Extended Forecast Discussion NWS Weather Prediction Center College Park MD 251 AM EDT Fri Oct 15 2021 Valid 12Z Mon Oct 18 2021 - 12Z Fri Oct 22 2021 ...Overview... A large surface high should be in place across much of the eastern U.S. for the beginning of next week, bringing more fall-like weather compared to the unseasonable warmth and humidity in recent days. A storm system is expected to organize across the central Plains by Tuesday and track in the general direction of the Great Lakes by around Thursday, with a trailing cold front that should reach the East Coast by the end of the week. Meanwhile across the western U.S., a large scale trough over the northeast Pacific will increase the potential for unsettled weather for the West Coast region, with a weakening storm system by midweek and potentially a stronger storm system to close out the week. ...Guidance Evaluation/Preferences... Model guidance for the 00Z cycle is in good agreement on the synoptic scale pattern on Monday. One of the more uncertain aspects of the forecast has been the timing of the upper low crossing the Rockies, and the guidance has come into better agreement compared to the 12Z Thursday guidance in portraying a slower solution. The 00Z ECMWF has trended in line with the slower GFS/UKMET. At the time of this discussion, the 12Z ECENS is the most progressive solution here, although this is also likely to adjust farther to the west. By the end of the forecast period, the GFS is more offshore with the West Coast storm system, and farther east with a closed low over southern Canada compared to the ECMWF/CMC. The WPC forecast was primarily derived from a GFS/ECMWF/CMC blend through the majority of the forecast period, with increasing contributions from the ensemble means by Thursday and Friday. ...Weather/Hazards Highlights... Valley rain and mountain snow associated with the potential closed low over the northern Rockies on Monday is expected to reach the northern Plains by midweek, with perhaps up to an inch of rain on the north side of the surface low across portions of northern Nebraska and South Dakota, and several inches of snow across the higher terrain of Idaho, Montana, and Wyoming. Showers and storms will likely increase in coverage ahead of the trailing cold front from Texas to the Ohio Valley, and this should reach the Appalachians by Thursday as the front steadily progresses eastward. The next storm system reaching the West Coast by the end of the week will likely bring moderate to locally heavy rain for coastal areas from northern California to the Olympic Peninsula, although placement specifics for heaviest rainfall remain uncertain. Temperatures are expected to be 10-20 degrees below normal to start the week across portions of California and Nevada in the wake of the cold front, and about 5-15 degrees above normal across the northern Plains and Upper Midwest. The cooler readings reach the Plains by midweek with the Ohio Valley and East Coast region getting a modest warm-up ahead of the next front, with anomalies only on the order of 5 to 10 degrees above normal. By the end of the week, most locations should be within 5 degrees of their climatological averages with perhaps some greater warm anomalies over portions of Montana and Idaho. Hamrick 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