Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jun 25 2023 15:57:41 FOUS30 KWBC 251557 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Sun Jun 25 2023 Day 1 Valid 16Z Sun Jun 25 2023 - 12Z Mon Jun 26 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE GREAT LAKES...OHIO AND TENNESSEE VALLEYS...AND NORTHEAST... ....Northeast... Deep southerly flow will remain in place across the Northeast today, ahead of the approaching deeper closed low tracking into the Great Lakes, and with the anomalous moisture in place (upwards of +2.5 standard deviations) and expected daytime heating/instability, scattered to numerous thunderstorms are expected across mainly the interior Northeast. Mean flow will be relatively weak (under 10-15 kts) and warm cloud depths will continue to be relatively high, supporting efficient rainfall producing thunderstorms and a localized/isolated threat of flash flooding. A quick look at the 12Z NAM...12Z neighborhood probabilities from the HREF and a few of the most recent runs of the HRRR suggest only a subtle southward expansion is needed (mainly along and near the I-95 corridor). 12Z soundings showed tall, skinny CAPE profile into the Mid-Atlantic which reinforced the idea that local downpours are possible. ....Great Lakes to Tennessee Valley... The deep closed mid/upper low will move from western Wisconsin this morning toward the Great Lakes by early Monday morning. Ahead of that, ongoing convection will slide eastward into portions of Wisconsin and Michigan, where greater forcing will align with an axis of higher moisture that has lifted northward. The latest guidance indicates upwards of 1.0-1.25" of PW present and with daytime heating today providing sufficient instability, some higher rain rates are likely with deeper convection just ahead of the surface low tracking through Wisconsin into Michigan. The 12Z HREF indicates some hourly totals up to 1-2" possible and storm motions may be slow or allow for repeating rounds, so a few instances of flash flooding will be possible. Farther south into portions of the Ohio Valley, Tennessee Valley and into Arkansas, the environment will much more conducive for deeper convection with a better setup for one or more complexes of strong/severe storms. With higher dewpoints in the lower 70s already in place and 12Z precipitable water values up to 1.75 inches embedded within a broader area where values were 1.5"...and plenty of instability...storms should quickly develop this afternoon into the evening and progress quickly east/southeast. Storm motions are likely to be quite progressive, limiting the duration of the intense rain rates and generally mitigating the excessive rainfall potential. However...a few colliding clusters or cell mergers may pose an isolated flash flood risk with isolated rainfall amounts up to 2". Even farther south...expanded the southern boundary of the Marginal Risk area a bit farther to the south in deference to latest HREF guidance. ....Central Washington... Isolated thunderstorms are once again expected to develop this afternoon across portions of north-central Washington and will have the potential to produce isolated rain rates up to 1"/hr per the latest guidance including the 00Z HREF neighborhood probabilities. Given some of the more sensitivity/vulnerable burn scars in the outlook area, particularly across western Okanogan County (Cedar Creek, Cub Creek, and Muckamuck), a Marginal Risk was introduced in coordination with WFO OTX. Taylor/Bann Day 2 Valid 12Z Mon Jun 26 2023 - 12Z Tue Jun 27 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS MID-ATLANTIC... ....Eastern U.S.... An anomalously deep upper low for late June is expected to be centered over central Michigan Monday with an associated surface trough extending southward into the Ohio Valley and Appalachians. Ahead of those feature, a relatively moist airmass will be in place characterized by dewpoints in the 60s to near 70F and precipitable water values above 1.5". Enhanced upper diffluence combined with the greater moisture and expected daytime heating instability will likely ignite a large area of convection across portions of the Northeast southward through the Mid-Atlantic and into North Carolina. The most recent forecast guidance has clustered the greatest rainfall amounts (pockets of 1-2", localized 3") and potential intense rain rates (1-2"/hr) from central North Carolina into portions of Pennsylvania and New Jersey. Over the last 7 days, the greatest precipitation anomalies have been across parts of Virginia southward into Georgia though Friday/Saturday there were pockets of heavy rainfall across the Mid-Atlantic. The Slight Risk area (central NC to eastern PA/western NJ) accounts for the best overlap of moisture and instability for the potential of isolated to scattered instances of flash flooding. Within the Slight Risk area, the southern half has certainly been the wettest with 7-day departures above 300 percent of normal and is somewhat more vulnerable to the expected rainfall amounts and intensity. ....Southern Montana into Northern Wyoming... A shortwave trough lifting through the Intermountain West will reach the Rockies late in the period while higher moisture being advected northward seeps into portions of Wyoming and southern Montana. With favorable forcing for ascent and daytime heating instability, a few complexes of thunderstorms are expected to initiate over southern Montana and northern Wyoming before tracking east/southeast late in the period. The most recent guidance shows potential for pockets of 1-1.5" over areas that have been considerably wetter than normal and a Marginal Risk was maintained this cycle for the potential of isolated instances of flash flooding. Taylor Day 3 Valid 12Z Tue Jun 27 2023 - 12Z Wed Jun 28 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHEAST AND NORTHERN HIGH PLAINS... ....Northeast... The closed upper low over the Great Lakes is expected to slow/stall during the period, gradually weakening and opening up through Wednesday morning. The latest model guidance indicates a piece of energy will rotate through the Ohio Valley and central Appalachians during the afternoon/evening hours, providing a focused area of forcing/lift across much of the Mid-Atlantic into the Northeast. A moist airmass is expected to remain place, with higher dewpoints in place and precipitable water values up to 1.5"+. Sufficient instability is likely to develop during peak heating and the 00Z deterministic and ensemble guidance shows potential for localized 1-2" amounts from the Northeast/southern New England southward into the Mid-Atlantic which could come down pretty quickly (rain rates up to 1-2"/hr) over the more urbanized corridor resulting in instances of flash flooding. ....Northern High Plains... Shortwave energy lifting through the Intermountain West will drive large scale forcing for ascent across much of the region and with daytime heating giving way to sufficient instability and enough moisture present across portions of southern Montana into the Plains, a few complexes of thunderstorms are likely to develop initially over portions of Montana/Wyoming then track east/southeast into the Dakotas. Moisture isn't overly impressive but likely enough to generate isolated 1-1.5"/hr rain rates that could lead to a few instances of flash flooding, especially given some of the saturated soils and recent heavy rainfall in the risk area. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4Ewa3rfuwdZ83qEimEDwgM0RcIW0BX_PPnxucQYYWSwq= nGUkiOlOZW4Y4h7DVBFTw6iE-LaRJJJ_OQnZlW2UQ57tMjQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4Ewa3rfuwdZ83qEimEDwgM0RcIW0BX_PPnxucQYYWSwq= nGUkiOlOZW4Y4h7DVBFTw6iE-LaRJJJ_OQnZlW2U9Pa7zQE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4Ewa3rfuwdZ83qEimEDwgM0RcIW0BX_PPnxucQYYWSwq= nGUkiOlOZW4Y4h7DVBFTw6iE-LaRJJJ_OQnZlW2U0nCaoRM$=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) .