Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jul 28 2023 00:56:30 FOUS30 KWBC 280056 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 854 PM EDT Thu Jul 27 2023 Day 1 Valid 01Z Fri Jul 28 2023 - 12Z Fri Jul 28 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF ARIZONA, THE UPPER MIDWEST, IN AND NEAR KENTUCKY, & FLORIDA... ....In and near Kentucky... Signals for localized 2-3" totals over a relatively short period into tonight remain in and near KY, where convection has been slowly shifting from east to west during the afternoon and evening hours as CIN sets in to the east. This is occurring within an area of low-level convergence with PWs of 1.6-1.9" with modest deep layer shear of 15-25 kts. ML CAPE is 2500-4000 J/kg in areas currently without CIN, which would support convective initiation, and these 2-3" totals could occur over as little as an hour or two. Corresponding FFGs (over 3-hr) are generally 2.0-3.0", therefore isolated instances of flash flooding are possible. As CIN continues to develop, the threat should lower as we head through the overnight hours. ....Upper Midwest and Great Lakes... The potential remains for more scattered areas of convection with backbuilding characteristics in association with a more subtle shortwave trough into the evening and overnight, which will eventually forward propagate southeast into IL. PWs of 1.4-1.8", MU CAPE of 6000-7000 J/kg upstream/to the west-southwest, and ~40 kts of deep layer shear expected to support heavy rainfall and help organize any convection that is able to develop into clusters -- mesocyclones cannot be ruled out. With backbuilding noted, Slight Risk impacts cannot be ruled out, although the 18z HREF isn't enthusiastic about the prospect for such. Once convection forward propagates, the excessive rainfall risk will be in decline. ....Southern AZ... Thunderstorms moving out of the mountains of southeast AZ should spread westward near the International Border with Mexico over the next several hours, presenting a highly localized risk of flash flooding. 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. Since thunderstorms with heavy rainfall have been on the wane elsewhere, significantly reduced the size of the risk area from continuity. ....Southern Florida and Florida Keys... Highly anomalous tropical moisture 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 and local 5"+ amounts.=20 So far, the threat has been isolated but where it has rained heavily near Fort Lauderdale, it has rained a good deal, with local 9" reports. Since the risk has been isolated, reduced the risk to Marginal through 12z. This is also supported by the 18z HREF, which shows minimal signal for heavy rainfall. Despite this, locally Slight to Moderate Risk impacts cannot be ruled out, particularly near the Florida East Coast. Roth 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 Valid 12Z Sat Jul 29 2023 - 12Z Sun Jul 30 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL APPALACHIANS AND UPPER OHIO VALLEY... ....Central Appalachians and Upper Ohio Valley... Upslope flow on strong westerly winds ahead of an advancing cold front will increase thunderstorm coverage and intensity into the Appalachians of WV. With pockets of QPF over 2 inches possible with the most persistent storms, combined with a wet footprint from previous rains across central WV, a Slight Risk area was introduced with this forecast update. ....South-Central CO and the Sangre de Cristo Mountains of New Mexico... An easterly LLJ is expected to impact the mountains of south-central CO into NM Friday afternoon. The added moisture in this small area should result in more widespread convection, that may lead to isolated flash flooding. Elsewhere in the 4 Corners Region, a few thunderstorms are possible but flash flooding risk is likely to remain under 5%. ....Northeast... A southward moving cold front will push the instability needed for strong thunderstorms and heavy rains south through the day. Thus, much of northern New England and northern New York were removed from the Marginal Risk, as the rainfall expected in this region is likely to be stratiform in nature, and therefore while widespread, is unlikely to produce rainfall rates high enough to result in flash flooding. Wegman ....Previous Discussion... ....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"). Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4YfpvkKlzUBn3h9Wxb3NXih9q4_S5cUD32vBIMiOx9P4= xjk3GOwWwemz44vfVoPsInPzGpA3KPNZRXbdiNe3Wsn8qGQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4YfpvkKlzUBn3h9Wxb3NXih9q4_S5cUD32vBIMiOx9P4= xjk3GOwWwemz44vfVoPsInPzGpA3KPNZRXbdiNe3diK3jtw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4YfpvkKlzUBn3h9Wxb3NXih9q4_S5cUD32vBIMiOx9P4= xjk3GOwWwemz44vfVoPsInPzGpA3KPNZRXbdiNe3mwbxiY4$=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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