Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue May 09 2023 00:45:29 FOUS30 KWBC 090045 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 844 PM EDT Mon May 08 2023 Day 1 Valid 01Z Tue May 09 2023 - 12Z Tue May 09 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM TEXAS TO THE MS AND OH VALLEY... ....Mid MS valley into OH Valley... Isolated flash flooding will be a concern across portions of southern MO for the next couple hours with slow moving convection along the front. However cells should begin to take on more of southeast progression as the evening progresses, lowering the flash flood risk with time. Otherwise a rather broad Marginal risk will extend eastward from there into portions of the OH valley and WV. At this time not expecting anything too intense over this region...but isolated to scattered convection should increase in coverage, and FFG is low enough that isolated exceedance can not be ruled out through the overnight hours. ....TX into Lower MS valley... A localized flash flood risk is ongoing across portions of far southeast TX and near the LA/AR border this evening. This activity should tend to fade over the next several hours as instability is eroded. Attention is then focused over south central TX where an impressive amount of instability remains. There is considerable model spread over this region lowering confidence in convective evolution overnight. The HRRR has been pretty dry, however the 23z run drastically changed to a more robust convective scenario...and several 12z HREF members were also showing higher QPF. There is a well defined shortwave moving east out of MX, which combined with aforementioned pool of instability, does suggest the possibility of organized convective development and some flash flood threat tonight. Thus maintaining a broad Marginal risk seems warranted. Chenard Day 2 Valid 12Z Tue May 09 2023 - 12Z Wed May 10 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST TEXAS... ....Texas... 21Z update... There was an overall eastward spread of the higher QPF approaching the Texas/Louisiana border with areal averages stills favoring the 1 to 3 + inch range. A couple pieces of guidance suggest local maximums of 5 to 6 inches near the western Gulf coast and adjacent points inland. The eastern boundaries of the Marginal Risk was expanded into southwest Louisiana and the eastern bounds of the Slight Risk was expanded to the Texas-Louisiana state line. Campbell A southerly low level flow combined with a forming mid-level cyclone should lead to ample moisture, instability, and effective bulk shear to support organized convection capable of producing heavy to potentially excessive rainfall. A few pieces of guidance indicate the potential for local amounts in the 5-10" range but don't show a great deal of overlap with their heavy rainfall signal. However, that overlap does exist across southeast TX so the threat level has been raised to Slight Risk in that region.=20 There remains dispersion on placement, but given time, the instability gradient should recede to the Gulf Coast and the best onshore flow appears targeted into portions of the Middle and Upper TX coast, which fits the guidance overlap and conceptual models. Should the signal become stronger, increased risk levels are possible in succeeding cycles. ....Central Plains... 21Z update... The environment and expected setup has largely remained unchanged for this part of the country. Minor westward and southern expansions were made to reflect the latest QPF trends. Consensus has the best potential for the highest QPF to occur over southern Nebraska to central Kansas during this period. Campbell A second focus for storms should extend from Nebraska across Kansas into northwest Arkansas in proximity to the surface boundary where any mid-level capping inversion appears weak enough to be able to be overcome by daytime heating. Enough moisture, low level inflow/effective bulk shear, and instability should exist to allow for convection with hourly rain totals to 2" and local amounts to 5" per the available model guidance. Portions of southern NE have seen some saturation from heavy rainfall over the past few days. With the best signal for heavy rainfall over the area which has been driest over the past week, thought it prudent to leave the risk as Marginal for the time being.=20 Roth Day 3 Valid 12Z Wed May 10 2023 - 12Z Thu May 11 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS IN A BROAD AREA AROUND THE ARKLATEX... ....Plains... 21Z update... There continues to be a signal for moderate to heavy rain to focus over southeast Montana, northeast Wyoming and western South Dakota, especially during the latter half of the period. Had considered raising a Slight Risk for this area however the potential is much better during the Day 4 timeframe. Further south, the models are suggesting a southward shift in the the placement of the highest QPF by roughly 100-200 miles. As such the Slight Risk area was adjusted southward, with the northern boundary over northeast Oklahoma and extreme southwest Missouri and the southern boundary now encompassing portions of southeast Texas coastline. Campbell The combination of a mid-level low moving 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. Five pieces of global guidance have a signal for local 5-8" amounts somewhere across the eastern half of TX within or near 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.=20 But, it is possible that portions of coastal TX could 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 and to some degree the 21z SREF surface-based CAPE...if the heavy rainfall signal seen on the 00z Canadian Regional and 00z UKMET becomes embraced by additional guidance, the Slight Risk could be extended down there in later cycles. 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 localized heavy rainfall. Southern NE has been wet lately, and the 00z Canadian Regional has local 6" amounts in that area, which keeps them in the Marginal Risk area. Precipitable water values near or above 1" in the High Plains are a big deal from a heavy rainfall perspective, so stretched the Marginal northwest to loop in the heavy rain signal seen across the available guidance towards and around the expected position of the 850 hPa low. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_xeJVfIaMJz-ZDjZOAnh_Jgf6vy16a7JAn345tccEAEM= Dz7gwnsRC7o80YWfcjSaYytCT1FsvvD-J0w7Q5fsNjJy9qs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_xeJVfIaMJz-ZDjZOAnh_Jgf6vy16a7JAn345tccEAEM= Dz7gwnsRC7o80YWfcjSaYytCT1FsvvD-J0w7Q5fsvdq3uzw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_xeJVfIaMJz-ZDjZOAnh_Jgf6vy16a7JAn345tccEAEM= Dz7gwnsRC7o80YWfcjSaYytCT1FsvvD-J0w7Q5fsK3rfwSo$=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) .