Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM 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... Model guidance from the 00/06Z cycle shows fairly good agreement with the broader pattern especially early on in the period, and the WPC forecast was able to use a composite blend of the 06Z GFS and 00Z ECMWF, UKMET, and CMC. The blend served to smooth out remaining differences with shortwaves and surface low pressure systems in the East. Guidance did continue its trend toward more amplified troughing coming into the West by Tuesday, which continues to be reflected in most new 12Z models as well (though the 12Z GFS ends up with an open 500 mb trough on Wednesday unlike the consensus closed low). This amplified flow trend may lead to a slightly faster track of the surface low through southwestern and south-central Canada and its associated cold front sweeping across the CONUS during the week compared to the previous forecast. By the latter part of the week, differences in the axis/tilt of the central U.S./Canada trough stemming from the low increase, as GFS and CMC runs bring shortwave energy through the southern part of the trough to produce a neutral-negative tilt, while the 00Z ECMWF holds onto a positively tilted trough Thursday. Given that this tilt in the EC showed almost the deepest solution compared to CMC/EC/GFS ensemble members in the southern U.S. Thursday and Friday, leaned away from that solution, and the new 12Z run trended away as well (though the 12/00Z EC runs have been flip-flopping back and forth on this type of solution). Farther upstream, flow out of the eastern Pacific into the West remains quite uncertain, with the potential for shortwaves to break through ridging at various times and areas in separate Pacific streams. Thus for the latter part of the medium range period, used a blend with increasing influence from the GEFS and EC means, lessening the ECMWF and GFS influence, which led to generally flat flow in the Pacific, 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 over areas that have been quite wet over the last week or so. 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. Then the potent surface low tracking across southern Canada Monday onward will bring a period of strong/gusty winds to northern tier areas, especially the Northern Plains on Tuesday into Wednesday. 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 record 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. The 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. Tate/Rausch 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