Big Snow Weekend in Cascades

December 28, 2007

SnapshotIt’s looking as if this last cold storm of the 2 week string will be the biggest for the Cascades.  Not such a big deal in the lowlands with less than 1" of rain though.  The 00z NAM shows 850mb westerly wind (around 4,500′) at or above 45 mph from 4pm Saturday to 4pm Sunday.  At times it averages over 55 mph!  This is very strong for the mountains and will probably squeeze out incredible amounts of moisture.  Our RPM model seems to agree.  It says 25-30" of snow accumulation in the Cascades by 7pm Sunday!  This is the most I’ve seen it forecast for any of these storms.  Sunday and Monday are going to be big powder days!  And you get sun with the powder on Monday as high pressure settles in.  I know thicknesses and 850mb temps support snow to 1,000′ or slightly below Sunday, but it’s a heck of an onshore southwest flow all the way through the evening.  Way too much mixing to get low level snow if you ask me…but at/above 1,000′ it’s more likely, especially in the orographic areas against the Coast and Cascade range foothills.  That’s why I downplayed the chance of snow in the metro area Sunday.  Moisture probably disappears soon after sunset Sunday too.

Speaking of high pressure, it quickly builds over the Intermountain Region later Monday and is quite strong by New Year’s Morning.  Looks like 10-12 millibars are likely through the Gorge on Tuesday and Wednesday with a very chilly airmass trapped in east of the mountains.  This will be the first time we see gusts over 70 mph in the Gorge this winter.  This is probably not the pattern that gives us downslope wind off the Cascades.  So not much east wind west of I-5…strictly a Gorge/East Metro event.

Now you’ll notice I put a 38 for a high Wednesday.   Assuming a system moves in from the west as forecast, the airmass east of the Cascades should be cold enough to produce freezing rain or snow in the Gorge.  It’s a bit too early for details, but some freezing rain is likely at the west end of the Gorge and POSSIBLY the portion of the metro area near the Gorge…definitely not a snow setup with warm air above.

The brief break in the weather sure isn’t going to last long!  Long range models throw us right back under the "storm train" beginning Thursday.  As always each model is different, but they all have powerful areas of low pressure moving quite close or over the West Coast.  Lots more fun on the way in this La Nina winter!  Next post on Monday…Mark Nelsen