Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 21 2024 21:02:27 FOUS30 KWBC 212102 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 502 PM EDT Thu Mar 21 2024 Day 1 Valid 01Z Fri Mar 22 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... ....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=20 FLORIDA... ....Southern Florida... Multiple rounds of organized deep convection currently located over the central Gulf Coast are forecast to expand and translate=20 eastward toward South Florida as surface low pressure consolidates=20 along the Gulf states tomorrow morning. Diffluent flow pattern noted over the Gulf will continue to intensify as a southern stream jet=20 streak over Baja California digs into the base of the upper-trough=20 axis, leading to cyclonegenesis and an associated uptick in low-=20 level 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=20 recent HREF guidance compared to last night. Given the uncertainty,=20 maintained a Slight Risk with a minor expansion westward as=20 locations that fall within the main axis of heavy rainfall will be=20 quite wet, with localized rainfall totals upwards of 6-7 inches=20 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=20 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 England=20 coastal plain. The guidance is quite impressive with the axis of=20 frontogenesis across this region, which coupled with the level of=20 low-level moisture convergence and divergent jet quadrants ahead of=20 the low may result in a southwest/northeast axis of rainfall rates=20 that reach as high as 0.50" to 0.75"/hour, and despite what should=20 be a general lack of instability. A stripe of 2 to 4 inches of rain=20 appears likely from the Delmarva up toward Long Island and areas of=20 southern New England where these enhanced rates tend to persist.=20 Accordingly, the Slight Risk of excessive rainfall was expanded=20 further northeast along the I-95 corridor given the 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=20 for the convective threat to then come to an end. Given the=20 conditional heavy rainfall threat in the prior period, any=20 additional rains Saturday morning may result in additional runoff=20 concerns. A Marginal Risk area has been depicted for the southeast=20 FL urban corridor as a 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=20 amounts are expected to be as much as 1.5 to 2.5 inches, with=20 potentially some spotty instances of 0.50"/hour rainfall rates.=20 Given some of locally heavy rainfall expected here before this=20 period on day 2, the antecedent conditions are expected to be rather wet, and the additional rains may pose some very isolated runoff=20 concerns. Asherman/Orrison Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8g2-DuZHBIbCCXfSUbbMy8Z4SvjEziWWjnmLNu0tODhJ= k8G_F2ZEAPURztI0ut78aTANjxdGH26AWZw37ZjNcnmqWTk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8g2-DuZHBIbCCXfSUbbMy8Z4SvjEziWWjnmLNu0tODhJ= k8G_F2ZEAPURztI0ut78aTANjxdGH26AWZw37ZjNS2dfcDI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8g2-DuZHBIbCCXfSUbbMy8Z4SvjEziWWjnmLNu0tODhJ= k8G_F2ZEAPURztI0ut78aTANjxdGH26AWZw37ZjNgVYiXoM$=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) .