Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 09 2024 19:56:21 FOUS30 KWBC 091956 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 356 PM EDT Fri Aug 9 2024 Day 1 Valid 16Z Fri Aug 09 2024 - 12Z Sat Aug 10 2024 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE MID ATLANTIC AND NORTHEAST... ....16Z Update... Debby's remnants continue to press rapidly to the north as the remaining core analyzed at 15z is rushing northward at just over 30 kts (35 mph). Bands of heavier rainfall continue to push north out of the Central Mid Atlantic with the heaviest rain focused near the remnant center of now Post-Tropical Cyclone Debby. The current forecast remains on track with little variance in the QPF and associated probabilities as rainfall rates of 1-2"/hr will be seen within the heaviest cores as the storm drives northward out of Northern New England by late this evening. The previous MDT risk was trimmed on the southern edge of the previous forecast to reflect the radar trends of the heaviest precip shifting north with more of a focus across Northern PA through Upstate NY with the max for the period likely situated across the North Country of NY state. A trailing cold front will allow for pockets of heavy rain to develop this afternoon across portions of the Central Mid Atlantic, but will quickly vacate eastward as low-level convergence along the front will sweep east with a degrading threat of heavy precip in wake of the boundary. Some of the SLGT risk across the interior Central Mid Atlantic was also adjusted given the environment improving with much of the heavier precip likely falling along and east of the Blue Ridge. The Eastern Carolinas exhibited very little in the way of variance from the previous forecast issuance with the afternoon and evening time frame remaining the period of interest with the convective development anticipated. Hi-res was in strong agreement on the placement of a line of convection extending from coastal SC up through Eastern NC with much of the heavier cores likely to fall east of the I-95 corridor. Embedded heavy thunderstorms within the main convective axis will likely produce a few cells that can reach upwards of 5" with a modest signature on the 12z HREF neighborhood probability for the threshold settling between 30-50% over a large area encompassing most of Eastern NC. HREF EAS signals for at least 2" is relatively high as well (20-30%) over the same areas, so the threat is likely on the higher-end of the SLGT risk threshold, especially given the antecedent saturated grounds after a multi-day onslaught from Debby. This led to an easy decision to keep continuity with locally significant wording reserved for any urbanized areas within the above region. Finally, out west the pattern remains on track with scattered thunderstorm development during peak diurnal max with a complex initiating over the High Plains of Southeast CO and Northeast NM, shifting east with the mean flow focused along the northern periphery of the mid-level ridge. Some pretty good signals for at least 2" given a solid EAS probability between 30-50% over a small zone between the NM/OK/TX borders. Neighborhood probability was bullish for the >3" signal running between 60-70% over the aforementioned area. There's little signal for >5", however so the threat is likely capped between 3-5" at peak QPF. That's still fairly prevalent to a localized flash flood concern within that portion of the Southern High Plains, so the risk is likely within the higher-end of the SLGT comparatively to what will occur over NM. Much of the impact will be focused in those burn scar areas where heightened sensitivity drives much of the risk. This was sufficient to maintain what was forecast previously with only some minor adjustments based on the latest HREF mean QPF footprint. Kleebauer ....Previous Forecast... ....Mid-Atlantic to Northeast... Debby and it's remnant plume of tropical moisture are forecast to lift northward today from the central Appalachians to the Interior Northeast, while producing a swath of heavy rainfall and the potential for numerous flash floods from near the West Virginia Panhandle early this morning to Upstate New York this afternoon. Precipitable water values of 2-2.5" stream along and just east of the system will equate to above the 95th climatological percentile and +3.0 standard deviations above the climatological mean. Additionally, ample shear surrounding the low center and MUCAPE values up to 1000 J/kg nearby along the Atlantic Seaboard being advected northward will allow for efficient tropical downpours within scattered to numerous showers and thunderstorms. A focus in the heaviest rainfall is likely on the northern and northeastern quadrant of the low pressure system as it transitions to an extratropical phase, becomes frontal, and begins to increase forward speed to the northeast by Friday night. A warm front lifting northward will add increasing frontogenetic forcing to the environment and lead to a more widespread precipitation shield today across parts of PA and Upstate NY, with embedded rainfall rates likely up to 2-3"/hr at times. The trend for this axis of heavy rainfall has been for it to occur slightly westward than previously forecast, now across far northwest NY. Additionally, the trend is for faster northward progression with this system. 12z NAEFS IVT valid 18z Fri is forecast to max all values in the 1979-2009 climo period, amplifying the moisture surge fueling today's heavy rainfall event. 00z HREF neighborhood probs for at least 3" in 6-hrs are high (60-80%) from central PA to the Finger Lakes region and northwest portions of NY State, which would also exceed local FFG for much of this region. As a result, the Moderate Risk was maintained from the previous forecast with only minor cosmetic changes. Numerous instances of flash flooding are expected in this area, with locally significant flash flooding possible. Elsewhere in the Northeast, convection developing along and ahead of a cold front should have enough forward motion to lead to only scattered to localized flash flooding concerns closer to the I-95 corridor, but with rainfall rates of 2"/hr possible. Areas that received recent rainfall and poor drainage locations could have a relatively greater flash flood risk, which supports the SLGT Risk extending to parts of New England and the Mid-Atlantic. ....Eastern Carolinas... A separate area along the East Coast at risk for flash flooding today will be associated with a convergence axis along coastal/eastern sections of North and South Carolina this afternoon and evening. PWs elevated and above 2.25", along with ample instability and mean winds parallel to the boundary/coastline will support slow-moving and/or training thunderstorms across saturated terrain. CAMs and the associated 00z HREF suite depict the potential for 5"+ amounts throughout eastern North Carolina and coastal South Carolina, but the expectation remains that these totals would be pretty localized. The SLGT Risk was expanded from the previous forecast across South Carolina given recent CAM output and very saturated antecedent conditions in place. Will monitor this region for upgrades in the future outlooks. ....Four Corners, Central Rockies, Central High Plains, and Southern Plains... Showers and thunderstorms are expected to fire up across the Southwest and Rockies as shortwave energy passes through and encounters the ongoing monsoonal moisture in place. There will be increased focus for more organization and higher amounts concentrating near the higher terrain of southern Utah, south/central Colorado, and northern New Mexico, where a SLGT Risk area remains in effect. Accumulations of 1-2" will be possible in the strongest cells, a stronger correlation to flash flooding concerns compared to historical precedence. The rest of the Southwest will still see periods of storms capable of localized flash flood concerns with the best threat located more into the Intermountain West and the Mogollon Rim in Arizona. Convection (possibly organized) will also spread eastward into western Kansas and the panhandles of Oklahoma and Texas, as well as northeast NM. Here, 00z HREF neighborhood probs are modest (20-40%) for at least 3" in 6-hrs. A broad MRGL Risk covers much of the High Plains. ....Central Texas... A MRGL Risk was introduced across parts of central Texas as convection develops along the tail-end of a lingering cold front and ahead of a stronger frontal boundary dropping south from the central Plains. Shear will be weak, but steep low-level lapse rates and sufficient moisture (PWs around 2") will allow for intense rainfall rates over relatively wet ground conditions and a historically flash flood prone region. 00z HREF neighborhood probs have increased and now show elevated chances (30-50%) for rainfall amounts exceeding 3" this evening. Snell Day 2 Valid 12Z Sat Aug 10 2024 - 12Z Sun Aug 11 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST, CENTRAL ROCKIES INTO THE PLAINS, AND THE COASTAL CAROLINAS... ....20Z Update... The previous forecast MRGL risks are still within reason with only minor changes to each respective risk for the period. The heavy rain signal across the Eastern Carolinas is more diffuse compared to previous days with a magnitude lower in the probability fields for heavy precip values, mainly anything above 3". There is still a formidable signal for isolated pockets of 2-4" in some storms as they move slowly within the mean flow. HREF neighborhood probabilities for 3" remain elevated 50-80% but have a significantly lower output for anything above 5". Best chance for flooding remains within those urbanized areas confined to the coastal plain, but with elevated small stream responses lingering from the past few days of heavy rainfall, some areas usually not as susceptible may be under a better opportunity for flash flooding if a heavy thunderstorms settles overhead.=20 Across the Plains and Western U.S, the signal for widespread heavy rainfall remains underwhelming, however another complex ejecting into the Southern Plains could offer a chance at a low-end SLGT upgrade if the mean QPF signal improves. The current indication is some scattered heavier cores within a progressive complex leading to modest probabilistic output within the 12z HREF. The area has been parched for precipitation the past several weeks, so the initial surge of rainfall will be a welcomed site with the FFG indices running very high for all 1/3/6 hour markers. The best threat for excessive rainfall will lie within more urban settings with some CAMs indicating a better chance within the OKC metro as well as a secondary batch just north of the DFW metro. Both areas lie within the MRGL risk threshold, but are not enough to warrant an upgrade given the antecedent conditions. Thus, maintained general continuity.=20 Kleebauer ....Previous Discussion... ....Southwest, Central Rockies, and Plains... A rather broad MRGL Risk was maintained extending from the Southwest to the central/southern Plains as diurnal heating and monsoonal moisture will keep showers and thunderstorms developing during the afternoon and evening hours for much of the Four Corners region. Meanwhile, an area of more organized convection triggered by shortwave energy from the D1 period will be making its way eastward into the foothills and spreading out into the high Central/Southern Plains. There is a fair amount of uncertainty with how far east into the Plains the convection reaches, location of heaviest amounts and the overall accumulations. Most of the consensus is suggesting 1-3 inches however a couple models have maxes near/over 4 inches. A Marginal Risk covers much of the Four Corners, eastern Wyoming, western Nebraska and Kansas, Oklahoma, northern Texas, southwest Missouri and northwest Arkansas. If amounts trend up then a Slight Risk area may be need with further updates. ....Coastal Carolinas... A lingering frontal boundary and diurnal sea breeze convection will continue chances for scattered thunderstorms across the Southeast and more specifically the eastern Carolinas on Saturday. Soils should remain saturated after prior heavy rainfall, increasing the flash flood threat across an area that isn't usually as prone to flash flooding. CAMs depict localized totals from pulse thunderstorms could reach 1-3", which is supported by PWs over 2". Snell Day 3 Valid 12Z Sun Aug 11 2024 - 12Z Mon Aug 12 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST, FOUR CORNERS, CENTRAL ROCKIES INTO THE PLAINS, AND THE EASTERN CAROLINAS... ....20Z Update... The best signal for heavy rainfall in the period continues to point towards the Southern Plains to Ozarks region with a fairly robust MCS potentially developing beginning early Sunday evening and moving eastward into the Ozarks area and Northeast OK by the back half of the forecast period. The potential for 2-4" of rainfall has been indicated via some deterministic, but the area has been void of precipitation for a while meaning some of the antecedent conditions favor some priming before a more significant flash flood threat can ensue. The ensemble bias corrected QPF is fairly aggressive and there's some consensus growing with the specific location, however we have seen these setups shift inside 48 hours in the past as a lot is contingent on the evolution of the previous days pattern. As of now, have maintained a MRGL risk, but the addition of a SLGT is high somewhere in-of the Southern Plains to the Mid-Mississippi Valley, pending the eventual track of the future complex.=20 Elsewhere, the Eastern Carolinas may see yet another round of enhanced convection, only this time the signal is for lower coverage and decreasing magnitude in the potential. The latest ensemble QPF output was lower within the southern half of Coastal SC leading to some trimming of the southern edge of the previous MRGL risk forecast. The remainder of the risk area was maintained with the best signal located across Eastern NC. Expect totals closer to 1-2" with upwards of 3" in the stronger cores this time around, a much lower threat compared to the previous several days.=20 Kleebauer ....Previous Discussion... ....Four Corners, Southwest, and Central Rockies... A deepening western U.S. trough by D3 will allow for another round of diurnal monsoonal convection across the Southwest and Four Corners region, with GEFS and ECENS depicting PWs above the 75th climatological percentile. Opted to maintain the MRGL Risk for now given the limited QPF signal in available guidance and the possibility for fast storm motions. ....Plains and Mid-Mississippi Valley... Northwest flow aloft and a gradually lifting warm front over the Southern Plains will support MCS potential into the overnight period on D3 along with increasing mid-level flow, which could produce training and intense rainfall rates within convection. Relatively high uncertainty remains with the exact placement of heaviest rainfall (greatest potential currently near the western Ozarks into northeast OK and southeast KS), which prompted the MRGL Risk as opposed to a SLGT for now. Additional convection is possible over SD and NE, with similar uncertainty in placement. ....Eastern Carolinas... Another day of diurnal convection along a nearby frontal boundary could be enhanced by an approaching shortwave diving over the Ohio Valley around a deep upper-level low over SE Canada. Similar to D2, rainfall amounts of 1-3" appear possible and are likely to occur over saturated soils. Snell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7Izotpj3J-VqHjEqikujVkfkbvUlSzZwgTU8rfvb68oF= -VbFr2WXwLUbYd_sW5OBa6Fu-2liONpUnTsRVWTtvAe3zVA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7Izotpj3J-VqHjEqikujVkfkbvUlSzZwgTU8rfvb68oF= -VbFr2WXwLUbYd_sW5OBa6Fu-2liONpUnTsRVWTtYhqlXqE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7Izotpj3J-VqHjEqikujVkfkbvUlSzZwgTU8rfvb68oF= -VbFr2WXwLUbYd_sW5OBa6Fu-2liONpUnTsRVWTtHRktZ3c$=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) .