[willamette-fcst] Forecast - Wednesday, February 17th, 2010

Wed Feb 17 09:19:02 PST 2010

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, February 17th, 2010 at 9:00am.

Burn Advisory:
     Agricultural burning is not recommended.
     Stack burning is not allowed.

Weather Discussion:
     A building upper-level ridge of high pressure, just offshore, will produce a dry northerly
     flow across the region today.  The air mass aloft is presently quite mild.  The freezing
     levels over Salem and Medford were measured at 8900 feet and 9500 feet respectively early
     this morning.  Clearing skies allowed overnight temperatures to drop well into the 30s
     across most of western Oregon and into the 20s and 30s east of the Cascades.

     Residual moisture, from the recent rainfall, helped fairly widespread fog to form in the
     valleys of western Oregon as well as in the river basins east of the Cascades.  Visible
     satellite imagery showed the areas of valley fog, on both sides of the Cascades, with
     mostly clear skies along much of the coast and over the Cascades.  The ODA surface
     analysis showed pressure gradients had become northerly across Oregon, with a building
     thermal trough along the southern Oregon coast.  Offshore flow had cleared skies, north of
     about Cape Blanco, along the Oregon Coast, with low clouds banking up along the southern
     Oregon coast.

     With northerly flow this morning, instead of southwesterly flow, temperatures are markedly
     cooler.  After starting out in the low 50s Tuesday morning, and hitting maximums near 60
     degrees, much of the Willamette was still in the upper 30s at mid-morning.  Even though
     the air aloft is warmer this morning, compared to Tuesday morning, the air mass below
     3000 feet is about 10 degrees cooler.  That will lead to lower mixing heights and poor
     ventilation conditions today.

     The morning fog and low clouds, across the western valleys, should give way to sunshine
     this afternoon, as the air mass continues to dry out.  Even though the air mass aloft is
     still quite warm, high temperatures will struggle to reach the upper 50s this afternoon,
     especially in areas slow to lose the morning fog.  The coastline, which was a few
     degrees cooler than the Willamette Valley on Tuesday, should be a few degrees warmer than
     the valley today, due to downsloping easterly winds and a full day of sunshine. 
     Highs along central coast should top out near 60 degrees.

Surface Winds:
     N 5-15 this morning, N 5-15 this afternoon.
Transport Winds:
     NE 15 this morning, NNE 15 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 2000 feet.  Ventilation index 30.
High Temperature:
     Salem\'s high temperature today will be near 58.
Humidities:
     Minimum relative humidity will be near 51%.
Sunrise/Sunset:
     Salem sunset tonight: 5:43pm; sunrise tomorrow: 7:08am.

Extended Outlook:
     The upper-level ridge of high pressure will continue to amplify on Thursday, with
     northerly flow aloft driving cooler Canadian air into the Comumbia Basin of eastern
     Washington and northeastern Oregon.  Increasing offshore flow, at the surface, will push
     the cooler and dry Canadian air through the Columbia Gorge and into the Willamette
     Valley...helping to clear skies and limit AM fog formation across the Willamette Valley.
     Brisk easterly winds will develop, at the western end of the Columbia Gorge.

     Drier air will make for a large temperature spread between overnight minimums and
     daytime maximums.  After starting off in the 30s, valley highs should climb to around
     60 degrees Thursday, before staring a cooling trend Friday. The offshore down-sloping
     flow will likely lift coastal temperatures into the 60-65 degrees range Thursday.  The
     air mass will progressively cool Friday and Saturday, with continued dry northerly flow.

     Some moisture may undercut the upper-level ridge and begin moving into southern Oregon
     by late in the weekend.  It is possible that moisture could advance northward, into the
     Willamette Valley, by early next week.  An increasing westerly flow aloft is forecast to
     bring back some rain and mountain snow by the middle of next week.

Tomorrow (18 Feb):  Patchy AM Fog...Mainly South.  Mostly Sunny and Mild.  34/62

Fri (19 Feb):  Mostly Sunny with Continued Offshore Flow.  34/58

Sat (20 Feb):  Mostly Sunny and Cooler.  33/54

Sun (21 Feb):  Partly Cloudy.  32/53

Mon (22 Feb):  Partly Cloudy.  34/54

Tue (23 Feb):  Chance of Rain and Mountain Snow.  Snow Level 3-4000 Feet.  40/54

Wed (24 Feb):  Chance of Rain and Mountain Snow.  Snow Level 4-5000 Feet.  38/55

ODA Meteorologist
weather at oda.state.or.us