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12/13/2018, 8:15 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.
12/12/2018, 8:40 am EST

U.S. Medium-range Forecast

Highlight: Extremely warm days 6-10; cold outbreak days 16-20. Chart of the Day: Madden Julian oscillation, or “MOJO” has loud voice Discussion: Stratospheric warming triggering an arctic outbreak in North America later this month is still on the table. As important is whether the MJO authorizes that (cold) outbreak. MJO is in the warm phase (for U.S. temperature) the next 2 weeks. If that warm phase fades as suggested by models, the arctic cold risk is more likely to occur. Medium-range 6-10 Day Forecast Valid Dec. 17-21, 2018 Medium-range 11-15 Day Forecast Valid Dec. 22-26, 2018 Discussion: An impressive HOT 6-10 day forecast which is warmer than yesterday is indicated. Developing new snow cover across the northern U.S. and southern Canada in the 11-15 day period enables a cold air mass in Canada to “bleed” southward (and into New England). Extended-range 16-20 Day Forecast Valid Dec. 27-31, 2018 Discussion: An arctic outbreak is cautiously in the forecast. %Normal Precipitation Forecast for Medium-range using the ECM ENS Discussion: The ECM ENS maintains a mostly dry day 6-10 precipitation anomaly forecast across the U.S. In the 11-15 day period the northern states to eastern Canada are snowy.