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February 2015
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Pattern Shift

Posted: February 23rd, 2015

 

Image via NWS Pocatello

Image via NWS Pocatello

 

The ridge of high pressure that has been keeping us warm and dry this winter has started to move. The regime, which for months has been blocking most storms, will position itself far enough out over the Pacific to allow storms to circulate over the top and affect us from the northwest. Reminiscent of La Niña, the pattern is welcomed because it means that rain and needed mountain snow are headed our way.

 

Not a lot of rain and snow, though. The bulk of this week’s precipitation is expected to fall on Wednesday and Thursday. Around a half-inch of rain is expected around Seattle, and around an inch of liquid over SPU’s Cascade Mountain reservoirs. Freezing levels will gradually drop as the retrograding ridge allows colder air to filter in. Around a half-foot of snow is likely above 4000 feet this week. (Our reservoirs are full thanks to near-normal precipitation this water year, but a little white above them will certainly be welcomed by many.)

 

Extended forecast models currently indicate that for at least a few weeks, cool, northwesterly flow will prevail over the ridge that has fought its way back all winter long. However, one- and three-month climate outlooks suggest otherwise. We shall see, but given how significant and severe atmospheric events have been elsewhere around our continent, it makes sense that the pattern would shift as we head into Spring. That said, Pacific Ocean surface temperatures—the most likely culprit in all of the above—will remain quite warm.

 

NWS Area Forecast Discussion
NWS Forecast Table Interface—Seattle
NWS Forecast Table Interface—Tolt Reservoir
NWS Forecast Table Interface—Chester Morse Lake
SPU Water Supply Outlook

 

JRH