Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jul 27 2023 20:19:26 FOUS30 KWBC 272019 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 417 PM EDT Thu Jul 27 2023 Day 1 Valid 16Z Thu Jul 27 2023 - 12Z Fri Jul 28 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH FLORIDA AND NEW ENGLAND... ....16Z Update... No big changes were made with the midday update. The Slight Risk areas in both New England and South Florida are unchanged with this update. ....South Florida... In South Florida, rainfall moving into the Space Coast has been largely stratiform, but the persistence of the rain led to a small expansion of the Marginal there. The Slight Risk area from the Treasure Coast through Miami remains the same, with the potential for slow-moving storms to redevelop this afternoon along the urban corridor, which has been hard hit in recent days so antecedent conditions favor the development of flooding should there be enough rainfall from the storms. The highest uncertainty even now is how much coverage of storms there will be in the area, with many of the CAMs suggesting coverage will be minimal. Thus, the Slight Risk area is currently considered a low end threat. That being said, and as mentioned in previous discussions, the availability of atmospheric moisture in this area is impressive, so any stronger storms that form have the potential to cause locally significant flooding since they will be slow-moving. ....New England... The well-advertised MCS moving across northern New England so far has been causing steady rain, but with only isolated instances of heavy rain. The most significant rainfall associated with it has ended across northern NY, so the Marginal Risk there was trimmed. Going forward through this afternoon, however, thunderstorms that will form as a result of daytime heating and aren't exclusively tied to the MCS should allow some backbuilding from far eastern and southern NY eastward through New England. A few of those storms are already starting to form over the Catskills and Mohawk Valley since the forcing is stronger closer to the MCS. Further south across the Mid-Atlantic and the I-95 corridor, the storms will form a bit later this afternoon once there's been sufficient daytime heating. The further north you go, the greater coverage of storms, so the inherited Slight Risk area looks on track. ....Ohio Valley... A weak upper level trough and the southwestern end of a front moving across the Northeast may trigger a few storms that may train along similar paths as each other this afternoon. There remains some signal for locally enhanced rainfall totals of up to 3 inches, and widely scattered totals to 2 inches. Much of the OH Valley, generally along the river separating OH and KY have been a bit wetter than normal, so soil moisture conditions are generally favorable for flash flooding development should training storms develop in this area this afternoon. ....Upper Midwest... The right entrance region of a 90 kt jet over Ontario will move across the region tonight. A local maximum of moisture on a 15-20 kt southwesterly LLJ will develop, supporting backbuilding storms over MN and WI through tonight. The storms should be moving fast enough that any flash flooding risk will remain isolated. ....Four Corners into Wyoming... Scattered convection is expected to develop this afternoon from the 4 Corners region northward through Wyoming. The greatest concentration of storms are expected from southern/southeastern CO north into WY. While many of the weaker storms are likely to remain dry due to dry air prevalent in the low to mid levels and high dew point depressions, the strongest storms, particularly those that may train from previous convection have the potential to produce local rainfall totals of 0.5 to 1 inch, which over particularly sensitive areas such as burn scars and slot canyons could result in localized flash flooding. As typical with the monsoon so far this year, the upper level ridging should keep those scattered storms to distinct areas, and the overall Marginal risk is considered on the lower side of the Marginal Risk category. Wegman ....Previous Discussion... ....Northeast/New England into the Mid-Atlantic... A potent shortwave trough (and the remnants 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 corridor 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 relatively 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 SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDWEST... ....2030Z Update... ....Midwest... Two rounds of storms are expected to develop in advance of a southward moving cold front across the upper Great Lakes on Friday. The first round of storms is likely during the peak heating of the day, where the storms will take advantage of MUCAPE values exceeding 3,000 J/kg. Since this round will be well ahead of the front, any storms that form in this region are likely to be fairly isolated, but since they'll have plenty of moisture and instability, they could grow to become intense. The far more significant round of storms will be the second one, which will impact the area during the overnight hours Friday night into Saturday morning. The storms will have the benefit of a stronger LLJ providing additional moisture, with the front backing up the storms with good forcing. With a swath of rainfall from yesterday averaging around 2 inches in the Slight Risk area, the soils are quite a bit wetter than normal for this time of year in the Slight Risk area. Thus, think this next round of rain, while areally expecting a bit less rainfall; a broad 1-3 inches of rain in this same area is likely to cause more widely scattered flash flooding. The area of highest threat is along the MI/IN-OH border, and into northeast OH and northwest PA. There is a bit more uncertainty both on the north side over central MI and also west of Lake Michigan over WI and IL, including the Chicagoland area. ....South Florida... A small expansion of the Marginal was made to include more of the Gulf coast of the FL Peninsula into the Marginal Risk, including the Tampa metro. The departing tropical wave and any potential local sea breezes should focus afternoon convection along the west coast of the Peninsula. While this area has been quite dry lately, the abundance of atmospheric moisture should allow any storms that form to have ample moisture to produce heavy rainfall. The inherited Marginal also looks good including southeast FL, which has been much wetter than the west coast, but also is expecting less coverage and intensity of storms Friday afternoon. ....Four Corners Region... Based on an analysis of recent verification and the relative minimum in expected coverage of storms across the Four Corners region, have opted to drop the Marginal Risk entirely from this area due to lack of signal. Any very isolated flash flooding possible will be less than the 5% coverage threshold for a Marginal Risk. Wegman ....Previous Discussion... ....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 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!6p8s5RvVm6FWUerx_u-L3Q8lIhzdqBuii7NKlDOdaCDe= U0RMo67MWDsAoqm2xWLLsA4aI8xjEcn01pEoPlLtQCuYeTg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6p8s5RvVm6FWUerx_u-L3Q8lIhzdqBuii7NKlDOdaCDe= U0RMo67MWDsAoqm2xWLLsA4aI8xjEcn01pEoPlLteD8nF88$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6p8s5RvVm6FWUerx_u-L3Q8lIhzdqBuii7NKlDOdaCDe= U0RMo67MWDsAoqm2xWLLsA4aI8xjEcn01pEoPlLt4PM-6CA$=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) .