Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu May 25 2023 08:50:14 FOUS30 KWBC 250850 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 449 AM EDT Thu May 25 2023 Day 1 Valid 12Z Thu May 25 2023 - 12Z Fri May 26 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF CENTRAL AND SOUTH FLORIDA, A LARGE PORTION OF THE PLAINS, NORTHERN ROCKIES, AND INTERMOUNTAIN WEST... ....Great Basin-Northern Rockies-High Plains... Amplifying longwave upper trough will allow for an expansive area of large scale forcing (upper difluence) eastward into the High Plains. In addition, the guidance continues to depict an elongated Warm Conveyor Belt (WCB) east of the upper trough, extending from northern Mexico through the Plains and into south-central Canada. Deterministic and probabilistic QPF from the models again support a localized or isolated flash flood threat possible anywhere within the broad risk area. We did give consideration for a Slight risk upgrade over portions of western KS into the OK/TX panhandle and adjacent portions of CO and NM. The HREF exceedance probabilities are a bit more elevated over these areas (though not in consecutive 3-hourly periods), and this is generally aligned with the area depicted as a Slight risk in the CSU ML output. However, the current pattern and continued spread in model QPF output do not convey the look of a more organized flash flood risk at this point, especially in the wake of the most recent MCS/MCV track and surface boundary locations from the convection Wednesday night. So while we could eventually need a Slight risk upgrade, feel the day shift will have an upper hand in honing in on the timing and location of any upgrade with the benefit of short-term trends and another cycle of HREF output. For now think the broad Marginal risk still represents the expected isolated flash flood risk best. ....Central and South Florida... Difluence aloft east of the upper level trough, along with continued cyclonic low-mid level flow will once again provide a favorable synoptic scale environment for convection more widespread than what would be typical for late May. This as the 00Z GEFS shows 500 mb height anomalies averaging 3 standard deviations below normal. The thermodynamic profile remains favorable for heavy rainfall as well, as mixed-layer CAPEs between 1000-2000 J/Kg and PWs averaging 1.6 to 1.7 inches are more than sufficient for hourly or sub-hourly rainfall rates of 2+ inches underneath the strongest storms. Thus like the past few days, localized areas of heavy rainfall again appear likely. The 00Z HREF neighborhood and EAS probabilities for Thursday are similar to what the HREF was showing for Wednesday. Hurley Day 2 Valid 12Z Fri May 26 2023 - 12Z Sat May 27 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS COASTAL PORTIONS OF THE SOUTHEAST AND LOWER MID ATLANTIC ALONG WITH A LARGE PORTION OF THE PLAINS, NORTHERN ROCKIES, AND INTERMOUNTAIN WEST... ....Coastal Portions of the Southeast and Carolinas... Stagnant upper level pattern with the quasi-stationary upper trough upstream and broadly difluent flow off the southeast coast will lead to coastal low formation/migration over a favorable area off the concave coastline. There is some spread in the guidance with respect to the QPF, though for the time being all models confine the heaviest rainfall offshore along with the more robust deep-layer instability. The need for a more expansive outlook area, and/or a more enhanced outlook (i.e. Slight Risk) will need to be monitored over the next couple of forecast cycles pending any potential shifts in the guidance, considering the pooling of highly favorable, low-level moisture/theta-e transport over the Gulf Stream very close to the Southeast Coast. Current GEFS output (00Z 5/25) indicates east-northeast 850 mb flow some 5+ standard deviations above normal creeping into the outlook area, with deep-layer thermodynamic parameters (especially moisture) supportive of optimal warm rain processes. For now given the spread in the guidance, and with the models keeping the more appreciable deep-layer instability offshore, have maintained the Marginal Risk that was inherited from yesterday's experimental Day 3 ERO. ....Great Basin-Northern Rockies-High Plains... Similar story on Day 2 (Fri-Fri night) in terms of the upper level pattern. Once again, the deterministic and probabilistic QPF from the guidance support a localized or isolated flash flood threat possible anywhere within the broad risk area. A more enhanced risk (potential Slight) may again be across central-eastern MT along/near the surface front, and again over the 5-state area (northeast NM, TX/OK Panhandles, southwest KS, southeast CO) with the upslope flow. Again though, the signal is not real clear in the deterministic and probabilistic guidance, as there continues to be quite a bit of spread with the max QPF areas and highest exceedance probabilities. Hurley Day 3 Valid 12Z Sat May 27 2023 - 12Z Sun May 28 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST AND LOWER MID ATLANTIC... ....Southeast and Lower Mid Atlantic... Slight and Marginal Risk areas inherited from yesterday's experimental Day 4 ERO were largely unchanged, owing to the continued spread in the guidance. There is a bit of a northward trend, at least in terms of the backside (southern periphery) of the rainfall footprint, and as such there was higher forecast confidence to remove the Marginal Risk area out of portions of GA. The only thing lacking in this setup is the deep-layer instability, as the guidance shows MUCAPEs of 500+ J/Kg remaining offshore for the most part. Despite this, strong low-level easterly inflow (40-50 kts), some 5+ standard deviations above normal, will lead to robust (anomalous) low-level moisture transport/flux. Meanwhile, the strength of the east-southeast flow, nearly parallel and around the same or exceeding the magnitude of the 850-300 mb flow, will favor cell training or repetitive banding as the surface low slowly migrates toward the lower Carolina coast. 00Z ECMWF indicated pockets of 3-4+ inches of rainfall on Day 3 within the Slight Risk area, with even higher localized totals (over 5") along the southern NC coast per the 00Z Regional GEM. ....Northern Rockies and High Plains... Setup very similar to Day 2, but with the back edge of the Marginal Risk area now east of the Great Basin. Once again per the stagnant upper level pattern, the deterministic and probabilistic QPF from the guidance support a localized or isolated flash flood threat possible anywhere within the broad risk area. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4jT7l1_HuTUrA3vkHCsorFs6hWhlmzZ9ASnf7TXaCI9f= kTPTmKfNaRHAuMLcO9wPgJusU-82mtelmPiE0Xnku4MjoWM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4jT7l1_HuTUrA3vkHCsorFs6hWhlmzZ9ASnf7TXaCI9f= kTPTmKfNaRHAuMLcO9wPgJusU-82mtelmPiE0Xnkq8rCt80$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4jT7l1_HuTUrA3vkHCsorFs6hWhlmzZ9ASnf7TXaCI9f= kTPTmKfNaRHAuMLcO9wPgJusU-82mtelmPiE0XnkNQFnbsU$=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) .