Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jan 09 2024 00:58:52 FOUS30 KWBC 090057 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 756 PM EST Mon Jan 08 2024 Day 1 Valid 01Z Tue Jan 09 2024 - 12Z Tue Jan 09 2024 ....A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS IN AND NEAR THE CENTRAL GULF COAST... Southeast/Gulf Coast... Ongoing forecast looks in pretty good shape. Recent HRRR runs and the 18z HREF continues to focus additional heavy rainfall over portions of the central Gulf Coast region overnight. Low level moisture transport and deep layer IVT both increase in magnitude tonight, and the axis/orientation of this increase indicates portions of southeast MS into central/southern AL and the far western FL Panhandle will see the longest duration of these enhanced values. Recent radar imagery shows some training/backbuilding of cells near the northward lifting warm front near the central Gulf Coast and similar activity is expected to persist into the overnight hours, with some uptick in coverage/organization likely given the aforementioned IVT uptick. Meanwhile more progressive convection along/ahead of the eastward moving cold front will move across the region from west to east tonight...and while the duration of this activity should not be all that long, it will likely produce intense hourly rates of 1-2"/hr, which in many places will be falling on top of increasingly saturated conditions from the warm frontal rainfall. Give all this, additional flash flooding is likely into the overnight hours across portions of eastern LA into southern MS/AL. Recent runs of the experimental WoFS suggest a QPF axis a bit south of recent HRRR runs...and recent radar trends seem to support this idea. The backbuilding convection may very well slow the northward warm frontal progression and support a max QPF axis a bit south of the HRRR. The main adjustment this update was to expand the MDT risk a bit eastward into more of southwest AL and far western FL Panhandle. This is based on the aforementioned 850mb moisture transport and deep layer IVT axis, along with following a slightly southward shifted HRRR/HREF QPF footprint. The expectation is for an additional 3-6" of rain over portions of the MDT risk area through 12z Tuesday. Otherwise a modestly faster progression of the rainfall warranted an expansion of the Slight risk into portions of the southern Appalachians, where some runoff concerns may begin to increase by 12z Tuesday. Pacific Northwest...=20 No major changes to the previous forecast as the overall synoptic evolution and expected rainfall remains on track. An additional 1-3" of rainfall is expected overnight along the favored coastal terrain. Chenard Day 2 Valid 12Z Tue Jan 09 2024 - 12Z Wed Jan 10 2024 ....A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE MID-ATLANTIC AND NORTHEAST, AS WELL AS A PORTION OF THE SOUTHERN APPALACHIANS... ....2030Z Excessive Rainfall Discussion... Not many changes made to the previously issued ERO. There was a slight eastward extension of the Moderate Risk area in Southern New England was warranted given the observed snowfall/snow cover from the storm this past weeked...and the added snow water equivalent that will be added to the volume of rainfall.=20 Overall...though...the large scale synoptics changed little with an amplified upper trough inducing a pronounced meridonal flow regime that draws deep moisture northward into the Mid-Atlantic region and Northeast United States. While there have been some minor run to run adjustments with respect to QPF...no major trend one way or another was noted and the changes to the ERO were minor in the Mid-Atlatic/Northeast and Southern Appalachian. Bann ....0830 Excessive Rainfall Discussion... ....Mid-Atlantic and Northeast... Amplified upper trough pattern across the central and eastern CONUS will induce a prolific meridional flow regime with ample moisture advection and ascent within the large area of difluence downstream of the mean trough, as well as between a 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 +7 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 that just passed through the Mid-Atlantic and Northeast only exacerbates the risk as any additional QPE within the time frame, enhancing 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 18" in portions of the Catskills and northeast PA. Much of the heaviest rain will occur on Tuesday 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 indications of 1-2" of rain likely in a 6-hour window between 00-06z Wednesday for the I-95 corridor from DC to near NYC. In coordination with local WFO's across the Mid-Atlantic and Northeast, was able to extend the previous Moderate Risk area farther west over much of eastern PA, northward into the Lower Hudson Valley, north to the Connecticut River, and eventually south to include the DC/Balt metro, northern VA, and adjacent areas within the Piedmont. Considering the CIPS analogs and general uptick in the forecast for the region, this is trending towards a more significant flood possibility just from within the means, not even including the signatures from current deterministic and eventual hi-res once they get into range of the event.=20 =20 =20 ....Southern Appalachians...=20 An emerging 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 has allowed for a targeted upgrade to a 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 60kts between 850-700mb which correlates well with a +2 deviation PWAT anomaly and saturation to the tropopause as indicated by all major guidance within the Tuesday AM time frame. Rainfall totals within a time frame of 00z Tuesday at the back end of D1, until 00z Wednesday are approaching 4" with a lot of the precip occurring within a 6-12 hr window as ascent is maximized within the large difluent signature ahead of the main trough axis. Probabilistically was likely the largest signal for elevated flooding concerns with the 00z HREF coming into view for the time frame of significance. The most gaudy of the signatures comes from both the neighborhood 5"/12-hr and 3"/24-hr EAS probabilities (00z Tue - 00z Wed) with a respective 80% and 80-90% for each probabilistic set of guidance. There is even some non-zero 8"/24-hr neighborhood probability signatures showing up in the latest data with 25-30% showing up over the area outlined. Given the topographic elements involved within the corridor outlined, as well as CIPS analog signatures pinning a "bullseye" of QPF max potential within the southern Appalachian front between Asheville, NC to Clayton, GA, have upgraded to a Moderate Risk within the above area with coordination and agreement from the Greenville/Spartanburg WFO.=20 =20 Kleebauer Day 3 Valid 12Z Wed Jan 10 2024 - 12Z Thu Jan 11 2024 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF COASTAL MAINE... 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 Main...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 Tuesday night into Wednesday with snow focused over Lewiston and points north. The upgrade to SLGT was a compounding factor of a multi-period extension of heavy rainfall at the end of D2 into D3 with the best flood potential likely occurring at the beginning of the D3 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 from both the GFS deterministic, as well as the NAEFS ensemble data set 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. Even at day 3, models are keen on a prolific heavy rain signature with rates of 0.5-1"/hr possible at times over the period from 06-15z before shoving off to the northeast. Multi-period totals are around 1.5-2.5" within the 18-hr period from 00-18z Wednesday before conditions improve as the low-level flow shifts everything out of the region. Bann/Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7H24DxXNqD5OAkRx9csUwK_7zcq3j8TqxoEv-9jQZ4xm= c4YBMQdmJy-6wT9HMIJr56kttYXhxonDd05_sGNV1JqTTho$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7H24DxXNqD5OAkRx9csUwK_7zcq3j8TqxoEv-9jQZ4xm= c4YBMQdmJy-6wT9HMIJr56kttYXhxonDd05_sGNVkQvqXc8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7H24DxXNqD5OAkRx9csUwK_7zcq3j8TqxoEv-9jQZ4xm= c4YBMQdmJy-6wT9HMIJr56kttYXhxonDd05_sGNVUf4ppis$=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) .