Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 22 2023 20:30:19 FOUS30 KWBC 222030 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 429 PM EDT Fri Sep 22 2023 Day 1 Valid 16Z Fri Sep 22 2023 - 12Z Sat Sep 23 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF EASTERN NORTH CAROLINA... 16z Update: With Potential Tropical Cyclone Sixteen, only minor adjustments were made. The 12z 3km NAM and ARW2 members of the HREF appear to be western outliers with the track of the system...with the 12z HRRR/FV3/Gem Reg a much closer match to the NHC forecast and the generally consistent global models. Thus ERO tweaks were made in the direction of these solutions...which resulted in a minor contraction of the Slight risk on the western and northern extent, and a minor westward expansion of the MDT risk over coastal NC. Guidance indicates the potential for a narrow swath of 4-8" of rainfall near the track of the system, as well as just to it's west (although a tight western gradient). This could be very close to the previous western edge of the MDT risk, so nudged that a bit further west to account for this risk. The ERO areas over the central U.S. looked in pretty good shape...however did go ahead and introduce a Marginal risk over portions of northeast IA, southeast MN, southwest WI and northern IL. Slow moving and periodically repeating convection should be a threat over this corridor into tonight. The 12z HREF probabilities have increased across this corridor...with 3" neighborhood probabilities now exceeding 60% in spots. A localized flash flood risk is possible this afternoon into tonight. Chenard ....Previous Discussion... ....Eastern North Carolina & Mid-Atlantic... Potential Tropical Cyclone (PTC) Sixteen is forecast to strengthen as it makes its way over the Gulf Stream and approaches the North Carolina coast Friday evening. The storm will not only have the Gulf Stream at its disposal for strengthening, but it will reside just north of a negatively tilted 250mb trough axis, thus fostering excellent upper level divergence aloft. Rain bands associated with PTC Sixteen will be making their way ashore Friday afternoon as 850mb theta-e advection increases. Moisture will be funneled into the Mid-Atlantic Friday afternoon and into Friday night thanks to a robust 750-1000 kg/m/s integrated vapor transport (IVT) that is surpassing the 97.5 climatological percentile from the Mid-Atlantic coast to as far inland as the VA/NC Piedmont. PWs are set to be highest east of I-95 in NC and southeast VA where 1.5-1.75" PWs will be most common, while the NC Outer Banks and Virginia Beach have the best odds of observing 2" PWs late Friday. As the storm makes its way ashore Friday evening, the axis of the heaviest rainfall will likely reside on the northern periphery of its circulation. 00Z HREF suggests 1,000 J/kg of MUCAPE will be located to along and east of PTC Sixteen's circulation and provide thunderstorms with a suitable thermodynamic profile aloft to generate Excessive Rainfall rates across eastern NC. Lastly, warm cloud layers from eastern NC on north to southeast VA are likely to be at least 12,000ft deep Friday night and in some cases could be as deep as 14,000ft. The 00Z HREF 24-hr probabilistic footprint is honing in on the lower DelMarVa Peninsula, southeast VA, and eastern NC that feature at least 60% odds of receiving >3" of rainfall through early Saturday morning. Of greater concern, there is a 70-90% area in eastern NC with rainfall totals >5". In collaboration with the Morehead City WFO, went ahead and upgraded to a Moderate Risk given their ideal placement along and just east of PTC Sixteen's path. Locally significant flooding is possible from North Topsail Beach on east to the Outer Banks Friday afternoon and Friday night. ....Northern Rockies across the Northern Plains... A closed low over the northern Rockies will slowly track east into the northern High Plains by Friday. Guidance remains in good agreement on the heaviest rainfall setting up to the north of the 500mb low, where PWs are >99th climatological percentile throughout central MT. These highly anomalous moisture values are a result of IVT levels over the Dakotas that are above the 90th climatological percentile. Easterly flow within the 850-700mb layer are also below the 1st climatological percentile. These percentiles being referenced are to show just how anomalous the setup is with exceptional moisture and the transport delivering it into the region. The frontal forecast shows a stationary front is likely to position itself over South Dakota where southerly 850mb moisture flux will intersect the boundary and aid in the formation of heavy showers and thunderstorms. The 00Z HREF shows central SD is likely to see the best available instability that combined with PWs up to 1.25" could result in 1.5-2"/hr rainfall rates. Meanwhile, MUCAPE of 500-1,000 J/kg and >1" PWs over central MT would support hourly rates as high as 1.5", which happens to be at or above 1-hr FFGs. For these reasons, the Slight Risk remains in place here while the Marginal Risk as far south and west as northern UT remains in place given the slower storm motions and ample atmospheric moisture present. ....Central Plains to the ArkLaTex... Not much change to the Marginal Risk as a stationary front will be the catalyst for scattered thunderstorms today. Prolonged southwesterly 850mb moisture flux will intersect the stationary front oriented north-south and act as a steady source of low-level inflow through the morning hours. There will also be 500-1,000 J/kg of MUCAPE at these storms disposal. Rainfall rates could reach as high as 1.5"/hr, and with the 00Z HREF hinting at multiple hours worth of Excessive Rainfall possible, 3-hr rainfall totals could surpass 3-hr FFGs that are as low as 2-2.5" in some locations. Storms should dissipate by Friday afternoon, although the front and resurgent LLJ Friday night could trigger additional storms over eastern OK and western AR late Friday night. Mullinax Day 2 Valid 12Z Sat Sep 23 2023 - 12Z Sun Sep 24 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEAST VIRGINIA AND SOUTHEAST KANSAS INTO EASTERN OKLAHOMA... 20z Update: ....Mid-Atlantic... Main adjustment was to add a targeted MDT risk over portions of southeast VA in coordination with WFO Wakefield. There is some spread in the guidance, both in regards to track and QPF with Ophelia. However we followed a solution that would match closest to the official NHC track...which has support from the more consistent global models. The 12z ARW2 and 3km NAM were outliers, and appear to be unlikely solutions. The 12z HRRR, FV3LAM and Gem Reg seemed to offer the most realistic solutions given the expected track/strength of the system. These models were also generally in line with the HAFSA and HAFSB hurricane models. These solutions favor portions of southeast VA for heaviest rainfall during this 12z Sat-12z Sun period. Current estimates are for storm totals of 3-6" across this area...with hourly rainfall rates peaking around 1". The expected QPF generally falls below 1,3,6 hr FFG...however there is an expectation that these FFG values will drop as heavy rain gradually saturates soil conditions. For most of the region this appears like a solid to high end Slight risk...however these totals and rates could cause some more considerable flood impacts over urbanized areas. Thus after coordination, we will go ahead and introduce a targeted MDT risk for southeast VA, which includes some more sensitive urban locations. Elsewhere the Slight risk looks in good shape from eastern NC across the Mid-Atlantic and into far southern New England, where 1 to 4 inches of rain is expected. ....Plains... The main adjustment here was to introduce a MDT risk across portions of far southeast KS into eastern OK. From later Saturday afternoon into the overnight hours the synoptic/mesoscale pattern appears favorable for potential training/repeat convective activity. A slowing front, broad upper level divergence, and decent southwesterly low level moisture transport into the area should result in multiple convective rounds. PWs increasing towards 2", CAPE of 2000-3000 J/KG and the potential repeat activity all support a flash flood risk. The HREF neighborhood probabilities of 3"+ are now over 60% across this corridor, with notable 2"/hr probabilities as well. Recent heavy rainfall over this area has also lowered FFG, making exceedance of these values increasingly likely. Thus do think the threat of flash flooding has increased enough to introduce a MDT risk with this update. The Slight risk over ND/SD/MN was adjusted some to match guidance trends, but overall the expectations here remain on track...isolated to scattered 3"+ amounts remain probable. Chenard ....Previous Discussion... ....Mid-Atlantic into Coastal New England... PTC Sixteen is forecast to track through eastern NC on Saturday delivering copious amounts to areas that stretch from the central Appalachians to the southern New England coast. The Excessive Rainfall threat is driven by an impressive easterly IVT that NAEFS shows values that surpass that 97.5 climatological percentile. PWs in southeast VA will likely top 2" to start the day, but values above 1.75" will be common from the central NC Piedmont north and east to Long Island. The highest concentration of CAPE will reside on the eastern flank of PTC Sixteen's track, where freezing layers will also be higher and result in warm cloud layers up to 13,000ft deep. When factoring the core of where the tropical moisture, instability, and forcing is most likely to reside, this would include central VA on east across southern MD, the DelMarVa Peninsula, and into southern NJ. There is the potential for a Moderate Risk being necessary somewhere within this region, but given lingering uncertainty in where the best confluence at 850mb sets up and how much instability will be available, have chosen to hold off on a Moderate Risk upgrade this forecast cycle. North and west of I-95 from the DC/Baltimore metro on north and east to southern New England, instability will be harder to come by and rainfall rates may struggle to surpass 1"/hr. Still, warm cloud layers up to 12,000ft deep in some cases will be plenty deep enough to support efficient rainfall processes, and localized flash flooding cannot be ruled out Saturday and into Saturday night. The Slight Risk encompasses southern coastal New England where some meager instability values may be present, while the Marginal Risk extends as far north as the Poconos, Catskills, and central Massachusetts. ....Northern Plains & Lower Missouri River Valley to eastern Oklahoma... A closed low traversing the Northern Plains will provide healthy upper level divergence over the Upper Midwest while, farther south, brisk 850mb moisture flux will be directed at a stalled frontal boundary over the Lower Missouri River Valley. Starting up north, a 50-60 knot 850mb jet will supply plenty of anomalous moisture into the Northern Plains. PWs will surpass 1.5" as far north as the Red River of the North, and as moisture wraps around the closed 500mb low, can result in PWs topping the 97.5 climatological percentile according to NAEFS. Farther south, while there is not as much synoptic-scale forced ascent aloft, there is an even greater reservoir of CAPE (up to 3,000 J/kg available) and PWs (reaching 2" in parts of central OK and western AR). Some portions of northeast OK, southwest MO, and northwest AR have been wetter than normal in recent days. This is depicted on AHPS where as much as 300-400% of normal rainfall has occurred in the past 7-days. There is the concern for repeating rounds of storms, especially in the Northern Plains as mean steering flow winds parallel the orientation of the occluded front Saturday evening. There is also the concern for the comma head on the western flank of the 500mb low to produce a slow moving, pivoting axis of storms over the central ND where 3-hr FFGs are <1.5" in some areas. Given these reasons, the Slight Risk remains in place from the Dakotas on south to eastern Oklahoma. Given the lingering upslope flow into the Bighorns along the MT/WY border and soils growing more saturated in wake of Friday's rainfall, have maintained the inherited Marginal Risk for that region. Mullinax Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4QulyUpwMDeF-2mXJXllqP0vdHsg9U2gtsRBGb-OTPDW= uMw00EUdbxTf2gsQcPxfNMM0Qbs87JVhDeoNSasiu8G_xc8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4QulyUpwMDeF-2mXJXllqP0vdHsg9U2gtsRBGb-OTPDW= uMw00EUdbxTf2gsQcPxfNMM0Qbs87JVhDeoNSasiYzNMOXE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4QulyUpwMDeF-2mXJXllqP0vdHsg9U2gtsRBGb-OTPDW= uMw00EUdbxTf2gsQcPxfNMM0Qbs87JVhDeoNSasizHbhWpo$=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) .