Winter Seasonal Text Alerts and forum
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*Indications of above or below average temperatures in the table above are relative to what is considered “normal” using data collected over the long term for a particular date. This is collected and maintained by the National Climatic Data Center in conjunction with the National Weather Service actual data from previous years collected at official ASOS/climatology stations across our coverage area. Also note that as time moves forward into a different period as shown above, average temperatures for those dates will also change. See the example below using Philadelphia, PA as the climo station:

The departure from normal uses the average temperature for the date, averaging temps over 24 hours for any given location, using both high and low temperatures hourly during any particular day. This outlook determines warm vs. cool periods relative to normal temperatures.

Technical discussion below will feature two (2) subcategories: Precipitation, and Pattern Discussion. Storm possibilities will be discussed exclusively in the Premium Forum with intense model analysis leading up to any major rain or severe events, not publicly. To join this discussion and hear our updated thoughts from our team, visit The My Pocket Meteorologist Page by clicking HERE and select the “Premium Weather Forum” option.

Precipitation:

We finally had a long stretch of 7-10 days of little to no precipitation across most areas, and now that stretch will come to an end this weekend. A system will bring rain showers to the region beginning Friday late afternoon or evening, continuing into Saturday morning. There may be a lull period in between waves, that allows for a break in the action temporarily Saturday afternoon and early evening, and then the 2nd wave moves in Saturday mid to late evening and continues into Sunday. Now Sunday is getting to be a bit interesting in the interior locations, mainly talking NE PA here, where high pressure to the north may come in just in time before the precipitation ends to provide some cold air damming. In that situation, Sunday rain would turn to ice and then snow before ending with minor accumulations. We will continue to monitor this development over the weekend, but several models now show this idea, including the 12km and 3km NAM, which is now in range.

The next system may also be rather potent, and could affect the region late in the week. Modeling differences between the GFS and ECMWF make this either a non-event or a potent system that brings more rain, since it is coming at a time when temperatures are expected to be more than sufficiently above freezing. It’s just a matter of storm vs. no storm at this point, not a question of precipitation type looking at this at arms length a week or so away.

Next winter storm signal we identified is between Christmas Eve and December 30th. At this time tropical forcing is expected to become more variable, with +AAM anomalies returning to the subtropical regions. At the same time, a Stratospheric Warming event is expected to occur soon, which could aid in the cold air supply being displaced from the polar regions to lower latitudes in North America. With an active southern jet stream thanks in part to El Niño, this could lead to a winter storm threat or two during this period, and likely beyond this period if extrapolated. It is important to understand that you don’t necessarily need below average temperatures from late December through January as it is climatologically our coldest stretch of the year. Near average can be sufficient. With that said, early look at January suggests at least a slightly above average month for snowfall, with slightly below average temperatures in place.

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Breakdown of precipitation departures from normal over the next several weeks:

December 17th-December 23rd: Near average

December 24th-December 30th: Near to slightly above average

December 31st-January 6th: Slightly above average

January 7th-January 13th: Near average

Public/free available maps will be updated when a threat is imminent for significant weather or snowfall on the weather alerts page throughout the Winter:  http://epawaweather.com/weather-alerts/

Note:  Any image in the outlook is clickable for larger viewing

The first low that brings showers Friday night into Saturday AM moves offshore, then the 2nd low moves in Saturday overnight. As high pressure sets up to the north Sunday and the low passes, rain can end as snow in NE PA. 

The 12z 3km NAM from Friday morning shows a transition from rain early Sunday AM to a period of icing, and then ending as snow in NE PA. This is a result of cold air damming from the high pressure situated to the north.

Pattern:

The Madden-Julian Oscillation (MJO): The Madden-Julian Oscillation is currently in the phase 3 octant, and the best forcing is expected to propagate eastward to the Maritime Continent (phase 4/5) in the coming two weeks. While phases 4/5 are typically milder phases, at this point we expect the MJO to become more variable and less influential, and El Niño forcing begins to run the show. This also leads to a +AAM returning to the subtropics, which contributes to the colder pattern. Stratospheric warming expected in the week ahead will lend a hand to the cooler risks late month, but uncertain if that makes it to our region in time for a White Christmas possibility, or waits until the last week of December between Christmas and New Year’s. The MJO can be trumped in the case of strong stratospheric warming episodes disrupting the Polar Vortex (PV) away from the polar regions, and displacing the cold air farther south. With any stratospheric warming event, it is a wait and see game to determine how it affects the PV, whether by split or displacement, where it displaces to globally, and does the warming reach the lower altitudes of the troposphere. We’ll continue to monitor closely. Right now the last week or so of December looks colder than average, not warmer, and that continues right into January.

The GEFS and ECMWF/EPS could not be farther apart with the evolution late month and the first half of January. The GEFS makes the most sense with other things we are seeing observationally, and it appears the flip-flopping EPS is placing too much weight on the MJO phases. The GEFS are what our chart shows above, and the EPS weeklies are literally a torch for the next four weeks. We think this is an improbable outcome, and have sided with the former for the time being.

The pattern begins with a wetter period centered around this weekend. Snowfall in December we will we have maintained expectations to “slightly below average” for now, but more snow chances will present itself toward the end of December. Some uncertainty with late month evolution as described above, so we’ll adjust this as needed as this signal becomes more clear with time.

The December outlook suggests a near to slightly above average month (+0.0°F to +2.0°F) as a whole. The colder start will be offset by the milder risks coming into play in the nearer term. How long this period lasts will ultimately determine near to slightly above average vs. near to slightly below for the month as a whole. Early look at January suggests a slightly below average month (-1.0°F to -3.0°F) with slightly above average snowfall.

Note:  Any image in the outlook is clickable for larger viewing

GEFS ensembles show the milder period coming between this weekend and a few days before Christmas, with a few transient cooler days mixed in between. Temperatures, although above average, are not a torch.

After this period, we expect the pattern to turn colder, with near to slightly below average temperatures returning from right around Christmas to around or just before New Year’s, and snow risks coming into play.

Images below are clickable for better viewing

Days 1-5 – confidence VERY HIGH

Days 6-10 – confidence HIGH

Days 11-15 – confidence MODERATELY HIGH

Days 16-20 – confidence MODERATE

Below is a look at the latest Soil Moisture Anomaly for the Contiguous United States. This is used by NOAA/CPC for the purpose of drought monitoring and drought outlooks.

This image will automatically update daily.

The My Pocket Meteorologist text alert program is a year-round program, and seasonal options for Spring/Summer will be available soon. Click on the image below for more information.

Forecaster: EPAWA Meteorologist Bobby Martrich
Discussion last updated:  Friday December 14th, 11:10am