Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Fri Oct 29 2021 Valid 12Z Mon Nov 01 2021 - 12Z Fri Nov 05 2021 ...Overview... Expect a fairly persistent upper-level pattern during the medium range period. A strong mean ridge will be over western Canada with a more modest ridge over the western U.S., leading to a mean trough extending from the eastern half of Canada into the central and eastern U.S. The overall ridge-trough pattern should drift a bit eastward by Thursday-Friday. At least a couple Pacific shortwaves will reach the West Coast with some of the energy deflected northward by the Canadian ridge and the rest passing through the West. After midweek the Pacific energy may either temporarily dampen the western U.S. ridge or push it eastward into the Rockies/Plains. These impulses and associated frontal systems will produce rounds of precipitation over the West. Farther eastward, chilly surface high pressure behind a cold front will promote below normal temperatures across the central U.S. and increasingly more of the East. Waviness along the front may enhance rainfall over the Southern Plains and possibly into the Lower Mississippi Valley around midweek. ...Guidance/Predictability Assessment... Most guidance offers some similarities for the large scale pattern but there are a number of important detail differences that generally do not appear to have great predictability at longer time ranges. With respect to the Pacific shortwaves, the GFS is on the faster side of the spread of the leading feature that reaches the Pacific Northwest and vicinity during the first part of the week. Historical biases, as well as a recent tendency for any solution that eroded the Canadian upper ridge more than consensus to revert back eventually, would support less weight on the GFS scenario. Meanwhile the GFS becomes stronger and slower with the portion of the leading shortwave that continues along in the southern stream--leading to a stronger/slower southern tier wave by Thursday-Friday. Recent GFS runs have also been on the strong/fast side with what energy reaches the northwestern U.S./western Canada after midweek while being weaker than recent ECMWF runs and 00Z CMC with the southern part of the overall shortwave. Tendencies again favor a more robust Canadian ridge than in the GFS (12Z run trended stronger than the 06Z run) but otherwise the wide spread seen in ensemble spaghetti plots by late next week plus unsurprising changes in the 12Z runs leave little confidence in any specific solution. Due in part to these eastern Pacific/western North American differences, guidance continues to have difficulty in resolving the specifics of the central-eastern U.S. upper trough. Aforementioned traits of the GFS have led it to become weaker than consensus with the eastern trough. Other GFS/ECMWF/CMC runs have displayed either an open trough or an embedded closed low anywhere from southern Canada or the Great Lakes into the Midwest/Ohio Valley. Thus far no model has been very consistent. Recent trends have been toward enough eastern troughing aloft to keep the surface pattern suppressed over the eastern states (which the ensemble means have been more steady with over recent days), shifting the most likely surface low track into the western Atlantic by Friday. At that time the latest GEFS/ECMWF means depict a wave reaching offshore the New England coast. The 00Z ECMWF/CMC were closest to that idea while the new 00Z CMC is slower. Based on the above considerations, the 00Z/06Z operational model blend used in the first half of the period gave somewhat less weight to the GFS than might typically be the case. Then the forecast quickly adjusted to a blend incorporating 40-60 percent total weight of the 06Z GEFS/00Z ECMWF means with the remaining input from the 00Z ECMWF/CMC. ...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 and Rockies, with normal terrain enhancements and higher elevation snow. Some snow/rain may extend into the central High Plains on Monday into Tuesday but amounts remaining uncertain (most likely light to locally moderate). Then the combination of moisture and southern stream shortwave energy interacting with a frontal system across the south-central U.S. will increase rainfall chances Tuesday-Wednesday for the Southern Plains and possibly into parts of the Lower Mississippi Valley, with some heavy rain possible. Some precipitation could spread east/northeast during the latter part of the week, with the higher elevations of the Central Appalachians and the interior Northeast seeing some possibility for snow but with low confidence in the specifics. Behind the cold front and underneath high pressure, well below normal temperatures are likely across the central and eastern U.S. This will especially be the case for highs which should be 10-20F below average from the central and eventually southern parts of the Plains and spreading across more Mississippi Valley and eastern U.S. Chilly lows in the 20s may reach as far south as Kansas and Iowa, more moderate minus 5-10F anomalies that would still bring the first freeze around next Tuesday or so. Expect above normal low temperatures under the western ridge aloft but periods of unsettled weather will keep highs somewhat below normal over California on most days and near normal elsewhere. Rausch/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