Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun Oct 31 2021 Valid 12Z Wed Nov 03 2021 - 12Z Sun Nov 07 2021 ...Overview... An omega block initially centered over western Canada during the medium range period will be instrumental in digging shortwave energy toward a slow-moving broad trough across the eastern U.S. where the potential for cyclogenesis late this week will continue to be monitored. Meanwhile, unsettled weather will likely continue along the West Coast and especially the Pacific Northwest where shortwaves will attempt to pass under the omega block into the western U.S. Models are in disagreement as to how much of the shortwave energy will get through the West and eventually reach the Plains states late this week where potential exists for the development of an upper trough/low over the southern Plains. ...Guidance/Predictability Assessment... Model guidance this morning continues to maintain their own preferences as to how the amplified pattern described above will evolve through next weekend. The ECMWF solutions continue to indicate cyclogenesis near the East Coast late this week due to more aggressive digging of shortwaves into the northern part of the U.S. midweek. The deeper upper trough in turn opens the door for shortwave energy to penetrate the western U.S. under the omega block and phase with the eastern U.S. upper trough. The GFS continues to be much less aggressive with the deepening of the upper trough and therefore keeping the potential for cyclogenesis much lower and much farther off the East Coast. On the other hand, the CMC shows elements from both the ECMWF and GFS solutions, and offers an intermediate solution. Meanwhile, the 00Z EC mean takes the low center off the East Coast in contrast with its deterministic solution which has been indicating cyclogenesis near the coast. With these in mind together with past performances of these models, an intermediate solution was adopted to compose this morning's WPC medium-range forecasts. This solution was based on a blend of 35% 00Z ECMWF/EC mean, 35% 06Z GFS/GEFS mean, and 30% 00Z CMC/CMC mean, yielding a storm track off the East Coast. This solution is also reasonably compatible with the previous WPC forecasts (with a faster forward motion of the storm center) as well as the latest (12Z) ECMWF deterministic run. In addition, this blend allows for an intermediate solution regarding the uncertainty in the amount of shortwave energy traversing the western U.S. under the omega block. The GFS and GEFS have been more aggressive with developing an upper trough/low over the southern Plains late this week whereas the ECMWF shows only a modest indication of this feature. ...Weather/Hazards Highlights... Shortwave energy moving across the southern Plains with moisture overrunning a wavy frontal boundary will promote moderate to heavy rainfall and potential for thunderstorms Wednesday into Thursday across the region. Some wintry precipitation can be expected over the central High Plains and central Rockies Wednesday morning. The precipitation is forecast to 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. The possibility for cyclogenesis near the East Coast this coming weekend will continue be monitored. It appears that the storm track should stay off the East Coast at this time. Abundant moisture ahead of frontal systems and upper-level shortwaves edging in from the Pacific will result in periods of enhanced precipitation during the second half of the week near the West Coast. Northern California and the Pacific Northwest can expect the heaviest precipitation, with normal terrain enhancements and higher elevation snow. Lesser precipitation amounts are likely to spread into the Intermountain West and Rockies, particularly Thursday into early Friday. Behind the aforementioned front in the southern Plains across the Gulf Coast, a cold high pressure system will maintain well below normal temperatures across the central and eastern CONUS. This will especially be the case for highs which should be 10-20F below average from the central and southern parts of the Plains and spreading across the Mississippi Valley toward the eastern U.S. Chilly lows are also forecast, with temperatures about 5-10F below normal across the Great Lakes/Ohio Valley and stretching toward the south-central U.S. by Friday. 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 to slightly above normal elsewhere. Kong/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