Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jan 10 2024 20:49:58 FOUS30 KWBC 102049 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 349 PM EST Wed Jan 10 2024 Day 1 Valid 16Z Wed Jan 10 2024 - 12Z Thu Jan 11 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF COASTAL AND DOWNEAST MAINE... 16Z Update... Low pressure and warm front will continue to lift north/northeast through the rest of New England today with the final push of widespread, steadier precipitation quickly moving through Maine. The combination of some localized heavier rain rates (up to 0.5"/hr) over snowpack and wet ground conditions may lead to additional flooding and runoff over southern/southeast Maine, particularly through about 18-21Z before precipitation is expected to wind down completely. The ERO risk areas for this update were trimmed to account for ongoing radar trends. Taylor ---previous discussion--- Steady progression of heavy precip will move northeast through ME this morning, exiting by the afternoon time frame with only some remnant light precip prevailing after 18z. Heaviest rainfall potential will exist in-of coastal ME up through Downeast locations before exiting. A very consistent 0.75-1.5" is forecast from south to north within the 12-18z time frame before conditions improve, much of it falling in a 2-4 hr time. The current setup is a compounding issue of heavy rain just before 12z and just after within the beginning of this D1 forecast. Heavy rains and snowmelt will contribute to local flooding concerns until the system moves out of the area. The previous forecast SLGT/MRGL risks over ME were mostly kept with only some minor adjustments within Downeast ME. This will conclude a very active flood pattern over the past 60 hrs with copious amounts of heavy rainfall and flooding reports extending from the central Gulf Coast up to New England. Kleebauer Day 2 Valid 12Z Thu Jan 11 2024 - 12Z Fri Jan 12 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-MISSISSIPPI VALLEY THROUGH ARKANSAS TO THE ARKLATEX... 21Z Update: No significant changes were made to the existing Marginal Risk area across the ArkLaTex, Mid-Mississippi Valley, and Lower Ohio Valley area as low pressure organzing in the area draws northward Gulf moisture and overlaps with favorable forcign for ascent. Rainfall amounts of 1-2", greatest over Arkansas, remain likely/expected which could lead to some instances of flash flooding. Taylor ---previous dicsussion--- Much of the D2 period will be fairly quiet on the rainfall front as the synoptic pattern resets ahead of the next disturbance to impact much of the CONUS. The process begins Thursday evening as a digging upper trough exits the southwestern US and ejects into the southern plains. By the time it reaches the longitude of west TX, shortwave trough will begin tilting more neutral with a negative orientation transpiring by the end of the period. The area downstream of the mean trough will enter an area of difluence with flow increasing out of the south, advecting rich Gulf air back through the Lower Mississippi Valley and Arklatex. Surface low will spawn in-of central OK and begin intensifying as the upper pattern allows with instability fields gaining momentum on the eastern flank of the low. Showers and isolated storms will begin firing in east TX, LA, and AR by 00z Friday, continuing to expand in coverage and magnitude by the end of the period. Deterministic guidance remains steadfast with the coverage and totals within the time frame of 00-12z Friday with all guidance keen on 1 to locally as high as 2" within a short time after cell initiation with some indications of locally higher over northern AR where cell mergers take place. This is only the beginning process of a large scale rainfall footprint that will occur after this period, but the prospects of convective development in a maturing instability field with PWAT anomalies blossoming to 1-2 deviations above normal allows for some low-end potential for flash flood concerns, especially within any urban corridors or terrain (Ozarks). The MRGL risk was maintained across much of Arkansas up into the Mid-Mississippi areas of southeast MO and far western KY, as well as bisecting the Arklatex. Kleebauer Day 3 Valid 12Z Fri Jan 12 2024 - 12Z Sat Jan 13 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC AND NORTHEAST... ....Mid-Atlantic and Northeast... The latest guidance remains on track with a potentially significant rainfall/flooding event late Friday through Saturday morning across the Northeast and southern New England. Moisture anomalies associated with this approaching system will be 2 to 3 standard deviations above normal with a favorable position of the 850 mb moisture transport axis into coastal Mid-Atlantic through southern New England. The 12Z deterministic and ensemble guidance continues to point toward 1-2" (locally 2-3") rainfall amounts, generally in the 00Z-12Z Saturday range (Friday evening through Saturday morning). This rainfall is expected to fall over areas that are experiencing ongoing significant/major flooding from yesterday's event but also experiencing weeks of above normal rainfall and higher soil moisture. So while this additional rainfall and rain rates would normally be well absorbed, the ground conditions and higher streamflows are at a fuller capacity that even these expected amounts will likely cause excessive rainfall/runoff and worsen ongoing flooding. Some of the flooding from northern NJ through southern New England could be significant where the ensemble guidance shows the greatest probability of the localized higher rainfall amounts materializing. The Slight Risk was expanded some, especially across southern New England and the messaging associated with the risk level was bumped up to "higher end" Slight Risk range (25-40%). Otherwise, the previous discussion below remains valid. Taylor ---previous discussion--- A stout upper level disturbance will once again bring moderate to heavy rainfall across much of the eastern seaboard, but the progressive pattern will yield a setup a couple rungs lower in the impact department, albeit still impactful given the antecedent conditions leading in. Deep moist conveyor within the WCB will transport rich, Gulf air poleward with a modest rainfall footprint carried from the southern Appalachians to the Northeast U.S. The positive here is the steady nature of the forward propagation of the precip field will limit widespread impacts, unlike the prior cyclone that doused the everyone east of the Mississippi with several inches of rainfall. The best prospects for 1" or greater of rainfall just so happens to coincide with the area with the lowest FFG exceedance markers across the Northeast Megalopolis. A general 1-1.5" is forecast within the corridor from DC to Boston with extension inland over PA and the Lower Hudson Valley of NY state. These are the same areas dealing with extensive flooding from the previous rainfall and accompanying snowmelt, so the prospects of some flooding within the same areas has a lot of merit considering what type of soil saturation we will inherit by D3. Primary concerns will be urban flooding and some creek/river flooding continuing as rivers will just be coming off crest of the previous event. A Slight Risk was maintained as of the previous forecast, but extended the northern and southern periphery of the outlook area to reflect the latest trends. ....Ohio Valley... The deepening surface low over the Central U.S. and favorable moisture transport will bring an axis of 1-3" across portions of the Lower Ohio Valley and Mid-Mississippi Valley. Some training/repeating rounds of heavier rainfall and some thunderstorms may lead to localized higher rainfall amounts but for the time being, the Marginal Risk captures the excessive rainfall and flooding risk given some of the drier antecedent conditions in the area. Taylor ---previous discussion--- Intense surface cyclogenesis across the Mid-Mississippi Valley will push northeast into the Midwest by the end of D3 with a large heavy rain footprint forecast across the Ohio and Tennessee Valleys. Rainfall of 1-2" will be plausible within the area extending from northern MS and southern MO up to central IN/OH. This is correlated with the strong low to mid-level moisture transport on the eastern flank of the low. IVT signatures are once again fairly robust considering the time of year with indices approaching 1000 kg/ms within the latest GFS/ECMWF deterministic. This makes sense considering the stout cyclogenesis pattern with a link to the Gulf for moisture transport. The progressive nature of the pattern will aid in keeping the threat to a minimum, for the time being. Considering the forecast FFG guidance and forecast rainfall, some localized flooding concerns are possible within the area mentioned above. A Marginal Risk was maintained with slight extensions further into OH to reflect the bias-corrected QPF field showing some heavier rainfall signatures into southwestern OH. ....Pacific Northwest... Atmospheric river pattern across the Pacific Northwest will transpire on Friday into Saturday morning with bouts of heavy rainfall within the coastal plain extending from northwest CA up through Astoria, OR. Rates will be standard with a general 0.25-0.5"/hr rainfall through the course of Friday with totals ranging from 2-4" with max of 5" possible across the central and southern OR coast. IVT signature around 700-800 kg/ms is currently forecast by both the GEFS and ECENS with some minor variations within the deterministic. This is considered a borderline weak/moderate setup which historically can produce some localized flooding with the urbanized corridors along the coast, but nothing too extraordinary in the grand scheme. NBM probabilities for at least 2" are pretty high at 72 hr leads, so the prospects of heavy rainfall are well within the scope of the setup. A Marginal Risk was maintained from the inherited D4 iteration of the forecast with no change to the orientation given the latest guidance. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7zy-0fzA3PE620vXOPL3rOaBxPwZ8Fre8kMfscAqYslm= Y-5F7Y3MW7527kfAEfaUuEs6lRBsu0w64apDtYn6OaayNYw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7zy-0fzA3PE620vXOPL3rOaBxPwZ8Fre8kMfscAqYslm= Y-5F7Y3MW7527kfAEfaUuEs6lRBsu0w64apDtYn6ESnxW50$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7zy-0fzA3PE620vXOPL3rOaBxPwZ8Fre8kMfscAqYslm= Y-5F7Y3MW7527kfAEfaUuEs6lRBsu0w64apDtYn6XpyF0Ho$=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. 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