Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Sun Jul 23 2023 08:25:00 FOUS30 KWBC 230824 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 423 AM EDT Sun Jul 23 2023 Day 1 Valid 12Z 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... An area of heavy rainfall appears likely this morning and afternoon across portions of northwest FL, with ingredients in place to potentially support an organized area of training/backbuilding convection. An uptick in 850mb moisture transport and convergence, and a more favorable position of the upper jet to the north would support organized convective development. Meanwhile unidirectional flow parallel to the front could support a training and backbuilding threat. Most of the 00z HREF members do show organized convection with some training this morning. HREF neighborhood probabilities of exceeding 3" and over 80%, with 5" probabilities more in the 30-50% range. These numbers seem reasonable, and would expect a swath of 2-4" of rainfall with very localized totals of 4-7" possible. Contemplated a targeted Slight risk upgrade for this area, but opted against for now. Most of the high res guidance indicate the organized convection will eventually develop a southward push that should gradually accelerate with time...and this progression may make it difficult to exceed the very high FFG that is in place. Even with the impressive rainfall the HREF is outputting, HREF FFG exceedance probabilities don't get much above 5-10%. Thus while localized flash flooding is possible, tend to think the coverage will remain localized enough to keep the risk at a marginal level. By this afternoon we should see additional convective development over portions of the FL Panhandle into southern AL/GA closer to the actual surface front. Mean flow parallel to the front may result in some training here, but also expecting this convection to gradually push off to the southeast. Recent rainfall has lowered FFG to some extent across portions of this area, so do think at least a few instances of FFG exceedance are probable. 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. Did give some consideration to a Slight risk here as well, but there's some placement differences amongst the models, and overall tend to think the coverage of flash flooding will stay rather localized, thus think a Marginal risk should suffice. Convection is also likely across NC into VA with some development focusing along the terrain of the western Carolinas into southwest VA. As we go into Sunday night a weak wave of low pressure will move northeast 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. ....Southwest... The strong mid/upper ridging shifts a bit eastward compared to Saturday, which should allow the better moisture to migrate a bit westward. This lowering of moisture and increasing heights over NM should lower the flash flood risk there. Marginally above average PWs over AZ will persist and nose northward into southern NV, with another day of isolated to scattered diurnally driven convection. Any storms could drop a quick inch of rain (00Z 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. 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 $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .