Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jul 23 2023 15:54:13 FOUS30 KWBC 231554 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1153 AM EDT Sun Jul 23 2023 Day 1 Valid 16Z Sun Jul 23 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... Ongoing area of moderate to perhaps locally heavy rainfall moving through the FL Big Bend will continue eastward and southeastward along a pre-frontal trough across the northeast Gulf. Hourly rainfall rates of 1-2" will continue which is below 1-hr FFG (3-4"). Favorable convergent flow into the region with PW over 2" (near the 90th percentile at TAE this morning) will continue through the afternoon over North Florida/Nature Coast where the 12 HREF neighborhood probabilities of exceeding 2"/hr are roughly 10-50% with 6-hr totals perhaps 4-5" per the 12Z CAM guidance. Will retain the Marginal risk outline for this update. By this afternoon we should see additional convective development farther north closer to the surface front, currently analyzed just north of the FL Panhandle into southern AL/GA. Mean flow parallel to the front may result in some training here, but also expecting this convection to gradually push off to the southeast. Morning rainfall has lowered FFG to some extent across portions of Big Bend, but focus may stay just north of this area per the HREF consensus and totals to the north will likely stay below FFG values as well (2.5-3"/hr). Another area of locally heavy rainfall is expected across the Carolinas into VA. Central and eastern SC looks to be one area of focus near the stationary front draped across the area. This boundary should aid in the development of convective clusters, with local mergers potentially resulting in some higher rainfall totals. Enough shear is present to support some maintenance of convective cells, and with PWs forecast around 2" heavy rainfall rates are likely. 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. With overall coverage of any flash flooding rather localized, will maintain the Marginal risk. Convection is also likely across NC into VA with some development focusing along the terrain of the western Carolinas into southwest VA this afternoon. Tonight, a weak wave of low pressure will move northeastward along the stationary front, which may help nocturnal convection persist across portions of the central/eastern Carolinas into southeast VA. This activity may also pose an isolated flash flood risk into the early morning hours (06-10Z) per the HREF probs. ....Southwest... The strong mid/upper ridging has shifted a bit eastward compared to Saturday, which should allow the better moisture to migrate a bit westward. 12Z RAOBs showed PWs between the 25th-75th percentile from TWC-FGZ-VEF, with an increase northward through the CO River Valley today. This supports another day of isolated to scattered diurnally-driven convection, focused over the higher terrain of central/western AZ into southern NV initially with a movement toward the NNW/N given the ridge center near the Four Corners. Any storms could drop a quick inch of rain (12Z HREF probs for 1" amounts over the day 1 period peak around 25-40%), which could drive a localized flash flood risk within any more susceptible basins. Fracasso/Chenard 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 AND NORTH FLORIDA... ....Mid-Atlantic... 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, 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. Can not rule out needing a focused Slight risk, but for now still think the coverage of any flooding will stay rather localized given forward cell motions. Plus there is still some uncertainty on exactly how the details of this setup with the low and associated frontal structure plays out. Scattered convection is also expected along/near a surface trough north of the low extending across VA and into southern NY. Instability is forecast around or above 1000 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. HREF neighborhood probabilities through 00z Tuesday of exceeding 2" are over 30%, with some low 3" probabilities 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 into southern NY. ....IA/MO/IL... Still seeing decent model consensus for a nocturnal convective complex over portions of IA/MO/IL Monday evening/night. Shortwave energy riding over the southwest ridge and increasing low level moisture transport/convergence should be the driving factors behind the convective threat. These features are pretty subtle though, so there is some inherent uncertainty with both the placement and magnitude of any convective threat. Not looking at anomalous PWs in place (though they are relatively high this time of year anyway)...but we should have extreme instability...thus any upscale convective growth should be intense and heavy rainfall rates would likely transpire. The magnitude of any flash flood threat depends on the duration of these rates which is hard to pin down at this lead time. Although several indicators suggest a rather progressive convective complex off to the southeast...the presence of the extreme instability pool upwind of the low level inflow could support some backbuilding. All in all nothing jumps out as a high end flash flood threat, but at least a low threat would exist with any convective complex that develops. Given the fairly good model agreement on this evolution, think a Marginal risk remains warranted...with some location adjustments probable as we get closer to the event. For now, preferred something close to the consensus seen with the 00z ECMWF/UKMET/Gem Reg. ....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. ....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 what should occur Sunday. Global model QPF is inching up, and the HREF is also indicating some higher QPF potential. Just like on 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 may see heavy rainfall Sunday. Combine this potential factor with the overall persistence of favorable ingredients, and think maintaining the Marginal risk is the way to go for now. 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... The mid level trough axis and surface front will work together to trigger scattered convection across the Northeast on Tuesday. PWs looks to be marginally above average, and instability is forecast ~2000 j/kg. So seems to be a thermodynamic environment favorable for heavy rainfall rates, and would expect the potential to be there for cells to drop a quick 1-2" in spots. 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. In a typical year this type of setup would probably not result in a flash flood risk...but given the continued above average soil saturation and streamflows over much of this region, do think a localized flash flood risk is a possibility. ....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 drive 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. Thus continue to take the safer option of covering a lot of the region in a low end Marginal risk to account for the flashiness of some of the basins in the area. ....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 other models are not nearly as aggressive with this feature, and the 00z ECMWF mean is quite a bit lower, likely indicative of spread within the EC ensembles. So while there is a conditional localized flash flood risk, opted not to introduce any Marginal risk area at this time given the low predictability. 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. However the fact that there is at least some light QPF present in most solutions within such a highly unstable airmass with some forcing in the vicinity...does suggest that there is some potential for an over performing convective area. The 00z GEM reg is the lone aggressive QPF output, and while it seems like an outlier, something similar can not be completely ruled out. Not nearly enough confidence to introduce any Marginal risk at this time, but will continue to keep an eye on the area. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!58WAWvhFWe0qlaNCgFScPQDjonLteRZMuxBn60SgT-uU= ZCcSBC5ov_kSh6QhzbzA0TpcP6bfvoXZ-gsWoM4DGC6_OLw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!58WAWvhFWe0qlaNCgFScPQDjonLteRZMuxBn60SgT-uU= ZCcSBC5ov_kSh6QhzbzA0TpcP6bfvoXZ-gsWoM4DaYpG8mk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!58WAWvhFWe0qlaNCgFScPQDjonLteRZMuxBn60SgT-uU= ZCcSBC5ov_kSh6QhzbzA0TpcP6bfvoXZ-gsWoM4DehVukec$=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) .