Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Wed Jul 10 2024 Valid 12Z Sat Jul 13 2024 - 12Z Wed Jul 17 2024 ...Hot temperatures persist in the West and spread eastward, but not quite as extreme as recent/short range heat... ...Overview... A persistent upper ridge over the West will gradually drift east and at times combine with ridging stretching westward from a strong Atlantic upper high this weekend into early next week. This will cause heat to spread across the central and eastern U.S., but as the ridge weakens, temperatures should become slightly less extreme during the period. The ridge shifting east will allow for some monsoonal moisture to spread into the West. To the north of the upper ridge, progressive shortwaves moving across Canada and the U.S. northern tier will push possibly multiple surface fronts into the northern states. Accompanying rounds of showers and thunderstorms will be possible from the north-central U.S. to Northeast. ...Guidance/Predictability Assessment... Model guidance agrees fairly well with the large scale pattern evolution during the period, showing ridging across the western U.S. along with ridging stemming west from a strong Atlantic ridge (anchored by a 600+ dm high near 60W longitude) gradually weakening a bit by Monday-Tuesday. To the north of the ridge, shortwaves passing through the mean flow are lower predictability. Models have generally shown broader but shallow troughing coming into the north-central to northeastern U.S. setting up next week, but vary with the southern extent of the trough and with placement and strength of Canadian low pressure that should push surface front(s) into the northern tier. While there are model differences in the dynamical and AI models, no real outliers seem evident. Farther west, mean ridging looks to rebuild in the interior Northwest, but models like the ECMWF and CMC have shown more shortwaves disrupting the ridge at times. These smaller details may not be resolved for a while. With the general model agreement at least on the larger scale, a blend of deterministic models favoring the 12Z ECMWF and 18Z GFS was used early in the period. Included some GEFS and EC ensemble means by Day 5 and gradually increased to about half by Day 7 with increasing minor model spread. ...Weather/Hazards Highlights... Increasing moisture will feed into the Four Corners states in a monsoonal pattern over the weekend and beyond, as the upper ridge finally weakens some and drifts east. A Marginal Risk remains in place for much of Arizona into northwestern New Mexico and southwestern Colorado on Day 4/Saturday. A Marginal Risk expands somewhat to cover much of the Four Corners states by Day 5/Sunday as moisture continues to stream in and increase. Sunday's risk includes more sensitive areas like the slot canyons of southern Utah and the Sacramento Mountains of south-central New Mexico that have recent burn scars. This moisture plume is forecast to expand farther north into early next week for increasing rain/thunderstorm chances for the Rockies to High Plains. Through the weekend and early next week, the combination of northern tier U.S./southern Canada shortwaves and a couple surface fronts will promote one or more clusters of showers/storms from the Dakotas/Upper Midwest into the Great Lakes. There remains low predictability in the magnitude and location of heavy rain amounts, precluding any ERO risks at this point. But much of this region will be sensitive to additional rainfall after recent weeks of heavy rain, so will continue to monitor the potential for flash flooding. Ample instability will be in place, allowing for high rain rates, but storms look to be moving quickly/not training, so there are competing factors. Rounds of rain/storms will also progress into the Appalachians/Northeast by early next week. Lingering rain is possible across the Eastern Seaboard on Saturday, but the heaviest rainfall should be over by then compared to the short range period. Expect scattered shower and thunderstorm chances to continue and expand through the southeastern U.S. (Lower Mississippi Valley into the Southeast/Florida and the Carolinas) over the weekend into early next week, with locally heavy rain possible. The dangerous heat wave that has been ongoing across the West should be easing somewhat by the medium range period, but still remaining hot. Temperatures in the 110s are likely to continue across the Desert Southwest, a few degrees above average. Farther north, temperature anomalies will slowly trend down from around +10F above normal Saturday to near normal next week in the Central Great Basin, but in the Northern Great Basin the anomalies will only drop from around +15F above normal to +10F. Meanwhile, the northern half of the Rockies and Plains can expect temperatures around 10-15F above normal this weekend, with some areas showing temperatures over 100F. The northern areas are likely to moderate to near or slighly below average for highs after a cold front passes, but high heat indices are likely Monday-Tuesday from the central Plains and Mid-Mississippi Valley southward. Areas across the Ohio Valley into the Appalachians can expect above normal temperatures this weekend and expanding into the Eastern Seaboard early next week. Overall, most areas of the lower 48 will have at least some heat concerns during the period. Tate 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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