Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1050 AM EST Fri Nov 23 2018 Valid 12Z Mon Nov 26 2018 - 12Z Fri Nov 30 2018 Synoptic Scale Overview ----------------------- Active pattern for the West Coast and the Northeast U.S. next week as a large scale trough settles over the eastern Great Lakes and multiple Pacific lows impact the Pacific Northwest and eventually northern California. Colder than average temperatures are likely for the central states eastward but closer to average west of the Rockies. Model Guidance and Uncertainty Assessment ----------------------------------------- The deterministic and ensemble means have struggled for days with the strength of the blocking/retrograding low over the North Atlantic and the shortwaves within the mean trough over the eastern third of the U.S. Another recent challenge is the breakdown of the western ridge. Confidence with the timing, intensity and exact track of specific systems remain near average early next week and drops to fair/below average at best by the end of the extended forecast. Aside from the aforementioned challenges, the northern Mid-Atlantic/Great Lakes/Northeast will have a fairly robust winter storm move through. As cold Canadian air rushes in behind the central low pressure, winds will increase over the lakes and may lead to local enhancement in the favored lake effect areas. Northern New England may see several inches of snow while precipitation across southern New England and coastal areas of the Northeast/northern Mid-Atlantic may remain in the form of rain. Onshore flow with multiple systems approaching the West Coast will keep at least coastal areas in an active wet pattern as the trough, but precipitation extending beyond the coastal ranges may develop as the ridge gives way. Given lower than average confidence of the model guidance, the forecast relied on a fairly equal mix of the means and 00Z/06Z GFS and 00Z ECMWF/UKMET/CMC. Sensible Weather ---------------- A strong low pressure system crossing the Ohio Valley and Great Lakes region on Monday is expected to result in widespread rain from the Mid-Atlantic to southern New England. Snow is likely to the north and west of the surface low across Wisconsin and Michigan, and also for the higher elevations of interior New England and upstate New York. Triple point low should wrap across southeastern New England and into the Gulf of Maine by Tuesday before slowly heading eastward. The Tuesday through Thursday time period should be active for the Pacific Northwest with low elevation rain and higher elevation snow as a lead and secondary system come ashore. Precipitation will work its way southward through northern California by next Thursday. Temperatures will generally be below average over most of the continental U.S. by about 5-10 degrees, especially east of the Rocky Mountains. Campbell WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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