Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sun Oct 31 2021 Valid 12Z Wed Nov 03 2021 - 12Z Sun Nov 07 2021 ...Overview... The medium range period begins Wednesday with vestiges of an omega block over North America, as strong ridging takes hold over the western half of Canada and lower amplitude ridging extends southward into the western U.S., separating troughing in eastern Canada/U.S. and mean troughing across the eastern Pacific. This pattern should generally shift eastward through the latter part of the workweek, while shortwaves attempt to pass under the omega block into the western U.S. Uncertainty with the pattern is high by Friday into the weekend, but the current forecast suggests the potential for cyclogenesis to occur in the western Atlantic. ...Guidance/Predictability Assessment... After reasonable 12/18Z model agreement with the amplified pattern described above through Wednesday-Thursday, guidance quickly diverges by Friday regarding how to handle shortwaves digging through the western side of the trough in the East and their evolution in determining the upper pattern. The guidance generally remains in two camps--ECMWF/CMC deterministic and ensemble guidance and the GFS/GEFS runs. The GFS suite has been persistently more progressive with this energy and takes the troughing from the Great Lakes region quickly across the Northeast into the Atlantic Friday into Saturday, though the 18Z run was a bit slower than the 12Z. On the other hand, the EC/CMC spin up a closed upper low separating into the southern stream and tracking over the Ohio Valley toward the Mid-Atlantic for the latter part of the week, which produces a surface low off the East Coast. Additionally, model guidance varies with the handling of the pattern across the eastern Pacific by late week, which greatly affects the pattern across the western/central U.S. as shortwaves may or may not track into the region. GFS runs and GEFS ensemble members end up with flatter flow and low amplitude troughing over the northwestern/north-central CONUS by Saturday and create a closed low in the Northern Plains by Sunday, while the EC remains ridgy. The 12Z CMC actually pairs closer to the GFS runs for this feature, but not as aggressive. Given the small scale of these initial features growing in size and importance as the forecast progresses (a chaos theory/butterfly effect situation), predictability is low, which is shown by ensemble member spaghetti plots as well, with lots of spread and GEFS members generally having the flattest flow in both the West and East. But for this forecast cycle, the best approach (considering relatively typical biases of these models) appeared to be to lean heavily toward the well-clustered 12Z EC and CMC ensemble means by the weekend. This provided a solution that has separation in the northern/southern streams in the East, but not quite to the level of a closed low that the deterministic CMC and EC have, plus a bit faster solution than the particularly slow 12Z ECMWF. Complicating matters a bit, the new 00Z CMC came in with a faster track of the eastern low. The new 00Z ECMWF remains quite slow with a closed low. ...Weather/Hazards Highlights... A shortwave moving across the Southern Plains along with a wavy frontal boundary and moisture coming in from the Gulf of Mexico will lead to moderate to heavy rainfall and potential for thunderstorms Wednesday into Thursday. Some precipitation is expected 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 off the East Coast by the weekend will play a role in determining the amounts and placement of the precipitation in the East. Frontal systems and upper-level shortwaves passing through the West will result in periods of enhanced precipitation during the second half of the week. 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 on Thursday. Behind the aforementioned front in the Southern Plains stretching 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. 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