Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Dec 10 2023 08:30:25 FOUS30 KWBC 100830 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 329 AM EST Sun Dec 10 2023 Day 1 Valid 12Z Sun Dec 10 2023 - 12Z Mon Dec 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHEAST... ....Eastern U.S... A very dynamic system will impact the East Coast today into tonight, continuing to advance downstream from the Midwest and central Gulf Coast from Saturday. A cold front is forecast to be progressive across the southern U.S. as the base of the upper trough becomes negatively tilted as it swings from the Lower Mississippi Valley to the East Coast through the period, with a deepening surface low forecast to track from the Mid Atlantic into the Northeast. A Slight risk was maintained across portions of the Northeast...stretching from near NYC northeast into southern ME. Embedded within this broader Slight risk there is likely an area of greater potential across central/eastern CT into central MA. This is along the low track where low level convergence will be maximized for the longest duration. 24 hour HREF EAS 3" probabilities are 50-80% across this portion of CT/MA, with 5" neighborhood probabilities in the 40-60% range. These values support fairly widespread 3" rainfall amounts, with localized swaths as high as 5-7" appearing probable. HREF data depicts high probabilities of 0.5"/hr rainfall with this event, but only low chances of 1"/hr rainfall. This is likely indicative of the lack of instability that is forecast with this system. However instability is non-zero, and the strong frontogenesis in the low/mid levels combined with the favorable right entrance region of the upper jet and approaching negatively tilted mid level trough all do support embedded convective elements within the broader rain shield. So while 1"/hr may be hard to come by except on a localized basis, 0.5"-0.75"/hr seems probable at times by this evening into the overnight hours. All of this data supports an increasing flood threat across this corridor, and we gave some consideration to a MDT risk upgrade. Factors against the upgrade include the relatively low 10-20% 1"/hr and 10-20% 2"/3hr probabilities...both of which indicate this event is borderline flash flood and might be more longer duration areal flood. This seems to be supported by the fact that despite the aforementioned high QPF probabilities from the HREF, 3hr FFG exceedance probabilities peak around 15%, with 6hr exceedance near 30%...neither of which necessarily support MDT risk levels. With that said, this event does have some higher end flood potential. Widespread 3"+ totals and isolated amounts of 5-7" is an impressive event, and even if this happens over a longer duration some higher end impacts could occur. For now we will consider this a higher end Slight risk for the aforementioned portions of CT and central MA...and will continue to monitor trends through the day. Should we see an uptick in expected rainfall rates and/or higher coverage of 5"+ totals then an upgrade to MDT is possible. Elsewhere within the Slight risk area Rainfall amounts of 2-4" are likely. Soil information from NASA SPoRT and CREST suggests that after some initial infiltration a good chunk of this rainfall will become runoff. Thus would expect to see the potential increase for some flooding of streams and other low lying or flood prone locations...including poor drainage and urban areas. The system is trending colder on the northwest side, which should limit the rainfall and SWE loss over VT...thus the flood risk appears to be decreasing there. However still looking at some snow melt over portions of NH and ME that will add to excess runoff expected from the upwards of 2-4" of rain. We did expand the Slight risk into portions of southern ME with this update...generally aligning with where totals may at least locally exceed 3" through 12z Monday. Over the Mid Atlantic generally dry soil conditions and below average streamflows should be more of a limiting factor for flooding compared to areas of the Northeast described above. However still expecting a swath of 2-3" of rain, with narrow localized swaths of 3-5" possible. There is some uncertainty regarding this higher QPF axis...but the highest HREF probabilities from 00z generally stretch from north central NC into southeast VA towards the Delmarva. Where these higher totals occur at least a localized flash flood risk will likely evolve. We will continue to maintain a Marginal risk across portions of the Florida Panhandle into southern Georgia where there's some support for brief training convection. Generally think convection will be progressive, but there could be some brief training, and do note HREF neighborhood 3" probabilities get towards 30%. May not have introduced a risk here if we didn't already have one out...but given the signal (albeit a rather weak one) not comfortable removing the risk at this time. ....Northwest... Little change made to the existing Marginal Risk over western Oregon given the atmospheric river event will still be ongoing at 12Z Sunday. While IVT values are expected to wane through the period, continued upslope flow and a compact mid level shortwave will help maintain showery conditions. Probabilities of 0.5"/hr drop off after ~15z, but would still expect to see pockets of 0.25"-0.5" through ~00z. Given the southward progression of the system and the overall weakening of the IVT plume with time...we think a Marginal risk should still suffice. 24 hour rainfall totals in the 1-3" range are expected along the coastal ranges...with 2-4" in the Cascades. Chenard Day 2 Valid 12Z Mon Dec 11 2023 - 12Z Tue Dec 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF MAINE... The event described in more detail in the day 1 discussion will be ongoing Monday morning across portions of eastern New England. By 12z Monday the low pressure should be over southern New England with it continuing to deepen as it quickly moves northeastward into ME. A very strong low level jet ahead of the system will focus a corridor of strong low level convergence and frontogenesis spreading from eastern MA into southern and central ME. Despite limited to no instability, the dynamic forcing of the system should still support embedded convective elements with rainfall rates locally exceeding 0.5"/hr. Given the fast motion of the system by this time additional rainfall after 12z Monday should not be all that extreme...with the general consensus supporting 1-2". This alone would not pose much of a flood risk...however this will be in addition to what has already fallen over the day 1 period. Given event totals expected to reach 3-5" and the potential for the localized higher rates along and ahead of the low track...we think a continuation of the Slight risk from day 1 is warranted. Thus we will upgrade the day 2 to a Slight risk from coastal NH into central ME...generally covering areas where heavy rainfall should be ongoing Monday morning and where event total rainfall has the highest probabilities of getting into the 3-5" range. Chenard Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9ae2-bOIa4Yc8vGRhN4N_3Qxannkz_C4d-O9EYxz76oV= c97zVwPIHpUmAYTKBng-CiqLM4sUrpgQn005sBy8MXEp5ns$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9ae2-bOIa4Yc8vGRhN4N_3Qxannkz_C4d-O9EYxz76oV= c97zVwPIHpUmAYTKBng-CiqLM4sUrpgQn005sBy8Nl7AK20$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9ae2-bOIa4Yc8vGRhN4N_3Qxannkz_C4d-O9EYxz76oV= c97zVwPIHpUmAYTKBng-CiqLM4sUrpgQn005sBy8qdsnOj8$=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) .