Extended Forecast Discussion NWS Weather Prediction Center College Park MD 424 PM EDT Thu Nov 04 2021 Valid 12Z Sun Nov 07 2021 - 12Z Thu Nov 11 2021 19Z Update: The 12Z model guidance suite is in very good agreement for the beginning of the forecast period on Sunday, with differences becoming more apparent by Monday across south-central Canada and the West Coast. The GFS is stronger with the shortwave crossing eastern Canada and faster with the arrival of the next shortwave reaching the Pacific Northwest by Tuesday, and this quick progression continues across the Plains by the middle of the week, and ahead of its own ensemble mean. The UKMET is on the slower side of the guidance with the ECMWF/ECENS/GEFS in relatively good agreement on timing. This will have implications on the next storm system across the central U.S. and the extent/timing of heavier shower and storms. Otherwise, the forecast remains largely on track from the previous version, and the overnight discussion is appended below for reference. /Hamrick ...Overview... The medium range period will begin Sunday with an upper ridge across the central U.S., separating troughing in the East and along the West Coast. This pattern shifts eastward as the week progresses, as multiple shortwaves from the Pacific deepen troughing across the central U.S. by midweek. A series of fronts coming into the West Coast will keep precipitation chances high there, and precipitation is currently expected to increase over the central U.S. by Wednesday as a low pressure system consolidates in the Midwest. ...Guidance/Predictability Assessment... The 12/18Z model guidance continues to show reasonably good agreement at the beginning of the medium range period with the large-scale pattern, though some differences still remain with the position and track of the surface low moving northeastward through the western Atlantic Sunday into Monday with the eastern upper trough. The 18Z GFS appeared to be on the faster/more northeast side of the guidance envelope and was not particularly favored, while the 12Z ECMWF was on the slower side by Monday. Favored a position in between, which was closer to the ensemble means' position and other 12Z guidance including the GFS, and which fits in line with WPC continuity. Shortwave differences remain within the jet stream across the West into the north-central U.S. and south-central Canada. Initially these differences are nothing too egregious considering the time range, but start to affect the overall flow across the western and central U.S. by around Tuesday. The evolution of these shortwaves and differences in strength of an upper low rotating in the eastern Pacific and approaching the Pacific Northwest Tuesday cause downstream differences across the central U.S., in terms of amplification of troughing moving through and with whether shortwave energy combines or remains separate. A significant amount of run to run variability (including with the newer 00Z cycle coming in) and between models lead to low confidence. A forecast mainly based on the ensemble means, which show troughing across the central U.S. Wednesday/Thursday but not as amplified as the 12Z ECMWF, seemed like the best starting point at this time. Overall the WPC forecast utilized a multi-model blend of 12Z deterministic guidance early in the period, favoring the 12Z GFS more than the 18Z, and increasing usage of the 12Z EC and GEFS ensemble means as the period progressed while lessening weighting of the less agreeable deterministic models. ...Weather/Hazards Highlights... Some light rain may linger across eastern North Carolina into Sunday and possibly over coastal areas of the Mid-Atlantic and Northeast given the Atlantic surface low, but most precipitation should stay offshore with this low track. Abundant moisture ahead of frontal systems and upper-level shortwaves edging in from the Pacific will result in periods of enhanced precipitation for much of next week near the West Coast and inland. Through the period, the Pacific Northwest and northern California can expect the heaviest precipitation, with typical terrain enhancements and higher elevation snow. Precipitation is likely to spread into the Intermountain West and Rockies as well, with rain and mountain snow threats increasing by Monday and especially Tuesday into Wednesday. Then as a low pressure system is forecast to consolidate in the Midwest by midweek, rain is expected to spread to that area and south along its trailing cold front in the Plains/Mississippi Valley. Chances for snow on the backside of this low in the north-central U.S. will continue to be monitored. Temperatures could remain slightly below normal on Sunday for the Southeast due to a cold surface high to the west and the low offshore bringing northerly winds. Meanwhile, a persistent upper-level ridge over the west-central U.S. should lead to temperatures that are above average by 10-20 degrees over the Plains into the Midwest through early next week, with highs 5-10F above normal stretching toward the East Coast by Tuesday-Thursday. Periods of unsettled weather will keep highs modestly below normal over the West Coast on most days, eventually entering into the Great Basin and Rockies by midweek. Tate Hazards: - Heavy rain across portions of California and the Pacific Northwest, Mon-Wed, Nov 8-Nov 10. - Heavy snow across portions of California, the Central Great Basin, the Pacific Northwest, and the Northern Great Basin, Mon-Wed, Nov 8-Nov 10. - Heavy snow across portions of the Central Rockies, the Central Great Basin, the Northern Rockies, the Northern Great Basin, and the Northern Plains, Tue-Wed, Nov 9-Nov 10. - High winds across portions of the Mid-Atlantic, Sun, Nov 7. 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