Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Fri Nov 12 2021 Valid 12Z Mon Nov 15 2021 - 12Z Fri Nov 19 2021 ...Moderate to heavy precipitation over the Pacific Northwest continues into Monday... ...Overview... Expect the western ridge/eastern trough upper pattern as of the start of the week to transition toward progressive and broadly cyclonic mean flow. As this transition begins, Pacific energy about to enter western North America will direct an axis of moisture into the Pacific Northwest while under the eastern trough aloft the chilly low level flow around a system tracking into the Canadian Maritimes will promote lake effect/enhanced rain or snow. Over the course of the week the Pacific shortwave will continue across the continent, supporting a strong southern Canada surface low and trailing cold front that ultimately crosses much of the lower 48--along with some strong/gusty winds over parts of the northern tier and later some rain over the east-central U.S. Well above normal temperatures from the West into the Plains and cool conditions over the East early in the week should moderate later in the week with the flatter trend for the mean pattern. ...Guidance/Predictability Assessment... A composite of 18Z/12Z operational models provided a good representation of significant systems during the first half of the period and incorporating some 18Z GEFS/12Z ECMWF mean input along with the 18Z GFS/12Z ECMWF reflected the most common ideas of guidance thereafter. There are still some finer-scale details to be resolved for one or two low centers that will consolidate into a stronger Canadian Maritimes system by early Tuesday. Some spread persists for the upper trough crossing the continent and associated Canadian low pressure/lower 48 cold front but clustering has been gradually improving, with GFS adjustments over the past 12 hours generally contributing to that trend. The new 00Z GFS/GEFS mean are a bit on the amplified side as the trough comes into/through the West but the 00Z ECMWF has strongly adjusted this way too. Later in the week there is decent agreement for the cold front reaching the East Coast but not much support for the 18Z GFS wave that would produce significant rainfall over parts of Florida (so that aspect of the 18Z GFS was downplayed). However the flatter mean flow farther west rapidly reduces confidence in specifics, with individual model runs varying for details of shortwaves that could be embedded within separate Pacific streams. The preferred model/mean blend yielded a conservative approach while awaiting improved agreement. ...Weather/Hazards Highlights... A band of enhanced moisture along a wavy front heading into the Northwest will produce locally heavy precipitation into the early part of the week. Precipitation should be all rain even in higher elevations of the Olympics and Cascades into Monday but snow levels should lower Monday night as the cold front comes through. Expect the highest precipitation totals over the Olympics and Cascades with locally heavy rain possible. Some rain and higher elevation snow should spill into the northern Rockies late Monday through Tuesday. Meanwhile shortwaves moving through the southwest side of the initial eastern U.S. upper trough and cold northwesterly low level flow behind a consolidating system tracking into the Canadian Maritimes will produce lake effect/enhanced precipitation downwind of the Great Lakes through Tuesday. The surface system itself could bring light rain and higher elevation/interior Northeast snow. The surface low tracking across southern Canada Monday onward will bring a period of strong/gusty winds to northern tier areas, especially the northern High Plains on Tuesday. Moisture should increase ahead of the low's trailing cold front from midweek onward, leading to increasing rain chances for portions of the Mississippi, Ohio, and Tennessee Valleys along with additional precipitation for the Great Lakes region. Most of this activity should be light to moderate. Well above normal temperatures will prevail over the West and into the Plains Monday-Tuesday with a broad area of plus 10-25F anomalies. Some readings could reach daily record values, perhaps with a slightly better chance for warm lows relative to highs. At the same time the East will be on the cool side, with Monday seeing the best potential for some highs 10-15F below normal. Cold frontal passage will spread slightly below normal temperatures from west to east across the lower 48 mid-late week. Leading warm anomalies reaching the South and East Wednesday-Thursday will be only in the plus 5-15F range. Rausch/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, 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