Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Apr 10 2024 15:52:58 FOUS30 KWBC 101552 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1152 AM 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=20 given the recent radar trends and observations signaling rain=20 rates in excess of 2"/hr across portions of eastern LA within the=20 confines of Baton Rouge down into Lake Pontchartrain. Radar/sat=20 composite is quite impressive this morning with upscale growth of=20 convection across eastern TX now plowing eastward through LA with=20 sights on southern MS/AL within the course of the period. A well-=20 defined MCS/MCV is currently moving northeastward through southern=20 MS with rain rates generally 1-2"/hr as it moves off towards=20 Meridian, MS. Further south, backed flow ahead of the complex has=20 aided in rapid convective development in-of Lake Pontchartrain with observed rain rates of 2-4"/hr north of the Causeway. This has=20 spurred several FFW's already within the confines of New Orleans up through that corridor along I-12. Expect rainfall totals between=20 3-6" with locally as high as 10" over the above area with locally=20 significant flooding possible. The threat will expand eastward=20 through the morning and afternoon with heavy rainfall anticipated=20 basically anywhere along and south of I-20 with the heaviest rains=20 confined closer to the Gulf coast given the expected forward=20 propagation of our MCS, as well as the instability field being most favorable within the proxy of the Gulf with MLCAPE between=20 1000-1500 J/kg extending through southern LA through southern MS/AL and the FL Panhandle. Within this corridor, 3-5" of rainfall is=20 likely with 6-10" of rain expected within narrow corridors of=20 convection embedded within the main complex, as well as the=20 addition of discrete cells ahead of the system.=20 The MDT risk was expanded eastward to match the trends in hi-res=20 deterministic and the latest HREF blended mean QPF footprint with=20 higher totals forecast over southwestern GA, likely due to the=20 forward speed of the complex and secondary enhancement being=20 forecasted across the FL Panhandle into the area later this=20 evening. Areas along the Gulf Coast from Baton Rouge over into the=20 western FL Panhandle and southeast AL are considered within a=20 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=20 occurs over an area subject to higher runoff potential.=20 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=20 rainfalls bringing down the FFG indices closer to 1-1.5" over the 1 and 3-hr intervals.=20 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.=20 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 INTO SOUTHERN NEW YORK... ....Summary... Based on the latest guidance trends, have scaled back on both the Slight and Marginal Risk areas inherited from yesterday's Day 3 ERO. ....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, along with hydrologic factors (latest FFGs and relative soil moisture/soil moisture percentiles), have expanded the Slight Risk area slightly northeast across interior Maine. Otherwise, no other changes were made. ....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!-I3qjq6B2WnPo6-QaYrOLKipHYT5gh4F_37QnK-jUEbp= Ib9s2bC-hL7nRFYUHMhR8HHtexm9ikXZASe2NYJXONI6MkQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-I3qjq6B2WnPo6-QaYrOLKipHYT5gh4F_37QnK-jUEbp= Ib9s2bC-hL7nRFYUHMhR8HHtexm9ikXZASe2NYJXj9p3J8g$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-I3qjq6B2WnPo6-QaYrOLKipHYT5gh4F_37QnK-jUEbp= Ib9s2bC-hL7nRFYUHMhR8HHtexm9ikXZASe2NYJXOppzM8k$=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) .