After above normal temperatures through this weekend, temperatures slide down to near normal towards the start of the New Year, where they may remain for the first week of 2015. The pattern early next week to around New Year’s Day may be a stormy one, but the exact storm tracks and position of the rain-snow line are a big wild card. We can’t dismiss snow chances but can’t get too excited about them either.

Beyond around January 2, the pattern is very uncertain and – for now – we’re settling on near normal temperatures and chances for snow.

Highlights

December 25-December 31

Temperatures: Above normal

Precipitation: Near normal

Chance of snow: Near normal

The period will start off warmer than normal with maximum temperatures running in the low 50s through Sunday but then temperatures will start edging lower as the front shifts to our south. There is a lot of uncertainty about the timing and track of waves on a front that will settle to our south. Light rain looks like a decent bet on Sunday afternoon or night. However, by December 31, the temperatures might be just cold enough to support winter weather across portions of the area. However, the best forecast with any storm so far out in time is simply a chance of rain or snow if there is even a precipitation event.

January 1-January 7

Temperatures: Near normal

Precipitation: Near normal

Chance of snow: Near normal

The best chances for snow would be early in the period. By the 3rd that pattern is a huge question mark. I’m guessing that temperatures will rise to near or slightly above normal levels and that the pattern might go downhill for snow a bit. However, with so much uncertainty, the best call is usually to lean towards climatology or slightly above it.

Technical discussion

Why do you think there is a window within a day or two of New Year’s that gives us a chance at snow?

Both the European and GFS ensemble 5-day mean pattern is very similar to the one we had last year during a couple of our snow events. Meteorologists like looking at the mean pattern because the larger scale waves are more stable than the smaller scale faster moving ones. So there is more skill looking at the 5-day mean pattern than in looking at the pattern for any one day or trying to predict the evolution of a storm more than 5 days into the future.

The strong ridge (red area) over Alaska on the 500-mb map below is a big positive for those liking cold and snow.

The ridge helps force the development of the trough and below normal heights downstream from it that extend southward into our area. Cold high pressure systems are likely to form to the east side of that ridge in northern Canada and then are likely to plunge southward following the flow along the thin black height lines. They essentially drop from northern Canada into the Northern Plains and then spread eastward across the county with some of the cold air eventually reaching us.

The above normal heights across Florida suggest that the front will stall somewhere across the Southeast setting the stage for a possible overrunning precipitation event.

Remember that the flow aloft at our latitude generally runs from west to east and parallels the thin black height lines on the map above. Therefore, our flow aloft on the map above has a southwesterly component. Such a flow pattern is favorable for us getting some precipitation as weak lows move along the front to the south.

The map below displays the temperature difference from normal or anomaly pattern for the ten closest matches since 1949 to the 500-mb map shown above.

The coldest temperatures relative to normal are to our north and west where the cold coming from Canada is the freshest. As the cold shifts eastward it moderates but the colder than normal air still sags south of us. The fact that the colder than normal temperatures do not extend past North Carolina suggests the front is not that far from us, raising the potential for warmer air to be lifted across the front. Such a setup would give us a chance of getting precipitation – possibly in the form of snow if the cold air is deep enough.

Why is there so much uncertainty about the timing of any precipitation that might fall after Sunday?

The southwesterly flow aloft and above normal heights over the Southeast make it likely one or more of the waves gives us some precipitation. However, with such a flow pattern, the weak impulses that move rapidly with the flow are extremely hard for the models to time. If you change the inputs into the model slightly, the timing of the waves change along with which waves might clip us.

The figure below is plume diagram showing the precipitation forecast by last night’s various GEFS ensemble members.

The important thing to note is that at least one ensemble member forecasts some precipitation daily from Sunday through New Year’s Day. Those members forecasting precipitation towards the New Year display a better chance of being snow than the members that offer precipitation Sunday or Monday. The various members and the operational models have being varying the timing of when precipitation might occur from run to run. They also have varied on whether the temperatures would be cold enough to support snow.

Why are you so uncertain about how long the cold will last?

Last night’s model runs showed signs of a change towards a milder pattern towards the end of the first week of the New Year. They were in pretty good agreement that there would be heights building across our area.

Let’s compare last night’s European 11-15 day forecast (below) with the 6-10 forecast shown earlier in the article (the top image in this post).

While ridging is still evident over Alaska in the latter period (shown directly above), the axis of the ridge (where the black lines poke up to the north the most) has shifted from eastern Alaska to western Alaska. That shifts allows the heights to start rising over the U.S especially in the East.

Note that the warm colors that were confined to the extreme southeastern part of the country have expanded across much of the country with the strongest warm anomalies extending across the Southeast into our area. The below normal heights have receded into Canada. That suggests that the jet stream will shift back to our north and that storms will shift back towards the Great Lakes putting us on the warm side of any storms.

The temperature anomalies based on the 5-day ensemble mean pattern ending on the January (see below) shows effects of the change in the pattern and storm track.

The cold air shifts west and above normal temperatures dominate the East. After seeing the analog temperature anomalies below I was getting gung-ho for warmer temperatures making a return.

Another reason to think another period of warmer than normal weather might impact us is the Madden Julian Oscillation (MJO) and its associated convection. The MJO is forecast to be locked in phases 4 and 5 during the period which favor warmer than normal temperatures across the eastern U.S. I was almost sold.

Unfortunately, today’s GEFS run threw a wrench in the works. While last night’s model runs were bullish about the 5-day period ending on January 8 as averaging warmer than normal across our area. Today’s GEFS is much less so and has us averaging colder than normal by introducing a negative Arctic Oscillation.

Below is comparison of the temperature anomalies being forecast for the 24 hour period ending at 1 p.m. on January 7.

The GEFS is much colder across the U.S. than last night’s European ensemble mean.

I’m inclined towards the European forecast but not with enough confidence to vary the forecast much from climatology.

The bottom line

This is a low confidence forecast concerning what the pattern portends once the cold front sags south of us early next week. The area will probably have precipitation chances from weak low pressure systems passing to our south but whether any will have enough cold air to produce snow remains questionable especially around the city as we are on the southern edge of the cold air. Still, you can’t rule out snow.

The colder than normal shot will probably not be long lasting if the European ensemble mean is right but consider today’s GEFS run holds onto the cold air. The forecast beyond the Jan 3 is little better than a crap shoot.