Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Apr 10 2024 20:03:27 FOUS30 KWBC 102003 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 403 PM EDT Wed Apr 10 2024 Day 1 Valid 16Z Wed Apr 10 2024 - 12Z Thu Apr 11 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE LOWER MISSISSIPPI VALLEY ACROSS THE CENTRAL GULF COAST... ....16Z Update... Previous MDT risk was expanded a touch further west and south given the recent radar trends and observations signaling rain rates in excess of 2"/hr across portions of eastern LA within the confines of Baton Rouge down into Lake Pontchartrain. Radar/sat composite is quite impressive this morning with upscale growth of convection across eastern TX now plowing eastward through LA with sights on southern MS/AL within the course of the period. A well- defined MCS/MCV is currently moving northeastward through southern MS with rain rates generally 1-2"/hr as it moves off towards Meridian, MS. Further south, backed flow ahead of the complex has aided in rapid convective development in-of Lake Pontchartrain with observed rain rates of 2-4"/hr north of the Causeway. This has spurred several FFW's already within the confines of New Orleans up through that corridor along I-12. Expect rainfall totals between 3-6" with locally as high as 10" over the above area with locally significant flooding possible. The threat will expand eastward through the morning and afternoon with heavy rainfall anticipated basically anywhere along and south of I-20 with the heaviest rains confined closer to the Gulf coast given the expected forward propagation of our MCS, as well as the instability field being most favorable within the proxy of the Gulf with MLCAPE between 1000-1500 J/kg extending through southern LA through southern MS/AL and the FL Panhandle. Within this corridor, 3-5" of rainfall is likely with 6-10" of rain expected within narrow corridors of convection embedded within the main complex, as well as the addition of discrete cells ahead of the system. The MDT risk was expanded eastward to match the trends in hi-res deterministic and the latest HREF blended mean QPF footprint with higher totals forecast over southwestern GA, likely due to the forward speed of the complex and secondary enhancement being forecasted across the FL Panhandle into the area later this evening. Areas along the Gulf Coast from Baton Rouge over into the western FL Panhandle and southeast AL are considered within a higher threshold of the MDT risk and should carefully monitor radar trends as the combination of rainfall rates and potential training will allow for significant, life-threatening flash flooding if it occurs over an area subject to higher runoff potential. Further west, our broad upper low is centered over north-central TX, beginning to take on a more negative tilt based on the latest UA analysis. This will promote considerably backed flow across east TX and the Arklatex all the way out towards the the DFW metroplex and surrounding locales. This is very evident on the forecasted theta-E's within the hi-res suite and will play a role in having a sneaky convective regime crop up later this afternoon after peak diurnal heating. Most guidance now has scattered 1-3" totals spanning north-central TX up through southeast OK into Arkansas given the favorable upper forcing anticipated. 12z HREF EAS intervals of 1" are between 70-90% across northeast TX with the 2" marker a solid 20-40% with a bullseye of 50-60% over western AR south of Fort Smith. Considering the forecasted track of the shortwave trough and enhanced mid-level vorticity, this area extending from Dallas to east-northeast will be the secondary area of interest today for flash flooding concerns with the recent rainfalls bringing down the FFG indices closer to 1-1.5" over the 1 and 3-hr intervals. As our upper low treks off to the northeast, SLP over the southern plains will advance in tandem with significant strengthening expected as it moves into the southern Midwest by the end of the period. There will be a swath of moderate to locally heavy rain that heads into areas of MO/IL/IN that will induce some flood potential, especially within any zones that are currently dealing with higher stream flows after the previous event last week. As a result, have maintained the previous MRGL and SLGT risk extension further north with only some minor adjustments based on the probability and mean QPF fields within the latest HREF and RRFS suite. Kleebauer ....Previous Discussion... ....Summary... Based on the latest guidance trends, particularly the high-res CAMs, have shifted the Moderate Risk area a little farther south (closer to the Gulf Coast) compared to yesterday's Day 2 ERO. Slight Risk area was broadened a bit to including a little more of eastern OK-TX, as well as northward across parts of the Mid MS Valley/Midwest. ....Synopsis... Subtropical jet streak and associated shortwave energy entering the base of the longwave trough will allow for further amplification of the upper trough across the Southern Plains today, as 500 mb height anomalies increase to 2-2.5 standard deviations below normal per the ensembles. Toward the end of the period (by 12Z Thu), the trough will take on a more negative tilt as it pivots through the Lower MS Valley. QG forcing ahead of the trough, aided by ageostrophic circulations owing to the northern and southern stream jet streaks, will maintain robust deep-layer ascent and widespread showers/tstms over the broad outlook regions. ....Mid-South and Lower Mississippi Valley to the Southeast... The more intense rainfall rates will be confined south of the I-20 corridor close to the Gulf Coast, given the stronger deep-layer instability (mixed layer CAPEs 1000-2000+ J/Kg) and more anomalous low-level theta-e/moisture transport into the effective warm/stationary front along and north of the Gulf Coast. Per the ensembles, 850 mb southerly flow and moisture flux anomalies are 5+ standard deviations above normal, with mean integrated water vapor transport (IVT) values peaking between 1000-1250 kg/ms as PW values climb close to 2.00". Expect more numerous convective areas producing hourly rainfall rates up to 2-3 inches within the Day 1 Moderate Risk outlook area, given current observations north of the Upper TX Coast along with the aforementioned thermodynamic parameters. With the latest 24hr mean QPF of 3-5+ inches, 00Z HREF exceedance probabilities are not surprisingly highest within the Moderate Risk area, including 40-60% probability that the Day 1 rainfall will exceed 5 inches, with spotty 20-25% probs of 8+ inch totals. ....Mid Mississippi Valley to Western Ohio Valley... Farther north, the degree of deep-layer instability and moisture flux/transport is decidedly less favorable compared to areas farther south; nevertheless, the southerly influx of deep-layer moisture and some (albeit weaker) instability will be met with stronger frontogenetic forcing ahead of a slower moving surface front, as the favorable right-entrance region of the northern stream jet axis lifts slowly northward. QPF footprint (widespread 1-2+ inch totals) and anticipated 1-3 hourly rates given the thermodynamic parameters would support a Marginal to Slight Risk of excessive rainfall, with the lower FFG values part of the consideration for the Slight Risk areas. Day 2 Valid 12Z Thu Apr 11 2024 - 12Z Fri Apr 12 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE CENTRAL APPALACHIANS AND MID ATLANTIC, ALONG WITH PORTIONS OF THE OHIO VALLEY... ....Summary... Trimmed backed the eastern fringes of the previous Slight Risk across NJ and NY state. Extended the SLGT risk through eastern OH, western PA, and WV given the increasing threat of convection=20 within an area of lower FFGs. ....20Z Update... Recent trends in the synoptic evolution of our broad mid-latitude cyclone have allowed for some additions to the area SLGT risk from the previous forecast period. Large scale ascent on the eastern flank of the strengthening SLP over the central Midwest will increase drastically during peak diurnal instability across portions of OH/PA/WV with scattered to widespread thunderstorms=20 anticipated after 18z Thursday. 12z HREF probability fields=20 indicate a decent threat for rates to reach upwards of 1"/hr=20 (25-40%) with a low-end potential for 2"/3-hrs (5-10%) which would=20 meet the current FFG exceedance intervals located over eastern OH=20 and western PA, west of the Laurel Highlands. Surface low will be=20 pretty robust with very efficient moisture transport all the way=20 into the Ohio Valley and Northeastern US with regional environment=20 solidly within a bound of 1-2 deviations above normal for PWATs.=20 This is attributed to the strong low-level moisture advection=20 flanking into the eastern side of the low within the warm-conveyor=20 belt. This is not a textbook flash flood threat, but one that is=20 coincident with stronger surface cyclones (<990mb). HREF EAS=20 probabilities for at least 1" in precip are between 45-65% over the outlined area in eastern OH with a precipitous drop in=20 probabilities for at least 2". This caps the higher end outcomes=20 necessary for anything higher than a SLGT risk, but was warranted=20 given the setup and lower FFG intervals in place after last weeks=20 heavy rain episode.=20 Further east, very low FFG intervals for all 1/3/6 hr periods will lead to a heightened risk of flash flooding anywhere there a decent convective coverage. The urban corridor from NoVA up through DC/Balt into the eastern half of PA is the most susceptible to convective impacts. Guidance has somewhat trended away from the "worst case scenarios" as the primary 85H LLJ screams into the area, but quickly vacates to the northeast with best moisture flux aimed more at the adjacent Delmarva and shores of NJ up into LI/CT. Those areas have higher FFG intervals given the sandier soils, thus the threat for flash flooding is low enough to maintain just a MRGL risk for the area. As a result for the above, maintained mostly continuity from the previous forecast cycle with the largest shift being the addition of a SLGT out across OH, western PA, and northern WV.=20 Kleebauer ....Previous Discussion... ....Synopsis... As the mid-upper level trough becomes more negatively tilted, large-scale forcing and deep-layer moisture transport will remain favorable up the Eastern Seaboard. The main differences however include the increased forward speed of the upper trough and surface low/associated fronts, along with weaker deep-layer instability the atmosphere will have to work with. ....Parts of the Central Appalachians-Mid Atlantic... Per collaboration with WFOs RNK and AKQ, have removed the Slight Risk across much of central and southern VA. Rather, have confined the Slight Risk where the guidance means indicate greater coverage of 1-3 inches of rainfall in areas of decidedly lower FFG values (widespread 1hr FFGs of inch or less, 3hr FFGs between 1-1.5"). ....Elsewhere across the Eastern CONUS... Maintained the Marginal Risk area outside of the areas with lower FFGs, as 1-3" of rainfall within 2-3 hours would more likely lead to localized/isolated areas of flash flooding. Day 3 Valid 12Z Fri Apr 12 2024 - 12Z Sat Apr 13 2024 ....THERE IS A SLIGHT RISK OVER PORTIONS OF NORTHERN NEW ENGLAND... ....Summary... Based on the latest guidance trends and collaboration with the Gray WFO, have expanded the Slight Risk area northwest across western=20 ME and northern NH. This matches well with the Flood headlines in place due to the incoming heavier rain and snowmelt combination that will result in localized flooding.=20 ....20Z Update... In coordination with the Gray/Portland WFO in Maine, have expanded the SLGT risk from the previous forecast to the Canadian border to align with current Flood headlines in preparation for the incoming moderate to heavy rain threat that will couple with rapid snowmelt along the higher terrain of NH and western ME. Current forecast remains steadfast from previous runs within the ensemble suite with deterministic only showing minor shifts in the precipitation maximum located over the White Mountains into western ME. A solid SWE of 1-3" is positioned within the southern and southeast slopes of the northern Appalachians which is subject to rapid snowmelt due to the flux of higher dew points and expected heavier rains from=20 our sprawling surface cyclone to the southwest. Forecast precip of=20 0.75-1.5" when combined with the snowmelt will generate local flood concerns within the terrain as stream flows will take in a large=20 quantity of water over the 24 hour period. The MRGL risk area was=20 unchanged from previous forecast as the threat of locally heavy=20 rain from the setup will allow for a low-end flash flood threat=20 over New England.=20 Kleebauer ....Previous Discussion... ....Northeast... Anticipated mainly (at most) localized instances of flash flooding as the low lifts northward across western Quebec and the cold and occluded surface fronts push through. A big factor to the excessive rainfall outlook areas is the aforementioned wet antecedent soils, exacerbated somewhat by the ongoing snowmelt. Maintained the Slight Risk across eastern parts of NH into ME, given the heavier rainfall footprint (1-2") per the ECMWF and GEM. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5zfKlPNjXP10PazgQYa3CFem0opbNwUsxHoskk3nMUhH= BkFuw6P3s2znUBGGmkEmM8dVDPBkrXpnQZ1EJkPRm9Gil-E$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5zfKlPNjXP10PazgQYa3CFem0opbNwUsxHoskk3nMUhH= BkFuw6P3s2znUBGGmkEmM8dVDPBkrXpnQZ1EJkPRY6SvACs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5zfKlPNjXP10PazgQYa3CFem0opbNwUsxHoskk3nMUhH= BkFuw6P3s2znUBGGmkEmM8dVDPBkrXpnQZ1EJkPRI_GTQYc$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .