Wednesday, April 2, 2014

Unusually wet February and March

Greetings folks, sorry it's been a while since we last posted to this blog, but we have been focused on implementing a new computer system for our office used to compose our complete suite of products and weather grids. Now that we have gotten our feet wet, it's time to discuss the unusually wet February and March the Inland Northwest endured.

So the weather pattern that took a dramatic shift in February unexpectedly continued into March. What was the cause? Recall that a highly anomalous ridge was fixed over the extreme eastern Pacific through the first half of the winter. Here's what it looked like on a 500 mb chart (approximately 17-18k ft). Notice the strong buckling in the flow just off the west coast. This is rather unusual and resulted in almost a record breaking dry spell for the first half of a winter.
500 mb mean map for 10/1/13-1/15/14

Well as the calendars changed to February, the unrelenting ridge gave way and opened the doors to countless storms which surged through the Pacific Northwest for the following 2-month period.
The mean 500 mb charts for February and March exhibited this ridge flattening.
500 mb mean map for 2/1/14-3/29/14
Notice although the ridge is still present (albeit flatter), its actually a much more favorable setup for precipitation since the amplitude of the ridge allows sub-tropical moisture to be wrapped into passing weather systems. Here's is what the anomaly of the atmospheric moisture (called precipitable water) looked liked for the period. Notice the well defined connection of moisture extending from southwest of the Hawaiian Islands northwest to the coast from northwest California to southern Washington (red, orange and green shading).

Precipitable Water Anomaly 2/1/14-3/29/14

So how wet was the two month period? Very was the answer. The map below shows the departure from normal of precipitation for February and March. Every location saw wetter than normal conditions, with the Cascades and Idaho Panhandle mountains leading the pack with well over 10" more than the normal amount of precipitation.

Rainfall departure from normal February & March 2014

From a percentage standpoint vs. normal it was also an impressively wet period. Note the dark blues and purples over the Cascades and north Idaho mountains, indicating where precipitation amounts were 2 to 4 times the normal for that period.

Rainfall % of normal February & March 2014

The unusually wet period has also brought the snow water equivalent (amount of water in the mountain snow pack) to normal or just above normal over the entire region. Note that although conditions were moist over Washington, Idaho, and western Montana, things were still quite dry across Oregon and California (not shown).

Not only did a considerable amount of precipitation fall, but it also did so on an unusually high number of days. Here's a look at the numbers compared to the normals.

Days of measurable rainfall for various locations across the Inland Northwest Feb-Mar 2014

Over locations where the temperature was cold enough, most of this equated to snow. In Holden Village, WA the snowfall for the two-month period was 180.7". That shattered the old record of 166.7" (set in 1999). Not quite as impressive was the 14.0" which fell in Lewiston, ID. This was the most since Feb-Mar in 1985.

So will this unusually wet weather continue? Showers are still fairly common this time of year. In Spokane we typically see some precipitation about 50% of the time during the first half of April. That number drops to around 40% of the days by the end of the month.  In Wenatchee and Moses Lake, the percentage of wet April days is a mere 25%!

The 8-14 day outlook for precipitation shows near normal conditions for most of eastern Washington and north Idaho.

8-14 day precipitation outlook. 

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