Extended Forecast Discussion NWS Weather Prediction Center College Park MD 314 AM EDT Tue Nov 02 2021 Valid 12Z Fri Nov 05 2021 - 12Z Tue Nov 09 2021 ...Overview... The medium range period will be characterized by an upper ridge with its axis gradually shifting from the Four Corners late this week into the Plains early next week, separating mean troughing in the East and the eastern Pacific into the West. Periods of shortwave energy and fronts coming into the Northwest will keep precipitation chances high there, while a front and developing low pressure system off the East Coast will spread some rain to Florida and the Southeast coast. ...Guidance/Predictability Assessment... Model guidance has now been in better agreement for a few cycles now that a shortwave will move quickly through the Great Lakes and Northeast on Friday through the base of a northeastern North America trough. More differences arise with the evolution of shortwave energy that begins over the northern High Plains on Friday and tracks eastward/southeastward, as well as southern stream troughing across the Southeast. GFS runs take the combination of the energy/troughing more quickly offshore this weekend, with other deterministic guidance holding energy back over the Southeast. These differences also impact the track of a surface low developing off the Southeast coast in the western Atlantic. Considering the typical biases of these models as well as WPC continuity, favored an upper-level and surface pattern similar to the 12Z EC ensemble mean. The 12Z ECMWF and CMC may end up a bit slow and strong at 500mb with this feature by Monday, especially considering that the newer 00Z ECMWF especially is a bit faster/weaker than its previous run. A persistent train of shortwaves coming into the Pacific Northwest shows considerable variability in the shortwaves' track across the northern U.S. and southern Canada this weekend into early next week. This is to be expected for these small-scale features at this time range; however, it does affect the broader trough-ridge-trough pattern in some instances if the shortwaves are successful in breaking down the central U.S. ridge. This particularly occurred with the 12Z GFS run around next Monday, which seems to be an anomaly considering other recent deterministic and ensemble mean guidance. Maintaining ridging seems like the best bet at this time, though some suppression along the edges of the ridging is possible depending on the eventual strength of the shortwaves. Given the reasons described above, the WPC forecast began with a multi-model deterministic blend of 12Z/18Z guidance, transitioning to favor the ECMWF more than the GFS runs, and adding larger proportions of the EC ensemble mean and some GEFS mean as the medium range period progressed. ...Weather/Hazards Highlights... A front over the western Atlantic, Florida peninsula, and the Gulf of Mexico and a developing surface low along it will cause rainfall over much of Florida on Friday as ample moisture streams in. As the low tracks northeastward into the western Atlantic, some rain is expected on the backside for the coastal Carolinas and possibly into parts of the Mid-Atlantic and Northeast Friday into Saturday, but how far west precipitation reaches will depend on the surface low track in the western Atlantic. Current forecasts do not show rainfall reaching very far inland, given a more offshore track of the low. Abundant moisture ahead of frontal systems and upper-level shortwaves edging in from the Pacific will result in periods of enhanced precipitation during the latter half of the week into early next week near the West Coast. Through the period, the Pacific Northwest/far northern California can expect the heaviest precipitation, with normal terrain enhancements and higher elevation snow. Precipitation is likely to spread into the Intermountain West and Rockies as well, with amounts there increasing by next Monday into Tuesday. Behind the aforementioned front in the Gulf, a cold surface high pressure system will maintain below normal temperatures across the southeastern CONUS through the weekend, with highs 10-15F below average and highs/lows 5-10F below normal in southern portions of the Mississippi Valley into the Ohio Valley. Meanwhile, a more persistent upper-level ridge indicated with this forecast cycle over the west-central U.S. should lead to temperatures that are above average by 10-20 degrees over the Plains and stretching into the Midwest. Periods of unsettled weather will keep highs somewhat below normal over the West Coast on most days. 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