Extended Forecast Discussion NWS Weather Prediction Center College Park MD 425 PM EDT Wed Nov 03 2021 Valid 12Z Sat Nov 06 2021 - 12Z Wed Nov 10 2021 ...Overview... The medium range period will be characterized by an upper ridge with its axis gradually shifting from the central U.S. over the weekend toward the East by Tuesday-Wednesday, separating weekend troughing in the East with mean troughing in the eastern Pacific/West. A series of fronts and shortwaves coming into the West Coast will keep precipitation chances high there, while a front and developing low pressure system off the East Coast will spread some rain across Florida and up the Southeast coast this weekend. ...Guidance/Predictability Assessment... The latest model guidance begins the medium rnage period with reasonably good agreement this weekend, which indicates a shortwave over the Midwest/Great Lakes attempting to combine with southern stream energy over the Southeast. Differences in the details and placement continue off the Southeast coast regarding a developing and deepening surface low. The 00z ECMWF is the farthest west with the surface low, which obviously affects potential rainfall impacts farther inland across the Carolinas Saturday-Sunday. The rest of the guidance favors a track a little more offshore, which fits in line with WPC continuity as well. Across the West, a couple of shortwaves should move ashore this weekend and again early next week. The guidance shows better agreement with the first one as it shifts across the Northwest and into southern Canada, pushing a cold front across the north-central U.S. and the Midwest. Behind this, troughing amplifies over the West Coast at the beginning of next week as an upper low drops southward from the Gulf of Alaska. Uncertainties begin to arise by Tuesday/day 6 both with placement and strength. Much of the deterministic guidance and ensembles continues to show plenty of run to run variability suggesting the evolution and timing of this system is low confidence at best right now, especially as it moves into the West/Rockies on Wednesday. A blend towards the ensemble means seems to give the best starting point, amidst plenty of lingering uncertainty. ...Weather/Hazards Highlights... As a surface low pressure and frontal system develops near/over Florida Saturday and tracks northeastward into the western Atlantic, it will spread rain across Florida and into the coastal Carolinas and possibly the Mid-Atlantic region on the backside of the low over the weekend. Current forecasts do not show rainfall reaching very far inland, given a well offshore track of the low, but small shifts of the low track could lead to significantly more or less rainfall in coastal areas, especially for the Outer Banks of North Carolina. The deepening system should also bring a period of gusty winds to locations along the East Coast. Abundant moisture ahead of frontal systems and upper-level shortwaves edging in from the Pacific will result in periods of enhanced precipitation this weekend into early next week near the West Coast. Through the period, the Pacific Northwest and northern California can expect the heaviest precipitation, with typical terrain enhancements and higher elevation snow. Precipitation is likely to spread into the Intermountain West and Rockies as well, with rain and mountain snow threats increasing next Monday into Wednesday. Behind the low pressure system in the western Atlantic, a cold surface high to the west will bring northerly winds and high temperatures that are 10-20F below normal to the Southeast through the weekend. Lows could be 5-10F below normal farther north in this pattern as well, with temperatures in the mid to low 30s for much of the Ohio Valley Saturday morning, which could result in the first frost or freeze of the season for some places. Meanwhile, a persistent upper-level ridge over the west-central U.S. should lead to temperatures that are above average by 10-20 degrees over the Plains into the Midwest through early next week, with highs 5-10F above normal stretching toward the East Coast by Tuesday-Wednesday. Periods of unsettled weather will keep highs modestly below normal over the West Coast on most days, eventually entering into the Great Basin and Rockies by midweek. Santorelli/Tate Hazards: - Heavy rain across portions of California and the Pacific Northwest, Mon-Wed, Nov 8-Nov 10. - Heavy snow across portions of California, the Central Great Basin, the Pacific Northwest, and the Northern Great Basin, Mon-Wed, Nov 8-Nov 10. - Heavy snow across portions of the Central Rockies, the Central Great Basin, the Northern Rockies, the Northern Great Basin, and the Northern Plains, Tue-Wed, Nov 9-Nov 10. - Flooding occurring or imminent across portions of the Northeast and the Great Lakes. - Flooding occurring or imminent across portions of the Northeast. - Flooding likely across portions of the Middle Mississippi Valley. - Flooding likely across portions of the Middle Mississippi Valley. - High winds across portions of the Mid-Atlantic, Sat-Sun, Nov 6-Nov 7. 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