[willamette-fcst] Forecast - Wednesday, December 2nd, 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: 
     Wednesday, December 2nd, 2009 at 9:00am.

Burn Advisory:
     Agricultural burning is allowed.  Suggested burn times are from 10:00am until 2:30pm.
     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.  The air aloft is still very warm
     over western Oregon.  The freezing levels, over Salem and
     Medford, were measured at 9500 feet and 11,000 feet
     respectively early this morning. Meanwhile, cool and dry
     low-level air is filtering southward, across the Willamette
     Valley.  Strong low-level temperature inversions are still
     in place, but ventilation near the Columbia Gorge has
     significantly improved due to an increase in easterly
     winds...gusting to over 30 mph.  Northerly winds in the
     central and southern Willamette Valley are not conducive to
     good ventilation for stack burning.

     The cool and dry low-level air helped to keep skies clear of
     fog in the northern Willamette Valley overnight, but there
     were still some areas of fog in the south valley this
     morning.  Skies were sunny along the coast and over the
     Cascades.  Temperatures were much cooler this morning, with
     most of the Willamette Valley dropping to near or below
     freezing.  Mid-morning temperatures ranged from the mid 20s
     in the south valley to the mid 30s in the north valley.

     The ODA surface analysis showed pressures rising across
     eastern Washington and north-central Oregon, in response to
     the cool and dry Canadian air spilling into the region.  A
     weak thermal torugh was also trying to build along the
     southern Oregon Coast.  The offshore flow will promote
     further drying of the air mass across western Oregon.  Fog
     should begin to lift, from the southern Willamette Valley
     this afternoon.  Highs this afternoon will likely stay in
     the 40s in the south valley but may reach the low 50s in
     parts of the north valley and along the coast.

Surface Winds:
     N 5-15 this morning, NNE 5-15 this afternoon.
Transport Winds:
     N 10 this morning, NNE 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 49.
Humidities:
     Minimum relative humidity will be near 40%.
Sunrise/Sunset:
     Salem sunset tonight: 4:32pm; sunrise tomorrow: 7:32am.

Extended Outlook:
     The upper-level ridge is forecast to hold over the region
     Thursday, with another frosty morning and dry offshore flow
     providing plenty of afternoon sunshine to most of western
     Oregon.  There may still be a few pockets of fog in the
     southern Willamette Valley and the southwestern valleys. 
     Overnight temperatures will likely cool slightly, as the air
     mass continues to dry out, but daytime highs should recover
     close to normal.

     A cool weather system is forecast to move from southwestern
     Canada into Washington and northern Oregon by late Friday. 
     This cold front may be strong enough to bring some light
     rain and snow to the region, with snow levels dropping to
     1000 feet, or lower, by Saturday morning.  Much colder air
     is forecast to push southward into Washington and Oregon
     over the weekend, but the air mass is forecast to remain
     mostly dry during the transition into the Arctic air.  Quite
     cold and dry conditions will likely persist through early
     next week.

     A westerly flow aloft is forecast to undercut the offshore
     ridge and begin a warming trend around the middle of next
     week.  That process will need to be closely monitored, for
     the potential of significant snow and/or ice accumulation,
     even in the Willamette Valley, as Pacific moisture gets
     thrown over the existing cold dome of low-level air. 
     Temperatures will likely warm above freezing, in the lowest
     elevations, by the end of next week, with snow and ice
     turning to plain old rain in the lowlands.  Significant
     snows may continue in the mountains.

Tomorrow (03 Dec):  Areas of AM Fog South.  Mostly Sunny.  NNE Winds.  26/46

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

Sat (05 Dec):  Slight Chance of Light Rain or Snow Flurries.  30/44

Sun (06 Dec):  Partly Cloudy and Cold.  Slight Chance of Light Snow.  26/35

Mon (07 Dec):  Sunny and Cold.  22/33

Tue (08 Dec):  Sunny and Cold.  20/35

Wed (09 Dec):  Good Chance of Snow.  Significant Accumulations Possible.  25/33

ODA Meteorologist
weather at oda.state.or.us