Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Wed Aug 29 2018 Valid 12Z Sat Sep 01 2018 - 12Z Wed Sep 05 2018 ...Pattern Overview and Guidance/Predictability Assessment... Entering the month of September, a cascade of shortwaves will continue to push from western North America eastward toward the north-central U.S. Each of these perturbations in the flow are of rather low predictability as models exhibit poor run-to-run continuity. There are individual members within the ensemble systems which favor more amplified scenarios. All of these shortwaves should generally remain confined to locations near the international border with Canada unless one of these more amplified ensemble members were to verify. Among the global guidance, the 00Z CMC seems to be too quick with eastward ejection of these embedded impulses. Elsewhere, some models intermittently show a modestly defined shortwave approaching the northern California coast sometime between Day 4-6, September 2-4. Most notably, this appears in the 00Z GFS/12Z ECMWF output. Not enough confidence exists to fully support such a scenario at this time. Across the eastern U.S., after a temporary reprieve, a sprawling 594-dm ridge should return to the region by Monday into the middle of the week. This has seemingly been the new preferred location for a positive height anomaly to situate itself. Model agreement is solid with no significant differences evident. Within the subtropics, there remain continued hints at some tropical disturbance moving westward across the Florida Peninsula into the Gulf of Mexico. It is mostly a ECMWF driven solution although the 00Z CMC ensembles show a small number of members. Along the southeastern U.S. coast, the 00Z CMC/NAEFS ensembles favor an inverted trough early next week. They do seem to be on their own which casts doubt on their forecasts. A multi-model consensus solution was utilized with a general combination of the 06Z/00Z GFS, 00Z ECMWF, and their respective ensemble means. By Day 5/September 3 onward, the forecast was heavily driven by the 06Z GEFS/00Z ECMWF ensemble means given building uncertainty, particularly within the active flow along the northern tier states. ...Weather Highlights/Hazards... Given the return of upper ridging to the eastern states, temperatures will likely remain above climatology, particularly next week. The most anomalous readings are expected over the northeastern U.S. with numbers around 5 to 10 degrees above average. Meanwhile, cooler conditions should prevail back across the Upper Intermountain West/Northern Rockies and eventually into the adjacent Great Plains. The passage of the longwave trough should encourage a period of cold advection allowing for temperatures in the upper 60s to low 70s on Day 6/7, September 4-5. Regarding precipitation, heavy rainfall will be possible along the slow moving front, particularly across the Middle Mississippi Valley into the Upper Great Lakes. Over the southeastern U.S., depending on what emerges out of the tropics, increasing QPF amounts are expected next week while a return of convection to the Four Corners is anticipated with monsoonal moisture back in place. Rubin-Oster WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml