Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jul 24 2023 00:47:15 FOUS30 KWBC 240047 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 846 PM EDT Sun Jul 23 2023 Day 1 Valid 01Z Mon Jul 24 2023 - 12Z Mon Jul 24 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST, SOUTHEAST AND MID-ATLANTIC... ....Southeast into the Mid-Atlantic... Going outlook is in pretty good shape with radar showing one area moving northeastward along the South Carolina coast and activity being more widely scattered closer to the terrain in western North Carolina and Virginia. This was on track with some runs of the high-resolution convective allowing models and with earlier forecast reasoning Local downpours approaching 2 inches per hour should gradually fade...but an area of organized rainfall with embedded moderate to heavy rainfall should continue to move northeast into portions of North Carolina later in association with weak low pressure at the surface and weak shortwave energy aloft. The HREF and ARW both show the inland activity growing wetter and growing in areal coverage with the latest runs. 12Z HREF probs of > 1"/hr peak near 70% but only 20-40% for >2"/hr. Some 1-hr FFG values are as low as 2.5"/hr so any more vigorous development could exceed that, with urban areas most susceptible. Then the HREF probabilities for 1 inch per hour rates diminish after 07Z. Despite the more aggressive solution shown by the 18Z NAM NEST in terms of both rainfall amounts and areal coverage...maintained the Marginal risk areas thinking that the overall coverage of any flash flooding will be rather localized and in a region of fairly high flash flood guidance. Areas farther south...removed the Marginal risk area. Some higher reflectivity echoes were lingering on radar off-shore of the Florida panhandle. Conceivably...some of this rain could come on-shore later but the expectation is for little in the way of run-off concerns given the 1- and 3-hour flash flood guidance values being so high. ....Southwest... 00Z RAOBs showed precipitable water values ranging from 0.8 to 1.4 inches from TWC-FGZ-VEF...an increase of nearly one-quarter of an inch at FGZ and VEF since the 12Z upper air cycle. Slow moving convection moving off the higher terrain in northwest Arizona should begin to encounter the 1.0 to 1.4 inch precipitable water values sampled by VEF...along with some enhanced convergence with southeasterly winds at the surface. This should allow for an increase in locally heavy rainfall rates at could reach an inch per hour before tapering off by 05Z. For additional details..refer to Mesoscale Precipitation Discussion 788. Bann Day 2 Valid 12Z Mon Jul 24 2023 - 12Z Tue Jul 25 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST, MIDWEST, MID-ATLANTIC/NORTHEAST AND NORTH FLORIDA... ....Mid-Atlantic/Northeast... The broad longwave trough over the east will continue to drive a scattered convective threat on Monday as shortwave energy moves east within the trough axis. The approaching mid-level trough axis combined with a ~100 kt jet at 250mb providing right entrance upper divergence over the Mid-Atlantic should allow for a surface low to intensify and push off the VA/NC coast Monday. Enhanced convergence associated with this low may drive a more organized convective threat Monday morning/afternoon across northeast NC into southeast VA. Definitely some potential for 2-4" rainfall amounts with this activity as PWs approach 1.8" and instability remains over 1000 J/kg. Cannot rule out needing a focused Slight risk in light of some lower FFG values over parts of the region, but for now still think the coverage of any flooding will stay rather localized given forward cell motions. Plus there is still some uncertainty with the details of the low and associated frontal structure. Scattered convection is also expected along/near a surface trough north of the low extending across VA and into central/northern NY as well as in advance of a weakening front across Ontario/Quebec. Instability is forecast around or above 1000-1500 J/kg along this trough axis, and the lower level convergence combined with the approaching mid-level trough should be enough to trigger convective development. Cells should generally pulse up and down fairly quickly and not expecting much convective organization...however the persistent low-level convergence and deep layer southerly flow may allow for some brief persistence/repeat cells in spots...and rainfall rates will be high enough to drop a quick 1-2" of rain on a localized basis. 12Z HREF neighborhood probabilities of exceeding 2" approach 50-60% up to the NY Southern Tier, with some low 3" probabilities (<15%) as well. Not expecting widespread or high end flooding, but localized flash flooding seems plausible anywhere along/near the low level convergence axis from northern VA northward into southern NY and northeastward into VT where FFG values are lower after the recent wet period. Per coordination with BTV, extended the northeastward portion of the Marginal risk outline to the CT River Valley which fits with the latest SPC Convective Outlook as well. ....IA/MO/IL... Models still struggling with development of any early morning/overnight (Mon and into Tues) complex over portions of IA/MO/IL. Shortwave energy riding over the southwest ridge and increasing low-level moisture transport/convergence should be the driving factors behind the convective threat. Trend has been downward in explicit QPF from the guidance, but potential remains for higher rates due to strong instability. The presence of the extreme instability pool upwind of the low-level inflow could support some backbuilding, and will maintain a somewhat smaller Marginal risk area for this cycle. Will re-evaluate with the next CAM run to determine if this threat remains. ....Southwest... On Monday the mid-level ridge continues to build over the southwest, with height anomalies maxing out over CO and NM. This should keep the better moisture and forcing on the periphery of the ridge to the west of these areas...generally across AZ, southeast NV and into UT. PW anomalies are a bit higher compared to day 1 over AZ/NV/UT, which combined with subtle forcing on the periphery of the mid/upper level ridge, will drive another day of isolated to scattered diurnally-driven convection. Any storms could drop a quick inch of rain, which could drive a localized flash flood risk within any more susceptible basins including slot canyons in Utah. ....North FL... The Marginal risk over northern FL was maintained as the front should remain in the vicinity, with continued unidirectional flow and upper jet dynamics supporting a training convective threat similar to today/Sunday. The convection may tend to propagate southward with time enough to limit the magnitude of the flash flood risk; however, antecedent conditions may be a bit more sensitive by this time as some of these same areas are seeings modest/heavy rainfall Sunday afternoon. Combine this potential factor with the overall persistence of favorable ingredients, and think maintaining the Marginal risk is prudent. Fracasso/Chenard Day 3 Valid 12Z Tue Jul 25 2023 - 12Z Wed Jul 26 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST AND NORTHEAST... ....Northeast... Cut back the western portion of the Marginal risk area west of the CT River Valley. The mid-level trough axis and surface front will work together to trigger scattered convection across portions of northern New England on Tuesday. PWs looks to be marginally above average, and instability is forecast ~2000 J/kg. QPF from the global guidance has ticked a bit lower, but environment remains favorable for some heavy rainfall rates and a quick 1-2" in spots per the NAM nest and FV3. Not looking for convection that is all that organized given the modest mid/upper support and generally weak shear, so the coverage of any heavier rainfall rates should be rather isolated. Will maintain the risk area as the rest of the CAM guidance comes into view and because FFG values are lower due to above average soil saturation and streamflows over much of this region. ....Southwest... The core of the mid-level ridge remains centered over CO and NM, with enough subtle forcing on the periphery of the ridge to drive another day of isolated to scattered diurnally-driven convection. Any storms could drop a quick inch of rain, which could yield a localized flash flood risk within any more susceptible basins. The Marginal risk is quite large and pretty much covers most areas where there is at least some convective threat Tuesday. It seems unlikely that this area will technically meet the 5%+ coverage of a Marginal risk, but given the nature of convection in this region it's hard to pin down exactly where a localized flash flood could occur. Incorporation of the CAM guidance will likely refine this area into the Day 2 outlook. ....Elsewhere across the CONUS... The ECMWF has been rather consistent in bringing a shortwave over the ridge and into the Dakotas Tuesday, which would result in an organized convective threat. However, it can be too strong with such features. Other models are not nearly as aggressive with this feature, and the 95th percentile QPF is not as high as the ECMWF. Will re-evaluate with later guidance. There are also some subtle signs of a heavy rainfall potential across portions of IN/KY/TN and vicinity. There does appear to be an axis of enhanced instability in most of the guidance across this corridor, with some semblance of lower level moisture transport/convergence. Pretty weak QPF signal amongst most of the deterministic and ensemble guidance suggests the threat is pretty low, but the 95th percentile QPF does maximize around an inch in this area. Fracasso/Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5VajWTaXLmbFyrWulym-se_bA-7xc4YPSB2wUGRPK2cA= v6LoZVrnQh6bngYj-djba3eFguW6yoo219pbJtYv1zbFabA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5VajWTaXLmbFyrWulym-se_bA-7xc4YPSB2wUGRPK2cA= v6LoZVrnQh6bngYj-djba3eFguW6yoo219pbJtYv5Bcv7Ys$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5VajWTaXLmbFyrWulym-se_bA-7xc4YPSB2wUGRPK2cA= v6LoZVrnQh6bngYj-djba3eFguW6yoo219pbJtYvPXffF44$=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) .