544 FXUS02 KWBC 131954 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 354 PM EDT Thu Aug 13 2026 Valid 12Z Sun Aug 16 2026 - 12Z Thu Aug 20 2026 ...Hazardous Heat in the South-Central and Southeast U.S... ...Heavy Rain Threats for the Southwest, Central Plains, Midwest, Northeast and Mid- Atlantic through early next week... 20Z Update: The 12Z model guidance suite features good overall agreement across the Continental U.S. to begin the forecast period Sunday. Noticeable differences appear in the mass fields by early Tuesday across the Pacific Northwest with the GFS depicting a stronger upper trough moving inland across the region when compared to the CMC and ECMWF, and this in turn erodes the ridge across the Four Corners region faster than the model consensus. The GFS becomes an outlier solution by Thursday when it is at odds with the other guidance across the northern Rockies and the Dakotas with its amplified trough solution, so it was not used in the fronts and pressures by that time. The CMC holds onto the ridge across the Western U.S. the longest. The guidance is in better agreement across the Great Lakes and the Northeast with the broad cyclonic flow aloft over the region. The ensemble means were increased to about 40% by next Thursday, while still keeping some previous WPC continuity. The previous forecast discussion is appended below. /Hamrick ---------------------- ...Overview... The broad and persistent upper ridge across the southern tier continues to bring hazardous heat across the south-central U.S. and Southeast into next week, while a relatively active northern- stream flow remains established from the northern Rockies/Plains into the Great Lakes and Northeast. This will facilitate multiple rounds of showers and thunderstorms that may lead to flash flooding. Meanwhile, a upper trough will push into the West Coast bringing chances for unsettled weather across portions of the northwestern U.S. while monsoonal moisture will continue to bring heavy rainfall across the Southwest into the central Plains. ...Guidance/Predictability Assessment... The latest model guidance continues to support good confidence in the placement of the southern U.S. ridge and the persistence of the periodic shortwave energy across the northern tier. Uncertainty grows in regard to the timing, amplitude, and the interaction of individual shortwaves with frontal boundaries and the developing trough over western U.S. The GFS seems to be faster with some of the features that eject into the West, which will impact precipitation maxima, while the CMC seems a bit slower, especially towards mid-week. With higher confidence of a slower moving trough, the forecast initially weighted an even multi-model blend, while phasing out the deterministic models by early next week and incorporating the ensemble means for the remainder of the forecast. ...Weather/Hazards Highlights... The strong upper ridge remains positioned over the southern U.S. and will maintain anomalous warm temperatures from the southern Plains through the Lower Mississippi Valley, Tennessee Valley, and Southeast. Afternoon temperatures will commonly reach the upper 90s to lower 100s across the interior South, while high humidity and persistent elevated dew points will produce oppressive heat indices of 105-115 degrees. Record-breaking warm overnight temperatures will further limit recovery, particularly where several consecutive days of heat occur. Widespread Major to Extreme HeatRisk conditions will remain possible into next week. Several consecutive days of high heat stress can lead to heat-related illnesses. Monsoonal moisture will advect northward, bringing an active pattern across the western and central U.S. through next week. The deep subtropical moisture will continue to circulate around the western periphery of the southern tier ridge, supporting recurring thunderstorms across portions of the Southwest into the central Rockies/Plains. Slow storm motions may lead to localized heavy rainfall. Repeated convection will maintain a risk for flash flooding, especially over burn scars, steep terrain, slot canyons, and urbanized areas. This is highlighted in the WPC Excessive Rainfall Outlook (ERO) for Day 4/Sunday and Day 5/Monday where there is a Marginal Risk across portions of the Four Corners and central Plains, with a Slight Risk over portions of Colorado for Day 4. Farther north, an upper trough digging southward off the West Coast is expected to become increasingly influential toward the middle of next week. Although there is still some uncertainty on the evolution of this feature, chances for shower and thunderstorms increase for the Great Basin and the northern Rockies as the trough moves eastward. The moist and unstable air mass will remain positioned ahead of a series of northern-stream disturbances. Frontal boundaries and surface waves interacting with the anomalous moisture will support repeated rounds of showers and thunderstorms across the Midwest, Ohio Valley/Mid-Atlantic, Great Lakes, and Northeast on Sunday. With the combination of high low-level moisture and instability, training thunderstorms may develop, which can lead to flash flooding, especially over areas where soils remain saturated from previous rainfall. This is highlighted by a Marginal Risk over parts of these regions on the Day 4/Sunday ERO, with a Slight Risk over portions of Mid-Atlantic and Northeast. On Monday, the surface low and associated boundary continues to push northeast, as the trailing front moves southward across the Ohio/Tennessee Valley into the Mid-Atlantic and the Northeast coast. Continuous interactions with the unstable airmass along the boundary may lead to organized convection and excessive rainfall. Meanwhile, another upper-level disturbance will bring multiple rounds of thunderstorms and showers across the Northern Rockies/Plains into the Upper Midwest and will linger through mid- week. Therefore, the Day 5/Monday ERO highlights the flood risk across the Mid-Atlantic and portions of the northern Plains and Upper Midwest. Otherwise, showers and thunderstorms push into the Southeast on Tuesday as the frontal system dips farther south. Oudit Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw $$