Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sat Oct 30 2021 Valid 12Z Tue Nov 02 2021 - 12Z Sat Nov 06 2021 ...Overview... An established strong ridge over western Canada and troughing across the eastern half of Canada stretching into the north-central and northeastern parts of the U.S. will be persistent during the medium range period, while shifting a bit eastward during the latter part of the week. 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, which may dampen any ridging that expands into the western U.S. as well as causing rounds of precipitation there. 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. Enhanced rainfall is likely over the Southern Plains and into the Lower Mississippi Valley around midweek in the vicinity of that front. ...Guidance/Predictability Assessment... 12/18Z model 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. Differences arise as early as Tuesday-Wednesday with a southern stream shortwave moving across the Four Corners, with the 12Z CMC the weakest deterministic model with this feature, as well as shortwave energy coming to the Northwest, where the 12Z CMC is somewhat aggressive in this energy digging into the Northern Rockies compared to other guidance. For the most part a multi-model deterministic blend of the 18Z GFS and 12Z ECMWF and UKMET with less emphasis on the CMC served as a compromise to minimize individual model differences with these features. Meanwhile, shortwave differences also appear within the eastern trough. The ECMWF has shown run-to-run discontinuities in its solutions with the strength of energy digging through the western side of the trough, and the 12Z ECMWF shows a compact low closing off for a time as it drops through the Plains around Thursday, while the new 00Z run spins up a closed low in the Ohio Valley by Friday. GFS runs have shown less propensity for digging energy causing such a pattern and could be too weak, while the 12Z CMC is stronger but takes a different approach with handling it, separating a closed low more from the northern stream trough by Friday as it moves over the Great Lakes/Ohio Valley. The new 00Z CMC backs off from this solution though the new 00Z ECMWF is actually not too different from the 12Z CMC. The eventual evolution of this eastern trough will also determine how a low pressure and frontal system ends up evolving likely offshore of the East Coast in the western Atlantic. Overall confidence is low in these specifics given the differences in the deterministic guidance as well as a broad range of solutions shown in ensemble spaghetti plots. The WPC model blend for Thursday onward utilized the ECMWF/GFS/EC mean/GEFS mean, but favored an increasing proportion of the 12Z EC ensemble mean over the operational EC and GEFS, as it had some hint of digging energy on the western side of the trough without going overboard. ...Weather/Hazards Highlights... The frontal systems and shortwaves approaching the West will cause periods of enhanced precipitation during the second half of the week. Northern California and far southwestern Oregon should see the heaviest precipitation, with some spreading into the Pacific Northwest 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 Tuesday and Wednesday, but amounts remain 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 the Mississippi Valley and eastern U.S. Chilly lows in the 20s may reach as far south as Kansas and Iowa, which would only be minus 5-10F anomalies but 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. Tate/Rausch 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