Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed May 24 2023 20:30:56 FOUS30 KWBC 242030 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 PM EDT Wed May 24 2023 Day 1 Valid 16Z Wed May 24 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... 16z Update: Only minor changes to the Marginal risk areas, as the forecast generally looks on track. Contemplated an embedded Slight risk over portions of WFOs AMA and LUB where recent rainfall has lowered FFG and another day of organized convection is probable. However just not seeing quite enough clustering amongst the higher res guidance to have enough confidence at this point. We do expect there will be isolated flash flooding like the past few days, and the risk is right on the edge of Slight risk levels. But given the only modest HREF 2" EAS and 3" neighborhood probabilities, and the likely progressive nature of convection, think the Marginal risk still messages the isolated flash flood risk well enough. So opted against what would be a lower end Slight upgrade. Chenard ....Previous Discussion... ....Great Basin into the Northern Rockies and High Plains... Longwave upper trough just east of West Coast will migrate slowly into the intermountain West through the short range period (days 1-3). 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. At the same time, thermodynamical profiles (deep-layer moisture and instability) will remain above seasonal norms for late May, as PWs average 2-2.5 standard deviations above normal per the 00Z GEFS while and MUCAPEs are expected to peak between 1000-1500 J/Kg during the peak heating hours this afternoon and early evening. Cell motions will be rather progressive (forward propagating) given the low-mid layer shear profiles; however, the stagnant mid-upper level pattern will allow for some regeneration of cells upstream, thus favoring some training or multiple bouts of convection over the same areas this afternoon and evening. The CAMs show hourly rainfall rates peaking between 0.50-1.00+" underneath the strongest cells (isolated), which given the current FFG, could certainly lead to localized runoff issues. ....Central Rockies and High Plains into the Southern Plains... The upper flow across these areas will become increasingly difluent as the upper ridge axis pushes into the eastern (lower) Plains. Meanwhile, increasing low-level south to southeast flow (ageostrophic low-level response) ahead of the approaching trough will bring an elongated ribbon of increased deep-layer and more focused low-level moisture flux north-northwestward into the central High Plains and front ranges. As with the other outlook area upstream, the predominant along-stream, exit region upper level forcing east of the amplified trough will inhibit low-level frontogenesis, and as such the forcing is more diffuse and not focused over any particular area. Granted, convection moving east off the terrain may tend to exhibit some training/backbuilding as they encounter low level southeasterly inflow, with anomalous PWs and sufficient CAPE for heavy rainfall rates. However, as was noted in the previous discussion (yesterday's Day 2 ERD), while the ingredients and the CSU first guess fields support lower-end Slight Risk probabilities over the High Plains, the 00Z HREF QPF probabilities are not too impressive, and the model QPFs continue to show quite a bit of spread with the max amounts (i.e. more in line with a typical Marginal risk). Therefore given the uncertainty, opted to maintain continuity with the Marginal Risk with the anticipation of more isolated flash flooding than otherwise. ....Florida... Persistent mid-upper level trough coinciding with typical favorable deep-layer moisture and instability will again lead to more numerous thunderstorms than what is typical for late May across the FL peninsula during the afternoon and early evening hours. Sea/Gulf breeze interaction will aid to the development, while 0-6km bulk shear values of 20-30 kts should again favor at least some organization, i.e. more clustered convection that can persist a little bit longer than the more typical pulse type. At the same time however, the CAMs again do not show much overlap with the convective bulls-eyes, indicative of more broad (less focused) dynamical support. Given the CAM deterministic and probabilistic output, along with the CSU first guess fields, the Marginal Risk inherited from yesterday's Day 2 ERO still looks on track.=20 Hurley 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... 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 continue model spread in QPF placement. In addition the 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, increases. 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!55A2qZFheHZh8QDdoNRJWSVXNe_gG_JwZeRNgfDDpQ03= SyTn8-Xq6lvIYYggViYu0VwCcrD0VegquIkZ6_0x7qdVIGQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!55A2qZFheHZh8QDdoNRJWSVXNe_gG_JwZeRNgfDDpQ03= SyTn8-Xq6lvIYYggViYu0VwCcrD0VegquIkZ6_0xFi7HyyQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!55A2qZFheHZh8QDdoNRJWSVXNe_gG_JwZeRNgfDDpQ03= SyTn8-Xq6lvIYYggViYu0VwCcrD0VegquIkZ6_0xGOQJX9Y$=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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