Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 PM EDT Sat Oct 31 2020 Valid 12Z Tue Nov 03 2020 - 12Z Sat Nov 07 2020 ...Heavy precipitation for the Pacific Northwest... ...Overview... After cold upper-level troughing exits the Northeast Tue, generally zonal large-scale flow can be expected across the CONUS through about Thu, with the exception of some embedded shortwaves. Then, troughing comes into the West by the end of the week, spreading cooler temperatures and precipitation to a larger proportion of the West after some potentially heavy precipitation in the Pacific Northwest around midweek given moist inflow and a series of frontal passages. Other than that, most of the CONUS should stay dry through the medium range period, with the exception of rain over the Florida peninsula under easterly post-frontal flow well to the north of a possible tropical system that is being monitored by NHC for development in the Caribbean. ...Guidance Evaluation/Preferences... For the most part, 00Z and 06Z model guidance was well clustered with the overall pattern over the CONUS described above, including a shortwave moving across the central U.S. Wed/Thu. Thus a multi-model deterministic blend was able to be used for the first half of the forecast period, while transitioning to more usage of the GEFS and EC ensemble means for the mass fields by the end of the period to reduce influence of small-scale differences from the deterministic models. This method maintained good continuity with the previous WPC forecast. Recent GFS runs (00, 06, and 12Z) have been aggressive with bringing the possible tropical system from the Caribbean toward Florida by next weekend. At this point, there is little support for this solution from the 00Z EC ensemble members and other guidance, which push the potential system into central America earlier on. So these GFS solutions were not favored at this point. But this will continue to be monitored for future forecasts, as the tropical system evolution has been inconsistent in most models. Uncertainty remaining with a possible upper low/trough around the Gulf Coast next weekend also contributes to the uncertainty with the tropical system's movement. ...Weather/Hazard Highlights... The initial troughing in the East will help cause cold temperatures with potential for record cold highs Tue, while lake-enhanced snows linger downwind of Lake Ontario. Otherwise, widespread above normal temperatures are likely to spread across the western and central U.S. (where some record highs are possible) toward the eastern U.S. later in the week. The most anomalous temperatures are expected in the Northern/Central Plains, where highs should be 15 to 25 degrees above normal into the 60s and 70s. Temperatures are forecast to cool to below normal in the West Coast toward the Northern Rockies by Sat as the main upper trough comes in. Higher than normal moisture values with an atmospheric river coming into the Pacific Northwest Tue, as well as a series of frontal systems passing through, should lead to modest to heavy precipitation totals especially in the Cascades and Olympics. Given the troughing coming in late in the week, rain and lowering elevation snows are forecast to spread toward the Northern Rockies Thu/Fri and farther south by Sat. Rain is also forecast for southern and eastern parts of the Florida peninsula for the second half of the week, due to easterly flow to the north of a frontal system that may stall over Cuba during the period. Tate Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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