Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Apr 26 2025 19:00:17 FOUS30 KWBC 261900 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sat Apr 26 2025 Day 1 Valid 16Z Sat Apr 26 2025 - 12Z Sun Apr 27 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTH AND NORTHWEST TEXAS INTO SOUTHERN OKLAHOMA... ....16Z Update... The Slight Risk area was adjusted downstream of the ongoing convection occurring across far north Texas and central and southern Oklahoma. A well established Mesoscale Convective Vortex is approaching the Oklahoma City area as of the time of this writing, with training thunderstorms actively causing flash flooding south of the city. For that reason, a higher-end Slight is in effect roughly from the Oklahoma City south to the Texas state line along the I-35 corridor, extending west to about Wichita Falls, TX. Guidance is in understandably reasonable agreement that this MCS will persist over the next few hours, as the embedded MCV tracks along I-44. For now, the slow movement and lack of instability suggests metro Tulsa only has an isolated (and therefore, Marginal) flash flooding threat. The Slight and surrounding Marginal were trimmed on the northwestern side behind the MCS, as none of the guidance suggests that there will be afternoon convection in this area with all the instability having been used up. The Marginal is largely unchanged owing to poor agreement in the CAMs as to how new convection will emerge this afternoon across portions of central and northern Texas. The behavior of the MCS as a whole will play a big role in how any new convection develops this afternoon. Wegman ....Previous Discussion... ....Summary... Based on the latest observational and guidance trends, have expanded the Slight Risk from yesterday's Day 2 ERO a little farther west-southwest to include a bit more of the Rolling Plains in Northwest Texas. Otherwise, more minor adjustments were made to the surrounding Marginal Risk area. ....Southern Plains... MCS and resultant MCV currently traversing the Texas Panhandle early this morning will gradually track ENE along the quasi- stationary surface frontal boundary across much of the TX-OK Red River Valley today. By late morning, the weakening LLJ will allow the MCS to diminish, while later in the day (afternoon) a narrow broken line of convection develops farther upstream along the dryline across Northwest TX during peak heating. This second area of convection is expected to take on a more ESE trek during the afternoon and evening, thus the southwest 'bulge' to both the Slight and Marginal Risk areas across the Rolling Plains and (for the Marginal Risk) into northern portions of the Hill Country. Overnight, the surface stationary front will eventually lift northeast as a warm front Saturday night, taking the more organized/widespread convection with it. As with yesterday's Day 2 ERO, the Day 1 Slight Risk area was adjusted to capture areas where 1-2+ in/hr rainfall rate probabilities from the HREF/RRFS ensembles are highest, while also overlapping areas where heavy rainfall has fallen over the past couple of days (wetter antecedent interviews). Hurley Day 2 Valid 12Z Sun Apr 27 2025 - 12Z Mon Apr 28 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN AND SOUTHEASTERN MONTANA... ....1930Z Update... In coordination with BYZ/Billings, MT forecast office, a Slight Risk upgrade was introduced with this update. Increasing forecast rainfall (areal averages of 2-3 inches southwest of Billings) will combine with high snow levels and burn scars to locally enhance the flash flooding potential in this area of Montana. The combination of surface troughing ahead of a northward moving leeside low out of Colorado, an advancing upper level low and trough out of the Rockies, and the northern tongue of an LLJ that originates straight out of the Gulf will all combine their respective forcings to support the area of heavy rain across southern Montana Sunday and Sunday night. High snow levels will support rain falling on snow, locally increasing snowmelt. Burn scars from various large fires last year will also locally increase the flash flooding potential=20 due to locally enhanced runoff. FFGs in portions of the Slight=20 Risk area are under an inch per hour, which are rates that are=20 probable to be exceeded given the convergence of all the=20 aforementioned ingredients favoring heavy rainfall. Widely=20 scattered instances of flash flooding are likely.=20 Wegman ....Previous Discussion... Mid-upper trough across the southern Great Basin early Sunday will lift across the central Rockies by 12Z Monday. This in-turn will nudge the upper ridge axis east of the Northern Plains and Upper MS Valley late Sunday-Sunday night, with a quick onset of upper level divergence in its wake. Along with the more favorable dynamical forcing (upper divergence and deep-layer ascent), deepening southerly flow will tap the western Gulf and lead to an uptick in low-mid layer moisture transport, with TPW values climbing to 1.3+ inches. Deep-layer instability meanwhile will also be ramping up, to the tune of 1000-2000+ J/Kg MUCAPE per the 00Z models. To some degree, the latest models indicate MCS development along the western periphery of the upper ridge, tracking east to eventually southeast along the 850-300 mb thickness. For the most part, the forward-propagating Corfidi vectors would support a rather progressive area of any organized convection that can grow upscale, though much will depend on the strength of the LLJ. For now, have maintained a fairly broad Marginal Risk area given the potential of intense short-term (sub-3hr) rainfall rates. Hurley Day 3 Valid 12Z Mon Apr 28 2025 - 12Z Tue Apr 29 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN PLAINS INTO THE UPPER MIDWEST... ....1930Z Update... No significant changes were needed for the Marginal Risk area across the northern Plains and upper Midwest. Deep soils across the Dakotas and prevalence of lakes in northern Minnesota should largely preclude other than isolated flash flooding. The cold=20 conveyor belt of an intensifying low will run into a strong cold=20 front with deep cold air behind it, resulting in ample baroclinic forcing for steady light to moderate rain across the Dakotas and Minnesota. Meanwhile into Wisconsin the pre-cold frontal convection will be the driver for any isolated flash flooding. There is some potential that before snow levels crash in southeastern Montana that lingering rainfall from Sunday night may continue into Monday. This may resulting in a locally enhanced=20 flash flooding risk into far northern Wyoming with the burn scars=20 on the northwest facing slopes of the Bighorn Mountains. The=20 predominant northeasterly flow orthogonal to the range may locally=20 enhance upslope and heavy rainfall rates in the area.=20 Wegman ....Previous Discussion... Mid-level circulation will open into a shortwave trough across the northern Plains and Upper Midwest on Monday-Monday night. Ample pre-frontal low-mid layer moisture transport and PW values climbing to 1.5-1.75" will make for a favorable thermodynamic profile, along with deep-layer instability as MUCAPEs peak between 2,000-3,000 J/Kg (even ~1,000 J/Kg wrapping westward into the TROWAL zone). The biggest concern during this period will be with cell training, especially late Monday-Monday night across the Upper Midwest with the strengthening LLJ and weakening downwind Corfidi Vectors. For now, guidance spread with the heavier QPF is still rather high (not well consolidated to support a Slight Risk at this point). However, per the GFS/ECMWF and especially the GEM- Regional, localized totals of 3-5+ inches within 12 hours would appear likely in this setup. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt= __;!!DZ3fjg!_pyAgUFxwdnO3O47BgPhtqFIwaC-Mqz7SuiqjMIC0QbmIsBnCwZJzf7xiXJY_fQ= 3Nx_G_Bwg4XlzpqDlVpc6kPi5yZM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt= __;!!DZ3fjg!_pyAgUFxwdnO3O47BgPhtqFIwaC-Mqz7SuiqjMIC0QbmIsBnCwZJzf7xiXJY_fQ= 3Nx_G_Bwg4XlzpqDlVpc6Ma_7tAY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt= __;!!DZ3fjg!_pyAgUFxwdnO3O47BgPhtqFIwaC-Mqz7SuiqjMIC0QbmIsBnCwZJzf7xiXJY_fQ= 3Nx_G_Bwg4XlzpqDlVpc6hYK5QUI$=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) .