Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed May 10 2023 07:31:04 FOUS30 KWBC 100730 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 330 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 Valid 12Z Fri May 12 2023 - 12Z Sat May 13 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH-CENTRAL TEXAS... TX... An upper low is expected to move out of northern Mexico into TX to the west of the track of the system crossing the state at this time, strongly increasing low-level inflow up the Rio Grande and into portions of the Escarpment towards 60 kts at 850 hPa.=20 Precipitable water values rise to 1.75-2", while CAPE should be 3000 J/kg or so. Convection with heavy rainfall is expected Friday evening which should continue overnight. Conceptually, the initial convective area should take roughly four hours to move -- any veering in the 850 hPa inflow is modest and towards Saturday 12z. Given the available moisture, 2.5-3" an hour totals cannot be ruled out while the convection is stationary -- especially if mesocyclones form -- which could pile up totals quickly. An equally big concern would be if convection formed out ahead of the developing complex -- particularly along the Escarpment -- once a cold pool formed leading to cell mergers as the advancing organized convection moves in to any preceding thunderstorm activity. Other problems would be if a warm advection band formed to its east-southeast which should be the eventual direction of propagation parallel to the 1000-500 hPa thickness gradient or if convection starts training on its western flank -- it's unclear whether that would be in Mexico or South-Central TX closer to the Rio Grande Valley. Some of the model guidance indicate that local amounts in the 10" (00z Canadian Global, 00z NAM12, and 09/12z ECMWF) or even 20" (Canadian Regional) range are possible should the worst case scenarios play out. For now, an upgrade to a Moderate Risk is being made which appears to be a conservative move. Further elevation in the risk level is possible in later cycles should the guidance get less dispersive while continuing to show such significant heavy rainfall potential. The new Moderate Risk area was coordinated with the CRP/Corpus Christi, EWX/New Braunfels TX, and SJT/San Angelo TX forecast offices. Northern Plains... Heavy, convective rains within a comma head of a low are expected to drop heavy rainfall in and near southwest SD due to ample moisture and the available instability. The guidance shows a good signal for local 2-4" amounts, which when added to rainfall on the previous day should be becoming problematic across the area. A slight risk was added on this issuance. Ohio Valley... Moderate low-level inflow bringing precipitable water values of 1.5-1.75" up the Ohio River could lead to heavy, excessive rainfall in and around the Upper Ohio Valley. Any training convection would likely occur during the afternoon and evening when the greatest instability is available. While most of the guidance shows local amounts in the 2-3" range (outside of the dry 00z GFS), the 00z Canadian Regional was much more emphatic across much of KY/the Ohio Valley. While there's still a bit of dispersion in the guidance on location, the ingredients suggest that the 00z UKMET, 00z Canadian Regional, and 00z NAM may be on the right track for where the heavy rain axis would be favored. A small Slight Risk was introduced in this update covering where heavy rainfall is expected to fall over low to modest flash flood guidance values. Hourly rain totals to 2" and local amounts to 4" would be possible wherever any short training bands set up. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6LogNengwssLvFKnI3se3RTUuDhjxrdwxYaKXPneOd-x= 5tOkPDuKE50PiUrClvclpoZCQklOsmmegHkqnlerWG-ifdw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6LogNengwssLvFKnI3se3RTUuDhjxrdwxYaKXPneOd-x= 5tOkPDuKE50PiUrClvclpoZCQklOsmmegHkqnlerP477uFQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6LogNengwssLvFKnI3se3RTUuDhjxrdwxYaKXPneOd-x= 5tOkPDuKE50PiUrClvclpoZCQklOsmmegHkqnlervfRsCHg$=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) .