Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 23 2023 19:42:40 FOUS30 KWBC 231942 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 341 PM EDT Sat Sep 23 2023 Day 1 Valid 16Z Sat Sep 23 2023 - 12Z Sun Sep 24 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHEAST NORTH CAROLINA, SOUTHEAST VIRGINIA, AND EASTERN OKLAHOMA INTO SOUTHWEST MISSOURI AND WESTERN ARKANSAS... 16z Update: Not much change to the ERO areas associated with Ophelia. The MDT risk area still looks in good shape over northern NC into southeast VA. An additional 2-4" of rain remains possible within that area..with rain rates periodically peaking towards 1" in an hour. This should be enough to continue to pose a more concentrated flash flood risk over this corridor...with locally considerable impacts possible. Over the central U.S. we did make some tweaks to the MDT risk area over OK/MO. There has been an eastward trend in the 12z HREF guidance. Still think the biggest training/backbuilding risk is over eastern OK (so some of the newer guidance may be undergoing this potential to some degree), but there is certainly also an increasing risk a bit further north and east into northwest AR and southwest MO. Areas of 3"+ amounts appear likely within the MDT risk area...with localized totals of 5-6" possible as well. Hourly rainfall rates look pretty impressive with this setup..with 2-4" in an hour appearing probable. Thus a more organized flash flood risk still looks on track...with locally considerable impacts possible. Considered expanding this MDT a bit further north over western MO where guidance is trending up with QPF and FFG exceedance probabilities. However opted against this for now as soil conditions are a bit less sensitive with northward extent, and the better chance of more prolonged training/repeat activity is probably south within the current MDT risk area. So will consider this portion of western MO a higher end Slight for now and continue to monitor trends. Chenard ....Previous Discussion... ....Mid-Atlantic... Tropical Storm Ophelia is close to making landfall somewhere near the Crystal Coast of North Carolina and will continue to track north through eastern North Carolina today. Ophelia will contain an exceptional easterly Integrated Vapor Transport (IVT) ranging between 750-1,000 kg/m/s, which is above the 99th climatological percentile according to NAEFS for 12Z Saturday. As Ophelia weakens throughout the day, so will the IVT, but it will still routinely be above the 90th climatological percentile to go along with a large areal extent of 1.5-1.75" PWs. In fact, 1.5" PWs will likely be observed as far north as southern New England. Between 12Z Saturday and 00Z Sunday, PWs will hover around or slightly above 2" from eastern NC on north along the I-95 corridor into the Richmond metro area. As Ophelia continues undergoing extratropical transition, it will in effect develop an occluded front and a warm sector that will lift north through eastern NC and the VA Tidewater. Most CAMs agree as much as 500-1,000 J/kg of MLCAPE will be available while elevated freezing levels from the NC Piedmont to the DelMarVa Peninsula suggest war cloud layers could be up to 13,000ft deep. This is a recipe for highly efficient rainfall rates within thunderstorm activity. This is especially the case along the northern and western flanks of Ophelia's circulation where the aforementioned strong easterly moisture transport intersects the surface occluded front and ascends rapidly as a warm conveyor belt, leading to vigorous convection to the north and west of Ophelia's core. The 00Z HREF did depict 30-35% probabilities of rainfall totals >5" from south-central VA to central NC, but localized amounts could approach 8". In coordination with the Wakefield, Raleigh, and Morehead City WFOs, have expanded the Moderate Risk into central NC and southern VA. Farther north, hourly rainfall rates will struggle to top 1"/hr, but the relatively deep warm cloud layers will still support efficient rainfall processes. Instability (MLCAPE of 250-500 J/kg) will be highest over from the Eastern Shore and northern DelMarVa to southern NJ Saturday afternoon and into Saturday evening. The northern flanks of the Marginal and Slight Risks were pulled south slightly given the lack of sufficient instability into southern New England through Saturday evening. ....Upper Midwest to the Southern Plains... An approaching 500mb shortwave trough over the Four Corners region is set to eject into the Southern Plains Saturday afternoon. At the surface, a cold front racing south through the Central Plains will encounter another frontal boundary located north of the Red River and stretching south and east through the ArkLaTex. In response to the falling heights from both the upper level disturbance from the Four Corners and a more robust upper trough over the Northern Plains, an 850mb LLJ will introduce added moisture flux into the region. One the primary concerns is 850-300mb mean winds become oriented quasi-parallel to the front over the ArkLaTex, and with a prolonged LLJ present, backbuilding and training storms are becoming increasingly likely. PWs will approach 2" and surpass it as storms develop, plus there is likely to be no shortage of instability thanks to 2,000-3,000 J/kg of MLCAPE. The 00Z HREF probabilities for FFG exceedance have continued to rise, especially across eastern OK. HREF probabilities for 3-hr QPF > 3-hr FFGs are as high as 60-80% between 00-03Z and 03-06Z across eastern OK. Meanwhile, the longer duration into southwest MO and northwest AR is a concern with 6-hr QPF > 6-hr FFG probabilities up to 40-50%. Chose to expand the Moderate Risk a little farther east as a result. The last QPF forecast calls for 3-4" of rainfall, but localized amounts surpassing 6" is well within the realm of possibility as 24-hr QPF > 5" probabilities on the HREF are 40-45% in east central OK. Farther north, an upper trough taking on an increasingly negative tilt will spawn a strengthening low pressure system that looks to track into the Red River of the North by Saturday evening. An excellent fetch of 850mb moisture flux out ahead of the forming colluded front will wrap around the 850mb low and lead to a well developed comma head of storms on the 850mb low's northwest flank. 00Z HREF showed 3-hr QPF > 3-hr FFGs probabilities north of Pierre that were as high as 30-40% between 18-21Z Saturday. For the day as a whole, probabilistic HREF guidance showed up to a 60-80% chance for 24-hr rainfall totals surpassing 3". The 750 kg/m/s IVT over central MN will play a big role in supplying the anomalous moisture aloft and MLCAPE between 500-1,000 J/kg will provide sufficient instability to generate Excessive Rainfall Rates. The inherited Slight Risk remains in good standing, as does the Marginal Risk in southern MT and northern WY near the Big Horns. Adequate upslope flow and a complimentary tandem of available moisture/instability over saturated soils makes this area still prone to flash flooding on Saturday. Mullinax Day 2 Valid 12Z Sun Sep 24 2023 - 12Z Mon Sep 25 2023 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST AND ARKLATEX... 20z Update: ....Northeast... Only minor changes made to the Slight risk area...with it now stretching from far northeast MD, into eastern PA, much of NJ, far southern NY and most of CT. The flood threat does appear to have increased some across this corridor compared to earlier forecasts...but still solidly a Slight risk. As what is left of Ophelia slows down we'll have increasing and persistent convergence focused across this corridor. Instability continues to be a limiting factor, however this setup does have embedded low topped warm rain convective potential. So even some very weak instability could be enough to get hourly rainfall into the 0.5"-0.75" range on a localized basis (with the 12z HREF supporting rates of this magnitude as well)...with event total rain locally pushing into the 2-4" range. Do think this will be enough to produce some localized area of flooding, especially if higher totals end up falling over a more sensitive urban location. The HREF supports the greatest threat from northeast MD into eastern PA and NJ...however it's worth noting that the 12z GEM REG and ECMWF...along with both HAFS hurricane models are a bit quicker with the system...thus spreading the higher rainfall more into CT (and possibly RI) as well. ....Southern Plains to lower MS Valley... Not much change to to the inherited Slight risk here. Still looks like a higher end Slight with the potential for a targeted MDT still on the table with future updates. Some risk should be ongoing at 12z over the ArkLaTex area...with redevelopment of convection later Sunday near the TX/OK border which will then gradually spread east and/or southeast. For the risk to end up more at the MDT level we would either need overlap of convection and saturated soil conditions (which remains unclear) and/or a more pronounced training signal later Sunday. Thus for now sticking with the Slight risk seems to make the most sense, and we can continue to monitor trends. Chenard ...Previous Discussion... ....Northeast... Ophelia's remnant circulation by Sunday morning is forecast to cross the Potomac River and inch its way toward the northern DelMarVa Peninsula by Sunday evening. There remains no shortage of moisture with PWs of 1.75" likely to be measured from Northern VA and Northern MD on east across the DelMarVa and into southern NJ. Ophelia's 850mb low will continue to introduce added 850mb moisture flux directly into the pivoting frontal boundary tracking north through the Delaware bay and into southern NJ. From far southeast PA to southern NJ and along the southern New England coast, the 00Z HREF shows as much as 100-250 J/kg of MUCAPE available. While this amount of instability will limit high end rainfall rates, sampled model soundings from the Philly metro area around midday show warm cloud layers up to 13,000ft deep and highly saturated profiles. This is a recipe for highly efficient rainfall processes that could still result in 1.5"/hr rainfall rates within the strongest of storms. The Slight and Marginal Risk areas on the far northern and western flanks continue to be trimmed off to an extent given the lack of instability in the more interior portions of the Northeast. Urbanized areas that contain a greater concentration of hydrophobic surfaces will be more susceptible to possible flash flooding. ....Southern Plains to the Lower Mississippi Valley... The same setup that resulted in widespread thunderstorms on Saturday will continue to make its way south into central TX and as far east as the Lower MS Valley. Latest guidance is keying in on the ArkLaTex with the best odds of receiving Excessive Rainfall. An 850mb low bordering the Red River will direct a continuous fetch of 850mb moisture flux at a stalled front positioned somewhere near the ArkLaTex, resulting in PWs approaching 2" and as much as 2,000-3,000 J/kg of MLCAPE. Similar to Saturday, the steady stream of low level moisture becomes oriented quasi-parallel to the stationary front, providing a suitable setup for potential backbuilding and training storms. Storms will be more than capable of producing 2-3"/hr rainfall rates, and given the potential training of thunderstorms, localized amounts could reach 4-5". Did increase the footprint of the Slight Risk area from central TX to the northern AR Ozarks. Should confidence grow in a swath of 5+" amounts in new guidance over the next 12-24 hours, there is a chance a Moderate Risk may be needed for parts of the ArkLaTex in subsequent forecast cycles. ....Southwest Oregon... The leading edge of one of the upcoming cold season's first atmospheric rivers will be knocking on the door of the Pacific Northwest coast Sunday night. A narrow swath of heavy rainfall will be directed at the Oregon coast Sunday night thanks to a robust 750 kg/m/s IVT that is 6-8 standard deviations above normal for late September. Rainfall totals could range between 1-2" with locally higher amounts closer to 3", but the bulk of this is falling within a 6 hour span. Given those possible rainfall rates, have introduced a Marginal Risk with any nearby burn scars most at-risk for possible flash flooding. Mullinax Day 3 Valid 12Z Mon Sep 25 2023 - 12Z Tue Sep 26 2023 ....THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE PACIFIC NORTHWEST, SOUTHERN PLAINS, LOWER AND UPPER MISSISSIPPI VALLEY... 20z Update: No big changes to the risk areas with this update...just some tweaking based on the latest guidance trends. Some chance we may need to hang on to a Marginal risk into day 3 over portions of southern New England...but too much model spread at this time, so opted to hold off. Something to keep an eye on...but should be a localized threat at best. Chenard ....Previous Discussion... ....Pacific Northwest... A powerful upper low approaching the Pacific Northwest will direct an atmospheric river at the West Coast on Monday, delivering a soaking rain to residents of northern California on north to western Oregon and Washington. The upper low off the coast is very impressive for late September with heights within the 500-850mb layer are routinely 3-4 standard deviations below normal according to NAEFS. This powerful upper trough will direct a robust 500-750 kg/ms/s IVT (peaking around 8 standard deviations above normal) at northern CA and western OR where PWs will be as high as 2-3 standard deviations above normal. This is effectively the first bonafide atmospheric river of the season for the Pacific Northwest. Fortunately, parts of far northern California and southern Oregon could use the rain as parts of the region are in D0-D2 drought. The primary concerns are residual burn scars where soils will be more susceptible to possible debris flows. The Marginal Risk remains in place as rates do not appear to be too excessive, but detrimental impacts could be felt in burn scars. ....Southern Plains & Lower Mississippi Valley... A frontal boundary stretching from West Texas to the Ozarks is forecast to undergo frontolysis throughout the day while at the same time, southerly 850mb moisture flux is expected to wain through the day. These features will still be present at a time when a weak 500mb disturbance approaches from the north, providing some modest vertical ascent atop the atmosphere. PWs are currently forecast to reach as high as 1.75" (1-2 standard deviations above normal according to NAEFS) and MLCAPE between 1,000-1,500 J/jk from central TX to the Lower Mississippi Valley. The available PWs and instability should support >2"/hr rainfall rates within the more intense cells, and soils could be a little more saturated after recent rainfall on Sunday. The Marginal Risk remains in place with any consideration of a Slight Risk likely to be discussed in future forecast cycles when more CAMs guidance becomes available to key in on the more at-risk areas. ....Upper Mississippi Valley... A meandering closed low over the Upper Midwest provides a suitable environment for divergent flow atop the atmospheric column on Monday. PWs will be as high as 1.25" and MLCAPE is currently shown to range up to 250-500 J/kg. A steady funneling of 850mb moisture flux ahead of a dissipating occluded front should help to supply developing storms with enough moisture to generate storms containing rainfall rates >1.5"/hr. This round of storms is also coming on the heels of a 2-day stretch where as much as 2-3" or rainfall is forecast through Sunday night, so soils will be a little more saturated compared to earlier in the week. With that said, opted to introduce a Marginal Risk for Excessive Rainfall as flash flooding has a chance to occur in poor drainage areas, more urbanized communities, and where soils are most sensitive. Mullinax Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7wA-rgED7AdsZ-snhFYP5Gg5Q9iqI0dMC6VtyXxio0vS= QFP0T2zz1ipBAiLIIDP7rza9HGHz_X1FKL3Iodwbzt7GTBA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7wA-rgED7AdsZ-snhFYP5Gg5Q9iqI0dMC6VtyXxio0vS= QFP0T2zz1ipBAiLIIDP7rza9HGHz_X1FKL3IodwbAAMP8dY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7wA-rgED7AdsZ-snhFYP5Gg5Q9iqI0dMC6VtyXxio0vS= QFP0T2zz1ipBAiLIIDP7rza9HGHz_X1FKL3Iodwbjsm8FS4$=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) .