FOUS30 KWBC 220059 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 859 PM EDT Tue Apr 21 2026 Day 1 Valid 01Z Wed Apr 22 2026 - 12Z Wed Apr 22 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA AND NEAR THE TEXAS/LOUISIANA BORDER... ...California... Broad on-shore flow has weakened as one shortwave made its way inland from the central California coast...but the approach of another vort center from the eastern North Pacific precludes removal of the Marginal Risk all together. Steep mid- level lapse rates resulting from cold mid-level temperature at the time of maximum daytime have supported some downpours...especially in association with the shortwave trough and strong orographically- forced ascent upon encountering the Sierra Nevada range. The expectation that the coverage of higher intensity rainfall will diminish with the loss of daytime heating...but still be enough to result in localized flash flooding. Bann ...Texas... The forcing from middle- and upper-level shortwave energy seen in water vapor satellite imagery has finally moving away from the central part of Texas...ending the risk of continued/renewed excessive rainfall from the part of the state soaked in the past 24 to 36 hours. However...there has been a consistent signal appearing in the past 5 or 6 runs of the HRRR (beginning around 21/20Z) that isolated convection with some potential for upscale growth in intensity and coverage along the Upper Texas coast after 22/09Z. Both the HREF and RRFS note low probabilities for flash flood guidance being exceeded at 3 hours...although the RRFS seems to display its overconfidence in probabilities and broader areal coverage than the HREF. Low level forcing looks to be fairly weak with the 18Z global models only showing 10 to 15 knots of on-shore flow at 925 mb. However...those winds should be transporting an airmass with precipitable water values at or slightly above 1.5 inches into the region. The concern for excessive rainfall continues beyond the end of the Day 1 period at 22/12Z. Bann Day 2 Valid 12Z Wed Apr 22 2026 - 12Z Thu Apr 23 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER TEXAS COAST AND SOUTHWESTERN LOUISIANA... 21Z update... A very small adjustment was made to the western boundary of the Marginal across eastern Texas to reflect the latest trends and WPC QPF. Campbell Only minor tweaks were made to the inherited Marginal Risk area across far southeastern Texas (upper Texas Coast) and southwestern Louisiana. Steady southerly flow off the Gulf will continue to advect a plume of Gulf moisture northward across the Marginal Risk area. The flow will not be much to write home about, generally between 5-15 kts at 850 mb. In addition to the rather weak advection, it's notable that unlike Day 1/Tuesday, the upper levels will become decidedly less favorable for widespread rain across the region as ridging and associated subsidence builds in. Thus, there is good agreement that any heavy rains will be disorganized and widely scattered in nature, as compared with the rains further west across Texas the past couple days. Urban concerns remain a driving factor for maintaining the Marginal, which appears to be a lower- end risk. Wegman Day 3 Valid 12Z Thu Apr 23 2026 - 12Z Fri Apr 24 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER MISSOURI AND MID-MISSISSIPPI VALLEYS... 21Z update... The latest guidance had a modest westward shift with the QPF footprint across the central states, which resulted in westward adjustment with both the western and eastern boundaries of the Marginal Risk area. Campbell A plume of moisture originating from the Gulf will advect moisture and instability northward into the Missouri and Mississippi Valleys on Thursday. Meanwhile, a strong cold front emanating from a deep low over the northern Plains will sweep eastward across the same region. Where these two competing air masses meet, most likely in the Marginal Risk area, expect widespread development of showers and thunderstorms. The greatest clashes are expected after sunset Thursday night, as the low level jet advecting the moisture and instability strengthens with the normal diurnal cycle. As that warm, moist air collides with the cold front, it will stall the latter's southward progress, creating an east-west boundary, likely starting around the Kansas/Missouri border well south of Kansas City, then expanding eastward with the front. Ozarks topography will likely also support locally heavier rainfall on the south- facing slopes as well. Guidance is in very poor agreement as to exactly where and when this clash of air masses will occur, with the above-described scenario seemingly the most likely solution based on an ensemble of the guidance. Should the guidance come into better agreement with strength and timing of the heaviest rainfall with future updates, a Slight Risk may need to be considered. The still full rivers and saturated soils across southeast Kansas and southwest Missouri from recent heavy rainfall events will contribute to more widely scattered instances of flash flooding should the heaviest rainfall occur over this region. Wegman 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