Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jan 08 2024 15:59:10 FOUS30 KWBC 081559 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1058 AM EST Mon Jan 08 2024 Day 1 Valid 16Z Mon Jan 08 2024 - 12Z Tue Jan 09 2024 ....A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS IN AND NEAR THE CENTRAL GULF COAST... Southeast/Gulf Coast... ....1600Z Excessive Rainfall Discussion... The 08/12Z HREF showed little reason to make wholesale changes to the on-going Excessive Rainfall Outlook. Trends in radar imagery suggested a westward expansion of the Marginal Risk area over eastern Texas was in order. Mid-morning satellite imagery showed cooling cloud tops in a region of upper diffluence of the upper low coincided with the axis of 70 to 74 knot low level jet sampled by the morning soundings leading to an uptick in convection capable of producing local downpours. The HREF probability values for 1 inch through 3 inch rainfall amounts in 1-hour blocks and the probability of 3 hr QPF exceeding 3-hour flash flood guidance both peak during the 09/00Z to 09/06Z range from portions of eastern/southeastern Louisiana northward into southern Mississippi...on par with timing from the previous run and consistent with the best low level dynamics/moisture flux off the Gulf.=20 Bann ....0830Z Excessive Rainfall Discussion... Maturing upper level pattern across the central and southern plains will allow for a deep moisture advection pattern downstream of the mean trough with the central Gulf coast in the crosshairs for the most prevalent low-level moisture and accompanying instability thanks to the rich, Gulf air being tapped in the setup. Intense cyclogenesis pattern over the southern plains in tandem with backside flow from a 1030+ mb high pressure east of the Mississippi will create a strong south to southeast low-level wind pattern that will advect tropical air northward allowing PWAT anomalies to grow up to 2-3 deviations above normal within parts of the southeast up into the southern reaches of the Midwest. This amplified pattern will aid in period of very heavy rainfall to initiate over the southeast with the highest probability for heavy rainfall and flooding concerns over the central Gulf coast. A couple of major factors at play that will enhance the opportunity for flash flooding, as well as areal extent of the potential. The first is the highly anomalous setup at both the surface and mid-upper levels where elevated integrated vapor transport is all but certain as IVT values of over 1000 kg/ms are forecast off the latest deterministic guidance, a prolific 6-8 sigma above normal characteristic indicative of the tropical moisture source that will be available within the confines of the Gulf coast. This allows for PWAT indices to reach upwards of 1.6-1.8", something you see more of in the warm season comparatively which is genuinely when flash flooding concerns begin to rise. Considering the airmass, theta-E advection regime will be co-located within the right entrance of a strengthening mid-level jet pivoting around the base of the upstream trough. Instability will rise through the afternoon with 1000-1500 J/kg CAPE signatures being forecast within the zone from Lake Charles over into southern AL and as far north as Jackson, MS and Montgomery, AL. Within this area will be the highest risk for flash flooding concerns given the expected convective signatures both ahead of and along an advancing cold front that will quickly propagate eastward on the southern edge of the surface low to the northwest. A strong low-level convergence pattern will develop across the south with an axis of thunderstorms and very heavy rainfall likely to move from south-central LA through southern MS and AL by the end of the period. 00z HREF neighborhood 1"/hr rate probabilities are over 80% for a large area within the aforementioned area with 2"/hr rate probabilities solidly between 25-40% between the hours of 00-12z with emphasis over southern MS. HREF neighborhood probabilities of at least 5" during the period from 00-12z Tuesday are over 80% between Baton Rouge through the Mississippi Delta with 8" probabilities of 20-25% showing up in the probability fields. This is a significant flood potential from both the means and probability fields and that doesn't even touch the local maximums from deterministic output. Have maintained the previous Moderate risk area, but expanded to the east over southern AL given the latest guidance expanding the heavy rain QPF footprint and correlated probability increases within the zone. Slight risk was also expanded further north into northern AL to account for both guidance and analog output for the incoming event. Kleebauer =20 =20 Pacific Northwest...=20 No changes to the previous forecast as the overall synoptic evolution and expected rainfall remains on track. Precipitable water values of 0.75"+ move ashore on the heels of 50 kts of 850 hPa inflow out of the west to the south of a strengthening cyclone heading towards Vancouver Island. Two bouts of heavier precipitation are expected...a warm air advection precipitation pattern early on and then the main moisture plume near the cold front late in the period when some instability moves in from the Pacific. Between the degree of inflow, expected boundary layer moisture convergence, and moisture availability, hourly rain totals of 0.5"+ should be possible at times today into early Tuesday. An old rule of thumb based on the sustained onshore flow suggest local maxima in the 4" neighborhood, not far from the 00z Canadian Regional guidance. Enough was there to maintain the Marginal Risk and continuity was generally maintained. Bann 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 THE PARTS OF THE SOUTHERN APPALACHIANS... ....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 further 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 across southern and coastal ME through the first half of the period. Area of interest will be across southern ME 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. 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. In coordination with the local WFO in Gray, have upgraded the coastal areas from the NH state line up to near Belfast with a Slight Risk in conjunction to the signals for a period of heavy rainfall and anomalous moisture flux downstream of our amplified upper trough. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9YhnY_kSxIVhJwMV4fINxmkjodRQKh8Qo1n_R9o9eS96= HvdPaxfJ73evjyeG2lhuCtqy_7lpO36WeAyXTlMKGAGKlJI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9YhnY_kSxIVhJwMV4fINxmkjodRQKh8Qo1n_R9o9eS96= HvdPaxfJ73evjyeG2lhuCtqy_7lpO36WeAyXTlMKNfYgRT8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9YhnY_kSxIVhJwMV4fINxmkjodRQKh8Qo1n_R9o9eS96= HvdPaxfJ73evjyeG2lhuCtqy_7lpO36WeAyXTlMKM8NnDd4$=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) .