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Posted at 03:33 PM ET, 01/17/2012

What the Pacific Northwest weather pattern change means for D.C. snow

The evolving weather pattern in the Pacific Northwest is the latest punch to the gut for winter weather lovers around the D.C. metro region and for much of the Eastern U.S.

Through the rest of January and into early February, the stormy pattern in the Pacific Northwest is likely to lead to above average temperatures in the East along with below average snow. (Having said that, we still may have a chance for some winter weather Friday night into Saturday. We’ll discuss that possibility more in detail tomorrow if the threat still looks viable.) However, there are some initial, subtle signs the pattern may change to a more wintry one for our region in early February.

What follows is a detailed discussion of the past, present and evolving weather pattern in the West and how it’s shaping our weather in the East.


60-day precipitation deficit in the West. (See bigger) ( NOAA )
THE PAST 60 DAYS - A SUPER DRY WEST

Both the nation’s (outside Alaska, notably) broad expanse of warmer than normal temperatures and lack of snow cover this winter have been in the news. But the extremely dry conditions across the West are a lesser known impact of the past pattern. To the right, see the map that displays the precipitation deficits for the past 60 days.

Portions of the mountains east of San Francisco have precipitation deficits as much as 16 inches below normal. If you convert that to snowfall, some of the mountains have probably had 160 inches less snow than normal at this time of winter.

Donner Ski Ranch and Tahoe Donner are two ski resorts that are closed due to the lack of snow. A better known resort, Squaw Valley, only has a third of its lifts running and is reporting a base of only 12”.

You can get a feel for how little snow there is in that region by clicking onto two live camera link, one for the Gondola at Heavenly Valley ski resort) and another for the Mount Shasta region (the mountains at the north end of the Sacramento Valley). These same cameras will allow interested people to follow the changes in the snowpack that are expected over the next 10 days.

THE EVOLVING SUPER WET PACIFIC PATTERN

The lack of snow in the West has been the result of a weak upper level ridge (bump in the jet stream) that has been parked over the West or just off the West Coast through most of the season. Its location helped deflect weather systems northward into Canada. The Pacific North American pattern, a measure of where that ridge sets up was therefore weakly positive. However, the ridge was only strong enough a couple of times to pull cold air southward into the U.S.

When you shift that ridge westward to well off the coast, the index shifts to negative. That’s what’s happening now (and is forecast to persist for the next couple weeks). In this setup, Arctic air is drawn into the Pacific Northwest, but - usually - not much farther east.

Here’s how the shift to a negative PNA evolved...


120 hr GEFS 500mb height and anomaly forecast valid at 1 AM 22 Jan. (see bigger) (Penn State University)
When the ridge (red area west of Alaska to the right) gained amplitude it also shifted west and actually closed off an anticyclone (high pressure center) aloft just west of Alaska. When such a strong ridge builds, there is always a downstream response. As a result, a strong upper trough (dip in the jet stream) developed across western Canada and much of Alaska.

The close spacing of the 500 mb height lines (flow at 18,000 feet) on the image to right suggests that a strong upper level jet will be impinging upon the Pacific Northwest and California. Such a strong jet and trough usually leads to a series of storms battering the West Coast.

If you examine the 500 mb forecast to the above right and consider the location of the positive anomaly (red area), you would find its placement is very similar to other pattern configurations that led to historic heavy precipitation events over central and northern California. While this series of events may not be historic, the series of storms should go a long way towards alleviating the drought except for over the Southwest.


180 hr forecast accumulated precipitation map valid at 1 p.m. Jan 24 (see bigger) (NOAA)
The latest 1 a.m GFS run hammers the Pacific Northwest and northern California with heavy rain and snow over the next 180 hrs (see image below). More than 10 inches of liquid equivalent is predicted along the northwest coast of California with more than 5 inches forecast for the mountains near Lake Tahoe. That suggests that some of the ski resorts will probably see more than 5 feet of snow.

WHAT THE NEW PACIFIC PATTERN MEANS FOR D.C. WEATHER

Temperatures

The negative PNA pattern also has implications for our weather. Again, look at the 120 hourr 500 mb height forecast above right. The flow of the 500 mb winds are approximately parallel to the lines shown on the map with the speed being proportional to proximity of the lines to each other. Note that the northerly winds are aimed at Alaska and that the flow across the lower 48 is flat (west to east, or zonal) originating in the Pacific. This zonal flow keeps colder air out of the country and floods it with warmer air. The latest CPC 6-10 and 8-14 day forecasts are both indicating above normal temperatures extending across the country into our area and are also showing the Pacific Northwest being wetter than normal. The negative PNA pattern in the Pacific will be the overriding factor determining the weather over the much of the country over the next 10 days to two weeks.

However, the warmer than normal forecasts for the next 2 weeks does not preclude a day ending up with below normal temperatures but does suggest that odds are quite a bit higher that we end the month with above normal temperatures and below normal snowfall for the remainder of the month.

Snowfall

The Pacific North American pattern is forecast to be in its negative mode through the next two weeks. Such a pattern is not very favorable for snow especially for 4 inch or greater snowstorms. However, the last couple of runs of the GEFS ensemble mean have suggested that the PNA pattern may transition back to its positive mode towards the beginning of February as ridging redevelops along the West Coast. The evolution of the pattern in the Pacific may have important implication about our snow chances if the latter change does occur (a giant if this far out).

Below I’ve constructed a scatter diagram of 4 inch or greater snowstorms at Reagan National Airport (DCA) since 1950 showing the AO and PNA index values at the time of the storm.


4”+ snowstorms in DC according to the phase of the Arctic Oscillation and PNA pattern since 1950
Note that most of the snowstorms occurred when the PNA index was positive (to the right of the darker vertical line) and when the AO index was negative (areas below the darker horizontal line).

Only 16 of the 70 snowstorms (or 23%) were associated with a negative PNA index. The ridge position to our west is an important player in adding to our snow chances. Of those 16 events, only 8 (11%) percent happened with a positive Arctic Oscillation and negative PNA.

A negative PNA pattern really hurts our chance of getting a 4 inch or greater event. Until ridging starts developing over the western North America, our chances of a 4 inch snowstorm are low.

However, if the PNA pattern were to shift back positive towards the beginning of February like the latest run of the GFS ensemble mean , the pattern would be the most favorable one for snow that we’ve had this year. Not only would the PNA be positive but forecasts suggest that the Arctic Oscillation Index will also probably be negative. This would indicate that the polar vortex is now weaker than normal, increasing our chances of getting blocking over Greenland (forcing the jet stream south over the Eastern U.S.).

For the next two weeks our snow chances look bleak. However, if ridging develops near the West Coast and we maintain our negative AO towards the beginning of February, our chances of getting a snowstorm would rise. The latter change is highly speculative as models aren’t very accurate at such long time ranges. Still, the pattern is worth watching closely as we approach February.

Related: Arctic air on the move, but where is it going?

A pattern change, probably yes, but what it means for D.C. uncertain

By  |  03:33 PM ET, 01/17/2012

 
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