Extended Forecast Discussion NWS Weather Prediction Center College Park MD 310 AM EDT Fri Oct 29 2021 Valid 12Z Mon Nov 01 2021 - 12Z Fri Nov 05 2021 ...Overview... A fairly stagnant upper-level pattern will be in place for much of the medium range period, with a strong mean ridge over western Canada, a more modest ridge over the western U.S., and a mean trough extending from the eastern half of Canada into the central and eastern U.S., before ridging shifts slowly eastward into the Plains Thursday and Friday. At the surface, a chilly high pressure system behind a cold front should cause below normal temperatures across the central U.S., while the initially slow-moving cold front enhances rainfall in the Southern Plains and Lower Mississippi Valley. Rounds of precipitation are also possible in the West as upper-level shortwaves and frontal systems approach. ...Guidance/Predictability Assessment... Model guidance from the 12/18Z cycle is in reasonably good agreement at the beginning of the period with the overall pattern. Some minor differences in track and timing remain for shortwaves coming into the Northwest as well as pivoting within the central/eastern U.S. low amplitude trough, but do not appear to cause notable differences in the pattern until around Wednesday onward. At that point, shortwave energy within the eastern trough appears to pinch off the most in the ECMWF compared to other guidance, and then model differences arise with upstream energy in the west side of the trough. By Friday, the 12Z ECMWF has a closed low across the Ohio Valley separate from the Canadian trough, which is within the realm of possibilities dependent on the evolution, but at this point is the only deterministic model to do so, and the new 00Z run shows a different structure. Additionally, shortwave energy in a mean trough across the eastern Pacific varies quite a bit with position and timing from midweek onward. The ECMWF brings troughing onshore most quickly by early Thursday, while GFS runs are slow and strong with the energy and aggressively create a closed low across California by Friday. The 12Z CMC was in between these solutions. Despite the variations in evolution, at this point there were not clear outliers, and blending the different solutions together ended up creating a good compromise forecast. The WPC forecast utilized a deterministic model blend of the 12Z and 18Z GFS and the 12Z ECMWF/UKMET/CMC early in the period, adding the GEFS and EC ensemble means into the blend by day 6 and 7. ...Weather/Hazards Highlights... The frontal systems and shortwaves coming into the West will cause periods of enhanced precipitation--first on Monday and then Wednesday into Friday. Northern California and far southwestern Oregon should see the heaviest precipitation, with some spreading into the Pacific Northwest (especially during the latter period) and lesser totals eastward into the Intermountain West, with normal terrain enhancements and higher elevation snow. Some snow may extend into the central High Plains on Monday into Tuesday, but amounts remain uncertain. Then as moisture flows into the vicinity of a frontal system across the south-central U.S., rainfall chances will increase on Tuesday into Wednesday for the Southern Plains into the Lower Mississippi Valley. Some precipitation could spread east and northward for the latter part of the week, with the higher elevations of the Central Appalachians and the interior Northeast seeing some possibility for snow, but confidence in specifics is low. Behind the cold front and underneath high pressure, well below normal temperatures are expected across the central and eastern U.S., especially in terms of highs. Highs of 10-20F below average are expected across central portions of the Plains into the Middle Mississippi Valley and Ohio Valley. Chilly lows in the 20s as far south as Kansas and Iowa are forecast, which is 5-10F below average, and some areas may experience their first freeze around next Tuesday or so. Above normal low temperatures are expected under ridging in the West, though highs should be right around average. 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 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