Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 09 2023 00:37:51 FOUS30 KWBC 090037 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 736 PM EST Fri Dec 08 2023 Day 1 Valid 01Z Sat Dec 09 2023 - 12Z Sat Dec 09 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Taylor Day 2 Valid 12Z Sat Dec 09 2023 - 12Z Sun Dec 10 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE PACIFIC NORTHWEST... ....Pacific Northwest... An Atmospheric River will impact the Pacific Northwest Saturday, peaking in intensity between 00-12Z. Forecast IVT from the 12Z guidance supports peak values of 500-600 kg/m/s with localized values over 600 kg/m/s focused mostly along the central coast of Oregon but with significantly values into Washington as well. With flow relatively orthogonal to the coast, some heavier rainfall totals are expected along the Coastal Ranges into the Cascades downstream. Only minor differences were noted with the 12Z model suite and the hires model consensus showing 3-5 inches over the 24 hour period. Instability is not expected to be a factor and rainfall rates should generally remain below 0.5 in/hr, but localized hourly totals in excess of 0.5 inches will be possible for some of the favored west to southwest facing terrain. Flooding impacts will be increased within the Slight Risk area given the saturated soil conditions and above average streamflows over this region from the atmospheric river event from earlier in the week.=20 ....OH/TN Valley into the Southeast... A broad southwest-northeast axis of heavy rainfall is forecast later in the day Saturday through early Sunday from the the Ohio/Tennessee Valleys southwestward into the Mid-South and the Lower Mississippi Valley/central Gulf Coast as Gulf moisture returns northward ahead of an upper-trough/surface cold front pushing eastward. The 12Z model cycle has trended faster or remained steady between 00-12Z Sunday, but given the large upper trough downstream and lingering ensemble spread, cannot rule out a slower trend with future cycles. While the upper forcing and surface front are fairly progressive off to the east, which will be a limiting factor for excessive rainfall amounts, there are some areas of concern. As the base of the upper trough sharpens across the Southern Plains during the day, 925-850 mb winds are expected to increase across KY/TN with the nose of these stronger winds setting up a convergence axis that is forecast to align with the mean steering flow. This region will be coincident with divergence within the left exit region of a strengthening 200 mb jet max over the Lower Mississippi Valley. A limiting factor for higher rainfall rates will be limited instability with the forecast showing less than ~500 J/kg across southern portions of the Ohio Valley into Tennessee. Across the southern Appalachians, a favorable flow regime will set up with 30-50 kt of speed forecast at 850 with the 925-850 mb component of the flow oriented S to SSW. 1 to 3 inch rainfall totals are expected with localized maxima over 3 inches possible by Sunday morning. Instability will be somewhat limited here with forecast values less than 1000 J/kg in even the more aggressive model output but short term training given PWATs climbing up to 1.2-1.3 inches could yield some localized runoff concerns. While deeper layer mean flow across a broad section of the Ohio Valley to Gulf Coast will be oriented mostly parallel to the front, forward movement of the front and limited instability will be limiting factors as discussed above, but closer to the Gulf Coast, instability may eclipse 1000 J/kg given better access to low level moisture. Short term training may favor localized heavy rain with 12Z HREF probabilities of 40-50 percent for exceeding 3 inches and 20-30 percent for exceeding 5 inches of the 24 hour period ending Sunday morning over the western Florida Panhandle. The potential for higher rain overlaps with a portion of the Gulf Coast which received over 5 inches of rain in over the past week which could be a factor in increased flash flood potential. Otto Day 3 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 Sunday into Sunday night, 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, but a deepening surface low is expected to organize Sunday night from the Mid-Atlantic region to New England possibly becoming a bomb cyclone Monday night with 850 mb flow rapidly strengthening as a result of the deepening low. Strong low level flow directed into the urban corridor of the Northeast and New England terrain will overlap with PWAT anomalies of +2 to +3 (via 12Z GFS). Areas of heavy rain appear likely although weak instability (up to a few hundred J/kg) should be a limiting factor for higher rainfall rates developing and perhaps more of a longer duration rainfall event with 6-12 hour potential for increased rainfall intensities. Given increased soil moisture values as seen in the most recent NASA SPoRT imagery over the Northeast (0-40 cm RSM of 50+ percent for the region and 70+ percent into portions of New England with snow cover), A Slight Risk was introduced for this update from the New York City metro into central New England. GEFS/ECENS probababilities for 3+ inches were 45/30 percent (respectively), a relatively high value for the Day 3 time frame. Farther south, limited instability and the progressive nature of the front warranted removal of the Marginal Risk across portions of Georgia and South Carolina, but maintaining a risk across the eastern Florida Panhandle into southern Georgia where potential for training could yield localized flash flooding. Farther north, with generally dry soil conditions and below average streamflows over the Mid Atlantic, would tend to think most of this rainfall will be beneficial in nature. However if and where we start getting rainfall up over 2-3" there will probably be at least some runoff impacts. The threat of true flash flooding seems low given the the only modest rainfall rates...but some urban and areal flood impacts will be a possibility. ....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, high rainfall rates (near 0.5 in/hr peak values) will continue for the first 6-12 hours of the period prior to the moisture plume weakening and the Pacific front moving through the region. An additional 1-3 inches of rain will be possible on Sunday. While impacts from Saturday/Saturday night's rainfall may still be ongoing early Sunday, it appears that rainfall intensities will be lowering and not quite up to the Slight category at this time. However, as the event draws nearer, subtle waves along the frontal boundary may allow for higher rainfall to linger across western Oregon longer than expected or changes to model guidance cannot rule out the need for a short fused Slight Risk on Day 3/Sunday. Otto Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9tT4y93JDzU9bXO6rVYIR6UxZnD9Hfj9bHxUowwrB64x= N7BeUya5UmDM8g5nwRBTH209xFa3aaAHfcdkKhEWsa8oP5U$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9tT4y93JDzU9bXO6rVYIR6UxZnD9Hfj9bHxUowwrB64x= N7BeUya5UmDM8g5nwRBTH209xFa3aaAHfcdkKhEWh5fPaXM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9tT4y93JDzU9bXO6rVYIR6UxZnD9Hfj9bHxUowwrB64x= N7BeUya5UmDM8g5nwRBTH209xFa3aaAHfcdkKhEW4COV69U$=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) .