Extended Forecast Discussion NWS Weather Prediction Center College Park MD 459 PM EDT Fri Oct 15 2021 Valid 12Z Mon Oct 18 2021 - 12Z Fri Oct 22 2021 ...Overview... The main weather feature across the U.S. next week will be an upper low pushing a surface frontal system ahead of it and spreading precipitation west to east across mainly northern and central parts of the country. Ahead of this system, a surface high will lead cooler and dry weather across the East for the early part of the week. Meanwhile across the western U.S., a large scale trough over the northeast Pacific could increase the potential for rounds of unsettled weather in the West Coast, likely around midweek onward. ...Guidance Evaluation/Preferences... Model guidance for the 00/06Z cycle is in good agreement with the overall pattern through about Wednesday-Thursday. A few model cycles in a row now have shown good consensus with the track and timing of the main upper-level feature influencing the forecast: shortwave energy dropping into the West early in the week and moving east through the Intermountain West, northern/central parts of the Plains, and into the Midwest by Thursday, while pushing upper ridging eastward and flattening it. A minor change with this model cycle was to show that the upper low may become closed a bit earlier, around Monday. Thus for the early part of the period, a blend of the deterministic 06Z GFS and 00Z ECMWF, UKMET, and CMC was used. Then by Wednesday into Thursday, model differences and thus uncertainty start to increase with the pattern across the eastern half of Canada, with orientation of energy in broad cyclonic flow and the possibility of a northern stream closed low forming. This influences the track of the low and how/when it joins back up with the northern stream by Thursday into Friday. Given no clear trend or particularly preferred solution at this point, today's model blend transitioned quickly to a GEFS and EC ensemble mean-heavy approach during the latter part of the period to minimize individual model differences. While mean troughing in the eastern Pacific is likely, rounds of energy moving through it and periods of the low potentially closing off vary somewhat among guidance, especially by Friday. The trend for this cycle has been for a slightly slower approach of the trough eastward toward the West Coast Friday, which would imply a slower front and less precipitation expected to reach the Pacific Northwest and California. The model blend approach described above also worked for this aspect of the forecast. ...Weather/Hazards Highlights... Precipitation associated with the closed upper low and cold front is forecast to track east through the period. Snow accumulating to several inches is possible over higher elevations of the northern Rockies (mainly Idaho, Montana, and southwest Montana) Monday into Tuesday, with some chance of light snow for the northern/central High Plains. Lower elevation rain to perhaps an inch spreads across the northern and central Plains around midweek, before tracking into the Midwest and Great Lakes region and eventually the Northeast later in the week. Some rain showers are also possible farther south along the cold front from Texas into the Ohio Valley and Appalachians. In the West, cold fronts approaching the coast ahead of troughing in the Pacific could lead to increased precipitation especially for coastal areas from northern California toward the Olympic Peninsula, though there is uncertainty with how much precipitation will reach the coast. Additionally, a wetter pattern is likely to return to southern Florida for the latter half of the week. Cool temperatures are forecast behind the cold front in the West on Monday and Tuesday, with highs 10-20F below normal in portions of California and Nevada and 10-15F below normal into the Intermountain West. Temperatures should moderate closer to normal as the front weakens, though should spread some cooler conditions to the northern/central High Plains. Meanwhile ahead of the front, a modest warm-up is expected, with the most anomalous highs of 10F+ above average for the Midwest on Monday and Tuesday. At this point, temperatures look to ease closer to normal for the latter part of the week over most of the country, with perhaps some greater warm anomalies over portions of Montana and Idaho. Tate 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 Hazards: - Flooding possible across portions of the Southern Plains. - Flooding occurring or imminent across portions of the Southern Plains. - Flooding likely across portions of the Southern Plains. 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