Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu May 25 2023 15:48:47 FOUS30 KWBC 251548 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1147 AM EDT Thu May 25 2023 Day 1 Valid 12Z Thu May 25 2023 - 12Z Fri May 26 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR THE CALIFORNIA/NEVADA BORDER AND NORTHWEST KANSAS... ....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.=20 There are two areas where a Slight Risk upgrade has been made.=20 One is in and near western KS where an MCV from earlier activity and a nearby shortwave are expected to foster slow moving thunderstorm clusters with heavy rainfall. Effective bulk shear is currently ~35 kts in this region, so activity should have some organization. The guidance has shown an elevated signal for heavy rainfall here for well over a day, and used the 12z HREF probabilities of 3"+ and 5"+ in 24 hours to sketch in the Slight.=20 In and near the northern border of CA/NV another Slight Risk has been added. Slow storm movement under the base of the upper level low, particularly near and downwind of the Sierra Nevada, along with saturated soils, creeks, and rivers in the region and related sensitivity led to the upgrade. Effective bulk shear is approaching 25 kts, so any convection could try to organize briefly into banded structures which would enhance/favor locally heavy rains. It supports the new Flash Flood Watch issued by the REV/Reno NV forecast office. ....Central and South Florida... Difluence aloft east of the upper level trough, along with continued cyclonic low-mid level flow around a developing low offshore of Cape Canaveral and an incoming 850 hPa low from the southeast Gulf of Mexico will 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 rainfall totals to 3" underneath the strongest storms. Effective bulk shear appears to be under 25 kts -- low-level flow is quite weak -- so any cell organization that occurs is expected to be transient. Localized areas of heavy rainfall appear likely. The risk areas across the FL Peninsula are similar to continuity. Roth/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!8xNNSrjq8m-5mmmt9FOTMAfC-u4nDIfJGNtIkMeLbaoY= zQdipYDY8Uyzgvixp-ku13WLO0g936weunACpRDta6iQ2tM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8xNNSrjq8m-5mmmt9FOTMAfC-u4nDIfJGNtIkMeLbaoY= zQdipYDY8Uyzgvixp-ku13WLO0g936weunACpRDtY-gHRfk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8xNNSrjq8m-5mmmt9FOTMAfC-u4nDIfJGNtIkMeLbaoY= zQdipYDY8Uyzgvixp-ku13WLO0g936weunACpRDtRQJVyv0$=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) .