Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jan 09 2024 20:20:58 FOUS30 KWBC 092020 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 320 PM EST Tue Jan 09 2024 Day 1 Valid 16Z Tue Jan 09 2024 - 12Z Wed Jan 10 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC AND NORTHEAST, AS WELL AS FOR PORTIONS OF THE WESTERN CAROLINAS... ....16Z Update... For the most part, the forecast remains on track this morning as the leading edge of the rainfall pushes northeast across northeastern MD and central PA. The primary change with this update was to expand the Moderate Risk area along the Blue Ridge/Shenandoah Valley of western VA. 12Z HREF probabilities show a 50% chance of 5 inches of rain and a 25% chance of 8 inches of rain through 12Z Wednesday in this region. NOHRSC shows up to an inch of liquid equivalent in the form of recently fallen snow along the ridge tops, which should fully melt with the expected rainfall, which will only worsen resultant flooding in this area. Thus, portions of the central Blue Ridge corresponding with those highest HREF probabilities are considered to be in the higher end of the Moderate Risk category, so widespread flash flooding is likely across the area, especially where the heaviest rainfall occurs. The change was made in coordination with LWX/Sterling, VA forecast office. The Slight and Moderate Risk areas were expanded a bit across the western Carolinas and over to the TN side of the NC/TN border in the Great Smokies. Frequent issuance of Flash Flood Warnings has been occurring in this area over the past couple hours, so local upgrades to the risk areas were warranted as ongoing flooding in the area continues to worsen. The last change of significance was a bump northward of the Marginal and Slight Risk areas along the NH and far southern ME coasts. Guidance shows a very impressive plume of moisture characterized by IVT values well over 1,000, and 850 mb temps getting as warm as +6 by 12Z in association with the moisture plume. The caveat in this area is that it's quite cold there now with temperatures in the teens to near 20. The warm-up, while drastic, will be brief, so think any resulting flooding will be isolated. However, because of how quickly temperatures will rise, still think flash flooding is possible. This change was coordinated with the GYX/Gray, ME forecast office. Wegman ....Previous Discussion... ....Mid-Atlantic and Northeast... Amplified upper trough pattern across the central and eastern CONUS will continue as a prolific meridional flow regime with ample moisture advection and ascent will shift eastward within a broad area of difluence and intense 140kt jet coupling associated from the longwave trough and tail of a jet extension over the North Atlantic. Broad cyclonic flow around the upper trough over the central US will advect tropical air poleward with PWAT anomalies along the entire eastern seaboard reaching between 1-3 deviations above normal with IVT values of up to +8 sigma bisecting a large area from the Mid-Atlantic through the Northeast U.S. This type of pattern has historically produced flooding and flash flooding concerns just based off the premise of the anomalous moisture field and heavy rainfall within the population centers located between Raleigh up to Boston. The added concern of snow melt stemming from the significant winter storm from the previous 36 hours through the Mid-Atlantic and Northeast only exacerbates the risk as any additional QPE within the time frame enhances flood concerns within the locales that received snow, as well as downstream within river basins as the water dumps into the array of rivers and streams located within the Northeast. Current forecast of widespread 2-3" with locally as high as 4" presents a major problem for the megalopolis and adjacent areas, especially those with ample snow cover that will become part of the flooding equation. NBM probabilities for at least 2.5" through the period are over 90% for a very large area encompassing eastern PA through the Lower Hudson Valley into southwestern CT. This is the primary area of concern as the snow depth within this zone is a standard 6-12" with as much as 15" in portions of the Catskills and northeast PA. Much of the heaviest rain will occur this evening as the best ascent is realized with a strong PVA signature advecting northeast over the region. A line of heavy rain will extend from southern New England down through the southern Mid-Atlantic by that time with a growing consensus of 1-2" of rain likely in a 6 to 8 hour window between 22-06z Wednesday for the I-95 corridor from DC to north of NYC. Latest HREF probabilities have also increased in regards to total rainfall as 00z EAS probabilities for at least 2" are over 90% encompassing an area from southeast MA down the 95 corridor in CT, through the Lower Hudson valley, through Northern NJ, and eventually arching back into northeast PA. This is coincident with an area that received modest to significant snowfall from the previous event creating a targeted spot for the worst impacts possible. This is a higher-end Moderate Risk for those areas in question as snowmelt and ice jams from the snowfall along with the prolific rainfall will create a very tumultuous setup in regards to flooding concerns. Further south and west from the Blue Ridge up through central and eastern PA, rainfall will still be fairly significant with probabilities of 2" or more running between 40-60% within the lower elevations and over 70% within all of the higher terrain areas above 2500ft. This is likely due to the orographic ascent pattern caused from the prevailing southeast flow banking against the terrain to couple with the broad difluent pattern overhead. A growing consensus of impactful rainfall within the Blue Ridge of west-central VA was enough to warrant an extension of the Moderate to the terrain east of the Shenandoah. High-end Slight Risk is still anticipated for much of the southern Mid-Atlantic between Asheville up through Roanoke, especially within the terrain in those locations. 10-year ARI exceedance intervals were highest over the area east of the Shenandoah with some 100-year ARI interval probabilities showing up in the HREF fields, so that was a factor in limiting the extension further south, along with the higher FFG indices located to the east of Roanoke down to Danville. =20 =20 ....Southern Appalachians...=20 Signal for strong low-level convergence and orographic ascent within the confines of the southern Appalachians in northeastern GA up through the tri-state area of SC/NC/TN continues leading to a maintenance of the Moderate Risk across the aforementioned area. Amplified upper-level pattern upstream of the area will induce a strong low-level moisture advection regime stemming from backed 850mb flow out of the southeast. Guidance has ramped up the intensity of the anomalous flow pattern with boundary layer winds approaching 65kts between 850-700mb which correlates well with a +2-3 deviation PWAT anomaly and saturation to the tropopause as indicated by all major guidance beginning this morning and continuing through the early afternoon time frame. Latest ensemble probabilities for the area remain robust within both a spatial and numerical sense as the prospect for prolific rainfall exceeding 2.5" has not only maxed out in the probability fields, but also expands further east and northeast within the confines of the terrain. 00z HREF EAS probabilities for at least 3" of rainfall within the end of D1 into D2 are over 80% within the area encompassing Clayton, GA up through the terrain northwest of GSP into the mountains adjacent to Asheville in southwest NC. 5"/12-hr neighborhood probability is between 50-70% as well within the tri-state point of GA/SC/NC to the northwest of Greenville, a signature commonly seen in the higher impact events within the terrain focused areas east of the Mississippi. The expansion of the heavy rainfall footprint to include some of the population centers in the Piedmont allowed for a correlating expansion of the Moderate Risk further east and north, including the Greenville/Spartanburg metro area. This was done in coordination with the local WFO in Greenville/Spartanburg as the signatures for locally heavy rain and flash flooding concerns were very much at the forefront with the latest trends. Kleebauer Day 2 Valid 12Z Wed Jan 10 2024 - 12Z Thu Jan 11 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF COASTAL MAINE... ....2030Z Update... Only minor tweaks were made to the Day 2/Wednesday ERO with this update. In coordination with GYX/Gray, ME forecast office, the Slight was adjusted eastward due to a bit faster movement of the rainfall. For far southwestern coastal Maine, the rain should be just about over at the start of the period, despite picking up over an inch of rainfall from Day 1/Tuesday night. Thus, kept the Marginal over a much broader area since it will be the tail end of the event by the start of the Day 2/Wednesday period. Meanwhile further north, the Marginal was trimmed out of northern Maine. Any brief warm up Wednesday morning that would result in some rainfall should largely be absorbed by the existing snowpack. The cold front will race behind the warm front Wednesday afternoon, any brief period where temperatures are above freezing. The area from Augusta northward should be largely immune from any flash flooding threat with this event since the warm air will be so short-lived. Along much of the Maine coast, the combination of heavy rainfall with 850 temperatures rising to near +9, and much higher than normal tidal/coastal flooding should all combine to result in flooding concerns despite the short duration of the warm air. The Slight Risk for the coast remains in effect for the combination of these various factors. Wegman ....Previous Discussion... Anomalous low-mid level flow ahead of the broad upper trough will advect considerable moisture into northern New England with a heavy rainfall footprint extending across southern and coastal Maine...generally through the first half of the period. Area of interest will be across southern Maine near the coast where most of the precip will start and end as rainfall on tonight into Wednesday with snow focused over Lewiston and points north. The Slight Risk is a compounding factor of a multi-period extension of heavy rainfall at the end of D1 into D2 with the best flood potential likely occurring at the beginning of the D2 window given the amount of rainfall, plus the additional volume of water from snow-melt. PWAT anomalies of 3 standard deviations above normal are forecast within both the deterministic data sets, as well as the NAEFS ensemble output which is considerable given the time frame. Much of the ground is either snow covered, and/or frozen within the confines of southern ME leading to a higher probability of run off. Population centers within the outlined area also add to the urbanization concerns that arise in these setups. Prolific heavy rain signature with rates of 0.5-1"/hr will be possible at times over the period from 06-18z Wednesday before shoving off to the northeast. Multi-period totals are between 2-3" within the 18-hr period from 00-18z Wednesday before conditions improve as the low-level flow shifts everything out of the region. With guidance remaining steadfast in the overall evolution and expected rainfall, did not feel much of a need to make many changes to the previous forecast issuance outside a nudge on the northern extent of the SLGT up the ME coast. Kleebauer Day 3 Valid 12Z Thu Jan 11 2024 - 12Z Fri Jan 12 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDDLE MISSISSIPPI VALLEY THROUGH ARKANSAS TO THE ARKLATEX... ....2030Z Update... Very few changes were needed to the Day 3/Thursday period as conditions continue to quiet down from previous days. The Marginal Risk area was extended just a bit towards the northeast to cover more of southeast Missouri as any heavier rains track northeastward into a bit lower FFG area around Cape Girardeau, MO. Elsewhere, no changes were made. Wegman ....Previous Discussion... Much of the D3 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 TX 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 has ramped up the coverage and totals within the time frame of 00-12z Friday with some guidance keen on 1 to locally as high as 2" within a short time after cell initiation. 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). Enough confidence was generated to introduce a MRGL risk across much of Arkansas and the adjacent borders of Mississippi and Missouri. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9pLa7KXy3L5Oaw7wJ6Xnbr9rpJp63jUZDwL_3J4iS-I5= Xyl6BizvdJynezA6qCBM4YeVnbXu4kURQIBONmOxSAREQFg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9pLa7KXy3L5Oaw7wJ6Xnbr9rpJp63jUZDwL_3J4iS-I5= Xyl6BizvdJynezA6qCBM4YeVnbXu4kURQIBONmOxjL5CJ-Y$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9pLa7KXy3L5Oaw7wJ6Xnbr9rpJp63jUZDwL_3J4iS-I5= Xyl6BizvdJynezA6qCBM4YeVnbXu4kURQIBONmOxvTc11lo$=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) .