf Coast are forecast to expand and translate eastward toward South Florida as surface low pressure consolidates along the Gulf states tomorrow morning. Diffluent flow pattern noted over the Gulf will continue to intensify as a southern stream jet streak=20 over Baja California digs into the base of the upper-trough axis, leading to cyclonegenesis and an associated uptick in low-level=20 inflow, feeding a very moist and unstable airmass into the=20 convective complexes along a warm front. Uncertainty remains=20 regarding the track of the convection along the front tomorrow, with a noted southward trend to the axis of heaviest rainfall in the recent HREF guidance compared to last night. Given the uncertainty, maintained a Slight Risk with a minor expansion westward as locations that fall within the main axis of heavy rainfall will be quite wet, with localized rainfall totals upwards of 6-7 inches possible by Saturday morning.=20 ....Southeast into the Southern Mid-Atlantic... The deepening low pressure center will drive strong low-level=20 onshore flow (upwards of 50 kts at 850 mb) and moisture transport=20 from the Gulf of Mexico and Atlantic into the Southeast and Mid-=20 Atlantic region, ushering in precipitable water values over 1-1.25=20 inches to the region. Despite the anomalous moisture for the=20 Southeast (+1 to +2 sigma standardized PW anomalies), poor mid-=20 level lapses will limit instability to fairly weak values (up to=20 ~500 J/kg and perhaps near 1000 J/kg near the coast), which should=20 limit rainfall rates overall. A Marginal Risk remains in place and=20 was expanded slightly westaward into the Appalachians given the upward trend in rainfall totals corresponding to a more upslope component to the strong low level flow. ....Northern California into Southwest Oregon... A Marginal risk was addded as southwesterly onshore flow increases ahead of an approaching frontal system tomorrow morning. IVT values may briefly approach 600-700 ahead of the cold front, leading to periods of moderate to locally heavy rainfall within a band of anomalous PW values +1 to +2 sigma above climatology. Combined with post frontal shower activity, localized rainfall totals upwards of 3-4" appear possible along the coastline.=20 Asherman/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 forecast thinking maintains a southern stream area of low=20 pressure and energy associated with it advancing northeastward up=20 across the Southeast coastal plain and generally along the Mid-=20 Atlantic coast on Saturday before then impacting southern New=20 England by Saturday night. The advance of northern stream shortwave energy/troughing into the Northeast will gradually capture/deepen=20 the low center as it advances north- northeast across the region.=20 Very strong low- level forcing/warm air advection and moisture=20 transport out ahead of the advancing low center should favor an axis of heavy rainfall across the Mid-Atlantic and southern New=20 England coastal plain. The guidance is quite impressive with the=20 axis of frontogenesis across this region, which coupled with the=20 level of low-level moisture convergence and divergent jet quadrants=20 ahead of the low may result in a southwest/northeast axis of=20 rainfall rates that reach as high as 0.50" to 0.75"/hour, and=20 despite what should be a general lack of instability. A stripe of 2=20 to 4 inches of rain appears likely from the Delmarva up toward Long=20 Island and areas of southern New England where these enhanced rates=20 tend to persist. Accordingly, the Slight Risk of excessive rainfall=20 was expanded further northeast along the I-95 corridor given the=20 trend for higher QPF along the coastline since overnight.=20 ....Southeast Florida... Locally organized areas of convection may still be transiting areas of southeast FL early Saturday morning, as energy and moisture=20 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=20 convective threat to then come to an end. Given the conditional heavy rainfall threat in the prior period, any additional rains Saturday=20 morning may result in additional runoff concerns. A Marginal Risk=20 area has been depicted for the southeast FL urban corridor as a=20 result. ....Northwest California and Far Southwest Oregon... A Marginal Risk of excessive rainfall is maintained for areas of=20 northwest CA and southwest OR as additional Pacific moisture and=20 energy arrives in association with a larger scale upper-level trough gradually crossing the West Coast. Additional rainfall amounts are=20 expected to be as much as 1.5 to 2.5 inches, with potentially some=20 spotty instances of 0.50"/hour rainfall rates. Given some of locally heavy rainfall expected here before this period on day 2, the=20 antecedent conditions are expected to be rather wet, and the=20 additional rains may pose some very isolated runoff concerns. Asherman/Orrison Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-CW8U1OBBvRFug-FbjFrszHSDQb53A3ZHIxHrUIi65YT= VaZl63U3VTB-lBDyWdDincLAI4hSNdm9uZgNGfWA_ClhiX0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-CW8U1OBBvRFug-FbjFrszHSDQb53A3ZHIxHrUIi65YT= VaZl63U3VTB-lBDyWdDincLAI4hSNdm9uZgNGfWAChJ05nA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-CW8U1OBBvRFug-FbjFrszHSDQb53A3ZHIxHrUIi65YT= VaZl63U3VTB-lBDyWdDincLAI4hSNdm9uZgNGfWAWn8FsAU$=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) .