441 FOUS30 KWBC 181859 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sat Apr 18 2026 Day 1 Valid 16Z Sat Apr 18 2026 - 12Z Sun Apr 19 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE GULF COAST AND LOWER MISSISSIPPI VALLEY AS WELL AS FOR PORTIONS OF NEW YORK/PENNSYLVANIA... ...16Z Update... Both low-end Marginal Risks seem on track for today/tonight. For the PA/NY Marginal, minor adjustments were made to shrink the Marginal as models converge on position of the locally heavy rain near/ahead of a cold front. Fast movement of the band and lack of instability will be limiting factors to flooding concerns there though, despite strong forcing. Then for the southern Marginal, hi- res CAMs do continue to show a narrow axis of 2" to locally 3" totals across southeast Texas into Louisiana. Urban areas should have relatively higher concerns for flash flooding in an otherwise high FFG environment. Some convection may focus farther west in southwestern Texas as well this evening/overnight, but very high FFGs should likely preclude flooding issues, yielding a less than 5 percent risk. Tate ...Previous Discussion... ...Gulf Coast and Lower Mississippi Valley... A cold front sinking south across Texas on Saturday will run into an increasingly moist and unstable air mass located along the Gulf Coast Sunday afternoon. As the front lifts that air mass, rounds of showers and thunderstorms are expected to form and roughly parallel I-10 from near San Antonio east through southwestern Louisiana. The potential for multiple rounds of heavy rain across urban areas, particularly Houston, could result in isolated instances of flash flooding. Recent dry weather across southeast Texas and especially into Louisiana and southern Mississippi should work to limit the flash flooding potential there, whereas across central Texas, soils are a bit wetter from more recent heavy rainfall. Thus, FFGs precipitously increase with eastward extent along I-10. The inherited Marginal Risk was trimmed on the northern side, as the most persistent thunderstorms are likely to focus further south towards the Gulf Coast. The Marginal was expanded a bit west down I-10 towards San Antonio due to the aforementioned more favorable soil conditions. ...New York/Pennsylvania... A progressive line of thunderstorms associated with a fast-moving cold front will sweep eastward across northern Pennsylvania and western and central New York this afternoon and evening. The Marginal was introduced primarily because preexisting moist soil conditions across this region and the topography will both support the development of isolated instances of flash flooding despite less than impressive rainfall totals across this area. The line of storms will traverse the Marginal Risk region at peak heating late afternoon through the evening, which should allow the storms to also be at their peak in strength. Wegman Day 2 Valid 12Z Sun Apr 19 2026 - 12Z Mon Apr 20 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH TEXAS... ...1900Z Update... No major changes were needed to the Day 2 South Texas Marginal. Lack of instability will certainly be a limiting factor across the region, and HREF probabilities of exceeding 3h and 6h FFG are nil. But a low-end flash flooding risk could emerge due to the moist environment with stalling/slow-moving convection discussed below, so will give the Marginal another cycle in the outlook. It looks like convergence along coastal areas may produce locally higher rain totals during the day, while northwestern areas of the Marginal could see renewed convection overnight as the shortwave approaches. Tate ...Previous Discussion... The cold front from Saturday night should be just off the coast and into the Gulf Sunday morning, although a residual inverted trough extending from the tail of this front is expected to linger across South Texas much of the period. As the day progresses, weak shortwave energy embedded within a shallow southern stream trough will emerge from Mexico to produce modest ascent through divergence and PVA, combining with convergence along the aforementioned trough to create scattered showers and thunderstorms across the region. Downstream of this feature, subtle ridging will halt the progression of the surface trough, while additionally keeping PWs pooled across South Texas, which is reflected by NAEFS ensemble tables indicating that PWs will remain above the 90th percentile of the CFSR climatology. The overlap of this ascent into the moist column will manifest as heavy rain producing convection, with both HREF and REFS indicating a low end potential for 1"/hr rainfall rates. While the high-res simulated reflectivity still features wide variation in coverage, making confidence in the excessive rainfall potential a bit lower than usual, storm motions within any of this convection will likely be slow (0-6km mean winds of just 5-10 kts), so any of these heavy rates could be sufficient to overcome the FFG. The greatest potential may be along the immediate coast due to the overlap of slower storms (pinned) and lower FFG, but anywhere these slow storms develop, isolated instances of flash flooding could result. Weiss Day 3 Valid 12Z Mon Apr 20 2026 - 12Z Tue Apr 21 2026 ...THERE IS A MARGINAL CHANCE OF EXCESSIVE RAINFALL ACROSS PARTS OF NORTHERN CALIFORNIA AND ACROSS THE LOWER-MIDDLE TEXAS COAST TO THE HILL COUNTRY... ...1900Z Update... Both the northern California and Texas Marginal Risks for Day 3 still look on track. Little to no changes were needed based on new guidance, and the previous discussion's reasoning remains valid. Tate ...Previous Discussion... ...Northern California... Deepening upper level low eventually closes off late in the weekend off the northern CA coast. Afterwards, the guidance shows this broad, deep low pinwheeling slowly eastward on day 3, thereby allowing for a prolonged period of favorable forcing, i.e. upper diffluence/deep layer ascent and anomalous low-mid layer moisture transport. The latest GEFS in fact indicates 850-700 mb moisture flux peaking between 2-3 standard deviations above normal from the Bay area into the northern CA coastal ranges and Sacramento Valley. Due to the nature of the upper system (closed mid-upper low), this will not be a typical AR event, per the 'weak' AR classification from CW3E. However, Integrated water vapor transport (IVT) values do peak around 500 Kg/m/s, while the guidance (GFS-ECMWF-NAM) does indicate elevated CAPEs of 100-250 J/Kg. Areal average QPFs per the guidance on Day 3 (12Z Mon-12Z Tue) support widespread 1-2" rainfall totals, though higher-res members (including the RRFS) do show isolated higher totals closer to 3". Given the potential for at least 0.50"/hr rainfall rates on Day 3, the Marginal Risk area was maintained (and expanded) to account for the possibility of some excess runoff over burn scars. ...Lower-Middle Texas Coast to the Hill Country... Guidance continues to show subtle/flat southern stream shortwave traversing this area Mon-Mon night, which will likely become convectively-aided from any MCV(s) emanating out of northeast Mexico. Uptick in deep-layer forcing along with moisture pooling from the western Gulf (PWs increasing to 1.7-1.8") and some, albeit weak instability (MUCAPEs ~500 J/Kg) will lead to a more favorable environment for widespread rainfall with embedded heavier cores. We maintained the Marginal Risk area inherited from yesterday's Day 4 ERO, while expanding a little farther north to include parts of the Hill Country based on the latest guidance trends (deterministic and probabilistic QPF). In addition, the latest ECMWF Extreme Forecast Index (EFI) for QPF peaks between 0.6-0.7 during this event, with a shift-of-tails close to 1 along the coast. Hurley 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