[willamette-fcst] Forecast - Thursday, November 20th, 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, November 20th, 2008 at 9:00am.

Burn Advisory:
     Agricultural burning is allowed.  Suggested burn times are from 10:00am until 2:30pm.
     Straw stack burning is allowed, for dry stacks, from 10:00am until 2:30pm.

Weather Discussion:
     Clearing skies Wednesday evening allowed fog to reform
     across the central and southern Willamette Valley overnight.
      Easterly outflow, from the Columbia Gorge, kept fog from
     forming in the Portland area, north of about Wilsonville. 
     It was another relatively mild night, for mid-November, with
     temperatures staying mostly in the 40s.  Hillsboro was one
     of the colder spots with a minimum of 36 degrees.

     A cold front moved onshore early this morning and brought a
     brief burst of locally heavy rain and gusty south winds to
     the Willamette Valley starting just before daybreak.  The
     frontal passage produced south winds gusting to near 25 mph
     in the south valley and 30 mph in the north valley this
     morning.  Rainfall totals, by mid-morning, were generally
     around two-tenths of an inch across the valley.

     Satellite imagery showed the cloud-band associated with the
     cold front extending from the north coast through the
     northern Willamette Valley to the centra Cascades at
     mid-morning.  Doppler radar showed the heaviest rain from
     near Kelso, Washington to Salem, Oregon.  The entire frontal
     system was quickly moving northeastward.  The ODA surface
     analysis showed a fairly strong south-to-north pressure
     gradient along and west of the cold front, so it will likely
     stay a little blustery at times across western Oregon today.

     Significant cooling aloft will wipe out the low-level
     temperature inversion that has been over the valley for
     several days and will greatly improve ventilation conditions
     today.  It has also dropped to snow level from 7000 feet to
     about 4000 this morning.  ODOT cameras confirmed that rain
     had changed to snow, over the Cascade passes, by
     mid-morning, with pass temperatures in the low 30s.  A
     winter weather advisory is in effect for the Cascade passes,
     until 10 pm, for up to 7 inches of new snow.

     A cold upper-level trough will follow the cold front onshore
     this afternoon and keep showers going across western Oregon.
     There is even a slight chance of a thundershower.  High
     temperatures will struggle to climb much above 50 degrees,
     which is about normal for mid-November.  Southwesterly winds
     will be gusty near showers.  The showers should quickly
     taper off this evening, as the upper-level trough moves east
     of the region.  Clearing skies will allow areas of fog to
     form with mimimum temperatures mostly in the mid to upper 30s.

Surface Winds:
     SSW 10-15 G25 this morning, SW 10-15 G25 this afternoon.
Transport Winds:
     S 25 this morning, SW 28 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 3000 feet.  Ventilation index 84.
High Temperature:
     Salem\'s high temperature today will be near 51.
Humidities:
     Minimum relative humidity will be near 80%.
Sunrise/Sunset:
     Salem sunset tonight: 4:38pm; sunrise tomorrow: 7:18am.

Extended Outlook:
     A warm front will spread clouds across western Oregon
     Friday.  Rainfall appears as if it will stay north of
     Oregon, until a cold front approaches the coastline Friday
     evening.  The cold front will sweep across western Oregon
     early Saturday with a few showers lingering into Saturday
     night.  Rainfall from this system will generally be less
     than one-quarter of an inch across the Willamette Valley
     with the greatest totals in the northern Cascade foothills.

     A strong upper-level ridge will likely bring a return of dry
     weather Sunday and Monday with a little sunshine...mainly in
     the north valley.  Low-level temperature inversions will
     make for poor ventilation conditions and lead to valley fog
     formation.  Outflow, from the Columbia Gorge, should help
     keep widespread fog from forming in the north valley, but
     fog could become persistent in the central and south valley.

     The ridge is forecast to begin breaking down about Tuesday
     of next week with increasing southwesterly flow aloft.  A
     weak cold front may bring light rain to the valley Tuesday
     with another ridge bringing back dry weather for
     Thanksgiving.

Tomorrow (21 Nov):  Mostly Cloudy.  Rain Likely Late. Snow Level 5000 Feet.  36/50

Sat (22 Nov):  Light Rain AM...Decreasing Showers PM.  Snow Level 4-5000 Feet.  41/51

Sun (23 Nov):  Areas of Fog...Mainly Central and South Valley.  Partly Sunny.  33/51

Mon (24 Nov):  Areas of Fog...Mainly Central and South Valley.  Partly Sunny.  37/53

Tue (25 Nov):  Chance of Light Rain.  Snow Level 4-5000 Feet.  38/53

Wed (26 Nov):  Partly Sunny.  35/51

Thu (27 Nov):  Partly Sunny.  35/52

ODA Meteorologist
weather at oda.state.or.us