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12/13/2018, 8:14 am EST

U.S. Medium-range Forecast

Highlight: Very warm outlook followed by reluctant cold (which will be reliant on returning snow cover). Chart of the Day: Comparing extended-range upper air forecasts by GFS Discussion: The GFS OP from overnight indicated a “polar vortex” cold event across the East U.S. on Christmas Day (left). The just-received GFS OP offers the opposing solution featuring a warm ridge in the East on Dec. 27th (right). Obviously, forecast confidence is low in the Holiday period thermal pattern. The model is struggling as to whether stratospheric warming over northwest Canada is positioned correctly to ignite excessive cold. Medium-range 6-10 Day Forecast Valid Dec. 18-22, 2018 Medium-range 11-15 Day Forecast Valid Dec. 23-27, 2018 Discussion: The super-warm 6-10 day forecast erases snow cover. That’s a concern for extended-range cold forecasts. Lack of snow cover will bias the climate warm especially in an evolving El Nino climate. The 11-15 day forecast is cold in Canada stretching to New England but forecast confidence is lower. Extended-range 16-20 Day Forecast Valid Dec. 28, 2018-Jan. 1, 2019 Discussion: Cold is farther north; warmer trend South. %Normal Precipitation Forecast for Medium-range using the ECM ENS Discussion: The 06Z GFS OP day 6-10 and day 11-15 snowfall forecast is indicated. Regenerating snow cover is required to allow any important cold to return to the U.S. The forecast indicates marginal increase for Interior Northeast snow cover in the 11-15 day period (also including the southeast quadrant of Canada). There is increasing support for cold into the Northeast U.S. in the 11-15 day period but not overwhelming.