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Posted at 01:25 PM ET, 01/02/2013

Long range: Warmer than normal temperatures returning, snow chances low


As Dan noted, the weather this week is expected to be on the cool side but should remain dry as there are no storms on the horizon. The weather for Sunday’s Redskins game should be in the upper 30s to low 40s at game time.

Next week, a warming trend is expected with the high temperatures slowly climbing from the mid-40s on Monday into the 50s by Friday or Saturday.

Through January 15 or so, storms are expected to track well to our north - unfavorable for the development of winter storms. Such a track puts the area on the rainy as opposed to snowy side of storms as the counterclockwise flow around the low pressure systems supply breezes from the south until the storms pass by. Behind any storms, some brief shots of cooler air are possible.

The latest long-range forecasts from the National Weather Service Climate Prediction Center (CPC) do a good job of assessing the pattern and predict our area to have above normal temperatures during the January 7-11 and January 9-15 periods.

During this timeframe, the heaviest precipitation is expected across the Ohio Valley region into the Great Lakes. In our area, precipitation will probably average near or a little above normal to complement the well above normal temperatures. Suffice to say, the first half of January will leave snow lovers wanting.

Technical discussion

The 5-day mean 500mb height and anomaly pattern from the CPC super ensemble mean does a good job of showing why this looks to be a warm, relatively snowless pattern. Long-range model forecasts are not particularly accurate so they are often based on using the 5-day centered mean forecasts of the 500mb pattern like the one shown below. In these analyses, the very largest scale waves in the atmosphere are the slowest moving, show up the most clearly and are the ones that are easiest to forecast.

There are at least three signals on the forecast map below that argue for January 7-15 being a warmer than normal period.

First, the position of the positive height anomaly (upper level ridge) is well off the West Coast. Such a position almost always forces the downstream trough and below normal height (blue area) into the western U.S. putting the East Coast under southwesterly flow aloft with above normal heights (red area) and temperatures.

A second factor favoring warm weather across the Mid-Atlantic region is that the North Atlantic Oscillation (NAO) is forecast to be in the positive phase. Note that the below normal heights (blue area) extend across much of Canada, southern Greenland and across Iceland. With a positive NAO and a trough well to our west, the subtropical jet should not be much of a player and storms should track towards the Great Lakes rather than to our south.


Day 11 super ensemble composite for January 13. (Climate Prediction Center)

Finally, the lower than normal heights across Canada suggest that pressures across southern Canada will be lower than normal. Since air moves from areas of high pressure to areas of lower pressure, there is no mechanism to facilitate the southward movement of cold air into the Mid-Atlantic region. Instead, air will be generally be moving from south to north.

The European ensemble mean forecast of the mean sea level pressure for Monday night, January 7, at 7 p.m. (see below) illustrates why having low heights at 500mb and low pressure across Canada is not conducive to cold air feeding our area. Note the large area of low pressure across southern Canada northwest of the Great Lakes. The winds across the U.S. are roughly parallel to the isobars (pressure lines) that extend across the U.S. but cut across those lines at about a 20 to 30 degree angle aimed from higher pressure towards lower pressure. By next Monday night (144 hours into the future), the surface winds across most of the U.S. east and south of that low have a mild southerly component.


144 hour European enseumble mean sea level pressure and 850 mb temperature forecast valid at 7 pm Monday, January 7.

The European model ensemble mean pattern 240 hours into the future below demonstrates how a storm track towards the Great Lakes region can lead to a northward surge of warm air. Readers should not get hung up on the timing of the low as model forecasts of such a feature will vary significantly at that time range and frankly aren’t very good. What is important is that the basic pattern favors such a track sometime in the January 9-13 time period and that such a track allows deep mild southerly flow to develop.

The arrow on map shows the approximate corridor where the ensemble mean has winds at around 5000 ft of 30 knots or more. Note how the arrow crosses the temperature lines on the map. The southerly winds are advecting (pushing) warmer air northward in advance of the low and associated cold front. Such strong warm flow raises the possibility of our area having one day with temperatures approaching 60 sometime between January 11th and January 13th depending on how cloudy it gets. With all the warm advection, clouds could still hold the temperature into the 50s.


240 hour Europen ensemble mean sea level pressure and 850 mb temperature forecast valid at 7 p.m. Friday, January 11.

What’s disturbing for snow lovers is that the degree of similarity in the basic pattern being shown on the various ensemble members. They fairly consistently suggest that the southwesterly flow aloft and above normal heights hang around the East Coast 11 days or so into the future. The only ray of sunshine is that the models do start poking a ridge northward into Alaska suggesting that some cold air will start being pulled southward into the northern Plains, eventually.

Conclusion

Warmer than average temperatures will hold sway during the January 7-15 period as lows track to our north and keep our flow with a southerly component more often than with a northerly one. Daytime temperatures by the end of next week are expected to rise well into the 50s. We could even have a day in which the temperatures approach 60 degrees. The weather is expected to be on the dry side until sometime towards January 11-14 when a low or a series of lows passing to our north bring a front through the area. Even then, the heaviest precipitation is expected to be to our north and west. The lack of cold air suggests that snow is unlikely east of the mountains into mid-January.

By  |  01:25 PM ET, 01/02/2013

 
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