Extended Forecast Discussion NWS Weather Prediction Center College Park MD 151 AM EST Thu Nov 22 2018 Valid 12Z Sun Nov 25 2018 - 12Z Thu Nov 29 2018 ...Overview... Active pattern for the northern corners of the lower 48 next week as troughing settles into the eastern Great Lakes and another Pacific system or two pushes into WA/OR and eventually northern California. Colder than average temperatures are likely for the central states eastward but closer to average in the west. Lead system at the start of the medium range on Sunday will provide some wintry weather on its north side from Iowa into Wisconsin that will lift into the Northeast early next week. ...Guidance/Uncertainty Assessment... The 18Z GFS/GEFS and 12Z ECMWF/ECMWF ensemble mean and FV3-GFS provided a good starting point to the forecast for the Sun-Tue period, but with lingering differences in the east. The GFS/GEFS were generally a bit quicker than the ECMWF/ECMWF ensemble mean (and 12Z UKMET/Canadian) with the upper trough and surface low/front moving out of the mid-Mississippi Valley Sunday but multi-day trends still support a middle solution or just slower than down the middle given the progressive pattern still in place. That changes after Sunday as strong upper ridging asserts itself over northeastern Canada to block the flow out to the northeast. In addition, another system has had quite poor consistency in the guidance near southern New England on Sunday as an initial trough exits the coast with a southern system exiting through Bermuda. Again tried to stay near the ensemble mean consensus but the deterministic models have not settled onto a common solution. By next Wed-Thu, the eastern system will slowly exit to the east as the upper low gets trapped over Ontario and then reinforced from central Canada. This could produce some higher elevation snow and perhaps a somewhat localized heavy rain event for southern Maine. In the west, troughing will attempt to push into the briefly re-established ridge on Sunday into Monday but may take another push of upper troughing to finally knock it down by Wednesday and Thursday per the ensembles. The 12Z ECMWF seemed too aggressive to lower heights while its ensemble mean seemed more reasonable, though quicker than the GEFS mean. Split the difference since although the trend has been to weaken the ridge a bit quicker they sometimes find a way to remain longer than originally forecast. ...Weather Highlights/Threats... Sunday will see one system off the NY Bight with a potentially short-lived but impactful precipitation event - coastal rain but elevation/inland snow for southern/southeastern New England into Maine. Over Missouri, organizing low pressure will lift toward Lake Erie by Monday with rain along/ahead of the front but snow to its north -- perhaps several inches from Iowa east-northeastward. Triple point low should wrap across southeastern New England and into the Gulf of Maine by Tuesday before slowly heading eastward. Next Tue-Thu will be active in the Pac NW 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 lower 48 by about 5-10F. A couple brief exceptions would be just east of the Mississippi on Sunday ahead of the cold front and also southern Florida until about Tuesday when a cold front should settle into the Florida Straits. Fracasso 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