Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jan 09 2025 19:15:19 FOUS30 KWBC 091914 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 214 PM EST Thu Jan 9 2025 Day 1 Valid 16Z Thu Jan 09 2025 - 12Z Fri Jan 10 2025 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS THE MIDDLE AND UPPER TEXAS COAST AND WESTERN LOUISIANA GULF COAST... ....16Z Update... The overnight forecast remains on track with little adjustment made to the inherited Marginal Risk along the western Gulf of Mexico coast. The moisture advection associated with this upper trough is exceptional as NAEFS shows IVT values topping 1,000 kg/m/s over the Upper Texas coast around 00Z this evening, which is well above the maximum observed IVT in the CFSR database for this time of year. In addition, RAP soundings around 00Z this evening depict warm cloud layers as deep as 12,000ft in some cases along the Upper Texas coast. There is the potential for briefing training of convection along the Upper Texas coast as low pressure offshore organizes and the warm front lifts north this evening, and convection should contain efficient rainfall rates given these deep warm cloud layers. That said, instability aloft is very limited and should keep flash flooding concerns to those in highly urbanized locations and/or areas that have a history of draining poorly. Mullinax --Previous Discussion-- Increased mid and upper level forcing is ongoing over portions of TX thanks to the slow ejection of a potent ULL situated over Sonora. A digging shortwave trough analyzed over the Central Rockies will continue plunging southward with an eventual partial phase with the primary shortwave disturbance moving east into West TX. At the surface, a maturing surface low over the western Gulf will slowly wander northward towards the middle TX coast with expected moisture advection regime to be a significant player in the prospects of locally heavy rainfall in-of the immediate TX coast. As of this time, there is a general consensus on the heaviest precip being focused up the coast near Port Aransas up through the remainder of the middle and upper TX coasts, pinning a bullseye closer to the Galveston area and points northeast. This is where a frictional convergence regime with persistent easterly flow on the north side of the main surface low will help initiate a period of convection just off the immediate coast within the primary axis of higher theta_E's located on the western flank of the surface reflection. Convection will hug the coast with some of the heavier precip cores moving ashore creating opportunity for rainfall rates to reach 1-1.5"/hr at peak intensity and producing totals between 2-3" with locally as high as 4" along that immediate stretch of coast. The longwave evolution will generate ample ascent within the diffluent axis downstream of the potent mean trough, pulling moisture inland and generation pockets of heavy rain just inland, but to less of a degree of the immediate coast thanks to the local instability maximum likely hugging the coast with near 0 CAPE just inland. Recent probabilities from the 00z HREF are indicative of the locally heavy rain threat with the neighborhood probs of >3" up between 40-70% for the stretch of the TX coast beginning from Port Aransas up the Lower Sabine where TX/LA border along the Gulf coast. There's non-zero 5+" probs, but a much lower factor of probabilities comparatively (<20%) and mainly confined to a small area near Galveston. This has been the zone of heaviest precip potential and likely strongest low-level convergence due to the forecast proxy of the surface low and greatest theta_E advection regime. Considering all the above variables, there was little need to deviate from the previous forecast. The previous MRGL risk was maintained given the steady forecast continuity and recent favorable probability fields. Kleebauer Day 2 Valid 12Z Fri Jan 10 2025 - 12Z Sat Jan 11 2025 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL GULF COAST... ....20Z Update... The inherited Marginal Risk remains in good shape as the region will struggle to see a dearth of instability aloft and any storms/showers will be progressive in their easterly movement.=20 That said, NAEFS shows that around 12Z Friday, IVT values aimed at the central Gulf Coast will be as high as 1250 kg/m/s, which would be above all observed IVT values in the CFSR database for this=20 time of year. Similar to the Upper Texas Coast on the Day 1 ERO,=20 soundings are highly saturated and warm cloud layers are around=20 12,000ft deep. This should result in efficient warm rainfall that=20 may support highly localized flash flooding potential, especially=20 in urbanized areas that drain poorly along the I-10 corridor. But=20 the progressive storm motions and lack of modest instability should keep the areal extent and severity of the flash flood threat to a=20 limited number of at-risk urbanized areas through the early=20 afternoon hours on Thursday. Mullinax --Previous Discussion-- Our western Gulf disturbance will migrate east-northeastward with a focus of heavier precip across southern LA through the urban zones of southeast LA during the morning Friday. The system will be fairly progressive and the trends have brought down storm totals due to the progressive nature and limited instability factoring in the local maxima relegated <2". The expectation is for there to be a thin line of heavier rates along a developing cold rain band, something not too common at these latitudes. There is a textbook surface convergence pattern right within the north and western flank of the low center as it cross through the southeastern Parishes of LA and this aligns within a narrow tongue of elevated theta_E's that get advected just ahead of the surface reflection. This will lend of a period of potentially significant rainfall rates bordering 1"/hr at peak intensity, a factor that could be enough to generate some localized flooding within the urban zones centered around New Orleans and points north, west, and east. This is very much of a lower end MRGL risk considering the trends of the forward propagation and such a skinny axis of the heavier precip anticipated. This also aligns with more of a morning and early afternoon risk with a quick decay behind the low as light stratiform takes over, then dries out as drier air advects in behind the departing low to the east. The previous MRGL risk was maintained in full with an emphasis on flooding potential along and south of I-10/12 with the best prospects located around urban zones surrounding Lake Pontchartrain. Kleebauer Day 3 Valid 12Z Sat Jan 11 2025 - 12Z Sun Jan 12 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Mullinax Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4Obf_Jd9oxBk1NArpjLZcNR-rXJT0bgMm5Do-aBHMbje= a1EHVt_QzTUrFGpTPaN9VZlbd7KO07SvlSxhyQNGTZpTpes$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4Obf_Jd9oxBk1NArpjLZcNR-rXJT0bgMm5Do-aBHMbje= a1EHVt_QzTUrFGpTPaN9VZlbd7KO07SvlSxhyQNGlZ11xgM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4Obf_Jd9oxBk1NArpjLZcNR-rXJT0bgMm5Do-aBHMbje= a1EHVt_QzTUrFGpTPaN9VZlbd7KO07SvlSxhyQNGOoxU5nM$=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) .