Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu May 25 2023 00:58:30 FOUS30 KWBC 250058 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 857 PM EDT Wed May 24 2023 Day 1 Valid 01Z Thu May 25 2023 - 12Z Thu May 25 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF GREAT BASIN INTO THE NORTHERN ROCKIES AND HIGH PLAINS, CENTRAL FRONT RANGES AND HIGH PLAINS INTO THE SOUTHERN PLAINS, AND THE FLORIDA PENINSULA... ....Great Basin into the Northern Rockies and High Plains... Longwave upper trough just east of West Coast will migrate slowly into the intermountain West with a well defined shortwave trough embedded within the broad southwesterly flow. This will maintain an elongated, SSW-NNE area of broad-scale upper difluence and divergence across the western Great Basin into the northern Rockies and High Plains with the shortwave trough helping focus and sustain convection through into late evening. At the same time, thermodynamical profiles (deep-layer moisture and instability) will remain above seasonal norms for late May.=20 That...and sufficient instability...will combine to result in some cells being efficient rainfall producers for a few more hours with the resulting potential for isolated flooding problems.=20 ....Central Rockies and High Plains into the Southern Plains... On-going convection from the Central Rockies and adjacent High Plains will continue to push eastward with time...being fed in the low levels by broad southeasterly flow which was drawing in air with precipitable water values approaching 1.5 inches. As with the other upstream outlook area...the predominant along-stream, exit region upper level forcing east of an amplified trough will inhibit low-level frontogenesis, and as such the forcing is more diffuse and not focused over any particular area. The 24/18Z HREF QPF probabilities were generally under 15 percent for 1 inch accumulations in an hour persisting through the late evening.=20 Those probabilities may be a bit too low given MRMS amounts just west of the NM/TX border near the panhandle ending at about 0030Z. Both the HREF and prototype Warn on Forecast System show individual cells being fairly progressive which should somewhat offset the risk of excessive rainfall due to intense rainfall rates. Overall...the Marginal Risk should be sufficient. Bann Day 2 Valid 12Z Thu May 25 2023 - 12Z Fri May 26 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS SOUTHERN FLORIDA, A LARGE PORTION OF THE PLAINS, NORTHERN ROCKIES, AND INTERMOUNTAIN WEST... 20z Update: Added a Marginal risk across southern FL with this update. PWs may be on a bit of a downward trend compared to Wednesday, but while not all that anomalous, they should still be high enough to support heavy rainfall. Sufficient instability should still exist, and a frontal boundary will remain in the area. Thus like the past few days, localized areas of heavy rainfall again appear likely. The 12z HREF neighborhood and EAS probabilities for Thursday are similar to what the HREF was showing for Wednesday. Thus overall the setup is similar enough to past days to warrant an extension of the Marginal risk into this day 2 period. Gave 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 signature is a bit higher than Wednesday (especially over western KS) and this is generally aligned with the area depicted as a Slight risk in the CSU ML output. However this pattern and model QPF output still does not really have the look of a more organized flood event at this point and the specifics are likely at least partially dependent on MCVs and boundary locations from convection Wednesday night. So while we could eventually need a Slight risk upgrade, would prefer to wait another cycle and give the night shift a chance to evaluate trends and another cycle of HREF output. For now think the broad Marginal risk still represents the expected isolated flash flood risk best. Chenard ....Previous Discussion... ....Great Basin-Northern Rockies-High Plains... Upper level pattern shifts very little by Day 2 (Thu-Thu night), though again compared to Day 1 there is more support for expanding the Marginal Risk area westward toward the upper low to include more of the intermountain region (western NV and northeast CA). Deterministic and probabilistic QPF from the models again support a localized or isolated flash flood threat possible anywhere within the broad risk area. As was noted in yesterday's Day 3 ERD, once area to watch would be across central-eastern MT, where the presence of a surface frontal boundary (weak low-level FGEN) could focus a more organized area of heavy rain (upscale convective growth). As of now however, the guidance spread in terms of QPF (convective bulls-eyes) remains too high (forecast confidence too low) to include an embedded Slight Risk area.=20 Hurley Day 3 Valid 12Z Fri May 26 2023 - 12Z Sat May 27 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS COASTAL PORTIONS OF GEORGIA AND THE CAROLINAS ALONG WITH A LARGE PORTION OF THE PLAINS, NORTHERN ROCKIES, AND INTERMOUNTAIN WEST... 20z Update: Only minor changes to the inherited risk areas. Again gave thought to adding a Slight risk over portions of western KS into northeast NM and the OK/TX Panhandles. Another day of organized convective development appears likely, and multi day rainfall by this time should be adding up in spots. This should result in increasing soil saturation and increased flash flood susceptibility with time. But again opted to hold off for now give some continued model spread in QPF placement. In addition the environmental pattern and model QPF solutions again do not really appear indicative of an organized flash flood threat per se, so this Slight would have been more from the result of saturating conditions from antecedent rainfall. So would prefer to wait a bit longer for confidence in soil saturation, and thus flood susceptibility, to increase. For now the broad Marginal and a localized flash flood risk should cover the threat. Guidance is clustering a bit better in bringing heavy rainfall into portions of the Southeast coast by Friday night, so the Marginal risk looks pretty solid at this point. Timing could change, which may result in changes to this outlook, but for now the bigger risk appears to be after 12z Sat, which is covered with a day 4 Slight risk. For day 3 still think the threat is more marginal in nature and confined pretty close to the coast. Chenard ....Previous Discussion... ....Coastal portions of Georgia and the Carolinas... Models continue to agree with the formation of a low off the east coast of FL during Day 2, though with quite a bit of spread in terms of the location and track through Day 3 as the low migrates off the GA-SC Coast by Saturday morning. As such, there is also 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 days 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 5/24) 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 4 ERO. ....Great Basin-Northern Rockies-High Plains... Similar story on Day 3 (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. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_sNNDMCe2SgBf5Hrkr3RiRLJJL_fTc0MtAH89ho4CTF4= IBBhHWtNyBqiAO0490Uu4Eop_XORZrCqauXv69bB5mREpu8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_sNNDMCe2SgBf5Hrkr3RiRLJJL_fTc0MtAH89ho4CTF4= IBBhHWtNyBqiAO0490Uu4Eop_XORZrCqauXv69bBa7WAvCM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_sNNDMCe2SgBf5Hrkr3RiRLJJL_fTc0MtAH89ho4CTF4= IBBhHWtNyBqiAO0490Uu4Eop_XORZrCqauXv69bB1kUThR0$=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) .