FOUS30 KWBC 232330 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 730 PM EDT Thu Jul 23 2026 Day 1 Valid 01Z Fri Jul 24 2026 - 12Z Fri Jul 24 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN PORTIONS OF THE SOUTHEAST, CENTRAL ROCKIES, AND CENTRAL PLAINS... ...Southeast... Good agreement among recent HRRR and RRFS runs for heaviest precip through the overnight along an axis from northern AL through north Georgia/South Carolina and southern North Carolina. Precipitable water values remain quite high, around or above 2 inches along this swath, so rain rates should continue to reach 2 inches per hour in areas where sufficient instability remains. With deep layer mean flow continuing from a westerly direction, there will be some training of convective clusters and line segments and thus flash flooding. Southern NC remains the favorite spot from the HRRR for greatest overnight rainfall, potentially in the 3-6" range inland from Wilmington. The higher end (aka "enhanced") Slight Risk remains in effect for this swath from northern AL through southern NC. For further information over the Carolinas and north Georgia this evening, please see MPD 833. ...Desert Southwest, Central Rockies through Central Plains... 23Z UPDATE: The Marginal Risk was further expanded over northern Arizona for ongoing monsoonal activity. Two Slight Risk areas maintained for CO and northeast KS where CAM QPF remains maximized. Recent HRRR/RRFS runs also favor south-central KS overnight, but that area has high FFG, so a Marginal Risk remains. ...Bertha/South Texas... Bertha is onshore north of Houston with heavy, slow moving convective development on the west side of the Houston metro. This activity will continue slow west-southwestward progress overnight. The Marginal Risk was expanded well north and now includes the San Antonio metro area as well (But south of the Hill Country. MPD 834 has further information for this evening. Farther south scattered convective activity has continues northwest of the McAllen metro/Lower Rio Grande Valley and is shifting west. There is a risk for heavy rain between there and the western Houston metro area where the Marginal Risk now covers. Jackson Day 2 Valid 12Z Fri Jul 24 2026 - 12Z Sat Jul 25 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST INTO THE OZARKS AND LOWER MISSOURI VALLEY... ...Southeast... A stalled front will continue to snake across the Southeast, extending from northern Mississippi, into north-central Alabama and north-central Georgia, and then into the eastern Carolinas. This will be the focus for renewed convective activity on Friday. Subtle changes over the next 24 hours may actually make the atmosphere more supportive of heavy rain rates by Friday afternoon. First, precipitable water values are projected to increase, largely due to a steady increase in moisture in the 850-500mb layer. By Friday afternoon, most of the area along the stalled front should see PWATs in the 2.1 to 2.4 inch range. Second, the deep layer mean winds are expected to decrease from around 15-25 knots today, to around 10-15 knots by Friday afternoon, as an upper level ridge axis builds in and reduces upper level winds. This should create an enviornment that favors very high instantaneous rain rates and slow-moving, backbuilding, and training clusters near the front. This may be compensated by a weaker front and low-level convergence, which would tend to favor clusters and line segments that are more scattered across the region, rather than aligned in a specific corridor. Nevertheless, there will still be localized opportunities for periods of training convection that could result in flash flooding. The chances (based on hi-res ensemble data) of rain rates in excess of 2 inches per hour, and for 24-hour rainfall totals in excess of 5 inches, are highest about 40mi either side of a line from Birmingham AL, to Atlanta GA, to Aiken SC, to Myrtle Beach SC, to Morehead City NC. In fact, the 12Z HREF even shows potential for rain rates to exceed 3 inches per hour in this same corridor, especially in South Carolina (neighborhood probabilities as high as 30 percent in SC), so the potential for locally significant flash flooding remains. An upgrade to Moderate Risk cannot be ruled out if enough observational evidence and model guidance suggests a corridor of sustained training convection is becoming more likely. ...Ozarks and Lower Missouri Valley... An MCS may be ongoing at the start of the period somewhere in the vicinity of eastern Kansas and Missouri, near the nose of the low- level jet. The biggest change with this outlook update was to trim the Slight Risk out of eastern Missouri in collaboration with WFO St. Louis. A limited number of models still progress heavy rainfall further east in Missouri, but the consensus now points more clearly to eastern Kansas and western and central Missouri. This also aligns with a typical bias in modeled MCS activity in the central Plains -- it often ends up further into the inflow than models would suggest. There is more uncertainty about what happens later in the forecast period, from late Friday afternoon into Friday Night. Most hi-res models suggest the convective cluster will progress southeast into the Ozarks and potentially Mid-Mississippi Valley by 12Z Saturday. However, they increasingly disagree on placement of the heaviest rainfall, the convective evolution, and magnitude of the rainfall. From a pattern recognition standpoint, there is some concern about an increasingly favorable setup for a backbuilding MCS on Friday Night. The low-level jet will veer to be more westerly, and the 20-30 knot low-level inflow will exceed the strength of the increasingly weak 15-20 knot deep layer mean flow. In these situations, you will occasionally see a convective line backbuild into the unstable inflow for hours, on the periphery of the cold pool. The 12Z HRRR is the model that shows this scenario most clearly, and it generates a maximum of about 6 inches of rain in 6 hours near the AR-MO border late in the forecast period. Conversely, other hi-res models show scattered convection that is far less focused. To put it another way, the synoptic evolution would favor potential for backbuilding storms on Friday Night, but whether it happens will ultimately come down to specific convective evolution from the Central Plains over the 24 hours prior, and probably whether a stronger, more concentrated cold pool can develop. This area will be monitored closely in future updates. Lamers Day 3 Valid 12Z Sat Jul 25 2026 - 12Z Sun Jul 26 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE EASTERN CAROLINAS AND FOR PORTIONS OF NEW MEXICO... ...Southeast through the Tennessee Valley... The outlook for the Southeast was left fairly consistent with the previous version with a Slight Risk in the eastern Carolinas, and a broad Marginal Risk over the rest of the region in the general vicinity of the lingering front. A key uncertainty is the position of the front, as earlier models, and still global model guidance is positioned further north, while hi-res guidance beginning to come into view for the forecast period (3km NAM, RRFS, REFS) places the heaviest rain further south. For the time being, the Marginal Risk was positioned in an intermediate location and close to the corridor of greatest QPF on the WPC deterministic forecast. However, the positioning of the risk area may continue to evolve in future outlook updates. Overall, the environment will still be favorable for high rain rates in the most organized thunderstorms, with precipitable water values still expected to be above 2 inches. However, the expectation of convective activity being more scattered with an increasingly diffuse frontal boundary may keep any flash flooding impacts isolated-scattered as well. ...Southwest... The monsoon recharges and resets across the Four Corners region on Saturday. A Slight Risk was introduced primarily across New Mexico where available REFS, RRFS, and 3km NAM hi-res model guidance shows localized heavy rainfall in the same area as elevated values approaching 15 percent on the 12Z CSU machine learning ERO probability guidance. The Slight Risk was concentrated more in Central New Mexico as models show dry air advection into southern New Mexico for much of the forecast period. North of that, in C/N NM, precipitable water values will continue to be around or just above the 90th percentile for July, and supportive of high rain rates in any thunderstorms that can develop. Lamers Day 4 and Day 5 Valid 12Z Sun Jul 26 2026 - 12Z Tue Jul 28 2026 ...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL ACROSS THE FOUR CORNERS STATES, EXTENDING INTO THE CENTRAL PLAINS MONDAY AND OVER THE GREAT LAKES, EXTENDING INTO THE NORTHEAST ON MONDAY... ...Four Corners States onto the Central Plains... Monsoonal flow from the Desert Southwest to the central Rockies continues into early next week. 12Z EC/GFS PW anomalies are offset with the EC farther west (NM and CO) vs the GFS (over AZ/NV) Maintaining Marginal Risks for now, but will continue to note that shortwave impulses coming up from Mexico could enhance precip and warrant a Slight Risk. Was able to remove part of southwest WY given confidence on anomalous moisture being contained south from the Uinta Mtns. For Day 5 (12Z Mon-12Z Tue), the amendments to the Marginal Risk were terrain based on the western side and farther onto the central Plains (western Nebraska/Kansas) given output from the 12Z EC and EC-AIFS. However, just like on Day 4, the EC is notably farther east with anomalous moisture than the GFS. Either way, convectively-aided shortwave activity should still traverse W-E overtop the flattening mid- upper ridge into the central High Plains. ...Upper Midwest-Great Lakes-Ohio Valley-Interior Northeast... Global and AI guidance continue to show convectively-aided shortwave activity riding down the top side of the upper ridge axis over the western Dakota border in northwest flow. Guidance still shows these ridge crawlers becoming a bit better organized through the day 4-5 period, with an uptick in deep-layer QG forcing (Qs convergence) and increasing low-level moisture transport. Broadened the Marginal Risk on Day 4 to Duluth per the 12Z RRFS and into northeast Iowa per the EC-AIFS. There is potential for this western side to then proceed into Day 5/Monday in Illinois, but for now the Day 5 Marginal Risk was expanded west through Indiana and northern Kentucky. There is still notable spread in global guidance for both of these days, but the threat for the NWly flow to become parallel to any boundaries warrants the Marginal Risks given enhanced moisture accompanying this activity. Jackson/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 $$