Another Snow Forecast

February 9, 2009

Snapshot  Another fun evening with varying snow outlooks for tomorrow in local forecasts.  Apparently I am at the lower end of snow forecasts.  The NWS says 1-3" in the city with a high of only 35 tomorrow.   Trimet and other local agencies are basing their snow plans on this so busses are chaining up this evening.

So why do I forecast so little snow?

1.  Lack of moisture when it's cold enough to stick.  During the period from 4am-10am, very little precipitation falls on each mesoscale model (and the NAM).  That means two things: you can't get a good snow accumulation, and temperatures are allowed to rise at least a good 5 degrees from the overnight lows.  Remember that there is no cold air east of the Cascades…The Dalles was 47 today and a westerly wind continues through the Gorge right now.  So an east wind isn't going to help us.

2. Temps too warm when the moderate precip rates arrive 11am-4pm.  Now we sure don't get a significant southerly wind tomorrow, so I don't anticipate a big or sudden warm up.  But we don't start all that cold either.  After 1am or so temps should rise a few degrees, or at least the cooling will stop when cloud cover thickens up again.  By the way, I do think it'll stay cold enough for more significant snow during this period up in Northern Clark county, Cowlitz County, and maybe Columbia county in Oregon too.  St. Helens, Scappoose, Ridgefield, Woodland, La Center…you may all get a few inches of snow.  Possibly out in western Washington county too (Banks, Forest Grove).

3. Little or no evaporative cooling. No dry air over us or coming from any other direction means temps only head uphill after the precipitation starts.  We probably won't get the usual 1-3 degree drop (like a couple weeks ago when it snowed).

Other than these 3 points, the next 36 hours could have been just perfect for a snowstorm here in the Metro area.  A low approaching the coast, then curling down to the south into Northern California…very similar to what we saw with the big snow storm on the 21st of December.  Flow remains offshore from late tonight through Wednesday.  If only we could have had a cold airmass east of the Cascades tonight.  That CAN happen this time of year (February 1993).

The rest of the 7 day forecast still looks pretty dead.  Strong split in the jet stream continues and most of the energy is going down to our south.  Since there is no sign of a blast of cold air, I think it's just about time to say (for gardeners) that we're done with the possibility for extreme cold…Mark Nelsen

For posterity, here are the forecasts for tomorrow (in the city of Portland):

1. NWS  1-3"…35 high
2. KATU  1-3"…43 high
3. KOIN  T-1"…39 high
4. KGW  T-1" …37 high
5. KPTV 0-1/2"…40 high