[willamette-fcst] Forecast - Wednesday, December 17th, 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.

NOON UPDATE

Issued: 
     Wednesday, December 17th, 2008 at 12:00pm.

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

Weather Discussion:
     Surface and satellite observations showed a low-pressure
     center developing near Vancouver Island, British Columbia
     late this morning with a warm front pushing into western
     Washington and northwestern Oregon.  The air mass was cold
     enough for precipitation to begin as snow along the northern
     Oregon coast early this morning, but that quickly changed to
     rain, as increasing southlery winds warmed temperatures well
     above freezing.  Clouds had already advanced across the
     region by midday with a wintry mix of precipitation types
     across western Wsahington and northwest Oregon.  Light snow
     was falling over eastern Washington and northeastern Oregon,
     where the low-level cold air is still very entrenched.

     Southerly winds, locally gusting to 30 mph, had warmed
     Willamette Valley temperatures to near freezing by late this
     morning with a wintry mix of precipitation reported as far
     south as Salem.  Doppler radar showed precipitation
     extending south to near Corvallis.  Warmer air aloft was
     also making it into southwestern Washington with some
     interior locations there reporting rain mixing with the
     light snow.

     Snow was increasing in the Portland area late this morning.
     As the low-pressure area tracks southward, along the
     Washington Coast, this afternoon, precipitation will
     increase across western Washington and Oregon.   Brisk
     southerly surface winds should lift snow levels to between
     1000 and 1500 feet in the Willamette Valley later this
     afternoon.  A wintry mix is likely during that transition,
     along the valley floor.  Cold outflow from the Columbia
     Gorge will delay the transition to rain in the Portland
     area, where minor accumulations of snow and ice are
     possible.

     Some light snow and sleet may accumulate on the Willamette
     Valley floors early this afternoon, before the likely
     transition to rain occurs.  Valley temperatures should climb
     into the mid to upper 30s later this afternoon with
     continued southerly winds making for good ventilation
     conditions.  With snow levels not lifting above about 1500
     feet, significant snow accumulations are liekly at higher
     elevations.  The coast range will receive about 4-8 inches
     of new snow with over a foot of new snow in the Cascades.

     Rain and snow showers will increase across western Oregon
     tonight, as a vigorus upper-level trough drops into the
     region.  Snow levels will likely drop at least down to 500
     feet and perhaps locally to the valley floor, especially in
     the north valley.  A couple of inches of snow are possible,
     above 500 feet.  Temperatures will fall back into the low to
     mid 30s.  Heavy snow showers are likely in the coast range
     and in the Cascades with significant accumulations.

Surface Winds:
     S 10-20 G30 this afternoon.
Transport Winds:
     SSW 30 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 3000 feet.  Ventilation index 90.
High Temperature:
     Salem\'s high temperature today will be near 36.
Humidities:
     Minimum relative humidity will be near 78%.
Sunrise/Sunset:
     Salem sunset tonight: 4:32pm; sunrise tomorrow: 7:46am.

Extended Outlook:
     Another cold Arctic front is forecast to drop south across
     Washington late Wednesday and into northern Oregon Thursday
     morning.  The Arctic cold front will advance south across
     western Oregon Thurssday...dropping temperatures back below
     freezing and changing all precipitation back to snow.  Snow
     showers will continue into early Friday, with valley
     accumulations on the order of 2-4 inches liklely.  Another
     surge of cold Arctic air will pour into the region Friday
     afternoon and night, with skies gradually clearing. Increasing
     easterly outflow, from the Columbia Gorge, will create very
     low wind-chill values in the north valley, near Portland.

     Clouds will increase Saturday from the next weather system
     with increasing cold easterly winds pouring out of the
     Columbia Gorge into the north valley.  Temperatures will
     stay below freezing across the valley with a chance of snow
     by Saturday evening.  Western Washington and Oregon are in
     for a wintry mix of precipitation Saturday night and Sunday
     with easterly Gorge winds helping to keep the low-level cold
     air in place, especially in the north valley.

     The latest comuter models solutions are beginning to
     deverge, beginning Monday.  Some models show a warm-up early
     next week, while others bring a reinforcing surge of Arctic
     air back into the Willamette Valley.  My forecast reflects
     the later solution, but my confidence in the forecast beyond
     Sunday is below normal.

Tomorrow (18 Dec):  Rain or Snow Showers Turning to Snow Showers.  2-4 inches Likely.  31/38

Fri (19 Dec):  Snow Showers Ending...Becoming Partly Cloudy. Continued Cold.  18/27

Sat (20 Dec):  Increasing Clouds, Cold, and Windy.  12/25

Sun (21 Dec):  Snow and Windy...Possible Wintry Mix.  Accumulations Likely. 20/32

Mon (22 Dec):  Decreasing Snow Showers...Continued Cold.  25/29

Tue (23 Dec):  Partly Cloudy and Cold.  18/31

Wed (24 Dec):  Chance of Snow.  20/32

ODA Meteorologist
weather at oda.state.or.us