Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jul 24 2023 08:09:51 FOUS30 KWBC 240809 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 408 AM EDT Mon Jul 24 2023 Day 1 Valid 12Z Mon Jul 24 2023 - 12Z Tue Jul 25 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MID-ATLANTIC/NORTHEAST, CENTRAL PLAINS, MID-SOUTH, OHIO/TENNESSEE VALLEY, SOUTHWEST, AND NORTH FLORIDA... ....Mid-Atlantic/Northeast... A broad longwave trough across the eastern CONUS will continue to drive a scattered convective threat today 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 deepen a bit and push off the VA/NC coast late tonight into Tuesday. Enhanced convergence associated with this low may drive a more organized convective threat Monday morning/afternoon across northeast NC into southeast VA, as expressed by 00z HREF 40-km neighborhood exceedance probabilities for the 3" threshold (over 24-hr) as high as 30-50%. Definitely some potential for localized 2-4" rainfall amounts with this activity as PWs approach 2.0" and instability (MU CAPE) peaks between 1000-2000 J/kg, but the same HREF exceedance probabilities for 3-hr time intervals peak near only 10% (so these will be likely be longer-term totals driven over a relatively long period). Cannot rule out the introduction of a focused Slight risk in light of some lower FFG values over parts of the region (as low as 2.5"/3-hr over northeastern NC), but for now still think the coverage of any flooding will stay rather localized (as indicated by the HREF probability-matched mean QPF, with more widespread and organized 3" totals remaining offshore the NC Outer Banks). 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 (MU CAPE) is progged to peak between 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, as rainfall rates will be high enough to drop a quick 1-2" of rain on a localized basis. The 00z HREF 40-km neighborhood probabilities of exceeding 2" continue to approach 50-60% up to the NY Southern Tier, with some splotchy lower 3" probabilities (10-20%) as well. Still 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 (and particularly so near the PA/NY border and across VT). Cannot rule out a targeted introduction of a Slight risk here either, but not enough confidence in scattered coverage of 1.5"+ totals at this time (as the HREF probability-matched mean QPF is rather isolated with these totals, and the ensemble agreement scale, EAS, neighborhood probabilities for 1" exceedance peak at only 10-20% across those southern tier portions of NY). ....NE/IA/KS/MO into the Mid-South and OH/TN Valley... CAM guidance came into better agreement overnight in depicting convection breaking out in the vicinity of NE/IA/KS/MO, within a tight instability gradient (MU CAPE) of 1000-3000 J/kg and PWs of 1.3-1.6" (between the 75th and 90th percentile, per OAX sounding climatology). A subtle shortwave impulse riding over the southwest ridge may continue to organize this activity that is breaking out, but perhaps more significantly an compact low-level jet (20-30 kts at 850 mb) will drive continued moisture transport/convergence for potential backbuilding of rapidly propagating convection (with upwind propagation vectors favoring a near southerly progression, and the 850-300 mb flow favoring a more southeasterly progression). The bulk of the 00z HREF guidance kills this convection off between 12-15z, but the threat of lingering backbuilding/training suggests some localized flash flooding may persist beyond 12z in the vicinity of eastern NE, southwestern IA, northeastern KS, and northwest MO (with the potential for 2-3" localized totals). Farther southeast into northern portions of the Mid-South (southeast MO, southern IL, far northeast AR, northern West and Middle TN, and far western KY), the aforementioned shortwave impulse (in addition to potential enhancement from an MCV) may allow for scattered convective coverage into late morning and the early afternoon. Instability (MU CAPE) of 1500-3000 J/kg in the vicinity of PWs of 1.4-1.8" will be supportive of 1-2"/hr rates, and any training/repeating could result in localized instances of flash flooding (as 3-hr FFGs are as low as 2.0-2.5"). Farther northeast into portions of the OH/TN Valley, weak frontogenesis will likely help to focus diurnally driven convection (with instability and moisture near or slightly lower than the environment in the Mid-South). Storms will also likely initially be slower through this region, and increasing deep layer shear into the evening hours may support isolated convective activity into the evening to early overnight hours. Hard to pinpoint any particular area with high confidence, given the rather wide spread in the both the CAM and downscaled global guidance. The 00z HREF ensemble agreement scale (EAS) neighborhood probabilities certainly speak to this, peaking at only 10% (in the vicinity of the IN/OH/KY border region). ....Southwest... A mid-level ridge continues to build over the Southwest today, 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 than yesterday 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 (localized sub-hourly rates of 1"+), which could drive a localized flash flood risk within any more susceptible basins including slot canyons, dry washes, and burn scars. ....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. Churchill/Fracasso/Chenard Day 2 Valid 12Z Tue Jul 25 2023 - 12Z Wed Jul 26 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST AND SOUTHWEST... ....Northeast... A mid-level trough axis and surface front will work together to trigger scattered convection across portions of northern New England into Tuesday. PWs of 1.0-1.4" look to be marginally above average, and instability (MU CAPE) is progged to increase to 1000-1500 J/kg. QPF from both the downscaled global and available CAM guidance depicts the potential for 1-2"+ amounts (which could fall over a relatively short period), but the overall spread and inconsistency suggests those totals will be fairly localized. That makes sense, given that the expectation is for convection to lack organizion given the modest mid/upper support and generally weak shear. Will continue to maintain a Marginal risk area as we wait for the full HREF suite to come 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. Continued incorporation of the CAM guidance will likely refine this area for later updates. ....Elsewhere across the CONUS... The ECMWF has remained 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 (with only the UKMET giving a modest signal for 1-2" QPF amounts), and the 95th percentile NBM QPF is still lower than the downscaled ECMWF. Will continue to re-evaluate with later guidance. There are also continued signs of subtle signs of a heavy rainfall potential across portions of IN/KY/TN and vicinity, but the model spread remains quite wide and with lesser amounts than on Day 1. 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. The continued weak QPF signal amongst most of the deterministic and ensemble guidance suggests the threat is pretty low, and the 95th percentile NBM QPF has trended downward to near half an inch. Churchill/Fracasso/Chenard Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8sAbzmcqwAaKn7fgMRx6t5vjMSA62pFelxbo1UKTZSCp= USMtsXjrPpAcBHGWxabYZR8duyVmz520aVJdY7VSJ-8RBcA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8sAbzmcqwAaKn7fgMRx6t5vjMSA62pFelxbo1UKTZSCp= USMtsXjrPpAcBHGWxabYZR8duyVmz520aVJdY7VSB_ro5DU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8sAbzmcqwAaKn7fgMRx6t5vjMSA62pFelxbo1UKTZSCp= USMtsXjrPpAcBHGWxabYZR8duyVmz520aVJdY7VSynpZv9c$=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) .