Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed May 10 2023 01:02:27 FOUS30 KWBC 100102 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 901 PM EDT Tue May 09 2023 Day 1 Valid 01Z Wed May 10 2023 - 12Z Wed May 10 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN SOUTHEAST TEXAS...=20 =20 ....Texas/Louisiana... 0100 UTC Update -- Trimmed a bit of the Moderate and Slight Risk areas based on what has taken place thus far (over 12hrs into the Day 1 forecast period), and given the latest observational/mesoanalysis and rapid-refresh model trends. Many (most) areas within the Moderate Risk area have received at least 1.0" of rain thus far. While model QPF consensus overnight is still not very good, all of the CAMs have a signal for heavy rainfall within the Moderate area and western portion of the Slight. The location and track of the compact MCV will be key in terms of where elevated convection becomes better organized and grows upscale, both in terms of the best forcing (DPVA and divergence aloft) and equally important the nocturnally-enhanced uptick in S-SE flow above the surface layer. One aspect that the guidance has been consistent with over the past several hours is that the axis of heaviest QPF should remain west of Galveston and Trinity Bays. =20 ....Central Plains...=20 0100 UTC Update -- More significant modifications were made to the outlook area across this region, largely due to the MCS that has already developed across eastern KS and migrated to the MOKSAROK 4-state area. Maintained a small portion of the Slight Risk area, over south-central and southeast KS, given the redevelopment of more organized elevated convection behind the outflow boundary and along/ahead of the main (stationary) surface front.=20 Hurley Day 2 Valid 12Z Wed May 10 2023 - 12Z Thu May 11 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEAST TEXAS... ....2030Z Update... ....Arklatex Region... There remains significant uncertainty in all of the guidance as to how the convection expected across eastern Texas will develop, but there is some agreement that convection along the Gulf coast to start the day Wednesday will slowly drift north ahead of a slow moving upper level disturbance following behind the convection. The storms will have plenty of Gulf moisture streaming in from the south, along with MUCAPE between 1,000 and 2,000 J/kg will have no trouble keeping the storms going as they track north into Arkansas by Wednesday night. Convection is likely to develop across AR Wednesday afternoon as well. However, it's likely that any and all convection in this region will be slow-moving, which will give the storms opportunity to produce rainfall rates capable of producing localized flash flooding. Of course, the area of southeast TX that not only saw rain recently, is currently seeing storms, and will get more Wednesday will have a much higher likelihood of flash flooding, along with overall higher rainfall in the area. ....Eastern WY, Southeastern MT, Far Western SD, & Northwestern NE... Few changes were made to the inherited Slight Risk area. Light easterly flow will advect some Gulf moisture into the area, while most of it tracks north up the Plains, but that flow will provide the moisture and the slow steering flow to support slow-moving storms that are likely to be caught along the mountains and adjacent Plains. A small southwestward expansion was implemented following latest guidance trends. Wegman=20 =20 ....Plains...=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. Where those ingredients exist the most prolonged period of time is in and near the ArkLaTex, where a Slight Risk area remains. There is a general signal in the global guidance for local 3-5" amounts somewhere across the eastern half of TX within the Slight Risk area. The better signal in general heavy rainfall was where ingredients suggest it should be across eastern OK and western AR, which remain in the Slight. The mesoscale guidance shows that portions of the Upper TX Coast should be receiving heavy rainfall at the beginning of the period in an area where a CAPE gradient should have developed due to recent heavy rainfall per conceptual models near the base of the mid level low. Based on the pattern showing low-level confluence south of the upper level low, it's possible that this would be in the form of training bands coming in off the Gulf of Mexico which can be highly efficient so long as the effective bulk shear can remain at or above 25 kts. The 00z HREF shows high probabilities of 5"+ in 12 hours early in the period. Combined with Monday's and Tuesday's expected rainfall, a Moderate Risk upgrade was made on this cycle per coordination with the EWX/New Braunfels TX, HGX/Houston-Galveston TX, and LCH/Lake Charles LA forecast offices. =20 Across the Central Plains, northeast CO, eastern WY, and southeast MT, a combination of upslope flow and some low- to mid-level frontogensis acting in concert with sufficient instability should lead to heavy rainfall. Heavy rain -- both which has occurred and is expected on Tuesday/Tuesday night -- keeps areas in and around southern NE and 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. Closer to the 850 hPa low in WY, the guidance has a strong signal for local 2-4" amounts. A new Slight Risk area was added for southeast MT, northeast WY, and western SD.=20 =20 Roth Day 3 Valid 12Z Thu May 11 2023 - 12Z Fri May 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN HIGH PLAINS... ....2030Z Update... ....Northern Plains... Only a few changes were made to the previous Slight Risk area, namely a southern extension along the I-25 corridor into northern CO, to highlight at least the second consecutive day of rain into this normally dry region. There is significant disagreement on where the heaviest rain will occur, but there's general agreement on the higher rainfall amounts impacting the entire Slight Risk area. ....Mississippi Valley... A few small changes were made, namely shrinking the Marginal out of much of IN and portions of eastern TX, including Houston, as the rain will be focused more along the MS River and the Ozarks. Rainfall amounts will not be high, but any individual cell will have the potential to cause localized flash flooding if it develops in a particularly sensitive area. Wegman ....Northern Plains/High Plains... Moisture advection from the Gulf of Mexico increases ahead of a slowing frontal system over the central/southern High Plains and the upper trough/low approaches=20 from the West. Precipitable water values (PWs) of at or above 1" remain across eastern MT and northeast WY as a low pressure system slowly migrates through the central Plains. A decent bubble of 1.25" PWs exists across North Dakota within the system's comma head. 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.=20=20 The Slight Risk area has been maintained from continuity over parts of eastern Montana into northeastern Wyoming and western portions of the Dakotas with some reconfiguration. ....Mississippi Valley/Lower Ohio Valley... A long fetch of ~1.5" PWs with decently strong low-level inflow/effective bulk shear and ample instability should allow for another day of convection across the ArkLaTex which is expected to shift into portions of the MS and OH Valleys. There is significant dispersion in the heavy rain signal here, with the 00z UKMET over AR and MO, and the 00z GFS the most northerly in and near IL/IN where heavy rain fell recently. Not helping matters is the deep layer cyclone pulling away from CO into SD, which is leading to warming at 700 hPa for locations closer to the ArkLaTex which could lead to a stronger mid level capping inversion down to the south. PWs show a subtle decrease on Friday when compared to Thursday. Due to the dispersion in the guidance and the less favorable environment for heavy rainfall down south, left the risk level as Marginal. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7j2mp44j5-2XI4AXeFmA_0z404air1IDQiRab0jYJZ0D= ZfjHgFFIREIbW6nvmIZRxJ-TE-LQNWH71PtDklFSpGGgsjg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7j2mp44j5-2XI4AXeFmA_0z404air1IDQiRab0jYJZ0D= ZfjHgFFIREIbW6nvmIZRxJ-TE-LQNWH71PtDklFSTlvnnZg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7j2mp44j5-2XI4AXeFmA_0z404air1IDQiRab0jYJZ0D= ZfjHgFFIREIbW6nvmIZRxJ-TE-LQNWH71PtDklFSMvOVzKE$=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) .