Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Apr 09 2024 15:51:24 FOUS30 KWBC 091551 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1151 AM 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=20 upwards of 2.5"/hr given the latest hourly obs out of central TX.=20 Environment is rich with moisture as of the latest 12z soundings=20 out of SHV indicating PWATs up around 1.7" which is a solid 2.5-3=20 deviations above normal climatologically. Large scale ascent ahead=20 of a deep upper low over the boot heel of NM will continue through=20 the period with multiple shortwaves ejecting out of the base of the trough, interacting with the downstream environment and subsequent=20 boundary juxtaposed over the South. Latest 12z HREF blended mean is much more robust compared to the last run with widespread 3-5"=20 totals now encompassing east-central TX along and east of the I-35=20 corridor near Waco and points east-northeast. As a result, HREF EAS probabilities have soared for all three intervals (1"/2"/3") with=20 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=20 Shreveport with a large 20-30% area for at least 3" now expanding=20 south into central LA which is aggressive considering that area is=20 not anticipated to see the heavy rain threat until a little later=20 in the period. Neighborhood probabilities are more bullish given=20 the definition, but even those probabilities are very high=20 (80-90+") for at least 5" within that target zone between I-35 out=20 to Palestine, TX where 3+" fell last night and early this morning.=20 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.=20 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.=20 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 A PORTION OF THE MID-SOUTH TOWARDS THE FLORIDA PANHANDLE... 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... BLUF: Have scaled back on both the Slight and Marginal Risk areas compared to yesterday's Day 4 ERO. Guidance has trended down with the QPF across the outlook areas, despite the favorable deep-layer forcing ahead of the upper trough which will become more negatively tilted with time. Despite 850-700 mb moisture flux anomalies peaking aoa 4 standard deviations above normal, the swift progression of the upper trough and surface front will inhibit the potential and areal extent of flash flooding. By the same token, there is some deep-layer instability noted per the guidance, especially within the Slight Risk area (generally under 1000 J/Kg). This will allow for more intense short-term rainfall rates, which again could pose a localized (or slightly more enhanced) threat for short term runoff issues, despite the faster eastward progression of pre-frontal quasi- linear segments and thus a more limited risk for cell training during the period. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6cIMOtPLHxWSBkI9yQQczvJrDaal_7T6fYTVR6Pu0w7J= D97Ktklqx1R44gZ70tGtENJV6pg9GhqlTO9Z_ZXxfEXdU8w$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6cIMOtPLHxWSBkI9yQQczvJrDaal_7T6fYTVR6Pu0w7J= D97Ktklqx1R44gZ70tGtENJV6pg9GhqlTO9Z_ZXxeGfTimo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6cIMOtPLHxWSBkI9yQQczvJrDaal_7T6fYTVR6Pu0w7J= D97Ktklqx1R44gZ70tGtENJV6pg9GhqlTO9Z_ZXxWSE37xA$=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) .