Monday, December 8, 2008

A Radical Weather Shift on Friday



Well, the big story is the major weather transition on Friday. I mean major. Today we have a residual Puget Sound convergence zone that is now dying with some weak showers in the mountains. This event produced a dusting at Snoqualmie Pass and several inches at Stevens. The snowpack right now is very poor, with the higher terrain only gaining about 20% of normal for this time of the year (see figure).

Tomorrow a warm front will bring showers for mainly the northern portion of the state...and some light snow to the N. Cascades about 3000 ft. This precipitation will continue on into Wednesday. Thursday will be generally dry. The big story in Friday. There will be a major reconfiguration of the large scale atmospheric pattern. The ridge over us will strengthen and move westward and we will be thrust into much colder northerly flow (see upper air chart for 7 PM on Friday night)--and a trough of lower pressure will move southward over region. We are pretty much guaranteed much, much colder air over us...and you should prepare for it. This kind of pattern makes plumbers very happy, as bursting pipes bring lots of business.
The mountains will get significant snow as the trough moves through. The question, of course, is lowland snow. I have studied this issue for a long time....and right now the flow situation is not ideal for much snow at low levels on Friday over Puget Sound. The threat is actually much higher to our south...Portland and the Willamette Valley. Anyway, no use speculating more on this until we have more confidence in the forecast...that will be Wednesday.

7 comments:

OWSweather said...

Pattern is pre-1950. You are looking at a snow problem up there, but the weather weenies will enjoy it.

Cliff Mass Weather Blog said...

Pre-1950?

Michael said...

For anyone who wants to see what -19C 850mb temperatures over Seattle looks like on the WRF, the 00z GFS-based WRF shows it:
http://img241.imageshack.us/img241/4545/850t1800000kp3.gif

Michael said...

And here's what that looks like at 925mb (with SLP):
http://img178.imageshack.us/img178/5692/slp1800000gm1.gif

OWSweather said...

Pre-1950 as in such a pattern not seen since before then. Not the general storm, but the overall pattern. Looks like a snow problem even down where I live as well. (SoCal)

Keeping one's head above water said...

please speak english to us who are clueless about weather

Keeping one's head above water said...

here is the statement that was at the NWS seattle website.



Special Weather Statement

--------------------------------------------------------------------------------

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE PORTLAND OR
105 PM PST MON DEC 8 2008

ORZ001>014-WAZ019>023-039-040-092300-
NORTH OREGON COAST-CENTRAL OREGON COAST-
COAST RANGE OF NORTHWEST OREGON-
CENTRAL COAST RANGE OF WESTERN OREGON-LOWER COLUMBIA-
GREATER PORTLAND METRO AREA-CENTRAL WILLAMETTE VALLEY-
SOUTH WILLAMETTE VALLEY-WESTERN COLUMBIA RIVER GORGE-
NORTHERN OREGON CASCADE FOOTHILLS-NORTHERN OREGON CASCADES-
CASCADE FOOTHILLS IN LANE COUNTY-CASCADES IN LANE COUNTY-
UPPER HOOD RIVER VALLEY-SOUTH WASHINGTON CASCADES-WILLAPA HILLS-
SOUTH WASHINGTON COAST-I-5 CORRIDOR IN COWLITZ COUNTY-
GREATER VANCOUVER AREA-SOUTH WASHINGTON CASCADE FOOTHILLS-
INCLUDING THE CITIES OF...ASTORIA...CANNON BEACH...TILLAMOOK...
LINCOLN CITY...NEWPORT...FLORENCE...VERNONIA...JEWELL...TRASK...
GRANDE RONDE...TIDEWATER...SWISSHOME...ST. HELENS...CLATSKANIE...
HILLSBORO...PORTLAND...OREGON CITY...GRESHAM...SALEM...
MCMINNVILLE...DALLAS...EUGENE...CORVALLIS...ALBANY...HOOD RIVER...
CASCADE LOCKS...MULTNOMAH FALLS...SANDY...
SILVER FALLS STATE PARK...SWEET HOME...GOVERNMENT CAMP...
DETROIT...SANTIAM PASS...VIDA...LOWELL...COTTAGE GROVE...
MCKENZIE BRIDGE...OAKRIDGE...WILLAMETTE PASS...PARKDALE...ODELL...
COLDWATER RIDGE VISITORS CENTER...MOUNT ST. HELENS...FRANCES...
RYDERWOOD...RAYMOND...LONG BEACH...CATHLAMET...LONGVIEW...KELSO...
CASTLE ROCK...STEVENSON...SKAMANIA...VANCOUVER...BATTLE GROUND...
WASHOUGAL...TOUTLE...ARIEL...COUGAR
105 PM PST MON DEC 8 2008

...TURNING EXTREMELY COLD WITH VERY LOW SNOW LEVELS THIS WEEKEND
OVER SOUTHWEST WASHINGTON AND NORTHWEST OREGON...

THE COLDEST AIR OF THE WINTER SEASON WILL BE SETTLING IN OVER
SOUTHWEST WASHINGTON AND NORTHWEST OREGON THIS WEEKEND...BRINGING
VERY COLD TEMPERATURES TO THE REGION AND SNOW LEVELS POSSIBLY NEAR
THE VALLEY FLOOR.

THE INITIAL SURGE OF COLD AIR WILL SPREAD INTO SOUTHWEST
WASHINGTON AND NORTHWEST OREGON FRIDAY NIGHT AS A STRONG COLD
FRONT MOVES THROUGH. WHILE AIR AHEAD OF THIS FRONT WILL BE
RELATIVELY MILD...VERY COLD AIR BEHIND THIS FRONT HAS THE
POTENTIAL TO DROP SNOW LEVELS QUITE LOW...NOT UNLIKE A SNOW EVENT
THAT OCCURRED IN MID NOVEMBER OF 2003 WHICH DROPPED SEVERAL INCHES
OF SNOW IN THE PORTLAND METRO AREA. WHILE THIS KIND OF DETAIL IS
NOT CERTAIN AT THIS POINT...THE SNOW LEVEL AT THE BACK EDGE OF THE
COLD FRONT FRIDAY NIGHT COULD TURN OUT TO BE QUITE LOW.

THE COLD AIR WILL CONTINUE TO STREAM ONSHORE DURING THE DAY
SATURDAY AND ANY SHOWERS WILL LIKELY DROP SNOW TO THE 500 OR 1000
FEET ELEVATIONS. THE ONSHORE FLOW WILL LIKELY LIMIT THE
POSSIBILITY OF THE SNOW REACHING THE VALLEY FLOOR DUE TO A
SIGNIFICANT OVER WATER TRAJECTORY OF THE AIR...AS THE WATER
TEMPERATURES OFF THE COAST REMAIN IN THE LOWER 50S.

IT APPEARS A REINFORCING SHOT OF ARCTIC AIR WILL THEN SPREAD
THROUGH THE GORGE AND DOWN THROUGH WESTERN WASHINGTON INTO
NORTHWEST OREGON SATURDAY NIGHT AND SUNDAY...AND BRING THE COLDEST
AIR TO THE REGION IN QUITE SOME TIME. AT THAT POINT...MINIMUM
TEMPERATURES COULD FALL INTO THE TEENS WITH DAYTIME TEMPERATURES
STRUGGLING TO REACH FREEZING. THE POSSIBILITY OF ANY SIGNIFICANT
SNOW ONCE THE MAIN SHOT OF ARCTIC REACHES OUR AREA IS UNCERTAIN AT
THIS TIME AS IT IS DEPENDENT ON WHETHER ANY ORGANIZED SYSTEMS WILL
SPREAD OVER THE AREA...BUT THE POSSIBILITY DOES EXIST.

BE SURE TO TAKE PRECAUTIONS THIS WEEK IN ADVANCE OF THE ARRIVAL OF
THE VERY COLD AIR...SUCH AS WRAPPING PIPES...COVERING FOUNDATION
VENTS...DRAINING OUTDOOR WATER PIPES...