Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jan 31 2025 00:48:30 FOUS30 KWBC 310048 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 748 PM EST Thu Jan 30 2025 Day 1 Valid 01Z Fri Jan 31 2025 - 12Z Fri Jan 31 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID- AND SOUTHERN MISSISSIPPI VALLEY AND ADJACENT PLAINS INTO=20 PORTIONS OF KENTUCKY AND TENNESSEE... ....Southern and Central Mississippi Valley Eastward... Main change to the on-going Excessive Rainfall Outlook was the removal of the Slight and Marginal risk areas from Texas and all but eastern Arkansas where the front has move through the area and rainfall had come to an end. Little additional rainfall is expected here as cooler and more stable air sweeps in behind the front. Ahead of the surface boundary...instability is limited but the=20 strength of upper level divergence of a 140 to 160 kt upper level=20 jet was helping offset that. Still maintained the Slight Risk from=20 parts of Kentucky and Tennessee where low level moisture continued=20 to be fed into the region by a 50 to 60 kt low level jet between=20 925 mb and 850 mb. The risk of excessive rainfall appears to be=20 highest from far northwest Tennessee and adjacent portion of far=20 southwest Kentucky given the amount of rainfall that fell over the=20 past 18 to 24 hours and at least some risk of another 1 to 2 inch=20 rainfall amounts in 3 hours before rain tapers off later in the=20 evening. Farther down stream, the risk of excessive rainfall should last deeper into the evening and overnight hours but rainfall=20 rates should gradually be diminishing in response to diminishing=20 instability.=20 A line of thunderstorms across the Southern Mississippi Valley should be progressive enough to preclude more than a few instances of=20 problems with run-off and flooding of low lying areas outside of=20 more urbanized areas, ....Parts of Iowa and Illinois... Maintained the targeted Slight risk area coordinated earlier in=20 the day to account for sensitive hydrologic conditions. With a deep frost layer...concern was that much of the rain that falls will immediately run off. Warm advection rain followed by a developing=20 deformation axis tonight should be accompanied by limited rainfall rates (remaining mostly below 0.5 in/hr) given a lack of=20 instability but steady rain of near 1 inch is expected to fall in a roughly 6 hour window which will likely result in some excess=20 runoff across portions of the Missouri/Iowa border into portions of northern Illinois. While this outlook ends at 12Z Friday, the=20 event will be ongoing at that time with a continuation of runoff=20 concerns extending into the Day 2 ERO period beginning 12Z Friday=20 into northern Illinois. Bann Day 2 Valid 12Z Fri Jan 31 2025 - 12Z Sat Feb 01 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF WEST VIRGINIA... Daytime update... For most areas, made only minor adjustments based on the 12Z guidance. Across portions of northern Illinois into far northwestern Indiana - given the reasons noted in Day 1, a Slight Risk was introduced for Day 2 as well. The new Slight Risk reflects an eastern extension of the Day 1 area and largely follows the higher HREF neighborhood probabilities for additional accumulations of an inch or more. Most of this is expected to fall within the first 6-hours of the period (12Z-18Z Fri). Pereira ....West Virginia... A Slight risk was issued for portions of central and eastern WV...primarily for areas where significant snowpack still exists. The latest NOHRSC analysis indicates upwards of 2-4" of snow water equivalent over the terrain of eastern WV and the latest model consensus is for 1-3" of rainfall over this area. With temperatures and dewpoints forecast to at least get into the 40s (maybe 50s in spots) the rain on snow should trigger a fairly efficient melting of the snowpack. This is likely to result in numerous instances of areal flooding and at least some river flooding. Rainfall rates are generally forecast to stay in the 0.25" to 0.5" an hour range, which is probably not enough to generate flooding of a more flash variety. However there are indications in both the 06z HRRR and 00z HREF that embedded convective elements Friday could locally push hourly rainfall a bit over 0.5". Given the amounts of runoff that will be occurring from the combined snow melt and rain, any of these higher rates could locally result in a flash flood threat as well. ....Northern Illinois and Indiana... A Marginal risk was maintained across this area as comma head rainfall drops up to 1.5" of rain in a 6hr period Friday. Typically this degree of rain would not result in impacts, however frozen ground will likely enhance runoff potential. Given the extra runoff this will generate, amounts around 1.5" in 6 hrs could be enough to trigger at least some flood concerns across this corridor. ....West Coast... An atmospheric river will bring heavy rainfall from western WA southward into northern CA Friday into Friday night. The IVT plume is generally progressive and weakening through the period, so not expecting major rainfall totals or impacts. Generally forecasting a 1-3" rainfall (locally higher in the favored terrain), which should mainly drive an isolated minor flood threat. The IVT axis does start to stall as it shifts into northern CA, and so could be looking at totals more in the 2-4" range across the terrain here. Will need to monitor for a potential Slight risk upgrade across portions of the northern CA coast, but for now will stick with the Marginal. Leaning towards this day 2 rain mainly helping saturate conditions leading towards a greater flood risk on later days. However should things trend quicker and/or wetter an upgrade may need to be considered. Chenard Day 3 Valid 12Z Sat Feb 01 2025 - 12Z Sun Feb 02 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA... Daytime update... Made only minor adjustments based on the 12Z guidance. Pereira Previous discussion... A pretty significant model change has been noted over the past couple cycles across the eastern Pacific into the west coast. The earlier model consensus was for a stalling of a moderate to strong AR over northern CA and southwest OR resulting in significant rainfall amounts. However recent model runs have unanimously shifted further south with this AR, now stalling it more over central to north central CA. There has also been a weakening trend in the AR, so QPF values are not as high as earlier forecast. However, with the southward shift the max rainfall axis is now over areas that tend to be more sensitive to heavy rainfall and related flood impacts. Thus even with the drop in QPF, think a southward shift of the Slight risk is warranted. The Slight risk now stretches from Mendocino county south into the San Francisco area and as far south as Santa Cruz county. It also extends inland across the Sacramento Valley and into the Sierra Nevada. Rainfall along the coast will start on Friday and continue into the day Saturday. A gradual saturating of soils are expected to lead to potential greater impacts on Saturday, warranting the Slight risk. Two day rainfall totals are currently forecast in the 2-4" range. Two day rainfall amounts of 1-3" are expected within Sacramento valley locations, with upwards of 4-6" within the Sierra Nevada (most of which will be rain given high snow levels). Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4UK_no7KzrChBh8mfWJy4gQHWBTOCR3zydvRKytnnoHb= xFycNI_XGWynEvAjBttAQfFweuVYD0PQnDPDQABAXulb2wg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4UK_no7KzrChBh8mfWJy4gQHWBTOCR3zydvRKytnnoHb= xFycNI_XGWynEvAjBttAQfFweuVYD0PQnDPDQABAqAiL2VI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4UK_no7KzrChBh8mfWJy4gQHWBTOCR3zydvRKytnnoHb= xFycNI_XGWynEvAjBttAQfFweuVYD0PQnDPDQABA49kEGMo$=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) .