FOUS30 KWBC 061937 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 337 PM EDT Thu Aug 6 2026 Day 1 Valid 16Z 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... 16Z Update.. Only minor changes were needed for this update. The MRGL Risk area was trimmed to remove parts of northern MO, given radar trends. The majority of rainfall associated with an overnight MCS has moved through northern and central MO at this time. A very minor expansion in the Northeast was also needed to include the entire state of VT as an influx of anomalously high PWATs (over 2 standard deviations above normal) fuels short-lived heavy rainfall. Latest runs of both HREF and RRFS neighborhood probabilities indicate medium-to-high chances (~65%) of 1hr rates exceeding 1". Given how extremely localized the heaviest rainfall will be, along with the short duration of rainfall, an upgrade was not considered, but isolated flash flooding here will remain a concern. Blanco-Alcala Previous Discussion... ...Central Plains into the Lower Missouri Valley... Organized convection is expected to fire up along the southward moving cold front. An increasing influx of moist, southerly flow over the frontal boundary overnight will lead to locally heavy rainfall, potentially accumulating 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. 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. 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. ...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 2 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... 1930Z Update.. Very minor changes were needed for the risk areas. The main change of note was a minor reduction to the MRGL area in central AR. Latest run of CAMs guidance suggest a progressive line of thunderstorms in the northern and central portions of AR, where rainfall will be heavy but relatively short-lived. FFGs are also high (~3"), and the progressive nature of the system should limit flash flooding potential. Blanco-Alcala Previous Discussion... ...Lower Lakes, Ohio Valley into New England... During this period a PW plume of 1.50-1.75 inches will be pooled across much of the central states to New England within the southwest flow sandwiched between the upper ridge over the Atlantic and a broad mean trough over the Great Lakes and Ohio Valley region. 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. ...Southern to Central Appalachians... PW values in the 1.5 to 1.75 inches range will hold steady across the region 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 Day 3 Valid 12Z Sat Aug 08 2026 - 12Z Sun Aug 09 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST, FROM PORTIONS OF THE PLAINS/MISSISSIPPI VALLEY AND ACROSS THE LOWER LAKES AND INTO THE NORTHEAST AND ACROSS THE CENTRAL TO SOUTHERN APPALACHIANS... 1930Z Update.. A minor reduction of the MRGL Risk area was needed for Iowa and northern Illinois as the QPF core becomes better defined. A progressive convective system is forecast to move through Iowa along the periphery of an upper ridge. PWAT anomalies, particularly in the ECMWF, are quite high at around 3 standard deviations above normal. SREF Spaghetti plots are also in agreement in a heavy precipitation core located in central to southern Iowa. As such, a targeted Slight upgrade may be possible in the area in future updates, but due to global model disagreement in terms on the location of the heaviest precipitation, a MRGL was maintained until CAMs guidance becomes available. Blanco-Alcala ...Lower Lakes/Upper OH Valley into northern New England... A frontal boundary will persist along the southern edge of the westerlies on the south side of a broad trof. Embedded shortwaves in this broad cyclonic flow will accentuate lift in the vicinity of this front where PW values are expected to rise to 1.50-1.75" ahead of each set of shortwave energy. Height falls moving along the southern edge of the westerlies will support widespread scattered convection from the Lower Lakes/Upper Ohio Valley, east northeastward into much of New York state and northern New England. Locally heavy rains and isolated runoff issues will be possible, thus maintaining a Marginal Risk area. ...Upper Mississippi 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 remains in effect for widespread convection. Although confidence in the location of higher amounts remains lower, PW values in the 1.5-1.75"+ range will support localized hourly rain amounts of 1-2"+ leading to potential runoff issues, particularly in urban areas and within areas of steep terrain. ..Southwest... A broad Marginal Risk area was maintained across the Southwest for monsoonal convection. Low confidence with QPF details persist however locally heavy rains and isolated runoff issues will continue in the axis of above average PW values across this area. Campbell Day 4 and Day 5 Valid 12Z Sun Aug 09 2026 - 12Z Tue Aug 11 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST, NORTHERN PLAINS, GREAT LAKES, MIDWEST, & NORTHERN MID-ATLANTIC STATES... 1930Z Update: The only notable change to both the Days 4-5 EROs was to cut back a little across the western portions of the Marginal Risk areas over the Northern Plains, specifically the Dakotas. This based on the 12Z guidance trends, including the 12Z AIGFS and ECMWF AIFS, along with the QPF exceedance probabilities from the 12Z GEFS and ECENS means. Hurley ...Northern Plains/Great Lakes/Midwest/Northern Mid-Atlantic states... Early next week, broad cyclonic flow expected to persist across the northern tier of the Lower 48 from the Northern Plains/Upper MS Valley, eastward through the Midwest/Great Lakes and into the Northeast, to the south of a closed mid-level low in Hudson Bay. A frontal boundary will lie along the southern periphery of this broad trough. Embedded shortwaves in this broad cyclonic flow will accentuate lift in the vicinity of this front early next week where PW values are expected to rise to ~1.75" ahead of each set of shortwaves. When looking at the ingredients, organized convection looks most likely south of the Great Lakes within portions of the Corn Belt, where hourly rain amounts to 2.5" are possible. However, there is a great deal of latitudinal spread in the axis due to differences between where the diffluent 1000-500 hPa thickness exists and where the instability gradient/better 850 hPa confluence/convergence could set up, which appears more poleward into the Great Lakes. When the guidance gets in better agreement at some future time, a Slight Risk or greater could be considered, but at the moment there's too much spread in the heavy rain signal to place anything more than a Marginal Risk. ..Southwest... Broad Marginal Risk areas exist both days across portions of the Southwest, Rockies, and Great Basin. While there continues to be low confidence with the details, locally heavy rains with the potential of 2" an hour amounts and isolated runoff issues will continue in the axis of above average PW values expected across this area. As usual, urban areas, arroyos/dry washes, box canyons, and burn scars would be most at risk for flash flooding. Roth 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 $$