[willamette-fcst] Forecast - Friday, December 12th, 2008

[Willamette Valley Ag/Burning Forecast]

Daily Smoke Management Forecast




Oregon Department of Agriculture
Smoke Management Program
Weather Outlook and Field Burning Advisory for Willamette Valley Growers and Fire Districts.

Issued: 
     Friday, December 12th, 2008 at 9:00am.

Burn Advisory:
     Agricultural burning is allowed.  Suggested burn times are from 10:00am until 2:30pm.
     Straw stack burning is allowed from 10:00am until 2:30pm.

Weather Discussion:
     Clearing skies, in the northern Willamette Valley and along
     the coast, made for a pleasant Thursday afternoon with high
     temperatures in the low 50s.  Low clouds were more
     persistent in the south valley, where highs only managed to
     climb into the low to mid 40s.  Light winds allowed
     temperatures to fall to near or slightly below the freezing
     mark across most of the valley overnight, with areas of fog
     and low clouds reforming.

     The Salem sounding, from 4 a.m., was similar to Thursday
     morning, with very warm air aloft and a strong low-level
     temperature inversion.  However, major changes were evident
     in the upper-level winds, which were much stronger than
     Thursday, in response to an approaching cold front.  By
     mid-morning, increasing southerly winds, at the surface, had
     lifted the fog from the southern and central valley and
     warmed temperatures into the low 40s.  South winds had not
     mixed the fog out of sections of the north valley, so some
     areas, like Hillsboro, were still in the low to mid 30s.

     A highly advertised change in the weather pattern is
     underway, with an amplifying upper-level ridge of high
     pressure, in the Gulf of Alaska, sending a strong and cold
     weather system down the British Columbia coastline towards
     the Pacific Northwest.  After many days with stagnant
     weather conditions, increasing southerly winds and rapid
     cooling aloft will combine to greatly improve ventilation
     across western Oregon today.

     Infrared satellite imagery showed a rapidly strengthening
     low-pressure system about 150-200 miles northwest of the
     northern Washington coast.  A broad cloud-shield extended
     across all of Washington and all but the southeast corner of
     Oregon.  Doppler radar showed light rain spreading across
     most of western Washington and moving onto the northern
     Oregon coast. The ODA surface analysis showed increasing
     southerly gradients across western Oregon.  Southerly winds
     were gusting to about 20 mph in the central and southern
     Willamette Valley and to about 30 mph along the northern
     Oregon coast.

     The low-pressure center is forecast to continue to
     strengthen and move inland, to near Seattle, by late this
     afternoon and to near Spokane by midnight.  That strom track
     will bring increasingly strong south and sothwesterly winds
     to western Oregon and southwestern Washington later today
     through tonight.  The National Weather Service has issued a
     High Winds Watch for the northern and central Oregon coast,
     for gusts to 65 mph, from 10 am today through tonight.  A
     Wind Advisory has been issued for the Willamette Valley,
     through tonight, for gusts to 45 mph.  The strongest valley
     winds will likely be from Salem north to Portland.

     A strong cold front will also bring increasing rain to
     western Oregon this afternoon, with snow levels rapidly
     lowering from more than 7000 feet this morning to less than
     1000 feet tonight. Valley highs should only climb into the
     mid 40s.  Heavy snow and strong winds will make for blizzard
     conditions in the Cascade passes later today and tonight.  A
     Blizzard Warning has been issued for the Cascade passes
     beginning at 10 a.m. today and lasting through Saturday
     morning.  Winter Storm Warnings have been issued, for the
     Cascade foothills and coastal passes, for this afternoon
     through Saturday.

     By tonight, snow levels may drop as low as 500 feet across
     western Oregon with continued shower activity.  Some snow
     accumulations are possible in the higher hills of the
     northern and central Willamette Valley.  Brisk southwesterly
     winds will keep overnight temperatures in the mid 30s on the
     valley floor, where accumulating snow is not yet expected...

Surface Winds:
     S 10-20 G25 this morning, SSW 15-30 G45 this afternoon.
Transport Winds:
     S 25 this morning, SSW 45 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 3400 feet.  Ventilation index 153.
High Temperature:
     Salem\'s high temperature today will be near 46.
Humidities:
     Minimum relative humidity will be near 68%.
Sunrise/Sunset:
     Salem sunset tonight: 4:31pm; sunrise tomorrow: 7:42am.

Extended Outlook:
     In the wake of the cold front, cold and unstable
     northwestery flow aloft will bring heavy snow showers to
     both the coastal and Cascade passes Saturday and Sunday.
     Total snow accumulations could exceed two feet over the
     Cascade passes with up to a foot of snow over the coast
     range passes.  Be prepared for extreme winter driving
     conditions over any mountain passes this weekend.

     Colder Arctic air will slowly sag southward, into
     Washington, Saturday and northern Oregon as early as
     Saturday night. Ahead of the Arctic cold front, The cold air
     will be pulled out over the warmer ocean and then returned
     inland in the form of rain and snow showers, with the snow
     levels progressively lowering. Sticking snow will probably
     get as low as 500 feet Saturday, especially in the northern
     and central Willamette Valley, and perhaps to the valley
     floors, in the north valley, by Saturday night.

     The snow level will drop to the valley floors, from north to
     south, on Sunday, as the cold Arctic air begins pouring
     directly into the valley, via the Columbia Gorge. 
     Accumulating snow, on the valley floors, is likely Sunday
     through early Monday with the potential for the most
     significant snow accumulations since January, 2004.  Most,
     if not all, of the valley will likely get at least an inch
     of snow by Monday morning with significantly more in the
     hills.  It is possible that areas of the valley floor could
     accumulate several inches of snow, Sunday through early
     Monday, depending on the speed of the Arctic cold front and
     the amount of available moisture.

     Drier and cold Canadian air will continue to move into the
     region Sunday night and Monday, with snow showers tapering
     off and temperatures dropping well below freezing.  Brisk
     easterly winds will develop in the Portland area with
     northerly winds in the remainder of the valley.  The next
     weather system is forecast to drop down the British Columbia
     coastline around the middle of next week.  With cold air
     already in place, precipitation from that system will fall
     in the form of snow.  It is still too far out to forecast
     how much snow may fall, but it is looking more likely that
     the valley will get measurable snowfall again late Wednesday
     and/or Thursday of next week, with continued cold weather
     into the weekend.

Tomorrow (13 Dec):  Rain or Snow Showers.  Snow Level 500-1000 Feet.  35/41

Sun (14 Dec):  Snow.  Valley Accumulations Likely.  32/34

Mon (15 Dec):  Decreasing Snow Showers.  Clearing and Turning Much Colder.  23/28

Tue (16 Dec):  Mostly Sunny and Cold.  12/25

Wed (17 Dec):  Snow Likely Late.  12/28

Thu (18 Dec):  Snow Likely.  24/28

Fri (19 Dec):  Snow Ending.  Becoming Partly Cloudy and Continued Cold.  17/23

ODA Meteorologist
weather at oda.state.or.us