Monday PM Update

December 27, 2010

Will there be enough, or any, snow to go sledding on Wednesday morning here in the lowest elevations?  Seems unlikely right now, I’d give it about a 20% chance fo “sleddable” snow here in the Valley.  It IS a tough forecast the next few days. 

Apparently we’re back to the usual “will the moisture and colder air mix it up just right” game for the first time this season.  In November we had an amazingly cold blast of air come in from the north and northwest…remember how easily we changed to snow and then a hard freeze overnight that Tuesday?  Then a few times this month we’ve had chilly air in place and moisture arrive over the top of it.  Evaporative cooling dropped temps down enough to bring at least the sight of snowflakes into the city with those. 

But this will be the first time this year (I think) that we’ve been close to snow with the cold instability showers behind a cold front.  And those of you who’ve been on the blog for several years know how frustrating it is to get decent showers coming in from the northwest.  Often the Coast Range eats up a good chunk of the moisture.   The biggest issue is that the cold air in this case is travelling over about 2,000 miles of mild Pacific Ocean water before it gets here…really tough to get snow to sea level in these situations for that reason alone.

Not much happening in the next 24 hours as a juicy cold front sits over us.  A 2nd wave renews the rainfall tomorrow afternoon and evening.  Our 18z RPM had shown steady and heavy precipitation lingering with that wave until the middle of the night tomorrow night as the snow level lowered dramatically.  The 00z version doesn’t show that…it just shuts down the precipitation after 8pm or so.

Our best chance for lower elevation snow will probably be late tomorrow night or early Wednesday morning as the cold upper-level trough slides through.  Models (and satellite) show a vigorous batch of showers passing overhead between midnight and 8am.  I think that guarantees a decent 1-3″ at/above 1,000′, and maybe something all the way down to sea level if the precipitation is heavy enough.  In our favor is no mild southwest wind.  So I’ll say zero to a trace at the lowest elevations, but 1-3″ at/above 1,000′. 

As I’ve mentioned numerous times, the elevation distinctions are approximate, especially with the showery nature of the precipitation coming through.

During the day Wednesday, with sunbreaks and lighter precipitation, temps should rise to around 40 degrees or so.

Thursday morning COULD be interesting, but the mesoscale models keep the moisture to our south as a wave slides down the Coast.   The much lower resolution GFS has precipitation over us, but since we’re at the edge of the precip…we’d be arguing over grid points on that one.  Either way generally dry from Wednesday evening through sometime early next week.  Colder too as higher pressure settles east of the Cascades beginning Thursday.  Not a big blast by any means, but seasonably chilly for late December or early January.

One other weather forecasting issue:  New Year’s Eve and/or New Year’s Day.  GFS has been very insistent in pushing a weak system through the developing ridge overhead.  The system holds together far more on this model than others.  The ECMWF has a full-on split and takes all precipitation to our south.  If the GFS verifies, we would probably have some snow on New Year’s Eve.  At least a trace, maybe 1-2″?  The GFS would also warm things back up to normal quicly either Saturday or Sunday…warmer at least than our 7 Day shows.  I’ve left the forecast dry, hoping for a change on either the 00z ECMWF of 12z GFS.

Chief Meteorologist Mark Nelsen

Snow & Cold Sandbox

December 27, 2010

This post is only for hard core weather discussion, rants, or arguing about models.

Bypass this post if you are a casual visitor, want to know what it’ll do in your neighborhood or have any other questions.  Keep all other discussion in the posts below or I may delete your comment.

Only a few minor rules:

1. No swearing or foul language.

2. No personal attacks.  By that I mean comments about personal life etc… If you think someone’s forecast is horrible, go ahead and point that out.  More important would be to point out why you think so.  I also don’t care if you rip apart my forecast…I’m a big boy.

3. No complaining about someone picking on you in here, if you don’t like it don’t come in.