FOUS30 KWBC 280734 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 334 AM EDT Thu May 28 2026 Day 1 Valid 12Z Thu May 28 2026 - 12Z Fri May 29 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE SOUTHEAST TO MIDDLE MISSISSIPPI VALLEY, SOUTHEAST FLORIDA, AND FOR PORTIONS OF THE NORTHWEST... ...Central Plains to Southeast... A bit of an omega block pattern is shaping up across the CONUS with a broad upper trough axis fixated over the Central and Southern Plains with ridging in place over the top. This will cause a rather chaotic flow under the mean trough and several smaller mid- level perturbations basically "stuck" within the overall trough axis. Ample instability with deep moisture presence will certainly continue to support at least a scattered heavy rain footprint across areas underneath influence of the trough. Scattered elevated probabilities between 30-70% exists for localized amounts >3" from KS all the way into the Southeast U.S. with the maxima split between eastern KS to northern AR and the Central Gulf coast. For KS/AR, parked underneath the upper trough, upper forcing and stagnant flow are more favorable for heavier rainfall totals. Maintaining a marginal risk at this time as the heaviest rainfall over the past 12 or so hours was more southwest of where the guidance is showing today and the rest of the region is still somewhat dry and can probably support some heavier totals. Along the Gulf Coast and into the Southeast, instability will be maximized in this area with weak flow supporting slow- moving convection and heavier rainfall over those isolated areas. Antecedent conditions across much of this region are also very wet, so it may not take much to cause issue. Given the still fairly isolated nature, maintaining a broad marginal risk, with any possible upgrades today stemming more from near term radar trends. ...Florida... An active sea breeze pattern with weakening steering flow will lead to periods of heavy rainfall across southeast Florida this afternoon. Greatest concern remains over the South Florida metro corridor from West Palm Beach to Miami. Latest HREF is robust with showing greater than 50% probabilities for 3" in 24-hours, with even some modest, albeit localized, probabilities for 5". This correlates well with CAM guidance, with some indicating bullseyes of greater than 3" within the larger QPF footprint. Locally significant rainfall is likely in this relatively classic pattern with ample moisture and instability present. A marginal risk remains in effect across Southeast Florida. ...Washington/Oregon... A deep upper level low over the West will allow for a broad axis of diffluent flow across the interior Northwest. Precipitable water values 1-1.5" are in the 90-90th percentile, and with sufficient instability, should support convection capable of producing heavy rainfall. Flooding and flash flooding are a concern for mainly the more vulnerable areas - urban, small stream, and low lying areas. HREF 24-hr probabilities for greater than 3" are between 40-70% for especially the Oregon cascades. Given the favorable environment though, a marginal risk extends all the way from far northern California northward to the eastern half of Washington state. A previous marginal risk for parts of the northern Rockies was trimmed out for this update given the lack of QPF signal. Santorelli Day 2 Valid 12Z Fri May 29 2026 - 12Z Sat May 30 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE NORTHERN ROCKIES AS WELL AS MUCH OF THE SOUTHEAST TO MIDDLE MISSISSIPPI VALLEY... ...Eastern Washington to Northern Rockies... A marginal risk is in place across far northeast Washington to northwest Montana due to an increase in the heavy rainfall signal for this period as the upper low over the West on Day 1, pivots slightly northeastward, placing this region in an area with better upper forcing and advection of moisture. The presence of anomalous moisture and some instability may present a threat for localized heavy rainfall and flash flooding, especially for burn scar locations within this region. ...Middle Mississippi Valley to the Southeast... Strong Canadian high pressure pressing in from the north will allow for a slow moving cold front to press south over the course of Thursday into Friday with convection likely to spawn along and ahead of the front and dry air advecting behind the front leading to a quick cutoff of activity. The front will arc back into the Tennessee Valley to Middle Mississippi Valley, supported by the persistent troughing over the south-central Plains. A moist and unstable environment near and south of this front will support scattered convection, with some heavier cells likely within the pattern. It is still too hard to pick out any discernible areas of interest, so maintained a very broad marginal risk over this area which is quite wet and will continue to be in the Day 1 period. Santorelli Day 3 Valid 12Z Sat May 30 2026 - 12Z Sun May 31 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE NORTHERN ROCKIES TO HIGH PLAINS AND THE SOUTHEAST... ...Northern Rockies to High Plains... The upper level low over the West during the Day 1 and 2 periods will weaken and shift northeastward, moving over the Northern Rockies to High Plains region on Day 3/Saturday. Moisture anomalies will continue to be in the 90-90th percentile, supporting moderate to heavy rainfall, with widespread QPF greater than 0.5-1" likely across western and central Montana. There may not be a lot of instability to work with, so any flood or flash flood threats should be mainly localized with particular attention to burn scar locations. A fairly broad marginal risk is in place across much of western and central Montana, into parts of northern Wyoming as well. ...Southeast... Same story as Days 1 and 2, with ample moisture and instability present near and south of a cold front that may stall over the region on Saturday supporting convection, across much of the Southeast. Some of this region, especially western parts of South Carolina and into the Appalachians has been very wet as of late and is well primed, so may be more favorable for flooding concerns. The marginal risk though continues towards the coast due to the presence of a weak frontal wave in the region. The Marginal Risk extends from the Southeast coast, westward into northeast Alabama and southeast Tennessee. Santorelli 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