427 FOUS30 KWBC 060037 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 837 PM EDT Wed Aug 5 2026 Day 1 Valid 01Z Thu Aug 06 2026 - 12Z Thu Aug 06 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL PLAINS TO MIDDLE MISSISSIPPI VALLEY AND ACROSS SOUTHERN ARIZONA... ...Central Plains to Mid/Upper Mississippi Valley... 2100Z Update: Based on what has already occurred today, along with the latest observational and guidance (CAM) trends for this evening and overnight, have pared a bit of the Slight Risk area, removing portions across IA and north-central to northeast IL. Elevated >2"/hr rainfall rate probabilities per the 18Z HREF begin to percolate across eastern KS-western MO after 05Z, then spread into central and northern MO thereafter. ...Southern to Central Appalachians into the Northern Mid- Atlantic... 0100Z Update: Continued to trim the Marginal Risk area, leaving in areas only where observational trends (radar, satellite, mesoanalysis) still support a short term risk for flash flooding (through ~0600Z). Most extensive convective coverage early this evening is across the higher terrain (western slopes of the Appalachians in particular), where some spotty FFWs are still being issued. Given the weak mid level lapse rates (~6 C/KM between 700-500 mb) and thus decreasing deep-layer CAPE trends following sunset, expect the already minimal flash flood risk to diminish towards midnight EDT or shortly thereafter. ...Southwest/Southern High Plains... 2100Z Update: Per collaboration with WFO Tucson, we have included a Slight Risk across south-central and southeast AZ in this special Day 1 ERO update. This corresponds to Tucson's flood watch, which is in effect until 06Z this evening. Pattern has not changed much overall compared to the past few days (previous Slight Risk days), with the moisture anomaly present and mesoscale enhanced/induced shortwave energy moving westerly underneath the flat mid/upper ridge. While not ideal thermodynamically (mixed layer CAPEs peaking around 1000-1500 J/Kg, PWs ~1.25 to 1.5"), when combined with the forcing between 2300-0600Z, the mesoscale guidance (all CAMs) show concentrated areas of heavy rainfall within this Slight Risk area, with areal average amounts of 1-2.5" and localized totals of 3-5" (NAM CONUS Nest and RRFS being the heaviest. Latest HREF and REFS QPF exceedance probabilities have also come up as a result during the late afternoon and evening hours. Elsewhere surrounding the Slight, the Marginal Risk area remains sufficient, as areas of heavy (potentially excessive) rainfall will be isolated at best. Hurley Day 2 Valid 12Z Thu Aug 06 2026 - 12Z Fri Aug 07 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE SOUTHWEST THROUGH THE CENTRAL/SOUTHERN PLAINS, MIDWEST, AND EAST... 21Z Update.. The previous outlook remains in good shape, and virtually no change was made to the inherited risk areas. Latest HREF neighborhood probabilities over the southern Appalachians indicated medium chances of 24hr rainfall exceeding 3", and low chances of precip exceeding FFGs which may bring isolated flash flooding. However, CAM guidance suggests a rather short period of heavy precip, and there was a general lack of agreement regarding the location precip core, and QPF totals. Blanco-Alcala Previous Discussion... ...Central Plains into the Lower Missouri Valley... Organized convection is expected to fire up along the southward moving cold front. During the overnight hours, moist, southerly flow will be increasing over the frontal boundary which will lead to locally heavy rainfall, potentially accumulation around 2.5 inches. ...Lower Lakes, Ohio Valley into the Northern Mid-Atlantic... A slow-moving frontal boundary expected to persist as shortwave energy track east along the southern edge of the Westerlies, helping to accentuate lift in the high PW axis in the vicinity of this front. Moderate to locally heavy rain totals expected with hourly rain amounts up to 2.5" possible, supporting at least isolated runoff issues in urban areas and places with some degree of soil saturation. ...Southern to Central Appalachians... A broad Marginal Risk covers the Central to Southern Appalachians for widespread scattered convection. Lasted guidance continues to have a fair amount of spread in QPF amounts and placement of the highest accumulations. Although the details remain uncertain, PW values are expected to remain in the 1.5-1.75"+ range so hourly rain amounts upwards of 2.5" could lead to runoff issues, particularly in urban areas and within sharp relief. ...Southwest/Southern High Plains... Monsoonal convection will continue through this period thus the Marginal Risk was maintained for much of the Southwest and Southern Rockies. While there is continued low confidence with details, locally heavy rains with hourly amounts up to 2" could lead to isolated runoff issues. Campbell Day 3 Valid 12Z Fri Aug 07 2026 - 12Z Sat Aug 08 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF THE LOWER AND MID MISSISSIPPI VALLEY EAST AND NORTHEAST INTO THE MID ATLANTIC AND NEW ENGLAND, ACROSS THE UPPER MISSISSIPPI VALLEY, AND OVER THE SOUTHWEST... 21Z update... Virtually no change was made with this update, other than cosmetic changes with the updated suite of global and ensemble guidance. Blanco-Alcala Previous Discussion... ...Lower Lakes, Ohio Valley into New England... An axis of high PW values...1.50-1.75"+ expected to continue to stretch from the Lower Mississippi Valley, northeastward into New England in the southwest flow between the western Atlantic upper ridge and the broad mean trof across the Great Lakes into the Ohio Valley. Shortwave energy in this southwest flow will help accentuate lift in this high PW axis, supporting widespread scattered convection across these areas. There may be better organization to the convective activity across the Lower Lakes/Ohio Valley region in the vicinity of a slow moving frontal boundary expected to stretch west southwest to east northeast across this area. The risk level was maintained at marginal, but with FFG values relatively low across the eastern Lakes/Northern Mid- Atlantic, an upgrade is possible, especially as more hi res guidance becomes available. ...Southern to Central Appalachians... PW values in the 1.5 to 1.75 inches range will persist across the region during this period aiding in the local enhancement of the widespread scattered convection. A broad Marginal Risk covers the Central to Southern Appalachians where localized hourly rain amounts of 1-2"+ could lead to runoff issues, particularly in urban areas and within areas of steep terrain. ..Southwest... While there continues to be low confidence with QPF details, locally heavy rains and isolated runoff issues will continue in the axis of above average PW values expected across this area. A Marginal Risk area remains in effect for much of Arizona and New Mexico. Campbell/Oravec Day 4 and Day 5 Valid 12Z Sat Aug 08 2026 - 12Z Mon Aug 10 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST, FROM PORTIONS OF THE NORTHERN PLAINS ACROSS THE LOWER LAKES AND INTO THE NORTHEAST AND ACROSS THE CENTRAL TO SOUTHERN APPALACHIANS... 2000Z Update: Only minor (cosmetic) changes were made based on the latest deterministic and probabilistic guidance, including the 12Z AIGFS and the ECMWF AIFS. Hurley Previous discussion below.. For both the day 4 and 5 period, broad cyclonic flow expected to persist across the northern tier of the CONUS from the Northern Plains/Upper MS Valley, eastward through the Lower Lakes and into the Northeast. A frontal boundary is the mean will persist along the southern edge of the westerlies on the south side of this broad trof. Embedded shortwaves in this broad cyclonic flow will accentuate lift in the vicinity of this front this weekend where PW values are expected to rise to 1.50-1.75" ahead of each set of shortwave energy. ...Lower Lakes/Upper OH Valley into northern New England... For day 4...a lead area of height falls moving along the southern edge of the westerlies will support widespread scattered convection from the Lower Lakes/Upper OH Valley, east northeastward into much of NY State and northern New England. Locally heavy rains and isolated runoff issues possible. ...Upper MS Valley into the Mid-West... This will be followed by a second area shortwave energy pushing east southeast from the Northern Plains into the Upper MS Valley/Mid West region Saturday night into early Sunday. The low level southerly to southwesterly flow is expected to strengthen into this front, increasing PW values to 1.50-1.75"+ and supporting heavy to locally excessive rainfall totals in what may be an area of organized convection. The marginal risk area was drawn to encompass the current spread in model qpf axes. ...Southern to Central Appalachians into the Southeast... A broad marginal risk is depicted through the Central to Southern Appalachians into the Southeast day 4 and across the Central to Southern Appalachians day 5 for widespread scattered convection. Not a lot of confidence in details, but with PW values expected to remain in the 1.5-1.75"+ range, localized hourly rain amounts of 1-2"+ could lead to runoff issues, particularly in urban areas and within areas of steep terrain. ...Northern Plains/Upper MS Valley into the Lower Lakes... No changes to the large scale pattern for day 5, as shortwave energy continues to push east southeastward along the southern edge of the westerlies. The combination of the lead area of shortwave energy that moved into the Upper MS Valley/Mid West region day 4 moving downstream into the Lower Lakes and additional upstream shortwave energy moving from the Northern Plains into the Upper MS Valley day 5, will support a widespread region of potentially locally heavy rains and isolated runoff issues. Confidence is low with qpf details, but there will be the potential for organized convection in the vicinity of the west to east oriented front, supporting locally heavy rains and isolated runoff issues. ..Southwest... No significant changes to the broad marginal risk area across the Southwest for monsoonal convection both day 4 and day 5. While there continues to be low confidence with qpf details, locally heavy rains and isolated runoff issues will continue in the axis of above average PW values expected across this area. Oravec 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 $$