Exciting (and Snowy!) Saturday Morning Ahead

December 14, 2012

I just checked the latest maps & models, and tomorrow morning screams “SNOW IN THE AIR” here in the metro area.

For those just wanting the basics, here you go:

MarkSnow_Outlook_Extended

  • As precipitation arrives between 7-10am, there is a very good chance it’ll be a rain/snow mix or even all snow IN THE AIR to the lowest elevations from about Wilsonville north to Longview.
  • Brief sticking (maybe turning white) is possible anywhere above 500′ and even below that elevation on the far westside of the metro area (Forest Grove, Banks, Vernonia etc…)
  • This means the West Hills, Bald Peak, Mt. Scott, Sandy, and northern Clark County will see the first snow of the season.
  • It won’t last long (1-2 hours) as a warming south wind arrives soon after midday.
  • If you live near/above 1,500′, you could see a quick 2″!

9:30pm Update:

We have 2 two chances for snow in the lower elevations over the next 7 days with a strong wind event sandwiched in between. 

FIRST SNOW TOMORROW MORNING

A  bit more on why I’m so bullish on at least seeing snow in the air tomorrow:

1. This morning we saw a lower snow level than expected with sticking snow briefly in spots down around 1,000′ or even lower.  I had my first trace of the season and the trees just above my house at 1200′ were all white.   All model data points to a slightly colder atmosphere tomorrow morning and heavier precipitation.

2. Cold night tonight with temps down around freezing and/or well below 40 degrees.  We’re set for a 7-9am precip arrival time, the timing doesn’t get any better than that.

3. Calm wind doesn’t bring in any warmer air from any direction until after 10am.  Both our model and WRF-GFS show basically calm wind through the metro area north of Wilsonville all morning.  THEN the south wind kicks in.

4. Steady precipitation and drier air up around 1,000-3,000′ should allow some evaporational cooling.  Mesoscale model soundings show this occurring soon after the precip gets going.

5.  Models actually generate sticking snow near the Coast Range.  On the higher resolution WRF-GFS it’s obvious it wants to stick snow down to around 1,000′, notice Bald Peak and West Hills stick out.  Also, the 12z 1.3km version showed other high spots getting a dusting.

rpm4kmsnow

STRONG WIND:

Now let’s move on to the stronger storm later Sunday and early Monday.  All models in agreement now bringing a major storm into the northern part of the Pacific Northwest.  The surface low (down around 970-980 mb) moves into either Vancouver Island or extreme NW Washington.  The orientation of the isobars (east-west across the region) is always good for a strong southerly wind up the Valley.  This low is actually weakening a bit as it moves onshore.  That plus the fact that models don’t want to get too crazy with the valley wind tells me we’ll see gusts maybe in the 40-50 mph range in the metro area.  Most likely the peak will be late Sunday evening through daybreak Monday.  Note a big windstorm, but it’ll match the strongest wind we’ve seen so far this season.

SECOND CHANCE FOR LOWER SNOW MONDAY NIGHT & TUESDAY

This is a totally different low elevation snow setup compared to what’s coming tomorrow.  This time it’s the usual cold and showery airmass coming in off the Pacific behind the early Monday storm.  This pattern is notorious for getting our hopes up for sticking snow to the lowest elevations.  Then we find out in the end it was a little too warm due to a southwest wind bringing in too much mild air off the ocean.  That “onshore flow’ is often a snow killer for the lowest elevations and it may be this time around too.  On the positive side, in the past 24 hours models have shown a slightly cooler solution with 500mb thicknesses down around 520 (very good) and 850mb temps down around -7 deg C.  This is definitely in the “snow possible” category for the lowest elevations.  If so, then it comes down to how much moisture is available.  I think it’s a little too soon to tell.  So I’ll punt on this one for now.

Beyond next Tuesday, all models show warming with a trough digging much farther offshore or some sort of annoying splitty flow as we head towards Christmas.  Notice the 12z ECMWF 850mb ensemble temps all move upward closer to normal after bottoming out Tuesday.  This general progression has been shown on this chart for almost a week:

tseries_850t_000-360_Portland

Chief Meteorologist Mark Nelsen


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