Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jul 09 2024 19:58:23 FOUS30 KWBC 091958 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 358 PM EDT Tue Jul 9 2024 Day 1 Valid 16Z Tue Jul 09 2024 - 12Z Wed Jul 10 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDWEST, MID MISSISSIPPI AND OHIO VALLEYS, AS WELL AS FOR THE SOUTHEASTERN LOUISIANA... ....16z Update Summary... Two primary updates to the previous D1 to reflect changes in guidance and to update on the recent radar reflection going forward. The SLGT risk across the center of the country into MI was expanded to the east over southern IN and further northeast across much of southern MI. The second update was across the Central Gulf Coast where the previous SLGT risk was scaled back in size to mainly encompass the New Orleans metro and surrounding locales given the recent radar trends and CAMs output through the period. More details were added to the respective sub-headings below.... Elsewhere over the CONUS, the other notable change exists across FL where the MRGL was cut over much of the western coast and over far southern FL due to the setup for flash flood potential being primarily confined to the FL Panhandle, FL Big Bend, and the eastern coast given the flow pattern conducive for localized enhancement from the Atlantic sea breeze. Spotty max totals over 3" are forecast within CAMs, but the placement is very variable, so wanted to maintain some coverage to account for the threat, but local uncertainty. The setup across New England, Southern Appalachians, and portions of the Southwestern US are all fairly similar with regards to the threat being localized for flash flooding within the respective terrain. The best threat remains over the burn scars in NM where any convective impact over the affected areas will be susceptible to flashy returns. The other areas within New England and the Southern Appalachians lie within the lower-end of MRGL, but non-zero considering the environment and terrain focus. Kleebauer ....Mississippi Valley and Midwest... ...16Z Update.. Increasing signals for heavy rainfall from the remnants of Beryl will impact some of the urbanized areas extending through the Detroit Metro with a greater convective concern located over southern IN. Southern IN will be within the best instability axis in the Ohio Valley with several small mid-level perturbations advecting overhead this morning through the evening around the broad periphery of Beryl's circulation. Recent HREF mean QPF across the area has really ramped up with storms already impacting the southern fringe of the state with locally heavy rain as rates top 1.5-2"/hr in the recent radar QPE output. A second round is expected this evening in wake of better instability leading to a multi-wave threat of convection within portions of the area. Considering the priming this morning and eventual impact later, the threat for flash flooding has increased enough to warrant the SLGT expansion. Across southern MI, the forward speed of Beryl's remnants will allow for a solid 925-700mb moisture flux through the region with sights on the Detroit metro by the end of the period. HREF neighborhood probabilities for at least 3" have shot up to 40-60% across portions of Southeast MI, including Detroit with signals over Southwest MI growing to 80-90% for at least 3" and 25-40% for potential of 5". This was a significant enough signal to warrant the SLGT expansion across Southern MI through the Detroit Metro with agreement from the local Detroit WFO. Kleebauer ...Previous Discussion.. Tropical moisture associated with Beryl will spread a swath of heavy rain across the Mississippi Valley and into the Midwest as a cold front advances east through the region. The last few runs of guidance have trended toward a more narrow axis setting up right along the front and QPF values increasing further northeast across Illinois/Indiana and southern Michigan. A sizable swath of 3+ inches continues to span from the Missouri Ozarks to northwest Indiana with local maximums upwards of 8 inches. A Slight Risk extends from central Arkansas to southern Michigan. A Marginal Risk stretches from the central Arkansas northward to southern Michigan and central Ohio. Campbell ....Central Gulf Coast and Southeast... ...16Z Update.. Recent trends in the enhanced QPF focus across the Central Gulf coast have led to the New Orleans metro and surrounding locales over Southeast LA as the focal point for heavy rainfall today into the evening. Considering very high FFGs located over the coastal portions of LA/MS/AL, the threat will be mainly an urban flood threat with the New Orleans metro easily being the target for the current setup. HREF probabilities for at least 2"/3-hrs is highest across Southeast LA with values mainly between 25-40% over a span of several hrs beginning now through 06z with the spatial coverage centered right over NoLA and surrounding Parishes. This allowed the previous SLGT to shrink in size to mainly encompass the New Orleans metro and Parishes just off to the south and southwest. Kleebauer ...Previous Discussion.. Deep, onshore flow will continue to drive convection to form along the Gulf Coast, with the potential for heavy down pours. Areal average rainfall is expected to be in the 1 to 2 inch range however the CAM guidance suggests very localized maximums of 3 to 5 inches possible. A Slight Risk is in effect for portions of the Gulf Coast from eastern Louisiana to the Florida Panhandle. Campbell ....Previous Discussions... ....Southwest... Isolated to widely scattered convection is likely to form along the mountains once during the afternoon and evening, which could very well move over flood-sensitive areas. A Marginal Risk remains in effect for portions of Arizona, New Mexico, southern Colorado and extreme western Texas. ....Rio Grande of Texas... Convection that forms along the mountains of Mexico will have enough of an eastward push that some may cross into the Del Rio and Eagle Pass areas. Isolated instances of flash flooding may develop, therefore a Marginal Risk remains in effect. ....Southern Appalachians... Storms are likely to form in the deep tropical moisture indirectly associated with Beryl with the mountains acting as the primary forcing. Numerous flood sensitive areas in western NC may support flash flooding with any slow moving or stationary storms. A Marginal Risk area remains in effect. ....Northeast... Deep tropical moisture pumping north well ahead of Beryl will make for anomalous PWATs across New England. The Appalachians of this region may also act as forcing for widely scattered thunderstorms capable of heavy rainfall. A Marginal Risk area is in effect for across this region. Campbell Day 2 Valid 12Z Wed Jul 10 2024 - 12Z Thu Jul 11 2024 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF UPSTATE NEW YORK THROUGH MUCH OF VERMONT AND NORTHERN NEW HAMPSHIRE... ....20Z Update... A Moderate Risk was introduced across portions of Upstate New York through much of Vermont and Northern New Hampshire. More on this setup within the the "Northeast" sub-heading below... Elsewhere, changes were minimal within the SLGT risk over south- central NM as locally heavy rainfall over the Sacramento Mountains is increasingly likely during tomorrow afternoon. This will interact with a highly sensitive area due to multiple burn scars within the terrain. A higher probability for impact reflects the necessity for a maintenance of the SLGT risk in place. The stalled front over the Central Gulf Coast will lead to a focal point for training convection as thunderstorms continue to fire within proximity of the boundary. Very high FFGs within the encompassing MRGL risk will limit flash flooding to very isolated and mainly within larger urban zones like New Orleans and towns based in the southeastern Parishes. Guidance remained consistent on the potential within that area of the CONUS, so maintained continuity for the time being, but could see this downgraded if the threat shifts further south, off the coast. ....Northeast... Remnant moisture and mid-level energy from Beryl will translate=20 northeastward out the Ohio Valley and Great Lakes creating a=20 heightened threat for widespread heavy rain and flash flooding=20 across Southeast MI through NY state and Central/Northern New=20 England. At the surface, a warm front will bisect much of NY=20 through Central New England, marked very well by a sharp theta-E=20 gradient within the latest deterministic suite. This will be a=20 focal point for convection prior to the main circulation moving=20 overhead, as well as the axis of where the highest PWATs will be=20 confined during the unfolding of the synoptic scale event. Low-=20 level instability will be highest within the confines of the warm=20 front and points south with the best upper level forcing likely=20 along and north of the boundary creating a dynamic scenario with a=20 zone of highest heavy rain potential where all three=20 characteristics overlap. The signal is becoming more pronounced=20 after the latest 12z suite with the highest heavy rain prospects=20 lying within the Adirondacks and points east into north-central VT=20 and NH, including the Green and White Mountains in the respective=20 states.=20 12z HREF EAS probabilities were much more aggressive in the signals for at least 2" and 3" across the aforementioned areas with a 50-80% probability for at least 2" within the Adirondacks to just south of the Champlain Valley, a strong signal for higher totals given the necessary overlap of CAMs to exhibit such a larger probability. Historically, when an EAS signal is above 70%, the expectation for widespread coverage of that value of rain or more is very certain and regardless a higher risk consideration if the areal FFG indices allow. HREF EAS for 3" was not as strong, but still manages a large coverage of 20-35% with the highest potential across the Adirondacks and northern Mohawk Valley. This is right within the inflection of where the warm front is forecast to reside, creating a zone of higher confidence for heavy rainfall. The main prospects for flash flooding will occur between 18z Wednesday until about 06z across NY state and till the end of the period for points further north and east.=20 Heavy rain will be possible all the way into ME where the elevated PWAT anomalies between +2 to +3 standard deviations will be recognized leading to a higher end SLGT risk residing from Southeast MI all the way into western and central ME. The area of highest potential is not characterized by a Moderate Risk upgrade located across much of Upstate NY and Vermont, as well as Northern New Hampshire with emphasis on the White Mountains where complex terrain and topographic ascent will create a potential QPF maxima away from the primary zone. Totals of 2-4" are anticipated with local maxima to 6" plausible within the above areas in the MDT risk. 1-3" will be possible as far south as central and northeastern PA up through the NY Capital District due to convection developing along and ahead of the trailing cold front moving through the area tomorrow afternoon and evening.=20 Kleebauer ....Previous Discussion... ....Central Rockies and portions of the Southwest... Continuation of the monsoonal moisture and diurnal heating will keep convection possible across portions of the Central and Southern Rockies and southwest New Mexico during this period. Higher QPF is expected to concentrate in the vicinity of the Sacramento Mountains where there have been a few recent wildfires. The burn scar complex (Blue-2, South Fork, Salt and McBride) is=20 very sensitive to anything more than a gentle, light rain. QPF is=20 forecast to range from 0.25 to 1 inch across portions of central=20 New Mexico which elevates the threat for flash flooding and debris=20 flows. A Slight Risk was raised for this part of the state given=20 the aforementioned sensitivity. ....Central Gulf Coast... A stalled frontal boundary along the Gulf Coast may continue to provide focus for convection capable or producing heavy rainfall. A Marginal Risk area remains in effect. Campbell Day 3 Valid 12Z Thu Jul 11 2024 - 12Z Fri Jul 12 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST THROUGH THE MID ATLANTIC, AS WELL AS THE SOUTHWEST U.S... ....20Z Update... The SLGT risk from previous forecast was downgraded due to significant changes from previous deterministic output as moisture and ascent from the remnants of Beryl will be exiting quickly out of New England through Thursday morning. More of the change in the "East Coast" sub-heading below... Elsewhere, monsoonal activity will be focused across portions of AZ into NM where a continuation of flash flood prospects within area burn scars and smaller urban corridors will maintain some chance for flooding over the course of the period. Local totals of up to 1" signal a lower-end threat, however it remains within the MRGL risk threshold leading to little change in the previous forecast. ....East Coast... Remnant moisture and ascent from Beryl will continue to push out of New England with the main threat confined to the morning hours of Thursday across Northern New England. Additional totals of up to 1-1.5" over the northern tier will lead to some local flash flood potential before the event subsides, so wanted to cover bases for the threat. The SLGT risk from prior was not conducive given the current QPF forecast and trends within the ensemble fields to limit the threat closer to a MRGL risk. Further south, a cold front trailing the primary low across Quebec will be the focus for convective potential with the highest risk=20 of flash flooding aligning within the Delmarva coastal plain down=20 through the eastern Carolinas. An approaching upper level disturbance of the Atlantic will round the western fringe of the=20 ridge out in the western Atlantic leading to an advection of enhanced upper forcing and moisture as it moves closer to the Carolina coast. Surface convergence pattern along and ahead of the slow- moving frontal boundary will create a period of enhanced=20 rainfall after the diurnal destabilization window reaches peak and=20 ignites a widespread area of thunderstorms within an above average=20 PWAT environment (+2 standard deviations). The slow forward=20 propagation of cells will create instances of training storms=20 capable of totals reaching 2-4" within the area mentioned above in=20 the Mid Atlantic. There is an opportunity for an upgrade across the VA Tidewater down through eastern NC if the signal remains=20 consistent or enhances further as the combo of ascent from the front and the upper level disturbance will create a formidable window of opportunity within that region.=20 Kleebauer ....Previous Discussion... ....Southwest... The monsoonal pattern of diurnal convection will persist thus maintaining an elevated threat for localized flash flooding. A Marginal Risk remains in effect for portions of eastern Arizona and much of New Mexico. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6IJy9cM69cnt_hWwS2j0Ky6hofFNzqwwtibEpTwdsTPG= O-UGWtgeurpnd1LpHMMWkSqX9J5yIe-uET0Ml5GRoo55oWw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6IJy9cM69cnt_hWwS2j0Ky6hofFNzqwwtibEpTwdsTPG= O-UGWtgeurpnd1LpHMMWkSqX9J5yIe-uET0Ml5GRh3xx1EI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6IJy9cM69cnt_hWwS2j0Ky6hofFNzqwwtibEpTwdsTPG= O-UGWtgeurpnd1LpHMMWkSqX9J5yIe-uET0Ml5GR9XeIGts$=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) .