Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu Oct 14 2021 Valid 12Z Sun Oct 17 2021 - 12Z Thu Oct 21 2021 ...Overview... The majority of guidance shows a progressive pattern across most of the lower 48 Sunday-Monday, with an eastern Pacific shortwave moving into the West while a ridge crosses the central U.S. and an eastern U.S. trough becomes more shallow. Then consensus shows an evolution toward a slower and more amplified regime, as an amplified mean trough settling over the eastern Pacific (consisting of leading energy that ejects north-northeast around Wednesday as upstream energy drops in) supports mean ridging over the western U.S./Canada and in turn troughing over east-central North America. Embedded within this pattern transition, the shortwave energy entering the West late Sunday continues to provide a significant forecast challenge for models and ensembles--keeping confidence moderate to low for the distribution and intensity of precipitation that may progress across the western two-thirds of the country during the period. There is somewhat better confidence in two periods of precipitation along the West Coast, one with the Sunday shortwave and the next from flow on the eastern side of the larger scale Pacific upper trough. ...Guidance Evaluation/Preferences... With each cycle there is a gradually increasing signal that incoming western shortwave energy will separate to form a closed low in some fashion. Recent ECMWF runs had joined the CMC which was the first model to suggest such an evolution (albeit in an extremely slow/deep form). More recently GFS runs adopted the closed low scenario, and finally in the new 00Z cycle the UKMET has switched from its prior open/progressive trough (which had been consensus a couple days ago) to a closed low. The basic evolution appears closer to being resolved but meaningful differences for track and timing of the upper low remain. Not surprisingly individual ensemble members from the various model systems vary considerably to yield an open trough. CMC runs have tended to be on the slow side--still a question mark given the large scale pattern favoring a building western ridge. Given the spread that existed through the 12Z/18Z cycles, the preference for the updated forecast was to keep adjustments fairly modest by way of starting with a blend of the 12Z/18Z GFS and 12Z ECMWF on day 3 Sunday and then some gradual incorporation of 18Z GEFS/12Z ECMWF means with those models. The result was to maintain a conservative depiction of some flow separation while keeping progression fairly close to the means. Over eastern North America there has been a notable trend over the past day or so toward a more persistent upper low over extreme northern New England and the Canadian Maritimes through the first half of next week, due to stronger ridging over and southwest of Greenland. The increased blockiness seems to provide some forecast complications over southern Canada later in the period as well while western Canada shortwave energy approaches. By day 7 Thursday the 12Z GFS and new 00Z CMC end up with an upper low well west of trough axis in all of the ensemble means. 12Z GFS weight in the aforementioned blend was kept low enough not to have an adverse impact on the forecast. In general there is better agreement and continuity with the amplified mean trough aloft that develops over the eastern Pacific. With typical timing differences for six days out in time, consensus shows north-northeastward ejection of leading energy near the West Coast around Wednesday as upstream energy digs in to reinforce the mean trough. ...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. However there is still considerable uncertainty over the strength, track, and timing of this upper low so confidence in the details remains 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. 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