Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Mar 15 2025 00:57:24 FOUS30 KWBC 150056 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 856 PM EDT Fri Mar 14 2025 Day 1 Valid 01Z Sat Mar 15 2025 - 12Z Sat Mar 15 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER TO MID MISSISSIPPI VALLEY INTO PARTS OF THE OHIO AND TENNESSEE VALLEYS... Widespread thunderstorm development is expected tonight ahead of a cold front moving through the Midwest into the Lower Mississippi Valley, associated with a powerful closed low currently over Kansas. 00Z data showed anomalous moisture extended northward=20 ahead of a cold front/dryline, from the central Gulf Coast into=20 the Upper Midwest, and while the greatest moisture anomalies are=20 across northern locations (90th+ percentile), storm motions are=20 likely to remain progressive from roughly central Missouri to north of the Ohio River. The greatest flash flood risk for these=20 northern locations will be from temporary training with line=20 orientation briefing matching the mean steering flow, supporting=20 potential for 0.5 to 1.0+ in/hr rainfall rates atop relatively low=20 flash flood guidance values. Farther south, low level convergence/confluence out ahead of the cold front (which is forecast to stall in the ArkLaTex tonight) is likely to help focus thunderstorms over the Lower Mississippi=20 Valley through 12Z Saturday. Strengthening 850 mb winds to locally 50+ kt over Mississippi/Alabama will aid in moisture transport and precipitable water values exceeding 1.5 inches in a few locations=20 across the Gulf Coast states, with overrunning of resultant cold=20 pools. With the forecast eastward movement of convection expected=20 to be quicker to the north than south, this may set up a favorable=20 orientation for SW to NE training, from roughly northern=20 Mississippi into portions of Tennessee, Kentucky and perhaps=20 northern Alabama. Boosting lift across the region will be increasing upper level divergence and diffluence within the left=20 exit region of a 120-130 kt upper level jet max over the central=20 Gulf Coast. Otto Day 2 Valid 12Z Sat Mar 15 2025 - 12Z Sun Mar 16 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE TENNESSEE VALLEY... ....Ohio and Tennessee Valleys... The upper-level pattern will remain amplified Saturday across the central U.S. as one upper-low lifts northward into Canada while a second upstream shortwave/upper jet dives southeast into the southern Plains on Saturday morning and spreads eastward over the Lower Mississippi Valley/Southeast into Saturday afternoon/evening. A surface wave developing along the leading system's trailing cold front will help focus deep Gulf moisture tapped by a very strong 850 mb jet while supported aloft by increasing upper-level divergence ahead of the approach of the upper jet. This will help lead to the continued development and maintenance of both open warm sector convective development along the leading edge of the moist warm air advection/850mb jet as well as along the slowing eastward moving cold front. The updated 12Z CAM guidance supports the most likely corridor of overlap between the warm sector development as well as potential repeated/training rounds of storms along and just ahead of the cold front across portions of the Tennessee Valley from northeastern Mississippi east-northeastward into northern Alabama, south-central/southeastern Tennessee, and northwestern Georgia. This region also coincides with areal average QPF amongst the CAM guidance of 3-6", with locally higher totals of 8"+, and HREF probabilities of greater than 5" of rainfall over 50%. These very heavy rainfall totals, supported by downpours of 2"+/hour, will have the potential to lead to widespread, significant instances of flash flooding, with an upgrade to a Moderate Risk now included for the region. While there is greater uncertainty, the latest 12Z guidance also shows support for similar totals further northeastward into portions of south- central/southeastern Kentucky as well as further eastward into western North Carolina, which may require an expansion of the Moderate Risk if confidence increases. Areas of south-central/southeastern Kentucky as well as western North Carolina also remain more sensitive to additional rainfall. Regardless, a broader area of average rainfall totals of 2-3", locally higher, will contribute to additional scattered to potentially numerous instances of flash flooding across the Tennessee and Ohio Valleys within the Slight Risk. ....Pacific Northwest... A Marginal Risk remains in place for portions of the Pacific Northwest (mostly coastal/western Oregon and a bit of far northwestern CA) where guidance continues to indicate the potential for 2-5" localized rainfall totals along the coast (with the highest amounts indicated in the OR/CA coastal border area). This is due to a faster/stronger AR reaching into the Day 2 period from Saturday evening into early Sunday morning (as an inherited Marginal risk has been maintained for this update for Day 3) with IVT of 400-600 kg/ms indicated perpendicular or nearly perpendicular to the coast that will continue into Day 3. A targeted upgrade to a Slight Risk may be necessary on Day 2 and/or Day 3 as confidence increases in the timing of the heaviest rainfall given it may very well split different portions of both periods. Putnam/Churchill Day 3 Valid 12Z Sun Mar 16 2025 - 12Z Mon Mar 17 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE EAST COAST, FROM THE SOUTHEAST TO NEW ENGLAND, AND FOR PORTIONS OF THE PACIFIC NORTHWEST... ....East Coast... As the aforementioned low lifts northward into Canada, what's left is a weakening cold front as the upper-level pattern becomes a bit less amplified (shortwave over the Southeast lifting a bit and cutting off from northern/polar stream). The result looks to be scattered localized totals of 1-2" over a large region from the Southeast (central FL, southeast GA, and Carolinas) into the Mid- Atlantic and portions of the Northeast/New England. The 12z GFS/ECMWF as well as GEFS and ECENS both show the potential for locally higher totals of 2-3", with more confidence in these totals being realized over portions of the eastern Carolinas, and then less likely further north into the Mid-Atlantic/Northeast/New England as well as further south into northern Florida. Regardless, these totals look to remain somewhat limited as the front begins a faster progression towards the Atlantic Coast. ....Pacific Northwest... The aforementioned AR looks to bring in stronger moisture transport into Day 3 with the potential for IVT in the 600-800 kg/ms range near the OR/CA border region, peaking for a period early in the day on Sunday. Model consensus suggests an additional 2-4" of QPF through Day 3, and a targeted upgrade to Slight risk remains possible in the day 3 period (as well as day 2) in future updates depending on forecast consistency and greater confidence on the exact timing of the window of heaviest rainfall, which may not evenly split the day 2/3 period. Putnam/Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!976PAQhACaRK-3LIxwVPXrq6_QlYgXVROvhlvcc2nKwa= 7ObMqjkAwNO62tTGutN6UoCo2TNqWAY_DYHcsVCUBvjf2Ho$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!976PAQhACaRK-3LIxwVPXrq6_QlYgXVROvhlvcc2nKwa= 7ObMqjkAwNO62tTGutN6UoCo2TNqWAY_DYHcsVCUFuwhVjg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!976PAQhACaRK-3LIxwVPXrq6_QlYgXVROvhlvcc2nKwa= 7ObMqjkAwNO62tTGutN6UoCo2TNqWAY_DYHcsVCUOrlDuQI$=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) .