Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jul 27 2023 09:08:18 FOUS30 KWBC 270908 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 507 AM EDT Thu Jul 27 2023 Day 1 Valid 12Z Thu Jul 27 2023 - 12Z Fri Jul 28 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH FLORIDA & THE CENTRAL GREAT LAKES AREA... ....Northeast/New England into the Mid-Atlantic... A potent shortwave trough (and the remanants of an MCV) are rapidly translating eastward early this morning, currently located near Lake Erie and Lake Ontario. Lift in association with this vort max (via DPVA), along with upper-level divergence (via the right-entrance region of a 120 kt jet streak north of Nova Scotia), will drive convective development within an anomalously moist (PWs approaching 2.0", well above the 90th percentile) and increasingly unstable (SB CAPE rising to 1000-3000 J/kg) air mass. There is still a fair amount of uncertainty with regard to the ultimate organization and coverage/placement of convection, but it appears increasingly likely that discrete convection will begin to initiate around mid-morning across portions of Upstate NY (and possibly farther south along the I-95 cooridor into the Mid-Atlantic). This discrete convection may intensify and organize rapidly in the presence of 30-40 kts of effective bulk shear, and more organized rainfall activity (mostly stratiform initially) may already be ongoing farther northeast into VT/NH (in association with a leading MCV/vort max and associated surface low). As instability increase, discrete convective elements will be capable of localized hourly accumulations of 1-2", though rather progressive storm motions of 20-30 kts will should keep any localized training/repeating to a minimum. Due to the timing of the shortwave, these more intense cells will likely not come about until they've left NY State, which will bring the primary threat of flash flooding to VT and much of NH/ME/MA/RI (where an inherited Slight risk has been extended south and eastward a bit). While the overall coverage of cells is more uncertain to the south of New England, antecedent conditions are rather wet (with as much as 200-400% of normal rainfall over the past 14 days with most streamflows still highly elevated). Farther north, there is better CAM agreement for localized totals in excess of 3" (via HREF 40-km neighborhood probabilities for 3" exceedance as high as 20-30%, versus 10-20% farther south). Even so, overall coverage of excessive rainfall is likely to fall short of the Moderate risk threshold (40%), and the progressive nature of the system will keep the peak of the activity confined to a 6-hr window (likely 15-21z across VT/NH, eastern NY and western ME, and between 18-00z farther south and east). The 12-hr probability-matched mean from the HREF (ending 00z) captures the expected QPF fairly well, suggesting localized totals as high as 1-3" across the bulk of the Marginal risk area, whereas those totals (and some maxima as high as 4-5") look more common (i.e. scattered) across the Slight risk area. Chances for flash flooding appear maximized where the higher-end of these totals (3-5") occur over more vulnerable terrain (metro and other poor drainage areas). ....Ohio Valley... Signals have increased for a number of the CAMs (primarily the ARW, ARW2, and NAM-nest) to support localized 2-3" totals over a realtively short period this afternoon (primarily from 18-03z). This looks to occur within an area of low-level convergence with PWs of 1.6-1.9" with modest deep layer shear of 15-25 kts. Instability (SB CAPE) is progged to increase to 2000-4000 J/kg to support convective initiation, and these 2-3" totals could occur over as little as 2-3 hours (per the HREF PMM). Corresponding FFGs (over 3-hr) are generally 2.0-3.0", therefore isolated instances of flash flooding are possible. ....Upper Midwest and Great Lakes... The 00z HREF continues to suggest the potential for more scattered areas of convection in association with a more subtle shortwave trough from late afternoon into the evening and overnight. While highly organized convection looks less likely with this system then previous days (due to a lack of amplitude in the trough), disorganized areas of convection may be able to become more organized into the evening hours (and would likely favor portions of MN/WI/IA). Maintained (and expanded a bit) a Marginal risk across the areas most likely to realize isolated to scattered coverage of convection (with PWs of 1.4-1.8", MU CAPE of 1000-3000 J/kg, and 30-40 kts of deep layer shear expected to support heavy rainfall and help organize any convection that is able to develop into clusters). The 00z HREF signal is still fairly unimpressive at this point, depicting 2-3" localized totals (mostly over southeast MN into WI). This signal will be reevaluated later today. ....Four Corners... An increasingly elongated monsoon ridge will remain amplified near the Four Corners today, presenting very similar risks (i.e. highly localized risk of flash flooding) to many of the same areas as earlier in the week (though shifting mostly out of UT with shrinking area over AZ). The diurnally driven convection will continue to support rainfall rates of up to 1"/hr (with localized sub-hourly accumulations of 0.5-1.0"). More vulnerable terrain (burn scars and dry washes) will be most at risk for any localized flash flooding.=20 ....South Florida and Florida Keys... Highly anomalous tropical moisture and convective coverage (in association with an easterly wave and lingering TUTT) will continue (while spreading northward along the Treasure and Space coast) today. Similar to yesterday, any instances of rapid-onset flooding will likely remain confined to localized areas where highly efficient rainfall (with rates up to 2"/hr) are repeat. Storm motions will also becoming increasingly slower, which may result in some localized totals of >2" over the course of an hour. The addition of high-resolution guidance will provide more clarity in future outlooks, but even through 00z Friday the HREF neighborhood exceedance probabilities suggest meaningful odds (15-25%) of localized 5" exceedance (over 12-hr). Churchill Day 2 Valid 12Z Fri Jul 28 2023 - 12Z Sat Jul 29 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MIDWEST, GREAT LAKES, OHIO VALLEY, NORTHEAST, FOUR CORNERS, AND SOUTH FLORIDA... ....Upper Midwest, Great Lakes, Ohio Valley, into the Northeast... A localized flash flood threat (at a minimum) looks to continue across much of the northeastern tier of the CONUS on Friday into early Saturday, as a low-amplitude trough is progged to traverse rapidly east-southeastward from the Upper Midwest to the Mid-Atlantic, likely organizing into clusters of convection (or perhaps even an MCS or two). What locations are ultimately directly impacted remains rather uncertain, given the dependence on mesoscale details that are not known at this time (though currently available CAM guidance suggests that MN/WI have the best chance for an ongoing/developing cluster of convection during the morning to mid-day on Friday). Guidance overall remains decently well-clustered over portions of the Great Lakes (primarily from near Chicago eastward to near Detroit/Cleveland, where totals as high as 2-4" occurred over the past 24 hours). A blend of guidance (and the current WPC areal average QPF) suggests localized totals of 1-2" across this area. In reality, deterministic solutions continue to depict the potential for stripe of higher localized (especially if convection is able to organize into an MCS or two), and the details will still need to be worked out by the addition of more convective allowing guidance later today. The general pattern does conceptually support the idea of organizing convection in a relatively weakly forced environment, with northwest flow aloft and the favorable synoptic position for MCSs along the northern periphery of a dominate ridge (with all moisture and instability readily available, the mesoscale environment will need to bring the final ingredient of lift via cold pool forcing for better organization). ....Four Corners... Very similar to prior days (and across much of the same areas of NM/CO/AZ on Day 1), isolated to widely scattered convection will be favored beneath the core of a dominate monsoon ridge. This convection will likely be capable of rainfall rates of up to 1"/hr (with localized sub-hourly accumulations of 0.5-1"). ....Southwest Florida... The aforementioned easterly wave (and TUTT) with associated highly anomalous tropical moisture begins to shift west by Friday, as veering and weakening low-level flow likely brings decreasing convective coverage to the Florida Keys. Portions of mainland South FL look to remain somewhat favored for additional rainfall, likely driven primarily by diurnal convection in association with the sea breeze circulation (and amplified by lingering tropical moisture). Rainfall totals of 1-3" in a short period could cause localized rapid-onset flooding issues, particularly over poor drainage metro areas (and worsened by saturated soils/surfaces from prior days rainfall). Churchill Day 3 Valid 12Z Sat Jul 29 2023 - 12Z Sun Jul 30 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE FOUR CORNERS, THE NORTHERN AND CENTRAL PLAINS, THE MIDDLE MISSISSIPPI VALLEY, THE OHIO AND TENNESSEE VALLEY, THE MID-ATLANTIC, AND THE NORTHEAST AND NEW ENGLAND... ....Northern/Central Plains, Middle Mississippi Valley, Ohio and Tennessee Valley, and into the Northeast and New England... The synoptic pattern remains supportive of convection overriding broad ridging across the Southern Plains and Southeast US. In addition, a potent shortwave trough is progged to dig southeastward into the Northern Plains and Midwest/Great Lakes early on Saturday, providing large-scale lift (with sufficient instability and ample moisture for convective development). Early indications are that this will favor convection along a broad corridor from the North/Central Plains through the MS/OH/TN Valleys and into the Mid-Atlantic and Northeast/New England. Since mesoscale details are of great importance, its difficult to pinpoint regions that will be at elevated risk for organized convective activity (and potentially more scattered areas of flash flooding). The most vulnerable locations will likely be across portions of the Northeast/New England, but to what is extent is dependent on the yet-to-occur activity on Day 1. A blend of the available guidance (and the current WPC QPF) depicts the highest areal average totals (generally 1-2") over the central Appalachians to Mid-Atlantic and northern New England, and these signals will continue to be monitored for greater confidence (with the addition of some CAM guidance also providing additional relevant details). ....Four Corners... Isolated to widely scattered convection is expected again underneath the core of a dominate monsoon ridge (with coverage looking to spread westward a bit to encompass more of AZ and southeast UT). As was the case on prior days, this convection will likely be capable of rainfall rates of up to 1"/hr (with localized sub-hourly accumulations of 0.5-1.0"). Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7QBHSi5QfzZU_VpHTPPD7LcFQ7RFBxHs7wSmBCa5mkL5= 463-gxMtjDHm8inqEcJwh-xaTFRC9tIGfdVK0xKN3OZGgDw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7QBHSi5QfzZU_VpHTPPD7LcFQ7RFBxHs7wSmBCa5mkL5= 463-gxMtjDHm8inqEcJwh-xaTFRC9tIGfdVK0xKN0wGPngQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7QBHSi5QfzZU_VpHTPPD7LcFQ7RFBxHs7wSmBCa5mkL5= 463-gxMtjDHm8inqEcJwh-xaTFRC9tIGfdVK0xKNSsVJLMs$=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. 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