White Christmas?

December 17, 2007

Uswhitechristmas_2Don’t forget that whenever I insert images into the blog, you can click on them to get a larger view.  In this case it’s pretty important because it’s making my old eyes hurt just to squint at this image.  Notice how much company the west side of the Cascades has with the rest of the USA.  Most of the country has a pretty slim chance of seeing an inch of snow on the ground on the average Christmas Day.  And now with only 8 days to go, I think the chance for snow is VERY low for Christmas Eve or Christmas Day.  The reason is that the westerly flow with frequent mild Pacific weather systems will continue through the foreseeable future.

System #2 for this week is arriving tonight.  Earlier today it was accompanied by a 986mb (estimate) low.  That low tracks northeast into the Puget Sound area by early afternoon tomorrow.  Normally this could be a high wind threat for inland areas considering that by that time the isobars are lined up perpendicular to the western valleys.  That can be very good for accelerating wind straight up the valley.  BUT, in tomorrow’s case the low has weakened to 990-995 millibars.  You don’t get damaging, or really even strong, wind with a very weak low that’s weakening as it moves inland.  Models have been showing this for days now, so not really a surprise.

System #3 is racing across the Gulf of Alaska and will be here in 24 hours!  This one should have quite a bit more moisture with it.  Also, a much stronger surface low down around 980mb moves onshore.  This low moves into Central Vancouver Island…a bit too far north to give us damaging wind in the western valleys.  But this should give the strongest wind to the Coastline that we’ve seen in two weeks.  Peak gusts of 60-70 mph are likely out there very late tomorrow night.

A cold trough then follows storm #3 for Thursday and Thursday night.  Thicknesses drop to around 522dm and 850mb temps down to around -6 means snow will be quite close!  I put one little hidden snowflake in the 7 day forecast.   It’s cold enough for snow on the hills for sure.    00z NAM says the freezing level drops to right around 2,000′ Thursday and a bit lower Thursday night.  This pattern tends to be awfully dry, but it IS similar to the day after the big windstorm last December.  Remember what happened that day?  A line of showers in the late afternoon moved from the Scappoose/St. Helens area down through Vancouver and into outer N.E. Portland, giving those areas a good 1/2" or so of snow.  So, Thursday and Thursday night needs to be watched closely.

High pressure settles in to our east Friday for flow turning offshore and likely dry weather.

Saturday-Christmas Day returns us to frequent weather systems moving through.  It seems that the 00z models have pushed most of the action a bit farther offshore though…Christmas MAY end up being pretty quiet…(or may not)….Mark Nelsen