Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jan 29 2025 15:36:15 FOUS30 KWBC 291535 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1035 AM EST Wed Jan 29 2025 Day 1 Valid 12Z Wed Jan 29 2025 - 12Z Thu Jan 30 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTH TEXAS AND SOUTHEASTERN OKLAHOMA... ....16z update... Observational trends in moisture placement remain on track, a tad=20 faster than bulk of guidance suite with solid surface to 850mb=20 anomalous moisture values/flux already increasing out of the=20 western Gulf into central Texas. CIRA ALPW flux in this layer is=20 analyzed at the 99th to maximum percentile for the 10yr period of=20 record at this time of year. LPW also shows subtropical moisture=20 connection already in place across northern Mexico into southern=20 Texas at the apex of the subtropical ridge helping to result in=20 1.5" total PWats that will slowly increase toward 1.75" by the end=20 of the forecast period when 40-45kt LLJ is peaking.=20 Hi-Res CAMs waver a bit on magnitude of elevated instability layer to support stronger updrafts along the convergent portion of the slowly eastward drifting warm conveyor belt. Most generally range in the 750-1000 J/kg range and should likely support elevated cells capable of 1.5-2"/hr rates, mainly after 00z. Trends in guidance=20 are typical with a slight eastward/southward shift and the 16z=20 update of the Slight Risk adjusts accordingly , but also expands=20 along the narrowing axis of training/repeat cells southeastward=20 into the LLano Uplift region of central TX as well as into portions of western AR. 12z HREF probabilities agree with this small south=20 and eastward adjustments. Small increases in 3" and 5" 24hr=20 probability ranges also enhance confidence in Slight Risk=20 placement.=20 Gallina=20 ~~~Prior Discussion~~~ An upper level cutoff low ejecting out of the Southwest into the Plains will provide ample forcing for ascent (along with the left exit region of a ~120 kt jet streak) for a developing low over the Texas and Oklahoma Panhandles into tonight. Return flow of Gulf moisture, warmth, and instability ahead of the low will advect north across Texas and into Oklahoma and Arkansas today (and especially into tonight). The typical diurnal strengthening of the LLJ overnight will greatly increase the coverage and intensity of storms along the developing low's cold front across Texas (particularly after 06z), while the warm front locally increases lift into Oklahoma. The surface low will track north into Colorado, contributing to a winter storm there. Meanwhile much of the convection will be associated with the cold/occluded front as it pushes east towards the Mississippi Valley. Since the upper level low will be slow-moving (though increasing in forward speed with time), this will allow the storms that do form to have a high likelihood of training over the same areas. Training convection will be of greatest concern as the storms are first forming over North TX and into southeast OK (where a targeted upgrade to Slight risk was made, given 00z HREF 40-km neighborhood probabilities for 3" exceedance of 50-70%, and primarily over a relatively short period from 06z-12z Thurs). Soils in the area have been drier than normal, meaning most rainfall should (in theory) be beneficial, however the rapid development of rainfall rates of 1-2"/hr over the dry soils may locally exacerbate the flash flood threat (due to hardened and compacted soil resulting in reduced infiltration and increased runoff). Churchill/Wegman Day 2 Valid 12Z Thu Jan 30 2025 - 12Z Fri Jan 31 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE ARK-LA-TEX, OZARKS, AND MID-SOUTH REGIONS... Thunderstorms will likely be ongoing on the northern and western sides of the Slight and Marginal Risks at the start of the period Thursday morning. A vigorous upper level cutoff low will begin to rejoin the polar jet by Friday morning. Before it does so, however, it will provide the forcing for numerous showers and thunderstorms across much of the Mid-South and Ark-La-Tex region. A 50-60 kt LLJ will pump plentiful anomalously high Gulf moisture across the Ark-La-Tex region Thursday morning, as the trailing cold front provides ample forcing for showers and storms across Oklahoma and Texas (and depending on how much rainfall occurs in the 6-12 hours prior to the start of Day 2, some ongoing flash flooding for prior training storms may be locally significant in portions of North TX into southeast OK, where probabilities for excessive rainfall are towards the higher-end of the Slight risk spectrum, being 25%+). The main limiting factor will likely be decreasing instability (to around 500 J/kg of MU CAPE into Thursday morning), which looks to be maximized across southern and central Texas. The greatest forcing looks to be displaced farther north (from the Ark-La-Tex north and east into the Mississippi Valley). This disconnect may help to diminish the flash flooding potential a bit (though concerns are greatest where the best instability and forcing may meet over aforementioned areas of North TX into southeast OK). It appears that many of the storms will be associated with a line that moves east across the area. However, storms embedded within the line may move somewhat parallel to the line towards the northeast, this will allow for the potential for training farther upstream into portions of the Middle MS Valley and OH Valley. Since there will be plenty of Gulf moisture (PWATs above 1.5 inches, above the max moving average using BNA as a proxy sounding), the showers and storms will have the potential to cause isolated to scattered flash flooding. By Thursday night the flash flood threat should diminish as the last storms cross the MS River with eastward forward speed of the storms increasing along with continued decreasing instability, thus diminishing the flooding potential with time. Churchill/Wegman Day 3 Valid 12Z Fri Jan 31 2025 - 12Z Sat Feb 01 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE PACIFIC NORTHWEST COASTAL RANGES AND FOOTHILLS... A wetter pattern will take shape across the Pacific Northwest region beginning Thursday night, as a series of storm system look to impact the area (originating from a closed low from the North Pacific interacting with another closed low/digging trough moving south along the west coast of British Columbia). After an initial round of light to moderate precipitation late on Day 2 in the coastal ranges and foothills, rainfall will become moderate to heavy at times throughout much of Day 3. While forecast maximum IVT values are relatively weak overall (500-700 kg/ms), a longer duration event could lead to a more 'moderate' atmospheric river event. 24 hour rainfall totals are forecast to generally range from 1-3" (though may locally exceed 3" in favored upslope terrain). Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5VyAUajogAkeH40WlIN60jiXtI0RrIR9r8QxSBfxsgrQ= graJ1mMWKQC1WwzAz1CruhSF0juUJBzIxcNxoxF4kZXORV4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5VyAUajogAkeH40WlIN60jiXtI0RrIR9r8QxSBfxsgrQ= graJ1mMWKQC1WwzAz1CruhSF0juUJBzIxcNxoxF4gi8Bysg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5VyAUajogAkeH40WlIN60jiXtI0RrIR9r8QxSBfxsgrQ= graJ1mMWKQC1WwzAz1CruhSF0juUJBzIxcNxoxF45GvC8TA$=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) .