Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Apr 10 2024 00:48:33 FOUS30 KWBC 100048 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 848 PM EDT Tue Apr 9 2024 Day 1 Valid 01Z Wed Apr 10 2024 - 12Z Wed Apr 10 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF EAST TEXAS THROUGH THE SOUTHERN MISSISSIPPI VALLEY... 01Z Update: Key Message: Repeating rounds of thunderstorms producing very heavy rainfall over far eastern Texas through north/central Louisiana=20 and west-central Mississippi may lead to locally significant flash flooding into the overnight hours. Additional localized rainfall=20 totals of 5-7+" will be possible.=20 Current radar imagery shows well organized complex of=20 thunderstorms advancing eastward into west-central Mississippi this evening, after tracking through much of northern Louisiana=20 earlier. THis activity has laid down an outflow boundary to its=20 south and will likely be the focus for additional/renewed=20 convection into the nighttime hours with the developing low-level=20 jet. Aloft, there is broad southwesterly flow ahead of the digging=20 shortwave trough moving through Texas and this is pulling plenty of moisture into the region, characterized by PWs above 1.5". A tight gradient of available instability exists as well, 1500+ J/kg=20 MUCAPE from eastern Texas/northern Louisiana/far west-central=20 Mississippi.=20 The expectation through the night is that another round of deep convection will develop across far eastern Texas in a southwest to northeast line and track east/northeast back into north-central=20 Louisiana and eventually into west-central Mississippi. This is=20 likely to fall near/over areas that already have picked up=20 considerable amounts of rainfall in the past 24 hours and where=20 FFGs are much lower, though the latest trends suggest that the heaviest rainfall may end up being just south of the greatest rainfall footprint from the past 24 hours. The 18Z HREF and also=20 the past few runs of the HRRR show some concerning trends with the=20 neighborhood probability of 12-hr QPF ending 12Z Wed exceeding 8"=20 near 20-25 percent in west-central LA and between 10-15 percent=20 into west-central MS. The probability for 5" is up to 45 percent.=20 The past few runs of the HRRR are advertising some localized 6-8"+=20 totals in a narrow stripe.=20 With all of this in mind, will continue to advertise the higher end of the Moderate Risk range (55-70%) across this area for the potential of significant flash flooding into the early morning hours Wednesday. This is due to a combination of saturated ground conditions and additional rainfall totals (5-7"+) in some areas with rain rates exceeding 1-2"/hr at times.=20 For the Texas Panhandle, convection should be waning over the next several hours with the threat of any intense rain rates subsiding after 04-06Z. Some localized 1-2" totals may be possible so will just advertise a Marginal Risk.=20 Taylor 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 least 3" of rainfall in the period have risen sharply with a large swath of EAS probabilities now into the 40-60% range for the 3" marker with neighborhood probs exceeding 70% for at least 5" within that same corridor outlined by the MDT risk. The combination of the boundary, backed flow ahead of the strengthening 5H trough/ULL, and an environment conducive to prime updraft maintenance through the any convective life-cycle will enhance the risk for long-lived thunderstorm clusters and linear segments across the mid-South towards the FL Panhandle near the end of the period. 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. 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. Kleebauer ....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 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. With cases of flood concerns due to hydrologic responses from the previous disturbance and the anticipated rainfall, wanted to make 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- Mississippi Valley given the latest trends in the 5H evolution and convective scheme focusing further north within the tongue of theta-E advection on the eastern flank of the SLP. The SLGT risk was maintained from the previous forecast with only minor adjustments made to correlate with the forecasted rainfall and convective potential. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5PdWJ1lrU-uqi1c7E0tXBEMoV1FWPNiW9c6KeleRVOau= -G4eT2D8A9FSQ_z3Wd0v2Lo2ONkIDjuJhqkUJvhf0TreaE8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5PdWJ1lrU-uqi1c7E0tXBEMoV1FWPNiW9c6KeleRVOau= -G4eT2D8A9FSQ_z3Wd0v2Lo2ONkIDjuJhqkUJvhfkjdO9uk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5PdWJ1lrU-uqi1c7E0tXBEMoV1FWPNiW9c6KeleRVOau= -G4eT2D8A9FSQ_z3Wd0v2Lo2ONkIDjuJhqkUJvhfrLZ1HEc$=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) .