Potential for Overrunning Snow Late Tuesday into Wednesday...

SNOW or RAIN?

There is a POTENTIAL for decent snowfall tonight, but everything needs to line up just right (like always!)... The setup is due to the cold air we already have in place, and an approaching warm front, this front will eventually raise snow levels well off the valley floor, but when it does will be crucial to snowfall in the PDX metro. The most important thing we're currently seeing is a slight offshore gradient (meaning winds are not coming off the ocean but towards them). This is what gives us our east wind... ANY, and I mean ANY sort of east wind tonight will make a dramatic difference in snow chances. It is forecast to get up to a healthy gradient of -4 to -5 from The Dalles to Troutdale, which in the east metro may provide some slightly breezy east wind for a few hours, enough to hold onto the cold for that much longer, and with the approaching warm front that could be all we need. Below is the image that I'm referring to, it can be a bit confusing but it is read from right to left, the red and blue plots mark pressure at Troutdale and The Dalles respectively, as you can see in the last couple of hours the gradient has switched to offshore...

Now the front I'm talking about is just now pushing showers into the coast range, with the heavier precip forecasted for 2-4am... This is the time period to watch for the possibility of some decent snowfall, but like I said, it all comes down to how strong this cold air and offshore flow can hold on..

As you can see this setup is about as borderline as it gets... But if the factors line up right we may get a few hours of good snow out of it.. Ill leave you for now with both the Short Term portion of tonights AFD, as well as the Winter Weather Advisory that goes into effect at 2am

SHORT TERM...RADAR IMAGERY SHOWS SHOWERS HAVE BEEN TAPERING OFF THIS
EVENING. A COLD AIRMASS OVER THE AREA COMBINED WITH RADIATION COOLING
HAS RESULTED IN COLD TEMPERATURES ACROSS THE REGION. SURFACE
OBSERVATIONS INDICATE THE FREEZING LEVEL IS NEAR 1000 FEET THIS
EVENING. SNOW OR A MIX OF RAIN AND SNOW SHOWERS WILL CONTINUE TO
DECREASE TONIGHT.

SATELLITE IMAGERY SHOWS AN APPROACHING WARM FRONT AROUND 150 MILES
OFF THE OREGON COAST. THE LANGLEY HILL RADAR IMAGERY IS JUST SHOWING
RAIN OR VIRGA AHEAD OF THE FRONT AROUND 70 MILES OFFSHORE OF THE
WASHINGTON OREGON BORDER. WARMER AIR WILL EVENTUALLY MOVE INTO THE
AREA AND RISE SNOW LEVELS...BUT NOT LIKELY WITH THE INITIAL BAND OF
PRECIPITATION.

EASTERLY WINDS EAST OF THE CASCADES WILL PUSH MUCH COLDER AIR FROM
EASTERN OREGON THROUGH THE GORGE BEFORE THE ARRIVAL OF
PRECIPITATION...AND CAN EXPECT A GOOD DOSE OF SNOW FOR THE GORGE AND
UPPER HOOD RIVER VALLEY LATE TONIGHT INTO WEDNESDAY MORNING. SOME OF
THIS COLDER AIR WILL SPILL INTO THE NORTHERN WILLAMETTE VALLEY AND
SUPPORT THE POSSIBILITY FOR SNOW IN THE INTERIOR LOW LANDS AS WELL.

THERE IS FAIR CONFIDENCE THAT SNOW WILL IMPACT MUCH OF THE AREA
BEFORE THE WARM FRONT BRINGS WARMER AIR BUT THERE IS LESS CONFIDENCE
IN THE AMOUNT OF SNOW.PRECIPITATION LOOKS FAIRLY LIGHT WITH THE
INITIAL SLUG OF MOISTURE WHICH SHOULD LIMIT ACCUMULATING SNOW. HAVE
MAINTAINED THE CURRENT FORECAST OF HAVING LIGHT SNOW
DEVELOPING AFTER 3 AM...BUT NOT REALLY ACCUMULATING UNTIL AFTER 5 AM.
THE INTERIOR LOWLANDS SHOULD HAVE ALL SNOW EARLY WED
AM...TRANSITIONING TO SLEET AND THEN RAIN AND SNOW BETWEEN 7 AM AND 9
AM...THEN JUST RAIN. 1 TO 2 INCHES GENERALLY EXPECTED WITH A FEW
AREAS GETTING UP TO 3 INCHES. BUT IN CENTRAL AND SOUTH WILLAMETTE
VALLEY...ESPECIALLY FROM SALEM SOUTHWARD...THE WARM AIR ARRIVAL
SHOULD LIMIT SNOW AMOUNTS TO LESS THAN 1 INCH.

Winter Weather Advisory
URGENT - WINTER WEATHER MESSAGE
NATIONAL WEATHER SERVICE PORTLAND OR
307 PM PST TUE DEC 18 2012

ORZ005-006-WAZ022-039-191400-
/O.CON.KPQR.WW.Y.0033.121219T1000Z-121219T1800Z/
LOWER COLUMBIA-GREATER PORTLAND METRO AREA-I-
5 CORRIDOR IN COWLITZ COUNTY-GREATER VANCOUVER AREA-
INCLUDING THE CITIES OF...ST. HELENS...HILLSBORO...PORTLAND...
OREGON CITY...GRESHAM...TROUTDALE...LONGVIEW...KELSO...
VANCOUVER...BATTLE GROUND...CAMAS...WASHOUGAL
307 PM PST TUE DEC 18 2012

...WINTER WEATHER ADVISORY REMAINS IN EFFECT FROM 2 AM TO 10 AM
PST WEDNESDAY FOR THE NORTH WILLAMETTE VALLEY...CLARK COUNTY AND
THE I5 CORRIDOR...

A WINTER WEATHER ADVISORY FOR SNOW REMAINS IN EFFECT FROM 2 AM TO
10 AM PST WEDNESDAY.

THE NATIONAL WEATHER SERVICE IN PORTLAND HAS ISSUED A WINTER
WEATHER ADVISORY FOR SNOW...WHICH IS IN EFFECT FROM 2 AM TO 10 AM
PST WEDNESDAY.

* TONIGHT...MORE WIDESPREAD ACCUMULATING SNOW WILL DEVELOP LATE
  TONIGHT. GENERALLY...1 TO 2 INCHES OF SNOW ARE LIKELY FOR LATE
  TONIGHT AND WEDNESDAY. HOWEVER...A FEW LOCATIONS TOWARDS EASTERN
  CLARK COUNTY AND THE WEST HILLS MAY SEE 2 TO 3 INCHES.

* SNOW LEVELS TODAY...WILL LOWER TO NEAR VALLEY FLOOR LATER
  TONIGHT INTO EARLY WEDNESDAY MORNING. SNOW LEVEL WILL RISE TO
  2500 FEET QUICKLY WEDNESDAY MORNING.