Sunday, April 14, 2013

A Damaging and Deadly Convergence Zone

Most Puget Sound convergence zones are benign affairs that produce a band of cloud or showers between Seattle and Everett, and during the winter there is usually some enhancement of snow in the central Cascades.  But yesterday's convergence zone was particularly intense, enhanced by very unstable air aloft and a persistent flow approaching the coast from the northwest.  It resulted in the deaths of two individuals, the temporary closure of Snoqualmie Pass, and power outages in Seattle.

As many of you know, Puget Sound convergence zones usually occur when the flow from the northwest approaches the Olympics Mountains; some of it goes through the Strait of Juan de Fuca and the other portion goes around the south side (the Chehalis Gap.  An area of low pressure tends to develop on the leeward (eastern) side of the Olympics and the result is an area of convergence, in which the two air streams are forced to rise--producing the clouds and precipitation.

If the air is less stable (tendency to rise vigorously when lifted), the convergence zone is really juiced up.   Such "juicing" is more prevalent in spring when the atmosphere is less stable, since temperature tends to decrease more rapidly with height in this season.  The atmosphere is still cool from winter, but the strengthening sun warms the ground and the air next to it.  Big changes of temperature with height can result in instability and convection...just like your cereal pot when you turn on the burner.   A visible satellite picture yesterday clearly shows convective showers offshore (cold air over relatively warm water) and the solid clouds over the mountain and in the convergence zone.

The convergence zone is also clear in the radar--here is a sample from yesterday.  You can see the enhanced precipitation extending ESE towards the Cascades and I90.

We can get the total estimated precipitation using our weather radar for the entire "storm" and here is the estimate from the NWS Camano radar.   Suggests 2-3 inches of liquid water content over the central Cascades..multiply this by 5-15 for snow depth, depending on how wet the snow is.

Here is the measured snow depth at Snoqualmie Pass (Alpental).  Snow increased from roughly 91 inches to 115... TWO FEET of new snow.   And the snow was relatively wet.  No wonder there were avalanches and road closures.

 The  convergence zone produced some unusually intense thunderstorms over central Puget Sound, particularly in Seattle.   I counted 4 lightning strikes at my dog and cat headed for the hills!  Here are the lightning strikes in the middle of the most intense part of the event in Seattle, the half-hour ending at 5 PM.

The intense thunderstorms brought heavy rain and small hail.   Here is the proof at Shilshoe Marina in Seattle taken by Michael de Haan.

 Power was lost to nearly 15,000 city light customers (see map below) and resulted in loss of power to portions of the University of Washington, including my department (and its computers).

But the most serious aspect of this event was the tragic loss of life of two individuals that were snowshoeing in the mountains.  It can't be stressed enough how important it is to check the forecasts and particularly those provided by our excellent Northwest Weather and Avalanche Center. The forecasts were for heavy snow in the mountains, as illustrated by the UW WRF model 24-h forecast from 5 AM on Saturday.  1-2 feet were predicted in the central Cascades.

 The avalanche center had warnings of considerable danger (orange colors on the figure below)

 Please check with the Avalanche center before heading out to the back country and stay away from steep slopes if the danger is considerable or higher.  If you do, make sure you do not go alone and are equipped with an avalanche radio beacon and probe poles.


Rod said...

The Weather Service nailed the forecast. I am obviously sad about the loss of life, but this was nailed. Great forecasting.

mel said...

Cliff, thanks for your suggestion to stay out of avalanche terrian when the danger rating is considerable or higher. Certainly many people travel in avy terrain when the danger is considerable without incident. Some remain safe via luck and others remain safe through their knowledge.
The word considerable doesn't convey danger in my mind but from looking at avy stats on the NWAC website I see that most fatalities occur when the danger rating is considerable.

Doug H said...

Here is an interesting link to a slightly off topic web page which presents a visual depiction of winds aloft over the US. Please note that this is an art project but is based on actual forecasts. Be sure to click on the archives for winds during significant weather events of the past.

mhoonchild said...

Hi Cliff. Will you comment on the rainstorm that swept through Redmond this afternoon about 2:30? I looked at radar, and saw that it swept in from the northeast, which seemed very unusual to me.

Matt T. said...

Cliff, what is the difference between the "Western WA 24-hour model snowfall" that you show in the post, vs the "Western WA 24-hour snowfall" model?