Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 26 2023 19:33:32 FOUS30 KWBC 261933 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 332 PM EDT Sat Aug 26 2023 Day 1 Valid 16Z Sat Aug 26 2023 - 12Z Sun Aug 27 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID MISSISSIPPI VALLEY INTO THE MID-SOUTH... 1600 UTC Update -- Based on the latest observational (satellite/radar) and mesoanalysis trends with the MCS and evolving MCV, have hoisted a Slight Risk across eastern and southeast MO, southern IL, western KY, northwest TN, and far northeastern AR. Deep-layer instability along and south of the MCS are modest at best (mixed-layer CAPES ~1500 J/Kg), though along with PWs ~2.00", will foster hourly rainfall rates of 1.5-2.0" and 3-hourly rates up to 3" in areas of training convection along/near the surface front, as areas of cell training will be likely given the weak Corfidi vectors. The 12Z HREF 40km neighborhood exceedance probabilities have come up within this region compared to the overnight guidance, which aligns well with the observational trends. Hurley Previous discussion below... Maintained a corridor of a Marginal risk area across parts of Missouri east/southeastward across portions of the Tennessee Valley/lower Ohio Valley..with on-going moderate to heavy rainfall being on-going at the start of the outlook period followed by additional elevated convection forming along/near a quasi-stationary front in a region of weak upper level height/thickness diffluence between the contracting deep-layer ridge and digging upper trough. There still looks to be some enhanced forcing within the right-entrance region of a 70-90kt upper level jet streak traversing the Great Lakes region. Spaghetti plots from the SREF and GEFS still not showing much overlap of 1 or 2 inch contours with each other...although the HREF was beginning to show some clustering of 40 km neighborhood probabilities of 1 inch per hour from the Mid-Mississippi Valley into the Tennessee Valley by later this morning and probabilities of 2 inches of rain in an hours develop by afternoon, The signals are not that strong...generally 15 per cent or less. The overall threat for heavy rainfall gradually shifts eastward into the east end of the Tennessee Valley by late tonight. Made a few adjustments to the previously issued Marginal. Bann Day 2 Valid 12Z Sun Aug 27 2023 - 12Z Mon Aug 28 2023 ....THERE IS A SLIGHT RISK ACROSS PORTIONS OF THE SOUTHEAST INTO THE LOWER MID ATLANTIC REGION... 2000 UTC Update -- Outside of minor modifications to both Marginal Risk areas, the notable change to the Day 2 ERO was to introduce a Slight Risk across portions of the Carolinas, far southeastern VA and far eastern GA. The 12Z guidance QPF has come up a bit in this region, along/near the surface front, owing to enhanced upper divergence/low-level FGEN in the right entrance region of the 70-90+ kt upper jet streak, along with sufficient deep-layer instability (mixed-layer CAPEs 1000-2000+ J/Kg) and more-than-sufficient deep-layer moisture (PWs 2-2.25"). The guidance, particularly the CAMs, all show a heavy rainfall footprint, albeit there remains some areal spread with the max QPF axes. Still, forecast confidence for more organized, mutlicellular convection has increased, as 0-6km bulk shear values are expected to climb above 20-25kts by Sun afternoon. Furthermore, given the thermodynamic profile (high CAPE/PWs), hourly rainfall rates of 2-3+" are anticipated underneath the strongest cells, as indicated per the impressive (and unusual) 12Z HREF exceedance probabilities, which include 40-80+ percent probs of >2"/hr rates between 19Z Sun-05Z Mon, while probs of >3"/hr rates peak between 25-50 percent). 24hr neighborhood probabilities of exceeding 5" of rain range between 40-70% within the Slight Risk area, again quite impressive. Hurley Previous discussion... ....Parts of the Southeast into the Lower Mid Atlantic Region... Maintained the Marginal risk area over the Carolinas despite shifts from the ECMWF towards the GFS idea of limited coverage of rainfall being confined near the coast. The overall synoptic pattern...of an upper level jet over the Mid-Atlantic region helping provide upper level support for lift in a region where precipitable water values will generally be in excess of 2 inches...suggests that a broader area of 1 to 2 inch amounts seems plausible especially in light of the ARW5 developing 800 to 1500 J per kg of CAPE with little surface based CIN. Later outlooks can adjust later as needed. Of particular interest was the appearance of some 2 to 5 inch QPF bulls eyes across parts of the Tennessee Valley in the latest guidance from the ECMWF/UKMET/CMC and to a lesser extent by the NAM. Extended the Marginal Risk westward in deference to the number of model signals. Still believe that even the wetter solution in the Carolinas will be handled well given the antecedent conditions with only isolated run-off problems from the most active convection...and that what falls on Day 2 will act to prime the area for additional rainfall on Day 3. ....Rockies and High Plains... Another round of widely scattered afternoon and evening convection is expected along and near the terrain from the Central/Southern Rockies. Deeper moisture is expected to be in place...setting the stage for a few of the cells to produce local downpours from parts of Wyoming into New Mexico. Precipitable water values of 0.8 to 1.2 inches across the northern and western portion of the outlook area to nearly 1.25 inches on the plains from far southeast Colorado into northeast New Mexico and adjacent areas of Texas. Thinking is that at least some spots may get rainfall intense enough to produce run off related problems...with recent burn scars..normally dry washes and arroyos being most vulnerable. Bann Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4PQh9mOLhm0R-D5j0xkwOjRPRniLDD6hJ_oZxof-rKhf= 71e6jdwVcADlSV4LVsImsiCr8m8DG6s8kKrqhZUlvWu5PdY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4PQh9mOLhm0R-D5j0xkwOjRPRniLDD6hJ_oZxof-rKhf= 71e6jdwVcADlSV4LVsImsiCr8m8DG6s8kKrqhZUluNjRKuM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4PQh9mOLhm0R-D5j0xkwOjRPRniLDD6hJ_oZxof-rKhf= 71e6jdwVcADlSV4LVsImsiCr8m8DG6s8kKrqhZUlNA2PvVw$=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) .