Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat May 11 2024 23:26:40 FOUS30 KWBC 112326 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 726 PM EDT Sat May 11 2024 Day 1 Valid 01Z Sun May 12 2024 - 12Z Sun May 12 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN AND NEAR PORTIONS OF TEXAS... A vigorous upper level low moves eastward, with an influx of deep- layer moisture into the southern Plains/TX elevating over a returning front. Upper diffluence will increase as the upper level trough/closed low aloft approach TX. Thunderstorms near the front=20 in eastern NM have been trying to move eastward, which is advertised by the 18z HREF guidance. Forward motion will remain=20 progressive as 850 hPa inflow veers through the period, but=20 sufficient MU CAPE (up to ~1000 J/kg north and 2000+ J/kg south)=20 and precipitable water values (>90th percentile... ~1" north and=20 towards 2" south) will favor hourly rain totals up to 2.5" beneath=20 stronger cores over TX, particularly where any short bouts of training or mesocyclones manage to form. Farther north, storms=20 with heavy rainfall shift eastward from the High Plains while=20 weakening as they move into areas of lesser instability in OK.=20 Isolated risk of flash flooding summarizes the threat overall.=20 Farther east and later in the period (06-12Z Sun), advancing warm=20 front into southeastern TX will promote heavy rainfall into the=20 region with some occurring over urban areas, and some over more=20 sensitive areas that have seen 10-25"+ rainfall over the past 30 days, with much of the total occurring over the past two weeks.=20 A greater flash flood threat will exist after 12z, but is non-=20 zero before 12Z as HREF probabilities show a weak, but existent, signal for 3"+ by 12z, which led to an eastward extension of the=20 Marginal Risk area from continuity. Roth/Fracasso/Hurley Day 2 Valid 12Z Sun May 12 2024 - 12Z Mon May 13 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF EAST-CENTRAL TEXAS... ....Summary... Mid-upper level trough traversing the Central-Southern Plains north of the Red River will lead to a surge of deepening instability and moisture from the western Gulf of Mexico as a warm front pushes into eastern TX and southwest LA. Favorable dynamic and thermodynamic profiles will result in widespread convection across central-eastern portions of TX into parts of the Lower MS Valley, areas of which have seen well above normal amounts of rainfall. Upgraded a targeted area over east-central Texas to a Moderate Risk of excessive rainfall for the Sunday and Sunday night period within a broader Slight Risk outline that covers the general convective threat. ....Central-Southern Plains and Portions of the Lower Mississippi Valley to the Mid-Upper TX and western LA Gulf Coast... Upper low centered over CO Sun morning will continue eastward into KS with a modest/strong ~100kt upper jet moving across Texas. Convection may be ongoing around 12Z along/ahead of a warm front over southeastern TX and smaller vort maxes streaming northeastward out of northern Mexico/Rio Grande Valley which will be part of the impetus for afternoon/evening convection into east-central TX eastward. Both moisture flux and precipitable water value anomalies will rise to around +2 to +3 sigma as 850 winds increase to 25-40 kts. The duration of the best moisture transport/anomalous moisture flux within the lower- mid levels remains rather transient but could be enough to yield heavier rates in excess of lower FFG values near/south of Dallas. An uptick in low- level frontogenesis along the warm front is still anticipated via the right- entrance region of the upper jet streak traversing the Southern Plains and TN Valley. This will also help retard the warm front's east- northeastward progression. Largely elevated, convection will become more widespread downwind of the warm front in a favorable deep- layer warm/moist advection pattern off the western Gulf of Mexico. The lingering low-level frontogenesis/slow progression of the surface front will allow for some cell training, especially across parts of central-eastern TX which had received quite a bit of rain over the past week. Over this region eastward through western LA, believe the ERO risk will be more of a "high end" Slight (i.e. at least 25% risk of rainfall exceeding FFG within 25 miles of a point). Within the Slight Risk area, 12Z models show pockets of 3-5+ inches of rain during day 2 period within this area, but with continued latitudinal differences (GEFS/GFS/NAM generally north, Canadian south, ECMWF and many of the CAM guidance in the middle). With a bit more agreement on the heavier QPF axis over more sensitive areas (including where rivers are running high -- see the NWC FHO for more info), and in coordination with the FWD/SHV/HGX offices, felt the upgrade to Moderate was justified at this point. Fracasso/Hurley Day 3 Valid 12Z Mon May 13 2024 - 12Z Tue May 14 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE GULF COAST REGION TO SOUTHERN PORTIONS OF THE MID-SOUTH FROM EASTERN TEXAS TO THE FLORIDA PANHANDLE... ....Summary... The aforementioned mid-upper level trough and corresponding surface low/frontal systems are expected to pivot slowly into the Mid Mississippi Valley by Monday night will continue to foster a heavy rainfall -- which by Day 3 (Mon-Mon night) will shift a little farther east to the central Gulf Coast region. Slight Risk outline is centered on southeastern LA eastward across southern MS/AL where there are higher-end probabilities. A broader Marginal Risk areas extends farther north to the Midwest. ....Lower-Mid Mississippi Valley, Parts of the Midwest, Much of the Gulf Coast Region, Parts of the Southeast... Buckling upper flow ahead of the trough will help to bolster the low-level frontogenesis for a bit ahead of the advancing cold front, and while while the anomalies and percentiles have come up per the global guidance, forecast confidence for a prolonged period of strong low-level southerly inflow/moisture transport off the Gulf, which would help justify a Day 3 Moderate Risk ERO, remains low. Latest CSU ML first guess fields support that thinking as well -- showing higher probabilities within the Slight Risk outline that do not yet meet the criteria for a Moderate Risk. Could certainly=20 see at least 2 rounds of more organized convection; one more=20 elevated ahead of the advancing warm front, with the next perhaps=20 a QLCS ahead of the cold front. However in either case given the=20 shear profiles, the storms should be moving relatively swiftly=20 given the degree of forward propagation inferred with westerly=20 Corfidi Vectors averaging 20-30 kts through the Day 3 period.=20 Model guidance 1 and 3 hour QPF progs Mon-Mon night all show this. Also, there's a legitimate chance that the second round of=20 heavier rain (ahead of the cold front) may be farther south than=20 the first round -- hinted at by the north-south displacement in=20 the global models and CMCreg/NAM. ECMWF EFI shows values around=20 0.8-0.9 and a Shift of Tails of 1, but displaced farther north=20 than the GFS/Canadian/UKMET and especially the ECMWF-AIFS which=20 showed its QPF max along the coast (where FFG values are highest). Suspect ultimately there could be a Moderate Risk hoisted over a=20 relatively small/targeted area, but given the aforementioned=20 considerations along with the areal spread in the guidance QPFs,=20 the forecast confidence to hoist a Moderate Risk at this point=20 (Day 3 forecast) is too low. Farther north, a broad Marginal Risk is outlined in advance of the upper low with less instability but lower FFG values. Fracasso/Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4bvRbfw9Q5FPrklbThuPVnzMY5SMxq7UQmremk5Nywxj= EJ3Leyb2ouYyALv0FVAp7nsDuCEJdVoqQUce2jp2Ea2xIIs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4bvRbfw9Q5FPrklbThuPVnzMY5SMxq7UQmremk5Nywxj= EJ3Leyb2ouYyALv0FVAp7nsDuCEJdVoqQUce2jp2CBHoxJc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4bvRbfw9Q5FPrklbThuPVnzMY5SMxq7UQmremk5Nywxj= EJ3Leyb2ouYyALv0FVAp7nsDuCEJdVoqQUce2jp2Gojp-Ok$=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) .