Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jun 04 2024 08:47:46 FOUS30 KWBC 040847 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 447 AM EDT Tue Jun 4 2024 Day 1 Valid 12Z Tue Jun 04 2024 - 12Z Wed Jun 05 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS=20 OF OKLAHOMA, NORTHEAST TEXAS, SOUTHWEST ARKANSAS AND NORTHWEST LOUISIANA... We upgraded to a Moderate risk across portions of southeast OK, northeast TX, southwest AR and northwest LA with this update. This is with the expectation that these areas will most likely see two organized convective complexes today...one this morning and another tonight. Expect an MCS with some brief training will be ongoing at 12z across portions of southeast OK and northeast TX. Given the=20 favorable downstream environment, this convective complex is=20 likely to persist as it dives southeast across portions of eastern=20 TX, LA and southwest AR. The complex should become more progressive with time, however short duration heavy rates are still likely to=20 drive some flash flood threat. By later this afternoon into this evening additional forcing=20 moving into The Plains is likely to trigger convection over=20 portions of KS and OK. Given the degree of large scale forcing, the extreme instability forecast, and the strong low level moisture=20 transport...upscale convective growth appears likely. This complex=20 should track over similar areas of OK, TX, AR and LA tonight. Even=20 without these two complexes much of this region has been quite wet=20 of late...with elevated streamflows and well above average soil=20 saturation. Combine those antecedent conditions with the potential=20 for two more rounds of organized convection today...and scattered=20 to numerous instances of flash flooding appear likely. This=20 warrants the upgrade to a MDT risk, with some significant flash=20 flooding possible. This MDT risk is surrounded by a large Slight risk that stretches=20 from KS/MO all the way to MN and WI. Pretty strong large scale=20 forcing expected from IA into MN with a sharp mid level trough and=20 plenty of shortwave and upper jet energy to work with. Most=20 indications are that the front and convection will be progressive=20 in nature, which should end up limiting the upper bound potential=20 of rainfall totals. However also tracking a well defined shortwave=20 that is near the KS/MO border as of 08z. This feature will track=20 northeast today, and may act as a convective focus over portions of IA, IL and WI by later today into tonight. Thus some of these=20 areas could see two rounds of convection today/tonight, locally=20 increasing the flash flood threat. Still generally not the=20 strongest HREF signal...strictly looking at HREF QPF probabilities=20 would suggest this is a borderline Marginal to Slight risk.=20 However will also note that the IA to MN and WI portion of this=20 risk does already have above average streamflows and soil moisture=20 from past rainfall. Thus flash flood susceptibility will likely=20 also be higher than normal...which combined with PWs around the=20 climatological 90th percentile...suggests some flash flood risk=20 exists and the Slight risk remains warranted. A Marginal risk remains in place over much of the Southeast into the TN valley and southern Mid-Atlantic. One MCV over northern MS as of 08z, will track eastward and likely help trigger/organize some convection over portions of TN, AL and GA today. Another=20 shortwave/MCV moving in from the west may help focus convection=20 over portions of the Southeast and/or TN valley tonight. Otherwise plentiful instability and increasing moisture will support=20 isolated to scattered convection over a large geographic extent.=20 Still not seeing enough agreement on where any more organized=20 flood threat could evolve to upgrade any of this area to a Slight=20 risk. At this time think the broad Marginal risk is most=20 appropriate, with an isolated flash flood threat. Chenard Day 2 Valid 12Z Wed Jun 05 2024 - 12Z Thu Jun 06 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL AND EASTERN U.S.... The frontal system moves east on Wednesday bringing a convective threat to a large portion of the southern and eastern U.S.. A broad Marginal risk stretches from TX across the TN/OH valley and into portions of the Great lakes and Northeast...pretty much everywhere along and just ahead of this front. Moisture is plentiful with this system, with PWs over the climatological 90th percentile (especially over the Mid-Atlantic). Instability is not extreme,=20 but should be sufficient for deep convective development. The main limiting factor should be the progressive nature of the system=20 limiting the duration of heaviest rainfall. However we are likely=20 to see multiple low amplitude shortwaves moving east ahead of the=20 cold front, which should also act as a focus for convective=20 development. This suggests that some areas could see multiple=20 rounds of convection. Of course this also means a more messy=20 convective evolution, which may end up impacting destabilization.=20 All to say that while an isolated flash flood risk exists over a=20 large geographic extent, confidence on pinning down an area where=20 the flash flood risk is more organized remains low. So while an=20 embedded Slight risk may be needed somewhere from the TN Valley=20 into the Mid-Atlantic, still think the broad Marginal will suffice=20 for now. Will need to keep an eye on southeast TX into the lower MS Valley. It seems probable that an MCS will be moving across this area Wednesday morning, and several of the higher res models suggest this complex leaves behind a well defined MCV. This MCV could become the focus for additional organized convective development later Wednesday, which would result in a more focused flash flood risk. While we may eventually need a Slight risk upgrade to=20 account for this threat, think it's a bit too early for that at=20 this point...as this is conditional on how convection evolves=20 Tuesday and Tuesday night. So will keep things at a Marginal and=20 continue to monitor. Also extended the Marginal risk into portions of the Northeast=20 with this update. While the stronger forcing holds off until=20 Thursday, it looks like there's some weak shortwave energy over=20 NH/ME and about 1000-2000 j/kg of CAPE helping trigger isolated to=20 scattered convection Wednesday. Several HREF members have some slow moving convection developing, with 2" neighborhood probabilities=20 over 40%. This setup may lead to a very localized flash flood=20 threat.=20 Chenard Day 3 Valid 12Z Thu Jun 06 2024 - 12Z Fri Jun 07 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHEAST.... ....Northeast... An interesting setup for heavy rainfall over the Northeast on Thursday. Stronger mid/upper level forcing moves in from the west, and at the surface we'll likely have prolonged convergence along/near a north south oriented occluded front over NY/VT, and a west east stationary front over southern New England. The=20 persistence of these features suggest some duration to the rainfall is possible with this event. It seems like the main uncertainty=20 comes down to the degree of instability collocated with this better convergence/forcing. The CSU ML tool does suggest Slight risk=20 probabilities over portions of the Northeast with this event. This does not seem too unreasonable given the forcing and PWs in place. However given the uncertainty on instability and the fact that=20 these areas have generally been dry of late with below average=20 streamflows and soil saturation...think the best course of action=20 is to stick with a Marginal risk for now.=20 Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7KwJwWP7v4MS81Hm5GrLppLiArUoePgyNL071hVPE34u= -5jorGdAYwUuBnnGNitKcf7z8aL3YrAiEE96TBlJTkx-CKc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7KwJwWP7v4MS81Hm5GrLppLiArUoePgyNL071hVPE34u= -5jorGdAYwUuBnnGNitKcf7z8aL3YrAiEE96TBlJ5LbV7YA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7KwJwWP7v4MS81Hm5GrLppLiArUoePgyNL071hVPE34u= -5jorGdAYwUuBnnGNitKcf7z8aL3YrAiEE96TBlJLIjPXvc$=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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