Extended Forecast Discussion NWS Weather Prediction Center College Park MD 203 AM EDT Sat Oct 12 2019 Valid 12Z Tue Oct 15 2019 - 12Z Sat Oct 19 2019 ...Pattern Overview and Guidance/Uncertainty Assessment... A relatively amplified and progressive pattern will be in place across the CONUS during the extended period. A shortwave crossing the northern Plains/Midwest on Tue and the Great Lakes/Northeast Wed/Thu is forecast to gradually amplify, with a closed upper low developing across New England by Wed night/Thu morning. At the surface, a low pressure system and surface frontal system are forecast to sweep across the eastern half of the nation. Formation of a new low is expected across the Mid-Atlantic region on Wed, which is forecast to intensity into a relatively deep cyclone along the New England coast Wed night/Thu as the upper low intensifies. Farther west, an intense Pacific upper-level jet is forecast to reach the Northwest along with a train of shortwaves starting Wed night, which should quickly lower heights across the Northwest and eventually the northern Rockies as the series of systems moves inland. By the end of the forecast period (day 7/Sat), models show quite a bit of spread, but there is some consensus that the primary trough axis should be centered across the Rockies with a rapid succession of energy ejecting eastward across the central U.S. The 12Z ECMWF/UKMET were heavily used in the forecast during days 3-5 (Tue-Thu). Additionally, the GEFS mean was preferred over the GFS as the mean was not quite as fast as the operational run to lower heights farther inland across the Northwest/northern Rockies. There has been a trend toward faster progression of energy inland compared to this time last night, but the GFS remains on the fast side of the model/ensemble spread and was not preferred at this time. Later in the forecast period model spread and run-to-run variability increases significantly. The evolution of shortwave energy across the central U.S./Canada, as well as the amplitude of additional shortwave energy reaching the West Coast, during this time frame, are very much in question. Confidence in the specifics of the forecast by days 6-7 is quite low, especially across the western and central U.S. A blend of the ECENS/GEFS ensemble means was primarily used for the day 6-7 forecast to accommodate the lack of confidence in the specifics. ...Weather Highlights/Threats... A weakening/stalled frontal boundary lingering over the Lower Mississippi Valley to Southeast on Tue will be the focus for widespread showers and thunderstorms. Model consensus suggests that 2-3 inches of rain are possible across portions of these areas. As the area of low pressure develops mid-week over the Great Lakes and moves into the Northeast U.S., deeper moisture will be pulled northward, resulting in widespread rainfall, some of which could be locally heavy. Meanwhile, precipitation is expected to gradually increase across the Pacific Northwest starting Tue and continuing through the the week as multiple shortwaves/frontal systems affect the region. Snow-levels are forecast to gradually fall through the week as lower heights begin to press inland. Heavy precipitation is expected to become widespread across the coastal ranges of the Northwest as well as the Cascades by later in the week, with multi-inch liquid equivalents possible. By late next week, moisture return across the central U.S. ahead of the western trough could generate scattered showers across portions of the Mississippi Valley, meanwhile, a lingering surface front in the Gulf of Mexico may focus showers along the western/central Gulf Coast. Temperatures are forecast to initially be well below average across the north central U.S. in the wake of the current winter storm. Highs are forecast to be 15-25 deg F below average across portions of the northern Plains and Upper Midwest Tue-Wed, before temperatures begin to moderate somewhat later in the week. Cooler temperatures are forecast to spread into the eastern half of the nation by the middle next week as Canadian high pressure overspreads the region behind the cold front. High temperatures 5 to 15 deg below average are forecast across a large area from the Mississippi River to the Eastern Seaboard Wed-Thu, before temperatures return to near seasonal norms by next Fri-Sat. Below average temperatures will spread into the Pacific Northwest and northern Great Basin by late next week as upper-level heights fall and the series of frontal systems affect the region. Ryan 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 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