FOUS30 KWBC 021551 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1151 AM EDT Fri May 2 2025 Day 1 Valid 16Z Fri May 02 2025 - 12Z Sat May 03 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS, LOWER MISSISSIPPI, AND TENNESSEE VALLEYS... ...16Z Update... The most notable change with the midday update was to expand the Slight eastward to include more of Tennessee and Alabama. The Nashville, Chattanooga, and Birmingham metros are now upgraded to a Slight-risk, albeit a lower-end, lower confidence one. Numerous areas of thunderstorms are expected to develop this afternoon in response to growing instability and Gulf moisture across the southern US ahead of a sweeping cold front. These storms are forecast in the guidance to develop into several clusters. The westernmost one over Texas will have the greatest potential to cause flash flooding, as the southward push of the front is most likely to stall out there as compared with areas further east. Fortunately, once the current MCS over north Texas pushes further south in response to new thunderstorm development this afternoon, that should be the end of the heavy rain threat in that area for a couple days. The best chance for training storms will generally be from Austin east to roughly the Louisiana border, which has been consistent in the guidance for at least the last 24 hours. Only a small portion of this area was hit with heavy rain yesterday, and with individual cells moving along with some southward component to their motion, think the threat doesn't quite rise to the Moderate Risk level, so the area remains in a Slight Risk. Atmospheric ingredients remain favorable for the development of flash flooding, with afternoon MUCAPE values expected to approach 3,000 J/kg across the Mid-South and PWATs in most areas between 1.25 and 1.5 inches. This is about 1-1.5 sigma above climatology for this area. The PWATs will remain favorable as a 20-30 kt southwesterly flow at 850 advects more Gulf moisture into the storms that is lost from rainfall. Thunderstorms will be a bit more progressive further east into northern Louisiana and southern Arkansas, but that area has been hard-hit with heavy rains over the past week, and soil moisture levels remain quite high. Thus, with antecedent conditions more favorable, the higher end Slight remains in effect for this area with few changes. Further east, as mentioned above the Slight Risk was expanded east to include more of Alabama and Tennessee. The combination of urban concerns in Nashville, Chattanooga, and Birmingham, some potential for pre-line convection developing over those metros, and at or above normal soil moisture levels in those areas favored the upgrade to a lower-end Slight. The fast-moving progressive nature of all of the storms, especially any line segments, does mitigate the flash flood potential. Finally, further up the front into the upper Ohio Valley and New York, the storms will be moving even more quickly, with less Gulf moisture to work with, but recent heavy rains in this area have made antecedent soil moisture conditions very favorable for the development of additional flash flooding, so the Marginal Risk area was expanded northeastward, though the severe threat in this area looks to be much greater. No changes were made to the Marginal Risk across portions of central and western New Mexico with few changes there in the guidance. Wegman ...Previous Discussion... Another round of thunderstorms is expected to develop along and ahead of a southward moving cold front on Friday...with guidance still focusing the greatest chances for excessive rainfall from portions of the Mid-South into the Southern Plains Friday into Friday night as weak waves propagate northeastward along the front. Southerly low level flow will draw Gulf moisture northward...helping increase precipitable water values ahead of the front to 1.5 to 1.75 inches at the time of the peak heating in a stripe from Texas into southern Arkansas. Forecast soundings depict an atmosphere supportive of rainfall rates exceeding 2 inches per hour. As mentioned previously...the potential for a prolonged period of training is fairly minimal so a Slight Risk still seems to cover the potential. 00Z HREF neighborhood probabilities of 1-hour QPF exceeding 2 inches were over 30 percent in parts of southeast TX with lower probabilities extending into parts of Arkansas. A Marginal Risk area surrounding the Slight Risk area extends as far northeastward as parts of the Great Lakes. Precipitable water values were generally at or below 1.25 inches ahead of the advancing cold front but guidance still showed enough instability to support showers and thunderstorms over an area that was still had lingering hydrologic sensitivity following recent heavy rainfall. In addition...the area will be underneath a right entrance region of an upper level jet stream aiding upper level support during the afternoon. Farther west...there is also a Marginal risk of excessive rainfall from late day and evening convection over the terrain of New Mexico. The QPF is fairly modest but even modest rainfall rates could result in isolated problems with run off...especially if the rain falls on recent burn scars. Bann Day 2 Valid 12Z Sat May 03 2025 - 12Z Sun May 04 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE GULF COAST REGION INTO PARTS OF THE EASTERN GREAT LAKES... The focus for excessive rainfall will shift eastward as a closed mid-level circulation center takes shape over the nation's mid- section. The mid- and upper-level flow backs with time from the Gulf coast northward across parts of the Tennessee and Ohio Valleys. Initially...this allows deeper moisture to stream northward. But it also results in the surface/low level front to move eastward more quickly later in the period...with generally a low-end risk of flooding from the Gulf coast northward. The 02/00Z suite of numerical guidance hint that there could be some 1 inch to perhaps 2 inch amounts where frontogenetic forcing aligns favorably in between the Gulf and the Ohio Valley...but the signal for excessive rainfall showed little run to run / model to model consistency. There is also a Marginal Risk of excessive rainfall over far southern portion of Texas...where the southern end of the cold has stalled its southward progression and return flow of Gulf moisture has begun. Several of the High Res ensemble highlight this area with QPF in the upper percentiles although the consensus is less than unanimous in amounts or placement. With soundings showing warm cloud processes through a deep layer and the CAPE profile is tall and skinny...introduced a Marginal risk here. Bann Day 3 Valid 12Z Sun May 04 2025 - 12Z Mon May 05 2025 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE EASTERN UNITED STATES AND SOUTHERN ROCKIES ... The risk of excessive rainfall persists across parts of the Mid- Atlantic and Southeast US as a broad upper trough continues to make its way eastward on Day 3. Deepest moisture of 1.25 to 1.5 inches will be eventually be shunted off-shore late in the period. Until then...isolated downpours are possible with spaghetti plots of 2 and 3-inches of rain in 24 hours mainly being produced by models with the ARW core...but enough members from the NMM core and from the GEFS to warrant a Marginal. There is also a Marginal Risk area over parts of New Mexico again as moisture continue to spread northward and westward from the Gulf. After 00Z...the flow aloft between the westerly subtropical jet and a jet streak and low- and mid-levels become increasingly diffluent with height falls beginning ahead of an approaching closed low from the west. This...along with moisture and a low level boundary already in place...should support late day and evening storms that may produce locally heavy rainfall rates. Bann Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt