Extended Forecast Discussion NWS Weather Prediction Center College Park MD 343 PM EST Thu Nov 11 2021 Valid 12Z Sun Nov 14 2021 - 12Z Thu Nov 18 2021 ...Moderate to heavy precipitation for the Pacific Northwest Sunday-Monday... ...Overview... The medium range period begins Sunday with upper ridging over the Southwest, with onshore flow bringing enhanced precipitation to the Pacific Northwest, and general troughing over the eastern half of the country with lake effect/enhanced snow/rain showers. This pattern moves fairly progressively after that, as troughing comes into the Northwest by Tuesday, flattening the western pattern and producing a notable surface low moving across south-central Canada. This low should spread gusty winds to the north-central U.S. behind it as well as eventually some rain to the Mississippi/Ohio/Tennessee Valleys along its trailing cold front for midweek onward. ...Guidance/Predictability Assessment... 00/06Z model guidance is in generally good agreement with the large-scale pattern described above, though there are some differences in the details. Aspects of the pattern where models vary include the potential for and timing of a mid-upper low to close off within the eastern trough and potentially more impactful differences in strength/depth of shortwave energy coming into the Pacific Northwest tracking eastward. The latter creates differences in the surface pattern with the track of the southern Canada low--the ECMWF and CMC runs have appeared more consistent with the low track, while GFS runs have varied more, with the 00Z run farther north compared to consensus and the new 12Z run perhaps a bit quick/east. This has trended south a bit compared to the previous forecast. But overall deterministic models were fairly well clustered, and for the WPC medium range forecast, a mainly deterministic blend of the 06Z GFS and 00Z ECMWF/CMC/UKMET was able to be used, phasing out the UKMET and phasing in small proportions of the GEFS and EC ensemble means by the latter part of the period. ...Weather/Hazards Highlights... For the Pacific Northwest, a long fetch of Pacific moisture directed onshore will lead to moderate to locally heavy precipitation during the early part of the week. Precipitation should begin as all rain even in higher elevations of the Olympics and Cascades on Sunday into Monday, but snow levels should lower Monday night as a cold front comes through. Precipitation will be enhanced over the Olympics and Cascades, with locally heavy rain possible. Some precipitation is likely to spill into the northern Rockies late Monday into Wednesday, with higher elevation snow. Shortwave troughs moving through the southwest side of a broad trough over the eastern half of the U.S. and cold northwest flow will lead to lake effect/enhanced precipitation downwind of the Great Lakes through around Tuesday, along with a weak low pressure system bringing light rain and higher elevation/interior Northeast snow. Then as the surface low pressure system moves through southern Canada Monday through Thursday, gusty winds are expected on its backside, especially for 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. Mild temperatures that are considerably above normal are forecast for the western half of the country early next week, with temperatures 15-25F above normal for the High Plains on Monday and Tuesday. After the cold frontal passage, cool high pressure will drop into the western and central parts of the CONUS and drop the temperatures to slightly below normal. Meanwhile, expect temperatures of 5-15F cooler than average in the eastern U.S. early in the week to push eastward Tuesday and offshore by Wednesday, replaced by normal to slightly above normal temperatures. Tate Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Sun-Mon, Nov 14-Nov 15. - Heavy rain across portions of the Pacific Northwest, Sun-Mon, Nov 14-Nov 15. - High winds across portions of the Central Plains, the Northern Plains, and the Northern Rockies, Tue, Nov 16. 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