[willamette-fcst] Forecast - Tuesday, January 20th, 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: 
     Tuesday, January 20th, 2009 at 9:00am.

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

Weather Discussion:
     The upper-level ridge that has brought dry and unseasonably
     warm conditions to much of the region over the past several
     days had shifted east, to over the Rockies, this morning. 
     on the wet side of the ridge, the upper-level winds were
     southerly over Oregon and Washington with the air aloft
     slowly starting to cool.

     The Salem sounding this morning showed a few degrees of
     cooling, compared with Monday morning, starting at about
     2000 feet and extending all the way up to around 26,000
     feet.  The freezing level had dropped slightly, from 12,300
     feet to 11,000 feet.  There was still a strong low-level
     temperature inversion over the Willamette Valley this
     morning.  Surface temperatures were mostly in the mid to
     upper 20s, with 3-5000 foot temperatures between 50 and 55
     degrees.  That will lead to very low mixing heights and poor
     ventilation conditions again today.

     The ODA surface analysis continued to show offshore flow,
     with high pressure over eastern Washington and Oregon and a
     thermal trough of low pressure just off the coastline. 
     Offshore pressure gradients are weakening, but easterly
     winds were still gusting in excess of 40 mph at the western
     end of the Columbia Gorge at mid-morning with temperatures
     in the chilly mid 30s.

     Enough drier air filtered into the Willamette Valley, via
     northerly winds, over the weekend to help clear the
     persisten fog from the region.  Fair skies and light
     notherly flow overngiht allowed many areas to drop well into
     the 20s this morning.  Easterly winds were still dropping
     off the coastal range, to the coastline, keeping
     temperatures there near 40 degrees under mostly clear skies.

     East of the Cascades, winds were generally light with
     widespread fog and low clouds in the mountain
     valleys...especially in the Columbia Basin of northeast
     Oregon, the Hood River Valley, and south into Jefferson and
     Deschutes Counties.  Temperatures were mostly in the teens
     and low 20s.

     Infrared satellite imagery showed only a few high clouds
     moving northward across Oregon and Washington, with a
     splitting and weakening weather system still several hundred
     miles offshore. The air mass over western Oregon was still
     dry enough to limit fog formation this morning, so skies
     should be mostly sunny, from the coast to the Cascades,
     again today.  Temperatures should be a few degrees cooler
     than on Monday, in most locations.  Valley highs will be in
     the 40s with coastal readings climbing into the 50s. 
     Cascades pass temperatures will be near 40 degrees with some
     western foothills climbing into the low 50s.  

Surface Winds:
     N 0-5 this morning, N 3-7 this afternoon.
Transport Winds:
     N 5 this morning, N 5 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 800 feet.  Ventilation index 4.
High Temperature:
     Salem\'s high temperature today will be near 47.
Humidities:
     Minimum relative humidity will be near 51%.
Sunrise/Sunset:
     Salem sunset tonight: 5:04pm; sunrise tomorrow: 7:42am.

Extended Outlook:
     A weakening weather system will move onshore, into
     California, on Wednesday with continued southerly flow aloft
     over Oregon.  Further weakening of the offshore surface flow
     may allow the air mass to moisten enough over the valley for
     more widespread morning fog.  Middle and high clouds should
     increase across the region during the day, but it appears
     that shower activity will not make it as far north as the
     Willamette Valley until possibly Wednesday night.  The air
     aloft will continue to cool but not likely enough to
     eliminate the strong low-level temperature inversion, so
     ventilation conditions should remain poor through Wednesday.

     There is a slight chance of light showers by Thursday
     morning across all of Oregon, especially for the southern
     half of the state.  More cooling aloft will lower the snow
     level to around 5000 feet with impoving ventialtion
     conditions over the Willamette Valley.  Any moisture riding
     over the trapped cold air in the Columbia Gorge and eastern
     valley locations could result in a wintry mix of
     precipitation, but amounts should be very light.  A weak
     ridge is forecast to move back over Oregon Friday with the
     flow aloft becoming northwesterly and drying out.  Continued
     cooling aloft will help to further improve ventilation
     conditions.

     The long-range computer models have been inconsistent with
     their forecasts of a major weather pattern change, so my
     confidence in the forecast beyond Friday is below average. 
     It appears that the upper-level ridge of high pressure will
     rebuild in the Gulf of Alaska with a colder
     north-northwesterly flow aloft developing over the Pacific
     Northwest beginning this weekend.  That would bring much
     lower snow levels with some rain and snow to the region.  It
     is possible that much colder Arctic air may spill into the
     region by Sunday, but that is still questionable at this time.

Tomorrow (21 Jan):  Areas of AM Fog.  Becoming Partly Cloudy.  27/46

Thu (22 Jan):  Mostly Cloudy.  Chance of Light Showers.  33/46

Fri (23 Jan):  Slight Chance of AM Showers.  Mostly Cloudy.  35/47

Sat (24 Jan):  Increasing Chance of Rain.  Snow Level dropping to 2000 Feet.  32/45

Sun (25 Jan):  Chance of Rain or Snow.  32/40

Mon (26 Jan):  Partly Cloudy and Colder.  25/35

Tue (27 Jan):  Increasing Clouds Late.  25/40

ODA Meteorologist
weather at oda.state.or.us