Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jan 11 2024 20:06:05 FOUS30 KWBC 112005 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 305 PM EST Thu Jan 11 2024 Day 1 Valid 16Z 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... ....16Z Update... No significant changes were made to the Marginal Risk area. The guidance is in good agreement that any significant thunderstorm activity holds until after 00Z, with the 12Z HRRR as late as 03-04Z for significant storms to begin firing across Arkansas. This first round of fast-moving storms will zip to the northeast into southern Illinois by 08Z. A second round of storms develops over western Arkansas around 09-10Z and traverses the state through 12Z. This second round will be more expansive, with similar convective elements, but likely won't make it to northeastern areas of the Marginal until after 12Z Fri, becoming part of the Day 2 period. Thus, the higher flash flooding risk is definitely over Arkansas, becoming much lower end (through 12Z Fri) along the Mississippi River and into Illinois. Wegman ....Previous Discussion... The wave associated with the upcoming synoptic evolution across much of the eastern and central CONUS today is currently tracking through California and will progressive move east-southeast into the southern plains by this evening. Large scale ascent within a well-defined mid and upper difluent pattern will aid in a rapidly developing area of convection in-of northeast TX into the the Lower to Mid-Mississippi Valley. The process begins between 21-00z as the digging upper trough begins to tilt more neutral to negative as it traverses the southern plains over TX/OK. The area downstream of the mean trough will enter an area of enhanced low-level moisture advection in conjunction with a rapidly intensifying 850mb jet ramping to 50-60kts. Rich, Gulf air will be pulled poleward through the Lower Mississippi Valley and Arklatex, eventually making headway into the southern tier of the Midwest by the end of the period. 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 the early evening hrs, 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 2-2.5" within a short time after cell initiation with the max generally over northern and central AR where cell mergers take place. NBM thunder probabilities have really climbed over the past succession of runs with a large area of 50-65% now showing up in the 12-hr probability fields, indicative of the increasing convective risks abound within this setup. MUCAPE on the order of 800-1200 J/kg will be found across the Arklatex into AR after nightfall, coincident with the combo of positive theta-E advection and falling height pattern within the amplifying 500mb trough. PWAT anomalies will blossom to 1-2 deviations above normal, an environmental favor for some low-end potential for flash flood concerns, especially within any urban corridors or terrain (Ozarks) where convection is prominent. 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. A small extension back southwest was made in coordination with ensemble mean trends of cell initiation potential to the east of the DFW metro. Kleebauer Day 2 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... ....2030Z Update... ....Mid-Atlantic to the Northeast... A number of changes were made with this afternoon's update. For the Mid-Atlantic to the Northeast, The Slight Risk was trimmed a bit with this update. There were numerous factors that went into this change: QPF has generally been decreasing with time in the guidance. While the PWATs ahead of the front forcing much of the rainfall get as high as 3 sigma above normal, the front will be moving very quickly as a 150 kt southwesterly jet races over the Mid-Atlantic. Thus, for almost all areas, the steady rain will be through and done in 6-7 hours. This very fast movement of the rainfall will significantly limit rainfall totals, which in turn diminishes the flash flooding threat. Secondly, there are likely to be limited impacts. In a somewhat unusual event, the primary driver of any flash flooding from this event will be the already ongoing flooding from the much larger rainfall event a few days ago, rather than the expected rainfall. For much of the Marginal Risk area, forecasted storm total rainfall is expected to be up to an inch, with storm totals in the Slight generally between 1 and 2 inches. The added rainfall in the Slight Risk area will be primarily due to upslope in the southerly winds into the terrain. While rivers are full, and in some instances still in flood, the past 2 days with an additional 24 hours from now until the start of the event Friday afternoon and evening will allow for additional recovery in the rivers. An analysis of area forecast hydrographs shows that many of the river gauges will be at a lower level at the end of this event than they are now...and many more of the forecasts are for river levels to remain steady during the rain, but not rise at all. While FFGs are understandably low across much of the Mid-Atlantic and Northeast, and it's likely that all or nearly all of the event's rainfall will be runoff into those rivers, the short-duration and light QPF amounts should be handled by the "added space" in the river basins, such that even if they're still above flood stage, no locations were noted to be at a higher level after this event than at the end of the previous event, so any "re-floods" will be more minor and highly unlikely to have recovered from the rain a few days ago. Thus, any resultant flash flooding is highly unlikely to be impactful beyond delaying ongoing recovery efforts from the previous event. Given all of the above, the Slight Risk was trimmed out of MD and most of PA, including Philadelphia as total rainfall in these areas unsupported by upslope should stay at an inch or less. One area that could be considered for a very local upgrade would be along the east-facing slopes of the Blue Ridge of northern to central VA, where local southeasterly flow as the front moves by should locally upslope an additional 1/2 to an inch during this event. Heavy rainfall will also struggle to move into southeastern VA, so that area was trimmed out of the Marginal. Much of the Blue Ridge from southern VA into the Carolinas remains unchanged, along with much of New England. ....Lower Ohio Valley... Hydrology was the driver behind the changes here too, though in the opposite way as in the Northeast. Very dry soil conditions remain prevalent across much of the Marginal Risk area. The storms will be ongoing along the Mississippi River at the start of the period (12Z Friday), but will quickly race northeastward on the 100 kt southwesterly jet. While training is possible as storms redevelop in the Marginal Risk area, the fast movement of the storms will largely offset that. With a widespread 1-2 inches of rain expected, and much of it likely to be beneficial due to the dry soil conditions, a more conservative Marginal Risk was drawn to highlight the corridor of highest expected rainfall totals. Substantive rainfall should be done in most of the Marginal Risk area by 18Z Friday, with very cold air racing in behind the front potentially turning the final precipitation over to snow. ....Pacific Northwest... The Marginal remains largely the same, but in following with the latest guidance shifting the heaviest rainfall south a bit faster, the northern edge of the Marginal in Oregon was trimmed a bit. Wegman ....Previous Discussion... ....Mid-Atlantic and Northeast... 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 continue to transport rich, Gulf air poleward with a significant rainfall footprint carried from the southern Appalachians through the Northeast U.S. The positive here is the steady forward propagation of the precip field will limit widespread impacts, unlike the prior cyclone that doused 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 an extension inland over PA and the Lower Hudson Valley of NY state. Beginning to see some lower end probabilities of up to 2" locally within the NBM fields (15-25%), mainly within the area of southwest CT through the Lower Hudson into northern and central NJ. These are the same areas that were 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 D2. 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 want to highlight the aforementioned area as a higher-end potential within the SLGT risk area (25-40% chance of FFG exceedance). The progressive nature of the precip is the saving grace for this being another higher impact event, but will monitor trends closely in the coming forecast cycles to see if there's any potential for an upgrade. ....Ohio Valley... Intense surface cyclogenesis across the Mid-Mississippi Valley will push northeast into the Midwest by the end of D2 with a large heavy rain footprint forecast across the Ohio and Tennessee Valleys. Rainfall of 1-2" with locally as high as 2.5" will be plausible within the area extending from northern MS and eastern TN up to central IN/OH. This is correlated with the strong low to mid-level moisture transport on the eastern flank of the rapidly intensifying surface 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, however there are some signals for a zone of favorable heavy rainfall potential that spans both the D1 and D2 cycles; southeast MO into far western KY. 00z HREF neighborhood probability of at least 3"/12-hrs has escalated to 35-45% over an area encompassing Poplar Bluff and Van Buren, MO over to just west of Paducah, KY, and as far northeast as the IL/MO border south of St. Louis. This would be during the 06-18z time frame on Friday with the heaviest rain occurring the very end of D1 and the first 6 hrs of D2. The compounding factor is what allows the current MRGL risk area to remain with a central focus on that zone. Considering the forecast FFG guidance and forecast rainfall, some localized flooding concerns are possible within the area mentioned above along with a continuation northeast through central IN and southwestern OH. The Marginal Risk was maintained with only minor deviations in the periphery given the latest forecast QPF and bias-corrected ensemble means. ....Pacific Northwest... Leading edge of an 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 central OR. Rates will be standard with a general 0.25-0.5"/hr rainfall through the course of Friday with totals ranging from 1.5-3" with max of 3.5" 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. There has been a slight shift south within the means for the impact area in question with the northwestern corner of OR possibly dealing with more frozen precip and less of a risk of flash flooding. A Marginal Risk was maintained from the previous iteration of the forecast, but did trim the northern extent, mainly excluding any risk area that was situated within the Portland WFO. Kleebauer Day 3 Valid 12Z Sat Jan 13 2024 - 12Z Sun Jan 14 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL NORTHERN CALIFORNIA AND SOUTHWEST OREGON, AS WELL AS COASTAL NEW ENGLAND... ....2030Z Update... No significant changes were made to either of the Marginal Risk areas, as the forecast remains on track. With widespread 3-4 inch totals into the coastal ranges of CA, it is possible some of these areas may need to be considered for a Slight Risk upgrade with future updates. Into New England, the Marginal is also largely unchanged and looks on track. Due to the fast motion of the front driving the rain, the Marginal may be able to be cancelled early since most of the rainfall will be over by 00Z Sunday and much colder air folloing behind the front will change any lingering precipitation over to snow by Saturday night. Wegman ....Previous Discussion... ....Northern California and Southwest Oregon... Atmospheric river event continues through the weekend with an additional 2-4" of rainfall centered over the southern OR coast down the northern CA coast, just north of San Francisco. These areas will be will be monitored closely for a potential upgrade given the back to back nature of the heavy rain footprint with some areas reaching 5-6" of total rainfall in a 48 hr period. The trends have been for the heaviest rain to be focused further south with the ensemble spread beginning to shift closer to the San Francisco metro with the bullseye centered over the King Range down to Mendocino for the period. NBM probabilities for at least 3" are around 50-60% within the King Range for the D3 time frame with 2" having a large 50+% area encompassing northwest CA down to Mendocino, so the prospects have greatly increased for some heavy rain to impact the aforementioned area. Any upgrades will likely be due to the compounding effect of multiple days worth of rainfall. The Marginal Risk was mostly maintained outside some minor adjustments on the southern edge of the risk area. ....New England... Remnant moisture on the eastern flank of the broad cyclonic circulation over the Great Lakes will continue through coastal New England before exiting into Canada the latter half of Saturday. A very similar setup to the previous disturbance with heavy rainfall lingering over the coastal areas of MA up through ME with a standard 1" to locally as high as 2" possible before the precip exits. All the rainfall and snowmelt within the past series of events will culminate in some localized flood potential within the population centers of eastern MA up through southern ME. The current MRGL risk is an inheritance from previous forecast cycle and on the lower end of the spectrum for the time being. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6AD9CN8gUrrzf14CzE4PlUVzf_-fMq-MqasyPoPec6eo= duJYQEsgQIAROBXewYvaXHb7sTdw4-Cev5paI8j_IwGp2Kk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6AD9CN8gUrrzf14CzE4PlUVzf_-fMq-MqasyPoPec6eo= duJYQEsgQIAROBXewYvaXHb7sTdw4-Cev5paI8j_A1DP2Pc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6AD9CN8gUrrzf14CzE4PlUVzf_-fMq-MqasyPoPec6eo= duJYQEsgQIAROBXewYvaXHb7sTdw4-Cev5paI8j_4iY0jYs$=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) .