Wednesday, November 24, 2010

The Thanksgiving Forecast

Major changes are going to happen in the next day and the end of the cold temperatures and icy grip are in sight....but there is much to get through first.

You can see the changes in the sky. Clouds moved in overhead accompanying warming aloft.

An interesting observation...temperatures warmed a bit in the middle of the night as the cloud spread overhead. The temps at Seattle-Tacoma Airport show this:
Nothing changed at the surface. Why? The reason is that clouds emit infrared radiation better than the clear air, so when clouds came in aloft there was more infrared radiation directed down to the surface...and thus warming.

There has been some snow flurries with the clouds aloft (you can see the light stuff in the radar below)
Ok, lets talk about the forecast. This is a MUCH easier forecast than Monday--fairly straightforward system coming off the Pacific without the coastal troughing/low generation that made the Monday forecast so uncertain.

First, what we are SURE about? It will warm up tomorrow afternoon. The ice and snow will rapidly melt late tomorrow and Friday. And rain is coming back.

So lets talk it through, including where the uncertainties lie.

Today the temps will stay in the 20s, and with little sun roads will not improve much. Keep in mind that ice is much less slippery in the lower 20s than in the upper 20s or near freezing. So in some sense conditions are much better now than on late Monday or what will occur as melting starts. And here in Seattle most of the major streets are in good condition.

Tonight the air will begin warming more rapidly aloft, but it will take time to scour out the cold dense air near the surface. Our computer models tend to mix the cool air out to quickly...and we know that. Anyway, warm- frontal precipitation should enter the region between 6 AM and 10 AM. It could well start as light snow. But by noon or early afternoon it should switch to rain and decrease in intensity. Good chance the precip will stop in the later afternoon and early evening. The main cold front comes in Friday morning.

There should be very strong winds over NW Washington and along the coast later on Thursday and Friday morning.

So the bottom line:

today...what you see is what you get.
tomorrow morning will have the potential for some light am snow and increasingly slippery conditions of melting ice.
By late Thursday the warming should improve conditions substantially.
Friday we will transition back to normal with rain,typical conditions, and wind in the normal locations.

Just in time to go shopping for black Friday. Hopefully METRO will turn its bus-tracker website back on (much more that in a future blog!)

10 comments:

Rich said...

I am wondering if it will take longer to scour out the cool air near the Fraser outflow in Bellingham. Is there any chance we may see precipitation as snow a little later up here?

OffBeatMammal said...

and of course with the KCM feeds down everyone suffers - http://onebusaway.blogspot.com/2010/11/king-county-metro-snow-and-real-time.html
sad that GPS is 2 years away. I wonder what it would cost a local telco (looking at you T-Mobile based right here in WA) to supply some old GPS equipped phones and EDGE data plans and just have the bus "phone home" it's location automatically every 5 mins. Heck, each driver could use their own phone for that!

Dan said...

"The reason is that clouds emit infrared radiation better than the clear air, so when clouds came in aloft there was more infrared radiation directed down to the surface...and thus warming."

Would it not be due more to the fact that cloud cover insulates thermal radiation from earth into deep space? Radiation between objects of similar temperature is minimal, but increases rapidly as the temperature differential increases -> Stefan-Boltzmann Law.

Drew said...

Thanks for the forecast, Dr. Mass.

Any word on what mountain passes will be like over the next couple of days? Will this front bring considerable mountain snow? I'm in Yakima and travelling to Everett, and I'd prefer not to spend Thanksgiving stuck at the pass for hours and hours.

Douglas said...

Cliff, you may already be aware, but there is an alternative to Metro's Bus Tracker:

http://www.onebusaway.org/

Amy said...

Douglas - onebusaway uses the data that Metro provides - just in a much easier-to-use way (phone, text, smartphone app, web)

smokejumper said...

Here's kind of a quirky stat. I originally thought of Sea-Tac but it didn't quite work, then i tried my near town Yakima, and we have had 10 consecutive days cooler than the other. Ranged from 65 to -10 degrees during that span.

stuart said...

@Douglas: Sadly, OneBusAway relies on Metro's data API.

natchrl8r said...

To answer my own question:

Winter Weather Advisory

URGENT - WINTER WEATHER MESSAGE
NATIONAL WEATHER SERVICE SEATTLE WA
338 PM PST WED NOV 24 2010


.A WEAK WARM FRONT WILL BRING SOME LIGHT SNOW TO WESTERN
WASHINGTON ON THANKSGIVING MORNING. THERE WILL BE A CHANCE OF
LIGHT FREEZING RAIN OR LIGHT SLEET DURING THE TRANSITION FROM SNOW
TO RAIN AROUND MIDDAY. RAIN IS LIKELY IN THE AFTERNOON. A COLD
FRONT WILL FOLLOW THURSDAY NIGHT AND FRIDAY MORNING...BRINGING
RAIN TO THE LOWLANDS AND SNOW TO THE MOUNTAINS.


WAZ503-506-250745-
/O.NEW.KSEW.WW.Y.0017.101125T0200Z-101126T0200Z/
WESTERN WHATCOM COUNTY-WESTERN SKAGIT COUNTY-
338 PM PST WED NOV 24 2010

...WINTER WEATHER ADVISORY IN EFFECT FROM 6 PM THIS EVENING TO
6 PM PST THURSDAY FOR THE WESTERN WHATCOM COUNTY AND WESTERN
SKAGIT COUNTY...

THE NATIONAL WEATHER SERVICE IN SEATTLE HAS ISSUED A WINTER
WEATHER ADVISORY FOR WESTERN WHATCOM COUNTY AND WESTERN SKAGIT
COUNTY FOR LIGHT SNOW...WHICH IS IN EFFECT FROM 6 PM THIS EVENING
TO 6 PM PST THURSDAY.


* SOME AFFECTED LOCATIONS...MOUNT VERNON...BURLINGTON...
BELLINGHAM...LYNDEN

* ACCUMULATIONS...UP TO 3 INCHES OVER WESTERN WHATCOM COUNTY...AND
UP TO 2 INCHES OVER WESTERN SKAGIT COUNTY.

* WIND...RISING TO SOUTHEAST 15 TO 25 MPH.

* TIMING...THERE IS JUST A CHANCE OF LIGHT SNOW OR FLURRIES
TONIGHT. SNOW IS EXPECTED TO INCREASE LATE TONIGHT. SNOW
CHANGING TO RAIN OVER WESTERN SKAGIT COUNTY BY MID
AFTERNOON...AND OVER WESTERN WHATCOM COUNTY BY LATE AFTERNOON.


PRECAUTIONARY/PREPAREDNESS ACTIONS...

A WINTER WEATHER ADVISORY MEANS THAT PERIODS OF SNOW...SLEET...OR
FREEZING RAIN WILL CAUSE TRAVEL DIFFICULTIES. BE PREPARED FOR
SLIPPERY ROADS AND LIMITED VISIBILITIES...AND USE CAUTION WHILE
DRIVING.

Wx Enthusiast said...

Cliff, are there problems with the WRF-GFS surface temperature output (mostly the extended GFS)? They have looked very strange all week, mostly between the 44 and 36 degree isotherms from the west side of the Olympics south through Oregon.