Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 AM EDT Thu Oct 15 2020 Valid 12Z Sun Oct 18 2020 - 12Z Thu Oct 22 2020 ...Overview... A broad upper-level trough over the northern two-thirds of the U.S. may shift westward as the week progresses as shortwave energy reorients its axis. A front stalling in the central U.S. could cause rounds of rainfall along it through the week. Snow is forecast in the Northern Rockies, some of which could spread into the Northern High Plains Sun as colder than normal temperatures can be expected there. ...Guidance Evaluation/Preferences... The 12/18Z model guidance was agreeable regarding troughing across much of the CONUS Sun-Mon influenced by an upper low spinning near the Hudson Bay. Additionally, surface low formation north of the Great Lakes Sun morning appeared well handled by deterministic guidance. The WPC blend for the early part of the week favored the 12Z ECMWF and 18Z GFS with some 12Z UKMET influence. The 12Z CMC was an outlier by Mon as shortwave energy created a trough across the west-central U.S. earlier and deeper than other guidance. By Tue onward, after model disagreement over the past couple of days, guidance is coming around to the idea that the ECMWF runs have been consistent with--a shortwave moving through western North America that deepens the western side of the broad trough, reorienting its axis westward. The EC remains more aggressive than the GFS suite with this troughing in the western CONUS. Blended the operational 12Z ECMWF and ECENS with the 18Z GFS and GEFS mean for the later period to minimize individual models, but at least spread seems to be reduced compared to previous runs. ...Weather/Hazard Highlights... A potent cold front will cross the Northern Plains to Lake Superior this weekend, reinforcing cold temperatures over the north-central CONUS. Max temperatures in the 30s and 40s are likely across the Northern Plains to the Upper Mississippi Valley through Tue, which is 15 to 30 degrees below normal. Min temperatures in the teens and 20s are 10 to 20 degrees below normal. Precipitation is likely with this frontal system, and notable snow is forecast for the Northern Rockies and into the Northern Plains. This may be the first snow of the season for lower elevations of the north-central U.S. Then, light to modest rain totals are possible along the frontal boundary passing through the central U.S., but the timing of the front and thus the rain is questionable. Though the trend for the front has been slower, the threat for heavy rain remains quite uncertain. The Southwest from West Texas to California and the Central Great Basin will remain under an upper ridge through the early part of the week, causing dry conditions. Warmer than normal temperatures are forecast there, and Sun in particular could see temperatures hitting 100 degrees for the lower deserts of Arizona and California. The upper ridge shifts farther west late this weekend, making for more moderate temperatures by early next week, though may stay slightly above average. Meanwhile, tropical showers are expected across southern Florida as temperatures remain mild and a few degrees above normal. Tate 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