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Posted at 03:17 PM ET, 01/08/2013

After warm-up, a trend toward colder weather expected by later next week


As CWG’s Matt Rogers noted in this morning’s forecast, the weather through this weekend is expected to average well above the seasonal norm, and by the weekend highs could be well up into the 60s. The forecast beyond the weekend, however, is much less clear than last week’s it’s-going-to-get-warm no-brainer

The outlook for the Jan. 14-22 period is a low-confidence forecast. Temperatures during the first half of the period are still expected to run above normal.  But Jan. 17-22 temperatures are expected to average near or even below normal as the pattern appears to be evolving toward a colder one.  Cold air will be plunging into the middle of the country early next week, and that air will slowly edge east toward the D.C. area with models suggesting a cold front moving through sometime around Jan. 17, though the exact timing is still up in the air. The approach of that cold front also increases our chances of getting precipitation, probably in the form of rain, before the colder air arrives.

One thing is relatively clear: Temperatures next week especially beyond Tuesday or Wednesday will not be as warm as this week.  Highs by the middle of next week are more likely near average, in the low-to-mid 40s, rather than the 50s. We could even have a couple of days with highs only in the upper 30s toward the end of the period.

Of course, the advent of any colder air into the region would also start opening up the potential for snow opportunities, though right now nothing is on the horizon.

Technical discussion

The 240 hour ensemble mean forecast from the European model illustrates some of the players in the pattern that make this forecast difficult for the Washington-Baltimore area.  Remember on the map below that the upper-level winds tend to parallel the height lines.  The red arrow approximates the upper-level flow and steering currents for weather systems coming across western Canada. This big dip in the steering currents should help pull the coldest air of the season southward into the Northern Plains. 


A second important player is the area of above-normal heights across the Southeast which extend north to approximately our latitude, and the southwesterly flow aloft implied by the height lines across the Southeast.  That area of above-normal heights and southwesterly upper flow will be battling to hold the cold air north of us. Another issue for snow lovers is the massive below-normal height area across Hudson Bay, the Great Lakes and New England.  Usually, that leads to an area of lower pressure across the Great Lakes as any low from the south approaches, making it hard to hold in the cold air. 

Below I’ve posted temperature composites for D+10 and D+11 from the analogs based on today’s CPC super ensemble mean.  D+10 analogs would give a rough estimate of the temperature anomalies for 7 p.m. Jan. 17 and D+11 for Jan. 18.  The composites suggest that arctic air will plunge into the middle of the country Jan. 16-18, and that some of that air will slowly ooze toward our area. 

For those who like winter, the pattern certainly looks to be trending colder. I wouldn’t get too hung up on the timing of the cold air as the analogs really can’t provide that type of specificity.  All they can do is give an idea how the evolving pattern might bring cold air into the Plains and then eventually the East.   

The 06Z GEFS ensemble mean for Jan. 20 is forecasting above-normal heights across Greenland and below-normal heights across the Maritimes. That combination makes the pattern somewhat favorable to getting snow opportunities sometime after Jan. 20, providing the ensemble mean forecast is reasonably accurate. (Based on the spread of the ensemble members, that is a dubious assumption).


The 06Z GEFS 240 hour spaghetti diagram (below) illustrates the vast uncertainty inherent in any forecast of how quickly the cold air will reach our area.  Remember that such a diagram shows the same four 500 mb height contours for each ensemble member and also shows those same contours in white for the latest deterministic run of the operational GFS in white.

When all the green lines on a spaghetti diagram are closely packed together with each other and the GFS solution, the probability of the GFS forecast being right is fairly high.  However, the spaghetti diagram below looks like your 2-year-old might have taken crayons and scribbled randomly on a sheet of paper, suggesting the predictability of the pattern is very low. 


Note that the second white contour from the bottom (the GFS) is considerably different than most of the green lines, and that some of the latter have the trough axis well west of the GFS.  The diagram is suggesting a number of members have slower eastward movement of the trough than the GFS.  Delaying the eastward shift of the trough would tend to slow how quickly the cold air might reach the area. 

Conclusion

Temperatures during the first half of next week are expected to be above normal. The colder-looking pattern is emerging, but a great deal of uncertainty still exists as to how quickly the cold air will arrive in our area. From around Jan. 20 onward, our chances for snow rebound back to at least normal levels. 

By  |  03:17 PM ET, 01/08/2013

 
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