Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Thu Oct 14 2021 Valid 12Z Sun Oct 17 2021 - 12Z Thu Oct 21 2021 ...Overview... The medium range period will begin Sunday with troughing pushing a cold front offshore the Eastern Seaboard, with some lake effect rain showers behind, while another frontal system comes into the West ahead of troughing/energy expected to track across the central U.S. as the week progresses. Model guidance has become more consistent with this energy separating into the southern stream and disrupting upper ridging on either side of it. A cold front and light to moderate precipitation should track east to west over the western and central U.S. ahead of this feature. Persistent amplified troughing with multiple energy centers in the eastern Pacific may lead to precipitation coming onshore the West Coast for the latter part of the week. ...Guidance Evaluation/Preferences... After an agreeable start to the medium range period, model guidance signal continues to increase that the shortwave energy coming into the West should separate to form a closed low likely tracking through the Intermountain West and into the Plains Monday through Wednesday. After some waffling in guidance over the past couple of days, the consensus shows a slower track of this upper low for the most part--in terms of the 00/06Z solutions, the 00Z ECMWF was actually the fastest deterministic model at moving the small closed low out into the Plains, with the UKMET/CMC/GFS runs in better alignment. The new 12Z ECMWF has slowed down to be closer to consensus. Ensemble member variability does show that the timing and depth differences are not completely resolved yet, but today's forecast for the bulk of the medium range period used a blend of the deterministic guidance with less weighting on the 00Z ECMWF. By around midweek onward, increased weighting on the 06Z GEFS mean and 00Z EC ensemble mean, as greater differences arise with possibly absorbing the shortwave energy back into the northern stream. Other features of the forecast including the trough lifting out of the eastern U.S. but forming a closed low spinning over eastern Canada, as well as rounds of troughing across the eastern Pacific, seem to be in fairly good agreement and could be handled with the above blend as well. ...Weather/Hazards Highlights... The system affecting the Northwest through the weekend will bring mostly moderate precipitation to western Washington/Oregon and northern California on Sunday. It looks increasingly likely that there will be an axis of rain and higher elevation snow across portions north-central portions of the West as an upper low closes off and progresses eastward. Though confidence regarding the strength, track, and timing of this upper low is increasing, confidence in the details of the associated precipitation remain lower than desired. Some of this moisture should continue across similar latitudes of the central U.S. while some rain could develop along the leading cold front. Flow on the eastern side of the Pacific upper trough should bring one or more periods of rain to central/northern parts of the West Coast Tuesday onward. On Sunday there will be some lingering rain to the lee of the eastern Great Lakes and over favored windward terrain in New England. The Florida Peninsula may see an increase of rainfall toward midweek as low level easterly flow develops. Coverage of temperature anomalies 10F or more from normal should be relatively low through the period, pending resolution of some forecast details. Warm flow ahead of a front crossing the northern tier may bring area of plus 10F or higher anomalies to locations from Montana into the Great Lakes Sunday-Tuesday. Similar anomalies for lows may continue into the Northeast thereafter. Meanwhile central/north-central parts of the West may see some highs 10-15F below normal early next week with the upper low closing off and tracking over the region. Otherwise moderately cool temperatures over the South and East on Sunday will quickly moderate heading into next week, yielding near to slightly above average highs by midweek. Clouds/moisture will keep highs modestly below normal near the West Coast on some days. 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