Extended Forecast Discussion NWS Weather Prediction Center College Park MD 358 PM EDT Thu Sep 10 2020 Valid 12Z Sun Sep 13 2020 - 12Z Thu Sep 17 2020 ...Overview and Guidance/Predictability Assessment... A complex evolution over the eastern Pacific should yield a fairly deep trough and closed low that will approach the West Coast midweek next week. Meanwhile initial western U.S. and southeastern U.S./western Atlantic ridges should combine to produce a broad area of southern tier mean ridging, though with one or more embedded weaknesses near the TX/NM border. Additional tropical development may play a part over the northern Gulf of Mexico. Progressive flow will prevail across southern Canada and most of the northern U.S. until about next Wed/Thu when the flow may become more amplified into Ontario and the Great Lakes. Multi-model blend sufficed to start the period Sun-Mon CONUS-wide with expected differences in the guidance. Thereafter, the primary focus of guidance trends and spread is on the eastern Pacific trough along with some downstream effects across the northern U.S./southern Canada. Over the past two or three days there has been pronounced trending in the guidance, first toward a deeper trough and then for the closing of an upper low. Involvement of multiple pieces of energy (an initial East Pacific upper low and one or two digging shortwave impulses from the northeastern Pacific/Gulf of Alaska) constrains confidence in specifics. Broadening ensemble spread by days 6-7 Wed-Thu also highlights the uncertainty, but a trend in the last 24 hours points to a bit slower progression via an increased in meridional flow. The 00Z ECMWF/Canadian and most of the multi-center ensembles indicate a closed low to slowly move toward the Pacific Northwest next Tue-Thu rather than progress inland as quickly as the 00Z/06Z GFS runs. Preferred to trend the forecast toward the slower solutions via the non-GFS cluster with an increase in ensemble weighting to counteract the rather deep ECMWF/Canadian solutions over central Canada next Wed-Thu. Any tropical features (or surface troughs) in the Gulf were coordinated with NHC and will be reflected in the 2130Z update to the fronts. ...Weather/Hazard Highlights... The front crossing the eastern U.S. on Sun will bring a broad area of rainfall with some locally moderate-heavy amounts, followed by a period of drier weather over the northern half to two-thirds of the East as strong high pressure builds in. On the other hand the combination of initial southern tier moisture and the aforementioned front settling along the Gulf Coast for multiple days should ultimately promote periods of rainfall through the period between the Southeast and Texas with some significant totals possible. A weak upper feature/surface trough/low over portions of the northern-northeastern Gulf may further enhance rainfall. The overall precipitation area could expand northward over the Tennessee/Mississippi Valley regions by the latter half of the period depending on how the potential upper weakness over or near the Plains evolves and interacts with the Gulf feature. The upper trough/low evolving off the West Coast may bring meaningful rainfall to some areas along the northern half of the West Coast, perhaps as an initial wave via a lead front followed by another front via the upper trough. Ongoing spread/trends in the guidance continue to temper confidence in timing, coverage, and amounts though. As the initial western U.S. upper ridge moves into the Plains Sun into the next work week, expect warmest temperatures versus normal (up to plus 10-15F anomalies) to progress from the Interior Northwest into the Northern Plains followed by a slight decrease in greatest anomalies from the north-central Plains into the Rockies/Great Basin toward Wed-Thu. The approaching East Pacific upper trough will bring a cooling trend for afternoon highs perhaps by Wed or Thu. Morning lows should remain above normal over much of the West. Highs over the East will tend to be near to below normal due to the clouds/rain over the southern tier during the period and the front crossing the East early, where southerly flow ahead of the front will bring above normal lows over much of the East Sun-Mon. Cool high pressure building in behind the front may bring in the coolest air of the season to areas such as the Mid-Atlantic where 50s are possible Tue-Thu. Fracasso/Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy rain across portions of the Southeast, the Southern Appalachians, the Mid-Atlantic, the Southern Plains, and the Tennessee Valley, Sun-Mon, Sep 13-Sep 14. - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, and the Southern Plains, Sun-Thu, Sep 13-Sep 17. - Heavy rain across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Ohio Valley, and the Tennessee Valley, Wed-Thu, Sep 16-Sep 17. - Flooding likely across portions of the Southern Plains. - Much above normal temperatures across portions of the Central Great Basin, the Northern Rockies, and the Northern Great Basin, Sun, Sep 13. - Much above normal temperatures across portions of the Central Plains, the Northern Plains, and the Northern Rockies, Mon-Wed, Sep 14-Sep 16. - Much above normal temperatures across portions of the Central Plains, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains, Tue-Wed, Sep 15-Sep 16. - Heavy rain across portions of mainland Alaska, Mon-Wed, Sep 14-Sep 16. - Heavy rain across portions of the Aleutians, Sun-Mon, Sep 13-Sep 14. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml