Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed May 10 2023 07:26:35 FOUS30 KWBC 100726 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 326 AM EDT Wed May 10 2023 Day 1 Valid 12Z Wed May 10 2023 - 12Z Thu May 11 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL NEAR THE TEXAS/LOUISIANA BORDER...=20 =20 ....Western Gulf Coast across AR...=20=20 The combination of a mid-level low moving north out of TX into the Midwest and an incoming closed cyclone moving through the West should set the stage for good moisture transport through the Plains, divergent flow aloft, strengthening low-level inflow/effective bulk shear, and sufficient instability for organized convection. To the south of the lifting upper level low, training and efficient bands of convection are expected from the Middle and Upper TX coast north-northeast through the Sabine River into far southwest AR. The reconfigured Moderate Risk area was coordinated with the HGX/Houston-Galveston TX, LCH/Lake Charles LA, SHV/Shreveport LA, and LZK/Little Rock AR forecast offices. The mesoscale guidance shows that portions of the Middle and Upper TX Coast should be receiving heavy rainfall at the beginning of the period in an area where a CAPE gradient should be lifting north within a warm advection pattern behind the departing upper low. The 00z HREF shows non-zero chances of 3"+ in an hour and 8"+ in 24 hours within the Moderate Risk area, which should be a morning event primarily in Southeast TX and occur during daytime heating into the overnight hours for places farther north. The Slight Risk area has been expanded across much of AR -- local Moderate Risk impacts are possible Wednesday night into early Thursday where any efficient, training bands manage to form. =20=20 ....Central High Plains/Northern Plains... A combination of upslope flow and some low- to mid-level frontogenesis acting in concert with sufficient instability should lead to heavy rainfall. Heavy rain which has occurred the past several days keeps portions of southern NE and northwest KS in the Marginal Risk. Precipitable water values near or above 1" in the High Plains are a big deal from a heavy rainfall perspective.=20 Closer to the 850 hPa low in WY/MT, the guidance has a strong signal for local 2" amounts...local amounts to 4" can't be ruled out. There has been some reduction to the size of the Slight Risk to follow the trend seen on the most recent guidance. =20 Roth=20 Day 2 Valid 12Z Thu May 11 2023 - 12Z Fri May 12 2023 ....Northern Plains/High Plains...=20 Moisture advection from the Gulf of Mexico increases ahead of a slowing frontal system over the central/southern High Plains and as the upper trough/low approaches from the West. Precipitable water values (PWs) of at or above 1" remain across eastern MT and northeast WY as a pair of low pressure system try to migrate from the High Plains into the central and northern Plains. A decent bubble of 1.25" PWs exists across the Dakotas within the system's comma head. In the High Plains, widespread heavy rainfall is expected in a favorable upslope pattern with available instability to the east and within a region with decent low- to mid-level frontogenesis, over wet ground conditions due to moderate/heavy rainfall the prior day. The Slight Risk has been reconfigured some in order to account for the latest model guidance QPF output. =20 ....Mississippi Valley/Lower Ohio Valley...=20 Near the ArkLaMiss, heavy rainfall, possibly in training bands, is expected early on/Thursday morning prior to the onset of daytime heating -- the 00z HREF indicates a high risk of 5"+ amounts near Winnsboro LA which appears within reason. A long fetch of ~1.5" PWs with decently strong low-level inflow/effective bulk shear and ample instability due to daytime heating should allow for convection across portions of the MS and Lower OH Valleys. There has been some convergence in the guidance concerning the heavy rain signal near the confluence of the Ohio and Mississippi Rivers, with the guidance generally showing the potential for 2-4" of rain, which could fall quickly, perhaps as much as 2.5" an hour should there be any mesocyclones or short training bands. For both regions, Slight Risk areas were introduced.=20 =20 ....Florida... Convergence near an old front combined with easterly flow across the Peninsula, plenty of instability, and precipitable water values near 1.75" should set the stage for heavy rainfall for central and northern portions of Florida's West Coast. Hourly rain totals to 2.5" and local amounts to 5" (as implied by the 00z HREF) are possibilities in this region. Added a Marginal Risk area to accommodate this potential heavy rainfall threat. Roth Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4W3-PdHUSNAofYHqwqUFkT-YPOD0oqfVGxSwF_bTAUh3= b1Qso7vpxPeb9ZMwu9LSVyTer0oPv8nZ9jH0Qb4c5EF5lwk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4W3-PdHUSNAofYHqwqUFkT-YPOD0oqfVGxSwF_bTAUh3= b1Qso7vpxPeb9ZMwu9LSVyTer0oPv8nZ9jH0Qb4cJkQ6AWc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4W3-PdHUSNAofYHqwqUFkT-YPOD0oqfVGxSwF_bTAUh3= b1Qso7vpxPeb9ZMwu9LSVyTer0oPv8nZ9jH0Qb4ctfYHWpw$=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. 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