Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu May 25 2023 19:06:26 FOUS30 KWBC 251906 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 304 PM EDT Thu May 25 2023 Day 1 Valid 16Z 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 SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR SOUTHEAST MONTANA & THE TEXAS PANHANDLE... ....Coastal Portions of the Southeast and Carolinas... A 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/over the Gulf Stream. 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 in later 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. GEFS output 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 continuity. ....Great Basin-Northern Rockies-High Plains... A slightly progressive and broadening upper trough is forecast for the Western States. The deterministic and probabilistic QPF from the guidance support a localized or isolated flash flood threat possible anywhere within the broad risk area. Sufficient moisture combined with 2000+ J/kg of CAPE, and ample low-level inflow in the Plains will help aid the development of convection with heavy rainfall. A more enhanced risk lies in and near southeast MT along/near the surface front, and in and near the TX Panhandle with the upslope flow. Slight Risk areas were introduced to these regions per the agreement seen in the deterministic guidance, with the MT one supporting the ongoing Flash Flood Watch. The TX Panhandle in particular has broad areas of 300-600% of average rainfall during this past week, which made the decision for the Slight Risk upgrade more straightforward. Roth/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 STATES... ....Southeast and Lower Mid Atlantic... Slight and Marginal Risk areas inherited from continuity remain similar. MUCAPE based on the 12z NAM shows an area of 500-2000 J/kg moving ashore/up the coast, but the main issue is that the instability doesn't persist in any one spot for more than 12 hours. Strong low-level easterly inflow (40-50 kts), some 5+ standard deviations above normal, will lead to robust (anomalous) low-level moisture transport/flux. Since this is expected to be a cyclone with some depth of a warm core per cyclone phase space diagrams and conceptual models, the low-level inflow slightly exceeds the magnitude of the 850-300 hPa flow, which when combined with a period of instability would favor cell training, repetitive banding, and/or cell mergers as the surface low slowly migrates inland. Much of the guidance advertises local amounts near 5", particularly within the NC Coastal Plain which has had above normal rainfall over the past week, particularly near Wilmington due to a preceding convective low. While there was some thought of an upgrade to a Moderate Risk, the progressive nature of the instability field argued against such. ....Northern Rockies and High Plains... An upper level trough across the region broadens and shows a little bit of forward progression when compared to Friday. A localized or isolated flash flood threat possible anywhere within the broad risk area as CAPE rises above 2000 J/kg within a broad region of 0.75" (at elevation) to 1.25-1.5" closer to the 100th meridian. Montana will be aided by upslope flow for portions of the time frame. There's a somewhat coherent signal for heavy rainfall across western TX, but for the moment, enough spread to preclude the addition of a Slight Risk area. While there's better agreement on heavy rainfall near the NE/WY border, magnitudes in the guidance do not appear high enough at the moment for the inclusion of a Slight Risk area. ....In and near Northern California... A closed low drops into the region from the northwest. This feature will lure precipitable water values up to ~0.75" and increase CAPE to 500-1000 J/kg. The concern would be for convection tied or backbuilding back to the higher terrain. Local amounts in the 1-2" are anticipated, which would be problematic, particularly in areas with snowmelt. Added a Marginal Risk area to cover this scenario. Roth/Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4HSyJwu6z2acKcdDAzU5jx-sX_kShev2zCTdv3_HiWKh= lnpYS_X5xkUUsWR9gyBmXuG9x-lizw6t2ttWMYxt6yFMX7U$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4HSyJwu6z2acKcdDAzU5jx-sX_kShev2zCTdv3_HiWKh= lnpYS_X5xkUUsWR9gyBmXuG9x-lizw6t2ttWMYxtG-4q9p4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4HSyJwu6z2acKcdDAzU5jx-sX_kShev2zCTdv3_HiWKh= lnpYS_X5xkUUsWR9gyBmXuG9x-lizw6t2ttWMYxtGxYzHZY$=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) .