[willamette-fcst] Forecast - Thursday, December 4th, 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.

Issued: 
     Thursday, December 4th, 2008 at 9:00am.

Burn Advisory:
     Agricultural burning is allowed.  Suggested burn times are from 11:00am until 2:30pm.
     Stack burning is not allowed.

Weather Discussion:
     An upper-level high pressure area began building offshore
     Wednesday with the flow aloft drying out and turning
     northerly.  A very slow moving modified arctic cold front
     brought drier Canadian air into Washington Wednesday and
     much of northern and eastern Oregon overnight.  That helped
     to clear the fog and low clouds from the northern Willamette
     Valley, but visible satellite imagery showed considerable
     low cloudiness continuing this morning in the western
     valleys in Linn and Lane Counties.

     With a little afternoon sunshine on Wednesday, north valley
     temperatures climbed into the low 50s, while cloudy and
     foggy areas in the south valley stayed in the upper 40s. 
     The drier air overnight lead to cooler minimum temperatures
     this morning...especially in the north valley.  Hillsboro
     dropped to 30 degrees, while clearing skies allowed Salem
     and Corvallis to fall to the freezing mark.  Clouds held
     temperatures in the low 40s in Eugene.

     The upper-level ridge will build into the coastline today
     with continued northerly flow aloft and increasingly dry
     offshore flow at the surface.  The Salem sounding this
     morning showed a strengthening temperature inversion from
     between 1500 and 2500 feet, so mixing heights should stay at
     or below 1500 feet this afternoon.  Forecast north-northeasterly
     transport winds are not good for  ventilating smoke from stack
     burns, so they are not allowed today.  

     The ODA surface analysis showed the weak arctic frontal
     boundary extending from the north coast, through the central
     Willamette Valley, to southeastern Oregon.  North of that
     front, skies were mostly sunny.  South of the front, valley
     locations, on both sides of the Cascades, still had
     considerable fog and low clouds.  As dry air continues to
     filter southward, across Oregon today, the low clouds should
     begin to clear from the southern valleys.  Mostly sunny
     skies will previal in the central and northern Willamette
     Valley, along the coast, and over the Cascades.  High
     temperatures should be near 50 degrees across the Willamette
     Valley and 50-55 along the coast.  Mostly clear skies
     tonight will allow temperatures to drop into the upper 20s
     across much of the valley.

Surface Winds:
     NNE 5-15 this morning, NNE 7-15 this afternoon.
Transport Winds:
     NNE 13 this morning, NNE 15 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 1500 feet.  Ventilation index 23.
High Temperature:
     Salem\'s high temperature today will be near 50.
Humidities:
     Minimum relative humidity will be near 50%.
Sunrise/Sunset:
     Salem sunset tonight: 4:31pm; sunrise tomorrow: 7:35am.

Extended Outlook:
     As the upper-level ridge moves directly over western Oregon,
     on Friday, the offshore flow is forecast to decrease.  That
     may allow the low-level air mass to moisten enough for areas
     of morning fog in the central and south valley.  The north
     valley should see another mostly sunny day.  The ridge is
     forecast to flatten Saturday, with a strengthening jet
     stream sagging south to over Washington and northern Oregon.
      That will bring a slight chance of light rain to the north
     valley.  A little stronger weather system is forecast to
     slide across Washington and northern Oregon Sunday, in a
     cooler northwesterly flow aloft.

     A transitory ridge of high pressure is forecast to bring dry
     weather next Monday.  A weak warm front may bring some
     sprinkles or light rain to mainly the north valley Tuesday. 
     The long-range models are finally coming into agreement,
     regarding a major change in the weather pattern, beginning
     next week.  It appears that the strong protective ridge of
     high pressure, that has brought unsesonably dry and mild
     conditions to Oregon since mid-November, will shift westward
     and allow a series of progressively stronger weather systems
     to drop into the region from the Gulf of Alaska.  That will
     bring a chance of rain back to western Oregon by the middle
     of next week with more significant storms possibly bringing
     rain and mountain snow to the region by the following weekend.

Tomorrow (05 Dec):  Areas AM Low Clouds and Fog South...Otherwise Mostly Sunny.  28/49

Sat (06 Dec):  Areas of AM Fog.  Mostly Cloudy.  Slight Chance of Light Rain North.  30/50

Sun (07 Dec):  Chance of Light Rain North.  Snow Level Dropping to 5500 Feet.  39/50

Mon (08 Dec):  Mostly Cloudy.  37/49

Tue (09 Dec):  Chance of Sprinkles North.  Snow Level Rising to 8000 feet.  40/50

Wed (10 Dec):  Chance of Rain.  Snow Level 6-8000 Feet.  40/53

Thu (11 Dec):  Chance of Showers.   Snow Level 6-8000 Feet.  38/50

ODA Meteorologist
weather at oda.state.or.us