Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 16 2024 15:53:56 FOUS30 KWBC 161553 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1153 AM EDT Fri Aug 16 2024 Day 1 Valid 16Z Fri Aug 16 2024 - 12Z Sat Aug 17 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEAST U.S... 16Z Update... Changes to the broad ERO areas were relatively minor with the morning update to account for modest adjustments related to 12Z CAMs.=20 The most substantial adjustment was to extend the MRGL risk back across the Ozarks and into far eastern KS/OK where the high- res has gotten into better agreement that an MCS will develop beneath a shortwave as it interacts with a sharp instability gradient along a stationary front. 20-30 kts of 850mb inflow will support expanding convection within this MCS, and propagation vectors aligned to the mean wind and front suggest training of 1-2"/hr rain rates. This could produce locally more than 3" of rain and instances of flash flooding. Considered a small SLGT risk as well for the urban SE coast of southern FL where 06Z HREF probabilities for 5" were notable. However, the synoptic pattern seems to support enough push SW that the SW coast would be more prone to isolated flash flood instances due to convergence along the sea breeze there. The 12Z HREF probs and CAMs did pivot subtly in that direction, so felt the MRGL risk was well placed and no upgrade was needed. Otherwise, any changes to the inherited were cosmetic and most of the overnight discussion was still valid. ....Southeast U.S... Convection across portions of the lower Ohio Valley early this morning is forecast to reach portions of southern/eastern Kentucky and Tennessee later today and eventually into northern Alabama/Georgia. There is lower confidence on how the activity will hold together or what additional line segments will develop but with plentiful moisture and sufficient instability in place this afternoon, several areas of heavier rainfall will be possible. Based on the latest guidance and 12Z HREF probabilities, pockets=20 of 1-2" hourly totals will be possible, most likely across portions of middle/eastern TN, far southeast KY, and into northern=20 AL/northwest GA where the Slight Risk in the Excessive Rainfall=20 Outlook exists. Some localized totals of 2-4" will be possible=20 within that area and could lead to scattered instances of flash=20 flooding. ....Great Lakes through the Ohio Valley and into the Ozarks...=20 For today and tonight, a cold front is expected to move eastward=20 from the Midwest through the Ohio Valley as the main upper level=20 shortwave energy digs further into the Great Lakes region. This=20 combination should provide plenty of support for scattered to=20 numerous showers and thunderstorms to develop along the advancing=20 cold front across the Ohio Valley into the Central Appalachians.=20 The activity should be fairly progressive and quick moving,=20 limiting the overall duration of any pockets of heavy rainfall at=20 any one location, but a few spots of merging storm cells or=20 training could develop that pose a flash flood risk.=20 Farther north into the Great Lakes (Wisconsin/Michigan), storm=20 motions under the core of the upper trough are likely to be slower=20 and when combined with the higher PWs and somewhat elevated soil=20 moisture percentiles (Wisconsin), a few instances of flash flooding will be possible. Farther south through the Ozarks, the front should align generally W-E while a shortwave moves atop it tonight. As the LLJ surges moisture into this boundary, an MCS is likely to develop with training of 1-2"/hr rain rates possibly causing instances of flash flooding. ....Southwest U.S... Increasing monsoonal moisture will begin to lift northward into portions of the region today, the highest PW anomalies will likely be tied to far southern Arizona but the 12Z guidance is keying on=20 enough moisture and sufficient instability developing today=20 combined with the subtle shortwave lifting through to spark at=20 least isolated thunderstorms into portions of southern NV, especially late this evening and tonight and southern/central New=20 Mexico, although instability may be somewhat marginal this far east. Some of the convection will be capable of producing intense rain=20 rates and could lead to flash flooding over the typical vulnerable=20 locations. ....South Florida... A trailing frontal boundary and trough over the region combined with a very high moisture axis will bring another round of scattered to numerous thunderstorms to South Florida later today through the evening hours. The combination of high PWs and sufficient instability will lead to very intense and efficient rain producing thunderstorms, capable of producing 2-3" hourly totals at times. The 00Z guidance was quite bullish for southeast FLorida=20 coastal areas with potential for some locations to see several=20 inches today, but the 12Z has backed off somewhat as the synoptic flow becoming more E/NE appears sufficient to push convection towards the Everglades and SW coast. There could be some slowing/training of intense rain rates on both urban coasts, but the signal shifting more towards the Everglades prevents any upgrades from the inherited MRGL risk at this time. Weiss/Taylor Day 2 Valid 12Z Sat Aug 17 2024 - 12Z Sun Aug 18 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE EASTERN U.S...SOUTHWEST U.S...SOUTH FLORIDA...AND PACIFIC NORTHWEST... ....Eastern U.S... The anomalous upper trough and associated cold front will make its slow progression eastward into the Eastern U.S. Saturday into Saturday night. Another day of mostly loosely organized convective segments are expected in the modest axis of higher moisture and available instability. THe latest guidance points toward areas of the OHio Valley, Central APpalachians, and Mid Atlantic being the focus for higher rainfall totals which could total a couple inches. Based on the environmental ingredients, pockets of 1-2" hourly totals will be possible where any boundary collisions or storm mergers allow for a slightly longer duration in rainfall at any one location but the storm motions overall should be fairly progressive. However given some of the terrain sensitivities and urban areas in the risk areas, a few instances of flash flooding will be possible. Further south, the lingering boundary across portions of the Southeast will again be the focus for another round of mainly afternoon/evening thunderstorms. Storm motions may be slightly less and with more robust moisture/instability, some of the rain rates may be rather intense but short-lived. This may lead to isolated instances of flash flooding. ....Southwest U.S... The return of deeper monsoonal moisture is expected across the region, initially late tonight across portions of southern Arizona, but by Saturday a plume of higher moisture surges north/northeast through the region. This is in response to the anomalous upper ridge axis becoming centered over eastern New Mexico and western Texas while a strong upper trough approaches the coast of California. This will set up a favorable flow of this higher moisture in between across much of Four Corners region during the period. PW anomalies reach +2 to +2.5 standard deviations above normal during the period while the latest model guidance is suggesting a subtle shortwave trough rippling through the flow, which will act to enhance the forcing for ascent across the region. Combined this with the expected instability during peak heating and the set up will become quite conducive for scattered daytime to evening showers and thunderstorms. The latest guidance is favoring northern Arizona and much of Utah for the greatest QPF and potential for higher rain rates, but the threat will exist across the larger region. Isolated to widely scattered instances of flash flooding will be possible, particularly for the more vulnerable locations and a Marginal Risk covers the region. ....Pacific Northwest... There is a increasing signal for heavy downpours across portions of the Washington and Oregon Cascades. During this period there will be and approaching trough along with a steady influx of PW values of 1+ inches. Some of the hires guidance project convection to spread/track northward along the terrain with hourly rain rates upwards of 0.75 inch/hour. In collaboration with the local forecast office a Marginal Risk area was raised to highlight the elevated threat for excessive rainfall and the potential impacts leading to debris flows on the recent burn scar areas from active wildfires. ....South Florida... Another round of deep convection capable of producing flash flooding is expected on Saturday, thanks to the lingering trough over the region. While the 00Z guidance isn't as robust for Day 2 as it is for Day 1 (and perhaps a bit to the south of the major urban areas), the repeating rounds / cumulative effect may bring a few round of flash flooding to the area again on Saturday/Saturday evening. Taylor Day 3 Valid 12Z Sun Aug 18 2024 - 12Z Mon Aug 19 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID ATLANTIC... ....Mid Atlantic and Northeast... By Sunday, the timing of the cold front and upper trough is expected to allow for another round of scattered to numerous showers and thunderstorms across the Mid Atlantic and portions of the Northeast. Based on the latest guidance, a focus area of locally heavy rainfall due to the combination of favorable dynamics, moisture (PWs near 2 std deviation above normal), and potential for training/backbuilding is setting up across portions of urban corridor from near Baltimore through southern New York. This is where the potential exists for a few rounds of heavier rainfall and given the anomalous moisture in place, pockets of intense rain rates will be possible over the more urban areas. Some areas may pick up a quick couple of inches. With this in mind, a Slight Risk was introduced for portions of the area where scattered instances of flash flooding will be possible. ....Southwest and Intermountain West... A continuation of the deep monsoonal moisture and daytime to evening showers and thunderstorms are expected across much of the Southwest U.S. and into portions of the Intermountain West Sunday into Sunday evening. Convection will be capable of producing hourly totals up to 0.75" at times, particularly from northern Arizona through Utah where the better instability is forecast to setup. This will bring another day of localized to widely scattered instances of flash flooding for the more vulnerable locations in the region. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7kmBF-AjYdUh1Qr-KqljvhX_RrKTW6kdTGHGkAGj5GQj= 09e7n3Dq8ImAeZZDKq2YRcFtyOu8hJSiGdOu0PwdXfPNKkQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7kmBF-AjYdUh1Qr-KqljvhX_RrKTW6kdTGHGkAGj5GQj= 09e7n3Dq8ImAeZZDKq2YRcFtyOu8hJSiGdOu0PwdYSOLo5A$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7kmBF-AjYdUh1Qr-KqljvhX_RrKTW6kdTGHGkAGj5GQj= 09e7n3Dq8ImAeZZDKq2YRcFtyOu8hJSiGdOu0PwdJQSh_PY$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .