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Posted at 10:00 PM ET, 01/30/2012

A change to colder next week: pattern to improve for getting snow

Last week, I issued a low confidence outlook for February calling for above average temperatures and below snowfall. But after reviewing new data since that time, I need to make some revisions.

My best guess now is that February temperatures will average near normal. That also raises the probability of getting a snowstorm so I’m upping projection for snow to near the February average.

If I had to choose between above and below average snow for the month, I’d still say slightly below because the pattern is likely to transition to a cold and dry one starting around February 6. But as I said before, it only takes one decent snowstorm to put us over the average, so any snowfall projections are little more than guesses.

Keep reading for my explanation...

TECHNICAL DISCUSSION

In Thursday’s discussion, I noted the the Pacific North American pattern and Arctic Oscillation (AO) would likely be significant players in the upcoming month. That still looks to be true but the AO has fallen precipitously and now is strongly negative (that favors cold over Eastern North America). When it falls this low, it usually has staying power for much of February. I now expect the AO to remain negative, more negative than I thought in my previous post.

At the same time, the Pacific North American ridge is forecast to shift into the western U.S. and is predicted to push northward into Canada. The ridge over the Pacific and negative PNA pattern that we’ve had is forecast to give way to a pattern with a strong ridge over the western U.S (northward push of the jet stream) well north into Canada and Alaska. With the upstream ridge, we should expect a big dip in the jet stream over the east. That combination suggests that after February 6, temperatures will probably average below normal for at least two weeks and possibly longer.


CPC ensemble forecast centered on February 10 indicating dip in jet stream (trough) over the East Coast and significant ridging over West Coast.
The CPC super ensemble mean 500 mb height and anomaly forecast centered on February 10 (to the right) is now showing a classic positive PNA pattern with a negative NAO. A mean 5-day pattern like the one below is usually easier for models to get right than a forecast for any single day. That’s the reason for using these kinds of diagnostics in longer range forecasting.

Note how far north the red area (ridge) over western North America extends into Canada. On the east side of that ridge, the jet stream (black arrow) plunges southward from northern Canada towards the East Coast. Essentially, systems will be diving southward from near the Arctic circle towards us from Canada delivering cold air.

The high heights and pressures across Greenland (red area) and lower heights to the south are also indicative of the North Atlantic Oscillation pattern becoming more favorable (i.e. negative) for locking in the cold for awhile. It’s also has a more favorable look for getting snow than anything we’ve seen yet this winter. That doesn’t mean we’ll see lots of snow. We had the same type of look to the pattern much of last winter and still ended up with below normal snowfall as the northern stream dominated the flow most of the season, robbing us of moisture. However, we did have the one memorable snowstorm on January 26.

Similarly, the pattern being forecast right now looks to be dominated by the northern stream which suggests generally cold and dry weather. However, at such a long time range, the models could change their look and allow more southern stream activity, or the trough (southward dip in the jet stream) along the East Coast could pull back west a little giving us a shot at tapping Gulf moisture. The latest GFS run does exactly that during the Feb 11-15 time range. Such a scenario would give us potential for a decent snow. However, such long range forecast of potential snowstorms are usually wrong. At such time ranges the models will typically not get the details right.


CFS2 model temperatures for February 6-12 (top) and February 13-19 (bottom). View bigger.
The CFS2 model pattern shown for the two weeks starting February 6 looks very reasonable based on the pattern shown on both the GEFS and European ensemble forecast systems. The temperature anomalies shown on it are in Kelvin. To get the temperature departures for Fahrenheit you need to multiply by 1.8. The CFS2 model essentially is saying for the two week period beginning February 6 our temperatures are likely to average 2 to 3.5 degrees Fahrenheit below normal. The climate model also suggests that the two week period will end up being on the dry side.

The positive PNA pattern being predicted now is a big plus for getting snow.

Below is graph showing all the 4” events plots based on their Oceanic Nino Index and PNA index. All the dots that are to the right of the vertical blue line occurred with a positive PNA index like the one that is being forecast for the upcoming period. All the dots below the horizontal magenta line occurred during a La Nina.


Four inch snowstorms in D.C. based on whether we have El Nino (above x-axis) or La Nina (below x-axis) and + PNA pattern (right of y-axis) or - PNA pattern (left of y-axis)
Compared to a negative PNA/La Nina combination which resulted in only four 4” snows in the historic record (lower left portion of graph), the positive PNA/La Nina combination (which had quite a few more) is a big improvement for snow lovers (lower right portion of graph). Still, even with a positive PNA pattern, getting a 4” or greater snowstorm can still be struggle in a La Nina compared to El Nino. Note how few dots are located below (La Nina) the magenta line than above it (during an El Nino) on the right side of the graph (+ PNA pattern).

Most snow events during La Nina winters are minor events. That’s the reason for not going hog wild for snow despite the improvement to the overall pattern. However, for being in a La Nina, this pattern is an above average one for snow lovers especially if the trough axis forecast to be along the coast shifts a little west.

The pattern has definitely changed to one that is more conducive to cold. Even the CFS2 is now forecasting a two week period of colder than normal temperatures. For that reason, temperatures are likely to average near normal for the month as the first 4 to 5 days of month are going to be pretty warm.

I’m still leery of going above normal for snow despite the better look to the pattern. We’re bucking La Nina climatology. The dry CFS2 forecast and the northwest flow both suggest a dry pattern. Still, I’d be surprised if we didn’t see a few minor snow events and there is the the possibility of getting a January 26 type of storm.

By  |  10:00 PM ET, 01/30/2012

Categories:  Latest, Winter Storms

 
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