928 FOUS30 KWBC 280101 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 901 PM EDT Mon Apr 27 2026 Day 1 Valid 01Z Tue Apr 28 2026 - 12Z Tue Apr 28 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDWEST AND FROM THE ARKLATEX TO PARTS OF THE MISSISSIPPI DELTA.. 01Z Update... Made a few notable updates to the D1 ERO Outlook valid through 12Z Tuesday. Hoisted a new Slight Risk from northeast TX east through the ArkLaMiss area, based on the current pre-convective environment over this area, along with recent satellite/radar/lightning trends with the convection growing upscale over northeast TX. Per the 00Z KSHV sounding, robust mixed-layer CAPEs 4000-4500 coincident with PWATs of 1.7-1.9 and surface dewpoints in the mid-upper 70s with increasing 925-850 mb flow will make for hourly rainfall rates between 2.5-3.0" underneath the strongest supercells. Area coverage for Slight-Risk ERO impacts may not be as high (neighborhood probability probably closer to 15% or on the 'lower end' of a Slight); however, as with the convection the past couple of nights, rainfall amounts for some could be quite prodigious. Per SPC mesoanalysis, S to SW 850 mb flow is already nearing 30kts in this area. Elsewhere, have continue to chip away the Slight and MGNL areas now west of the more organized areas of convection. Have also introduced a MGNL over a large portion of south-central TX. Expect isolated supercells moving eastward from northeast NM, and as such expect the excessive rainfall threat to be localized. Hurley Day 2 Valid 12Z Tue Apr 28 2026 - 12Z Wed Apr 29 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE ARKLATEX INTO THE MID SOUTH... 20Z Update... The previous Excessive Rainfall Outlook still encapsulated the environmental conditions that favor numerous organized thunderstorms and the associated risk of excessive rainfall on Tuesday into early Wednesday. Some modest realignment was done to the eastern boundary of the Slight and Marginal risk areas in deference to the 12Z cycle of the ERO first-guess (which was already in good shape compared with the on-going ERO). Bann Previous Day 2 Excessive Rainfall Discussion... A favorable environment will exist for numerous organized thunderstorms over the southern U.S. on Tuesday, with a broad warm sector containing very strong instability (CAPE over 3000 j/kg), deep moisture (PW climbing into the 1.6 to 2.0 inch range), and strong deep layer shear (0-6km bulk shear 40-60 knots). In that sort of environment, rain rates should reach 2 inches per hour. The open question is what sort of mesoscale boundaries may exist to focus convective development, and whether or not some of these will be oriented in a west-east fashion (parallel to the bulk shear vectors). Any prominent boundaries that are favorably oriented could easily focus a corridor of significant rainfall and flash flooding in this sort of environment. However, an equally plausible scenario would be more scattered clusters and lines of thunderstorms that can produce localized heavy rainfall and flash flooding, but fail to focus the convection in a way that leads to more significant totals. For now, it seems reasonable to maintain the broad Slight Risk and wait for greater confidence in the mesoscale details. Lamers Day 3 Valid 12Z Wed Apr 29 2026 - 12Z Thu Apr 30 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM CENTRAL AND SOUTHEAST TEXAS TO THE CENTRAL GULF COAST... 20Z Update... Maintained the Marginal Risk across the southern US due to the 12Z cycle. Numerical guidance from 27/12Z remained fairly modest even though the guidance also kept a fetch of moisture originating in the tropics streaming into the region overhead at/or above 3 km altitude. An upgrade remain possible. Bann Previous Day 3 Excessive Rainfall Discussion... The threat of excessive rainfall and flash flooding seems to be increasing in the most recent set of model guidance, due to a continuation of strong instability and the arrival of a plume of unusual amounts of mid-level moisture. Backward trajectory analysis indicates that moisture around 3km above the surface originated in the tropical central Pacific, advected into the southern U.S. by an anomalously strong subtropical jet (300mb winds around 60+ knots) extending about 6000 mi. from the Marshall Islands, to just south of Hawaii, all the way to Texas. This should push PW values over the whole region above 1.7 inches, and above 2 inches in portions of Texas, which is quite unusual. For instance, a 2+ inch PW has only been observed on an upper air sounding at Corpus Christi 3 times prior to May 1st (36 year period of record; since 1990). Although model QPF is not exceptionally high at this point, that may largely be a function of a lack of high-res guidance from convection-allowing models. The environment (when also considering strong bulk shear to around 50 knots) would favor organized thunderstorms with very high rain rates, potentially in excess of 2 inches per hour. Given that the previous outlook had probabilities below 5 percent, the main change with this update is to introduce a Marginal Risk, but a further upgrade may be required, especially across parts of Texas. Lamers 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