The Party Crashers; Weekend Cooler Weather Update

February 22, 2012

The  IT”S GOING TO GET COLD AND SNOWY IN THE LOWLANDS weather party is over;  crashed by models showing we have some normal lower late winter snow levels and just cooler than average temps on the way.  More on this below…first today:

This morning a nice cold front has moved south of our area and now quite a bit of clearing showing up from the Metro area north and west visually and on the satellite image as well.  Cooler air is moving into the Cascades after yesterday’s abnormally warm day.  After a few showers the rest of the day, the weather will be quite slow through Friday morning.  Another cold front slides in here Friday afternoon for renewed rainfall.

Now, what everyone really wants to know about for this coming weekend…
1.  Sticking snow is very unlikely where 90% of us live, below 1,000′ along the I-5 corridor in the western Valleys.  It’s okay if you removed your studs if you live in the lowest elevations.  I don’t see any traffic issues the next 5 days at least on area roadways.
2.  There is no unusual late season arctic chill on the way.  If you turned on water to your outside faucets?  They’ll be just fine.

So who will and when?
1. Above 500′ in the Coast Range some sticking snow is possible Saturday afternoon/night and into Sunday morning.
2. Near and above 1,000′ here in the Western Valleys, including the top of the West Hills, Mt. Scott, Sandy etc…most likely a little (less than 2″, or maybe just a trace) Saturday night or Sunday morning.
3. Up around 1,500′ and higher, 2-4″ looks good Saturday afternoon/evening through Sunday midday when it dries out.

The cold trough that swings in late Friday and Saturday bringing lots of showers also produces a strong south-southwest wind during the day Saturday.  That coupled with marginal 850mb temps -5 to -7 along with slightly too warm soundings is the reason I don’t think there’s any chance of lowland snow through Saturday evening.   You can see the strong wind right away Saturday morning on our RPM text output.  The surface flow turns light by Sunday morning as slightly colder air moves in (-7 to -8 @ 850mb.), but then the moisture goes way down for just light showers.  Check out the 24 hour rainfall forecast between 4am Sunday and 4am Monday off the UW WRF-GFS.   Only around .10″ in the driest parts of the metro area up to around .50″ in the wettest parts of the Coast Range.  One more reason to not get excited about anything other than flurries or light rain/snow showers under partly to mostly cloudy skies Sunday.  Plus, it’s the end of February and temps jump each afternoon unless you get steady precipitation and solid cloud cover.  It’ll still get up into the 40s. 

Another cool trough comes through next Tuesday and Wednesday, but this one will be marginal again for snow in the lower elevations; possibly a good shot in the foothills again.  You can see how cool it is through the whole Sunday-Wednesday period on the WRF-GFS cross-section.  I’ve added a solid blue line to show the 2,500′ elevation and the dotted line is the 32 degree temp.  If you want snow in Portland you want to see that line dip extremely close to (or down to) the surface.  If there is moisture for showers, the Monday morning dip is plenty chilly, and Sunday morning looks close as well.  Otherwise the freezing level is hovering up around 2,000′ most of the period.

Looking farther ahead, here is the 12z GFS ensemble chart and last night’s 00z ECMWF chart.  Lots of below average 850mb temps as we head into March, but generally nothing too extremely cold.  This could be the best snow of the winter (the next 10 days) if you live up around 1,500′!  It appears the mountains are FINALLY going to see their La Nina winter as we head into meteorological spring (March-May).

And one more chart just for fun, the mild winter may morph into a mild spring across the USA; check out the 850mb chart for Chicago.  Several periods of well above average temps, in fact most of the time it’s above average:

Chief Meteorologist Mark Nelsen