Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Tue Jul 10 2018 Valid 12Z Fri Jul 13 2018 - 12Z Tue Jul 17 2018 ...Overview, guidance evaluation and preferences... Model guidance this evening shows good overall agreement on the evolution of the synoptic pattern through next Tuesday although the spread has increased since yesterday. The main uncertainty will be across the Pacific Northwest eastward into the northern Plains this weekend into early next week as a couple of shortwave troughs are forecast to move along or just north of the U.S.-Canadian border. The GFS has introduced a digging and higher amplitude shortwave in the last few runs while the ECMWF keeps a less-amplified positively-tilted trough mainly north of the border. The GEFS on the other hand shows a less-amplified mean solution comparable to the EC mean. The ECMWF has actually shown a tendency for increasing the 500 mb height over the northwestern U.S. while the GFS shows the opposite. A general compromise between the 18Z GEFS and the 12Z EC mean mixing in with their deterministic solutions was used to handle this area from Day 5 through Day 7 (Sunday-Tuesday), which maintains good continuity with the previous WPC package. Otherwise, models continue to show excellent agreement on a subtropical high dominating the lower 48 later this week into next week. ...Weather highlights... Although the dominating subtropical high will bring heat across a large portion of the country through early next week, there will be bouts cool air intrusion from Canada following occasional frontal passages across the northern tier states. A cold front will initially bring showers and thunderstorms across the northern Plains into the Great Lakes Friday but as the front is forecast to become more diffused with time, more isolated coverage in the Northeast will be the result during the weekend. Meanwhile, the next front will move out of Canada this weekend with scattered showers and thunderstorms along/ahead of its boundary. Cool air in its wake will break the heat wave across the interior Pacific Northwest and is forecast to drop temperatures more than 10 degrees below normal across the northern Plains early next week. Ahead of the front, temperatures will be about 5-10 degrees above average (80s and many 90s), and initially near 100 over portions of the interior Pacific Northwest/Great Basin on Friday. But trip-digit heat is expected to continue over parts of interior California. From the southeastern U.S. westward along the Gulf Coast, a dissipating front will keep a chance of showers and thunderstorms in the vicinity through the weekend. Monsoonal moisture will linger across the Four Corners area where locally heavy downpours will be possible through early next week. Temperatures will generally be near to below average in the Southwest under limited solar insolation and a daily chance of showers/storms. Kong/Fracasso WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found at: http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 http://www.wpc.ncep.noaa.gov/heat_index.shtml