Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Apr 09 2024 19:52:03 FOUS30 KWBC 091951 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 351 PM EDT Tue Apr 9 2024 Day 1 Valid 16Z Tue Apr 09 2024 - 12Z Wed Apr 10 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF EAST TEXAS, THROUGH THE SOUTHERN MISSISSIPPI VALLEY... ....16Z Update... The general consensus this morning is for a greater westward expanse of the heavy rain threat as convection blossoms in-of east TX through northern LA along our stationary front currently positioned south of the I-20 corridor from the TX Hill Country to 50-75 miles south of the interstate line in LA. Significant thunderstorm development has occurred over the past hour with rates hovering between 1-2"/hr with a few overachieving cells reaching upwards of 2.5"/hr given the latest hourly obs out of central TX. Environment is rich with moisture as of the latest 12z soundings out of SHV indicating PWATs up around 1.7" which is a solid 2.5-3 deviations above normal climatologically. Large scale ascent ahead of a deep upper low over the boot heel of NM will continue through the period with multiple shortwaves ejecting out of the base of the trough, interacting with the downstream environment and subsequent boundary juxtaposed over the South. Latest 12z HREF blended mean is much more robust compared to the last run with widespread 3-5" totals now encompassing east-central TX along and east of the I-35 corridor near Waco and points east-northeast. As a result, HREF EAS probabilities have soared for all three intervals (1"/2"/3") with the 3"/24-hr contour now indicating up to 70-80% for that area near Waco to the northeast. 30-60% stretches all the way out to Shreveport with a large 20-30% area for at least 3" now expanding south into central LA which is aggressive considering that area is not anticipated to see the heavy rain threat until a little later in the period. Neighborhood probabilities are more bullish given the definition, but even those probabilities are very high (80-90+") for at least 5" within that target zone between I-35 out to Palestine, TX where 3+" fell last night and early this morning. Rainfall rates are sufficient for significant flood potential, especially for areas that have been hit overnight through the morning as areal FFG indices for 1/3/6 hr(s) have all taken a nosedive compared to where they were yesterday. HREF probabilities for 2"/hr or greater are between 20-40% this afternoon across portions of east TX and northern LA but ramp up further this evening thanks to an expected complex of thunderstorms initiating over south-central TX, growing upscale as it approaches the TX/LA border and continues its propagation to the east. This is due to a stronger mid-level vorticity maxima ejecting out of MX and helping organize the areal thunderstorm development within a very favorable environment. Given all the above trends and forecast probabilities, have expanded the Moderate Risk area further west into portions of central TX and points to the east-northeast. This threat will be on the higher end of the MDT threshold and some discussion of a High risk was entertained. This will be monitored over the course of the period, but have maintained the MDT for now. Further west over the TX Panhandle, severe thunderstorms due to the favorable ascent pattern downstream of the upper low has allowed for locally heavy rainfall across the northern Caprock. This convection is scattered in nature for a heavy rain threat as we are missing a primary focal point to pin for training potential that is customary for significant rainfall setups. Still, any smaller towns and low-water crossings across the area between Lubbock and Amarillo will have the threat for localized flooding concerns, thus maintained the SLGT risk in place. Kleebauer ....Previous Discussion... BLUF: Compared to yesterday afternoon's Day 2 ERO, there's a slight expansion of the Marginal Risk area across OK, northwest AR, and southern MO. Otherwise, the Slight Risk area was largely unchanged, while the Moderate Risk was shifted a tad south into the southern Piney Woods of eastern TX, though northern LA and into the ArklaMiss. The trends in the guidance, particularly the high-res CAMs, has resulted in a southward shift in the heavier QPF footprints, largely owing to the deep-layer CAPE trends (greater farther south) and thus the impact on short-term rainfall rates (highest farther south). Deep-layer forcing ahead of the amplifying southern stream trough pivoting into the southern Plains by Wednesday morning will cover quite a bit of real estate, with the broad diffluent flow aloft and coupled left exit/right entrance region forcing from the southern stream/northern stream jet streaks respectively. For this region the Marginal and even Slight Risk areas remain quite large. However, within the Moderate Risk area will be an overlap with a more favorable thermodynamic profile as 850 mb moisture flux/transport anomalies get closer to +3 standard deviations above normal, PWs to 1.75+ inches, and MUCAPEs closer to 1000 J/Kg. Within the Moderate Risk area, the latest (00Z) HREF indicates the highest probabilities...between 25-40+ percent...of 24hr QPF exceeding 5 inches, with 15-25+ percent probs of exceeding 8 inches. Hurley Day 2 Valid 12Z Wed Apr 10 2024 - 12Z Thu Apr 11 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER THE MID- SOUTH INTO PORTIONS OF THE FLORIDA PANHANDLE... ....20Z Update... Moderate risk was expanded back to the west to include eastern LA through the Lower Mississippi Delta into southern AL and the adjacent FL Panhandle. Recent trends on guidance are all in agreement on a significant heavy rain event forming this evening and propagating eastward along a stalled frontal boundary across the Southeast US. This will be the focal point for any convective organization and associated heavy rainfall as depicted by the latest HREF and hi-res deterministic suite. Probabilities for at=20 least 3" of rainfall in the period have risen sharply with a large=20 swath of EAS probabilities now into the 40-60% range for the 3"=20 marker with neighborhood probs exceeding 70% for at least 5" within that same corridor outlined by the MDT risk. The combination of=20 the boundary, backed flow ahead of the strengthening 5H trough/ULL, and an environment conducive to prime updraft maintenance through=20 the any convective life-cycle will enhance the risk for long-lived thunderstorm clusters and linear segments across the mid-South=20 towards the FL Panhandle near the end of the period.=20 HREF blended mean has jumped upwards of 4" for a large chunk of the southern MS/AL area with embedded QPF maxima between 5-6" characterized by the expected strong thunderstorms within the evolution of the overall pattern. Areas outside the MDT are still well at risk for localized flooding concerns due to the high-shear, moisture laden environment present across the Southeast extending north by the second half of the forecast period as our trough ejects to the northeast with flow becoming highly meridional. Rainfall amounts of at least 1" are now forecast all the way up into the central Midwest with areas through the Mid-Mississippi Valley up through IN being the beneficiaries of appreciable rainfall given the amplified upper pattern and strong low to mid- level moisture advection occurring east of the Mississippi. Some concerns linger to the north from previous rainfall that saturated the local river/stream sources, so the prospects for local flood concerns has more merit despite the heaviest rain likely over the Deep South. Regardless, the pattern being highly amplified with a deep moisture return over all locations east of the Mississippi will create an elevated flash flood threat with highest potential over the Southeast US thanks to the combo of primed instability and surface focus along a stationary front located north of the GoM.=20 The MDT risk area over the mid-South is considered the higher end of the threshold and an upgrade to High risk is possible pending convective evolution overnight into the morning. The best risk for heightened impacts will lie from central LA through southern MS/AL with the northern extent likely along the I-59 corridor to about Birmingham.=20 Kleebauer=20 ....Previous Discussion... BLUF: No significant changes made from yesterday's Day 3 ERO. Once again per coordination with local WFO's, given the continued progressive nature of the convective pattern from the latest 00Z guidance (including the 60hr FV3 and RRFS), have maintained a rather targeted Moderate Risk area over the area (including central AL that will most likely see significant rainfall during a larger portion of the forecast period). This area continues to align well with the highest areal-average QPF per WPC and the NBM (2.5 to 4+ inches). However this remains a low confidence ERO forecast, especially given the progressive nature of the convection as with the upper trough and surface front. In addition, some of the extended CAMs, including the 00Z FV3 and RRFS, are now farther south with the heavier rainfall footprint during the Day 2 period (12Z Wed-12Z Thu), i.e. closer to the Gulf Coast. For now, again per collaboration with the WFOs, have opted to maintain continuity with only minor adjustments, with the anticipation of better clarity in the guidance over the next couple of forecast cycles as the event gets within all of the high-res windows. Hurley Day 3 Valid 12Z Thu Apr 11 2024 - 12Z Fri Apr 12 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE APPALACHIANS TO WEST OF THE I-95 CORRIDOR FROM THE MID ATLANTIC TO NORTHEAST... Upper trough across the southern plains will tilt neutral to negative by Thursday morning and begin ejecting northeastward into the Ohio Valley area by the end of the period. Highly meridional flow ahead of the mean trough will allow for a poleward advancement of deep moisture from the Gulf all the way up through the northern Midwest and Northeastern US with a cold front extending from the base of a strengthening SLP moving in tandem with the primary vorticity associated with the shortwave trough. Scattered heavy rain instances will be plausible from the Big Bend of FL up through the Northeastern US with the best flooding risks within the terrain of the Southern and Central Appalachians and along the urban=20 corridor from DC to just northwest of NYC where FFG indices are notoriously low due to urbanization and the recent rainfall. Further to the northwest, our low pressure will deepen considerably before occluding once into MI. Axis of deformation will develop over the Midwest and locally heavy rain on the northwest side of the surface reflection will take shape bringing totals between 1-2" from Thursday afternoon through the end of the forecast period. Lower theta-E values indicate less of a prominent convective risk, however there will be enough forcing and mid-level instability to generate some bands of heavy precip for a time before the system begins scaling back as it becomes vertically stacked.=20 With cases of flood concerns due to hydrologic responses from the=20 previous disturbance and the anticipated rainfall, wanted to make=20 sure the low-end threshold for flash flooding was covered. The MRGL risk was expanded to include the central and northern Midwest with a subsequent northern shift in the SLGT risk into the Mid-=20 Mississippi Valley given the latest trends in the 5H evolution and=20 convective scheme focusing further north within the tongue of=20 theta-E advection on the eastern flank of the SLP. The SLGT risk=20 was maintained from the previous forecast with only minor=20 adjustments made to correlate with the forecasted rainfall and=20 convective potential. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4SNLVHOo8T1GFKO2WlxEbEDkyzWqvnms25v-F1G8Tw4q= GrYQkcZKc4DDylGjRGpnPLe9-Acjybo0arsb4j1kqMec4-I$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4SNLVHOo8T1GFKO2WlxEbEDkyzWqvnms25v-F1G8Tw4q= GrYQkcZKc4DDylGjRGpnPLe9-Acjybo0arsb4j1kiVM0DWI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4SNLVHOo8T1GFKO2WlxEbEDkyzWqvnms25v-F1G8Tw4q= GrYQkcZKc4DDylGjRGpnPLe9-Acjybo0arsb4j1kAQaeqrE$=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) .