Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Feb 26 2024 16:03:24 FOUS30 KWBC 261603 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1102 AM EST Mon Feb 26 2024 Day 1 Valid 16Z Mon Feb 26 2024 - 12Z Tue Feb 27 2024 ....16Z Update... The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman 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 VALLEY'S... 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 OHIO VALLEY, CENTRAL APPALACHIANS, AND WESTERN NEW YORK STATE, AS WELL AS THE COASTAL PACIFIC NORTHWEST... ....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!_LxVYLUqJW0pGU1ZJeIYowLBDBr3Q8xI0jaI2xmIszFL= h0Chwc_ZrGa4h9XEBr5R9Rl0fyISr63whDdK98AoADuekfA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_LxVYLUqJW0pGU1ZJeIYowLBDBr3Q8xI0jaI2xmIszFL= h0Chwc_ZrGa4h9XEBr5R9Rl0fyISr63whDdK98AolXHWNPQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_LxVYLUqJW0pGU1ZJeIYowLBDBr3Q8xI0jaI2xmIszFL= h0Chwc_ZrGa4h9XEBr5R9Rl0fyISr63whDdK98AomtH4KH4$=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) .