[willamette-fcst] Forecast - Thursday, December 3rd, 2009

[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: 
     Thursday, December 3rd, 2009 at 9:00am.

Burn Advisory:
     Agricultural burning is allowed.  Suggested burn times are from 11:00am until 2:00pm.
     Stack burning is not allowed.

Weather Discussion:
     An upper-level ridge of high pressure, centered just
     offshore, is producing a dry northerly flow aloft over
     Washington and Oregon today.  The air aloft is still very
     warm over western Oregon, with freezing levels around 10,000
     feet.  However, cool and dry low-level air has filtered southward,
     across the Willamette Valley.  That is maintaining a strong
     low-level temperature inversion over the valley.  

     It was cold again this morning across the state with Willamette Valley
     minimums mostly in the 20s and low 30s.  There were pockets of icy fog
     in the south valley, with visibilities less than one-quarter of a mile
     in the Eugene area.  In contrast, the the central and north valley were
     free of fog but getting some increase in high clouds from a weak weather
     system movning into southwestern British Columbia.

     The ODA surface analysis showed pressures continuing to rise
     across eastern Washington and north-central Oregon, in
     response to the cool and dry Canadian air spilling into the
     region.  That was combining with a weak thermal trough along
     the southern Oregon Coast to bring a dry offshore flow to
     western Oregon.  Easterly winds were gusting as high as 30 mph along
     the central Oregon Coast and to over 40 mph at the western end of the
     Columbia Gorge.  The few patches of fog in the southern Willamette Valley
     should give way to increasing high clouds this afternoon.  Highs this
     afternoon will likely stay in the mid 40s in the south valley but may
     reach the low 50s in parts of the north valley and along the coast.

Surface Winds:
     N 5-10 this morning, N 5-10 this afternoon.
Transport Winds:
     N 10 this morning, N 10 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 2000 feet.  Ventilation index 20.
High Temperature:
     Salem\'s high temperature today will be near 50.
Humidities:
     Minimum relative humidity will be near 33%.
Sunrise/Sunset:
     Salem sunset tonight: 4:32pm; sunrise tomorrow: 7:34am.

Extended Outlook:
     A cool weather system is forecast to drop southward, across
     Washington and northern Oregon, late on Friday.  It will
     have very limited moisture but will bring a chance of spotty
     light rain and snow to the region.  Snow levels will rapidly
     drop to around 1000 feet, or lower, Saturday.  An Arctic
     cold front is forecast to push southward across Washington
     and Oregon Saturday night and Sunday.  That system is
     associated with very cold air but limited moisture.  It
     should produce lots of clouds, and at least some snow
     flurries, as it pushes southward across the Willamette
     Valley Saturday night into Sunday morning.  It is possible
     that some valley locations could pick up enough snow to
     cover the ground, but significant accumulations are unlikely.

     Quite cold and dry conditions will follow the Arctic cold
     frontal passage later Sunday through Tuesday.  Cold easterly
     winds will increase in and near the Columbia River Gorge,
     making for very low windchill values.  Overnight minimums
     will drop well into the 20s, across the Willamette Valley,
     Monday and Tuesday (locally into the high teens).  Mostly
     sunny skies will help daytime highs recover into the 30s.

     A westerly flow aloft is forecast to undercut the offshore
     ridge and begin introducing moisture into western Oregon
     next Wednesday.  With cold air in place across western
     Oregon, there is a high potential for significant snow
     and/or ice accumulation Wednesday and Thursday, before
     warmer Pacific air can penetrate inland.  Temperatures will
     eventually moderate above freezing, from south to north
     across the Willamette Valley...most likely on Thursday.  The
     last place to transition back to plain old rain will be
     areas in and near the Columbia River Gorge.  Snow levels
     should lift to 4-5000 feet by Friday, with precipitation in
     the Gorge finally changing to all rain.

Tomorrow (04 Dec):  Increasing Clouds.  Slight Chance of Light Rain or Flurries Late.  26/45

Sat (05 Dec):  Partly Cloudy.  Increasing Clouds with Snow Flurries Late.  28/44

Sun (06 Dec):  Chance of Light Snow Early.  Turning Partly Cloudy and Cold.  27/35

Mon (07 Dec):  Sunny and Cold.  20/33

Tue (08 Dec):  Sunny and Cold.  21/35

Wed (09 Dec):  Snow Developing Late.  Possibly Turning to Freezing Rain or Rain South.  22/34

Thu (10 Dec):  Snow, Freezing Rain, or Rain...Turning to Rain From South to North.  32/44

ODA Meteorologist
weather at oda.state.or.us