Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Apr 08 2023 00:22:20 FOUS30 KWBC 080022 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 821 PM EDT Fri Apr 07 2023 Day 1 Valid 01Z Sat Apr 08 2023 - 12Z Sat Apr 08 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS THE LOWER MISSISSIPPI RIVER VALLEY... Another round of moderate to locally heavy rainfall is on tap again this evening and tonight across portions of the Texas Gulf Coast through the Lower Mississippi River Valley. A shortwave trough embedded within the southwesterly flow will help draw northward additional moisture over a coastal front draped offshore Texas and then into the central Gulf Coast. Fairly anomalous moisture is expected ahead of this shortwave, particularly from southeast Louisiana eastward where PWs approach or exceed 1.5" overnight. The best instability will likely be right along the coast of southeast Louisiana, where the best opportunity for intense rain rates exceeding 1-2"/hr are likely based on the 18Z HREF probabilities. Elsewhere from Texas to southwest Louisiana, the lack of deeper instability is likely to keep rain rates generally under 0.5-1"/hr. The Slight Risk was adjusted to these trends and based on recent radar imagery. Parts of southern Louisiana have received heavy rainfall, some significant, over the last couple of days, priming the soils for possible additional flash flooding. Moisture will continue to stream eastward toward Georgia and the Carolinas overnight with some potential for a moderate, long duration rainfall into Saturday. Isolated streaks of 1"+ will be possible. Another area of concern is across southeast Georgia where interactions with the sagging frontal boundary and existing outflow boundaries in the presence of higher instability and moisture could focus slow moving intense thunderstorms. The most recent runs of the HRRR have been hinting at a cluster of storms developing there and redeveloping on its southern flank well into the overnight hours. The 18Z HREF neighborhood probabilities are moderate for 2"+ in that region. Despite the higher flash flood guidance and overall drier soils, appears to be at least some threat of flash flooding so the Marginal Risk was expanded to include that area for this update. Taylor Day 2 Valid 12Z Sat Apr 08 2023 - 12Z Sun Apr 09 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM THE FLORIDA PANHANDLE TO THE CENTRAL CAROLINAS... ....2030Z Update... The marginal risk area was extended to the southeast coast and down into northern Florida to due the latest guidance signal for thunderstorms to spread into coastal areas on Saturday. Details outlined in the previous discussion are still in place, but some of the CAMs are suggesting that a complex of intense thunderstorms containing modest rainfall rates of 0.25-0.5"/hr will initiate over an area of favorable instability and moisture and impact parts of northeastern Florida between 20z and 01z Saturday evening. Any flash flooding threat from this would likely be isolated/confined to urban areas. Kebede ....Previous Discussion... Only very minimal adjustments were made to the on-going Marginal Risk as the previous Day 3 outlook becomes today's Day 2 outlook.=20 Global guidance largely remained consistent...although a bit of disagreement on the axis storm coverage and thus heavy rainfall threat lingered across the Southeast. Given southeasterly low-level flow setting up over the Atlantic and over the boundary will help to continue to support the threat for areal average rainfall totals of 1-2", with locally higher amounts prompting an isolated threat for flash flooding, especially given the 500 mb to 250 mb mean trough and weak shortwave energy embedded within the broader trough to provide some focus and support while the flow aloft is at least modestly difluent. Bann Day 3 Valid 12Z Sun Apr 09 2023 - 12Z Mon Apr 10 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE PACIFIC NORTHWEST... ....2030z Update... The marginal risk area along the Pacific Northwest coast was expanded southward a bit to encompass parts of Oregon's south coast. Details described in the previous discussion remain accurate. Mussie ....Previous Discussion... On-shore flow of deep-layered moisture will once again develop across portions of the Pacific Northwest on Sunday that persists into Sunday night/early Monday morning...especially in the coastal ranges from Washington into northern Oregon. Precipitable water values in the 1.00 to 1.25 inch range will be drawn from the eastern North Pacific to the coastline by low level winds of 30 to 45 kts in the 925 to 850 mb layer.. Global model QPF values were typically in the 1.5 to 2.75 inch range...most of that being focused in a 12 to 15 hour period. Given potential for the flow to interact to terrain...will highlight the excessive potential with a Marginal and reevaluate again in subsequent outlooks. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-fChB9UVml61AD3wNnODHYrBBGYzGexHFUem4ifLakEm= cVTR15eUDZKQFHWzL40mUk9EEaXeYtmgYezQHnre11AXTIg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-fChB9UVml61AD3wNnODHYrBBGYzGexHFUem4ifLakEm= cVTR15eUDZKQFHWzL40mUk9EEaXeYtmgYezQHnrewYvVsgw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-fChB9UVml61AD3wNnODHYrBBGYzGexHFUem4ifLakEm= cVTR15eUDZKQFHWzL40mUk9EEaXeYtmgYezQHnrez2thse8$=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) .