[willamette-fcst] Forecast - Thursday, December 10th, 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: 
     Thursday, December 10th, 2009 at 12:00pm.

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

Weather Discussion:
     A very cold and dry air mass remains in place over Washington and Oregon today.
     Satellite imagery and surface reports show mostly clear skies over both states.
     Light winds and clear skies allowed temperatures to locally drop into the single
     digits across the Willamette Valley early this morning. Eugene and Hillsboro both
     dropped to 8 degrees (after recording minimums of 7 degrees Wednesday morning),
     Salem and Corvallis both bottomed out at 10 degrees, for the second consecutive
     morning, and Portland hit 13 degrees (Wednesday\'s low in Portland was 12 degrees).

     Even the southwestern valleys dropped well down into the teens.  Medford recorded a
     minimum this morning of just 12 degrees and Roseburg dropped to 14.  The Arctic chill
     extended to the beaches.  Astoria had a morning minimum 20 degrees (Wednesday\'s low
     was 15 degrees), North Bend dropped to 23 degrees (Wednesday\'s low was 19 degrees).
     As expected, the coldest temperatures in the state were east of the Cascades.
     Many location were below zero this morning.  Redmond dropped to -10 degrees...the
     third straight day with a double-digit below zero minimum.  Pendleton also dipped
     below zero with a minimum of -1.

     Sunny skies had helped warm midday temperatures into the mid to upper 20s across the
     interior valleys of western Oregon, with readings in the 30s along the coast.  As a sign
     of things to come, Brookings, on the southern Oregon Coast, had warmed into the mid 40s.
     Midday temperatures were only in the teens and low 20s east of the Cascades...including
     in the Columbia Gorge.  The ODA surface analysis showed high pressure centered near the
     eastern end of the Columbia Gorge, with offshore flow continuing across western Oregon.
     
     The very cold northerly jet stream, that brought the Arctic air into the region, is
     slowly shifting east...allowing warmer air to move in aloft over the region.  The Salem
     sounding from early this morning showed several degrees of warming, between 2000 feet
     and 16,000 feet, but the entire column of air was still below freezing.  A strong
     low-level temperature inversion is making for poor ventilation conditions.  

     Another full day of sunshine is expected today, but the shortened day-length and low
     sun-angle will not generate enough surface warming to break the low-level temperature
     inversions.  Valley highs, once again, will struggle to get to the freezing mark.  Winds
     will remain light from the north-northeast, except for easterly gusts to around 30
     mph near the western end of the Columbia Gorge...making for very cold windchill values.

Surface Winds:
     NNE 3-8 this afternoon.
Transport Winds:
     NNE 6 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 1700 feet.  Ventilation index 10.
High Temperature:
     Salem\'s high temperature today will be near 31.
Humidities:
     Minimum relative humidity will be near 40%.
Sunrise/Sunset:
     Salem sunset tonight: 4:31pm; sunrise tomorrow: 7:40am.

Extended Outlook:
     Skies should remain mostly clear overnight across the Willamette Valley, making for
     very cold temperatures.  However, some changes will begin to show up on Friday.  The
     flow aloft over Oregon will turn westerly, with a weather system undercutting the
     offshore ridge and moving into California. Clouds will increase, from south to
     north, across Oregon during the day.  With surface low-pressure forecast to approach
     the northern California Coast, offshore gradients will continue to feed cold low-level
     air out of the Columbia Gorge and into the Willamette Valley.  Meanwhile, the air mass
     above 2000 feet will continue to warm and should climb above freezing...especially over
     the central and southern Willamette Valley.  That will set the stage for a wintry mix
     of precipitation later Friday through Saturday...

     Computer models differ considerably on both their timing and intensity of the precipitation
     across western Oregon.  Some begin the wintry mix, in the southern Willamette Valley, as
     early as late Friday afternoon...pushing into the north valley Friday night.  Others hold
     off on the precipitation until Friday night south and late-morning north.  Forecast
     precipitation amounts vary from around one-tenth of an inch (water equivalent) to around
     four-tenths of an inch.  Even small amounts of snow, sleet, and freezing rain can make for
     very hazardous travel conditions.

     Precipitation is expected to taper off Sunday, with the low-level flow turning weakly
     onshore across most of the valley.  That should help most of the Willamette Valley
     transition to just plain rain.  However, there could still be enough cold air coming out
     of the Columbia Gorge to keep wintry precipitation in the immediate Portland area. Another
     weather system is forecast to come onshore late Monday.  There could still be enough
     cold air remaining in the Columbia Gorge, and the immediate Portland area, for a wintry
     mix of precipitation, but the rest of the Willamette Valley should get just rain.  Even
     the Gorge should get scoured out by Tuesday, with warmer Pacific air moving back into
     the region.

Tomorrow (11 Dec):  Mostly Sunny and Cold Early.  Increasing Clouds.  15/34

Sat (12 Dec):  Snow, Sleet, or Freezing Rain...Likely Changing to Rain South.  28/35

Sun (13 Dec):  Showers...Possible Wintry Mix Near the Gorge.  32/42

Mon (14 Dec):  Mostly Cloudy.  Chance of Rain...Possible Freezing Rain North.  32/40

Tue (15 Dec):  Rain.  Possibly Starting as Freezing Rain North.  32/47

Wed (16 Dec):  Mostly Cloudy and Warmer.  Rain Likely Late.  37/49

Thu (17 Dec):  Rain Turning to Showers.  42/49

ODA Meteorologist
weather at oda.state.or.us