Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Feb 27 2024 00:48:02 FOUS30 KWBC 270047 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 747 PM EST Mon Feb 26 2024 Day 1 Valid 01Z Tue Feb 27 2024 - 12Z Tue Feb 27 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley Day 2 Valid 12Z Tue Feb 27 2024 - 12Z Wed Feb 28 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE OHIO AND MID-MISSISSIPPI VALLEYS AND THE CENTRAL APPALACHIANS... ....2030Z Update... Very few changes were made to the inherited Marginal Risk area. The pattern remains one quite favorable for severe weather, but nowhere near as much so for flash flooding. Despite PWAT values 2-3 sigma above normal, a 60 kt LLJ, and 500-1000 J/kg of instability noted in the latest guidance (with locally higher maxima) these ingredients favor wind-related impacts more than flash flooding impacts. The primary factor working against flash flooding is the incredibly fast motion of any storms that develop in this region. This will be supported by the 60 kt LLJ and a supporting 150 kt upper level jet. Much of the CAMs guidance that covers the Tuesday afternoon and overnight period suggests anywhere from 1 to 3 rounds of storms...separated by a few hours...will move across the Ohio Valley. These storms will be capable of heavy rainfall rates, with embedded hail with the stronger storms. Hail in this area will cut down on overall precipitation totals. The very rapid movement of the storms suggests any one area may see an hour or less of rainfall with any one storm, so it will likely take 2 or 3 rounds of rain for flash flooding to develop. The storms are likely to grow upscale into MCS's and lines rather quickly, so that will favor a bit more widespread rain, though not everyone will see heavy rain due to more widely scattered storms and hail. Secondly, atmospheric moisture. As noted, PWAT values are 2-3 sigma above normal. However as this is the dry time of year, that only equates to PWATS of 1 to 1.25 inches. This will therefore limit overall precipitation totals as well as more moisture goes into maintaining the storms with less resulting in rainfall. Finally we have very dry soil conditions, with NASA Sport suggesting soil moisture over the Ohio Valley near 50% saturation, which in some areas is less than 10% of normal. The empty river basins will be easily able to handle any and all precipitation. Flash flooding would most likely be in any local creeks, streams, and poor-drainage urban areas. Taking all of this together, think it will take multiple rounds of heavy rain (or mostly rain) moving over a poor drainage area to result in flash flooding. Thus, instances of flooding should be very isolated. The Marginal was considered for cancellation, but the prevalence of urban areas, somewhat low FFGs for Ohio and points east (1-1.5 inches per hour FFG values), and then terrain considerations in the east may be enough to support an isolated occurrence or two, as well as keeping with consistency. For western areas (IL/IN) where FFGs are a little higher, the storms are likely to be a bit stronger as they will move through closer to peak heating Tuesday afternoon and evening. Wegman ....Previous Discussion... A robust mid-level shortwave will propagate southeast out of the Pacific Northwest on Tuesday with increasing difluent flow ahead of the mean trough that will help initiate a round of convection within the confines of the Mid-Mississippi and Ohio Valley's. Focused meridional flow ahead of the mean longwave trough will advect deeper moisture and associated theta-E's poleward with a tongue of elevated instability building within the areal theta-E advection regime. Getting more into the hi-res time frame, we are seeing some forecast MUCAPE values approaching 500-1000 J/kg with locally higher lying along a projected warm front situated to the east of a developing surface reflection in the central Midwest. PWAT indices soar to 2-3 standard deviations above normal within a large zone spanning from the Mid-Mississippi Valley up into the Great Lakes by Tuesday afternoon as the enhanced low and mid-level flow usher Gulf moisture into the outlined area. By late-Tuesday afternoon, convective development is likely thanks to the approach a mid-level vorticity maxima moving out ahead of the shortwave trough as it migrates eastward. Models are keying on a band of heavy rainfall within the proximity of a warm front, denoted fairly well within the theta-E fields on guidance. The question that still needs to be solved is where that boundary will lie. Current consensus is north of I-70 with some guidance as far north as southern MI which leads this to be the focal point of expected convective development. Recent HREF probability fields for at least 1"/hr have risen to 25-30% within the confines of the IN/MI/OH border which is where most of the CAMs currently have the warm front situated when examining the surface moisture/temp fields. Further south will have the best deep layer moisture and elevated PWATs, but the lack of a focused boundary like the warm front in question will lead to more isolated flash flooding concerns, mainly within urbanized areas, and terrain as you head further east. Any convective development will continue into the overnight hours as we approach Wednesday with upscale growth of convection likely after 06z as the shortwave trough pivots east of the Mississippi and begins to take on a more neutral tilt by the end of the period. Large scale forcing will be at its maximum by the end of the forecast cycle with a line of heavy rain likely extending from eastern OH, south into the Tennessee Valley. This will continue to advance eastward and the focus shifts into the Appalachian front and terrain-centric areas in the Mid-Atlantic and Ohio Valley as we roll into D3. The Marginal Risk from the previous forecast was maintained, but adjusted to reflect the latest QPF trends within the convective-allowing environment being forecast. A Slight Risk upgrade is non-zero, but current forecasted rates within any convection keep this capped for the time-being. Kleebauer=20 Day 3 Valid 12Z Wed Feb 28 2024 - 12Z Thu Feb 29 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER OHIO VALLEY THROUGH NEW ENGLAND, AS WELL AS THE COASTAL PACIFIC NORTHWEST... ....2030Z Update... ....Appalachians through New England... A strengthening surface low starting the day near Lake Huron will track east on the Canadian side of the border until its absorbed into the cold front associated with a stronger low over northern Quebec by Wednesday evening. Convection ongoing Wednesday morning will reform in some areas and continue racing eastward into the west-facing slopes of the Appalachians Wednesday morning. This area is where there is the greatest confidence that isolated flash flooding will occur, when added to any rainfall from Day 2/Tuesday and any snowmelt contributions. For western portions of the Marginal Risk, the cold air will follow in earnest, with below-freezing temperatures ending both rainfall and flooding threats as early as Wednesday evening. East of the Appalachians into New England, this area will be in the warm sector for all but the wee hours of Thursday morning. However, despite the continued LLJ of 50+ kts, instability will be largely lacking, keeping any rainfall largely showery with some convective elements. Thus, for most of the I-95 corridor, rainfall rates are unlikely to materialize enough to constitute a significant flash flooding threat. As this period moves into the high-resolution models time frame, new details may determine that the Marginal Risk threat may need to come east, but for now all of the guidance suggests there will not be enough rain overall to result in even an isolated flash flooding threat. The Marginal Risk area was expanded northeast to include the rest of New York State and almost all of interior New England. The line was largely drawn following the NOHRSC depicted areas of snow cover. As temperatures rapidly warm well above freezing Tuesday and especially Tuesday night, snowmelt will become an increasingly significant factor driving the potential for flash flooding across New England. Rainfall totals don't vary all that much across New England except into extreme down-eastern Maine, where strengthening flow off the Atlantic will locally enhance rainfall totals. Thus, the flash flooding risk and the rationale for expansion of the Marginal is largely driven by snowmelt rather than rainfall, though up to an inch of rain on top of snow should be enough to cause flash flooding. The real question is how substantial the snowmelt component will be as temperatures remain above freezing for as little as 24 hours in some areas. ....Pacific Northwest... No changes were made to the inherited Marginal Risk area for coastal WA and OR. Widespread 2 to 3 inches of rain totals are forecast for this area. However, there are a couple points to note working against flash flooding. First off is low snow levels and cool air. This will be a cold rain for sure along the coast, as the area spends much of the day in between 2 separate reinforcing shots of cold air. Thus, a substantial portion of the precipitation that falls in the coastal mountains and definitely the Cascades will be snow, and therefore not contributing to rising river levels. Limiting the area of potential flooding to the coastal plain, which is largely able to handle large amounts of rain due to flora and a wet climate, even these larger amounts of rain will struggle to result in flash flooding. Thus, confidence is low for this Marginal Risk area, but any poor-drainage areas will be susceptible to flooding given the large amounts of rain forecast. Wegman ....Previous Discussion... ....Mid-Atlantic and Northeast... Potential mid-level shortwave trough will pivot eastward, taking on a neutral to eventually negative tilt as it progresses through the Great Lakes and Ohio Valley on Wednesday afternoon. Large scale ascent pattern will be maximized downstream of the mean trough with a line of rainfall extending northeast to southwest across the interior Northeastern U.S down into the Central Appalachians. Locally heavy rainfall embedded within the main line will allow for isolated flash flood concerns, especially within the terrain focused areas of western PA down into WV and eastern KY. A secondary area of interest will be across western NY state, mainly to the east of Lake Ontario where snowpack with modest snow-water equivalent (SWE), based on the latest NOHRSC data set will couple with warming temperatures ahead of the cold front and locally heavy rainfall on top of the snow causing rapid snow melt. QPF totals exceeding 1" are forecast within the Tug Hill and adjacent Adirondack area of Upstate NY where the highest SWE is located. Considering rates between 0.55-0.75"/hr possible along with the anticipated snow melt, this could lead to localized flooding as rivers and streams would have to take in a lot of the excess water and deep soil moisture is already running above 80% for much of the aforementioned area. QPF forecast of 1-1.5" is currently situated over the above area, as well as within Central Appalachia, but there is potential for higher QPF pending the evolution of the convection over the Ohio Valley as synoptic pattern is favoring fairly robust low to mid-level forcing within a moisture laden environment given the +2.5-3 deviation PWAT anomaly across guidance. There is some agreement within the spread on the heavy rainfall making it across the mountains and into the Piedmont of NoVA and MD up through eastern PA. If there is more consensus in future runs, would not be surprised to see the MRGL risk expanded to encompass part of the population centers west of I-95. For now, will maintain the Marginal Risk over areas west of the Blue Ridge in the central Mid-Atlantic and west of the Capital District in NY state. ....Pacific Northwest... Another atmospheric river will indulge on the Pacific Northwest coast with the primary concern focused over the coastal plain of the Olympics in Washington and coastal Oregon. Current AR forecast is for a minor impact event with IVT values between 550-650 kg/ms anticipated to advect over the aforementioned areas, protruding inland to the Cascades. This will aid in copious amounts of rainfall over the span of 24-36 hrs, but rates will be lacking as probability of exceeding anything over 0.5"/hr will be modest, at best. Totals in-of the coast will be between 2-3" with up to 4" possible over the northwestern OR coast near Astoria. Overall, a low-end MRGL risk area was maintained from previous forecast due to the rainfall totals anticipated, as well as the moist-leaning soils based off the recent NASA SPoRT soil moisture analysis (Mainly within coastal WA and northwest OR). Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!55OWe84M3JzyaisAkwKW9qGylXAaWXAnfwxu0_UNlrpt= g43-J2i4AdFBZ1Wl7VI0U32JtamZD6uyVgeI9Mbxn-qxY-c$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!55OWe84M3JzyaisAkwKW9qGylXAaWXAnfwxu0_UNlrpt= g43-J2i4AdFBZ1Wl7VI0U32JtamZD6uyVgeI9MbxDddks4Q$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!55OWe84M3JzyaisAkwKW9qGylXAaWXAnfwxu0_UNlrpt= g43-J2i4AdFBZ1Wl7VI0U32JtamZD6uyVgeI9MbxjGlHRVU$=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) .