[willamette-fcst] Forecast - Friday, December 19th, 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.

NOON UPDATE

   The ODA meteorologist is scheduled for vacation until Monday 12/29/2008.
   That may change depending on weather conditions.  You can access National
   weather Service forecasts 24 hours a day at: http://www.wrh.noaa.gov/pqr/

Issued: 
     Friday, December 19th, 2008 at 12:00pm.

Burn Advisory:
     Agricultural burning is allowed.  Suggested burn times are from now until 2:30pm.
     Straw stack burning is allowed from now until 2:30pm.

Weather Discussion:
     A cold upper-level trough maintained snow shower activity
     this morning across western Oregon with some areas getting
     minor accumulations, even on the valley floor.  The
     late-morning surface analysis showed a weak low-pressure
     center moving south, near Astoria, with a nearly stationary
     Arctic cold front extending from it eastward along the
     Washington/Oregon border.  To the north of the Arctic
     boundary winds are north-northeasterly across most of
     Washington.  Temperatures are in the low to mid 20s across
     western Washington and single digits in eastern Washington. 
     Most of Oregon remained just south of the Arctic cold front
     late this morning with light southwesterly winds helping
     temperatures stay mostly just above freezing on the valley floors.

     The colder Arctic air was able to seep into Astoria and
     Scappoose, west of the Cascades, where temperatures are
     below freezing with light snow.  Arctic air had also
     penetrated into the Columbia Basin, of northeastern Oregon,
     and into the eastern Columbia Gorge, where temperatures were
     in the teens to low 20s.  The morning sounding over Salem
     showed continued cooling aloft, since Thursday, with the
     freezing level near the surface.

     Satellite and Radar imaagery showed snow showers continuing
     to rotate onshore with upslope conditions enhancing the snow
     showers over the coast range and in the Cascades.  A strong
     north-northwesterly jet stream will actively keep snow
     showers falling today, from the coast to the Cascades, with
     local valley floor accumulations of 1-2 inches possible, and
     3-6 inches in the coast range and Cascades foothills.

     High temperatures will not make it above the mid 30s today
     and will likely hold steady or fall this afternoon in the
     extreme north valley, as the Arctic air slowly advances
     south into the Portland area.  Snow showers will slowly
     taper off this evening and tonight, as colder and drier
     Arctic air pushes further south into the valley.  It may
     take until early Saturday morning for the colder and drier
     Arctic air to make it all the way south to Eugene.  Skies
     will only partially clear tonight, but valley temperatures
     will fall well below freezing.  Wet road surfaces will
     freeze...making for hazardous driving conditions.

Surface Winds:
     SW 5-10 this afternoon.
Transport Winds:
     W 10 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 2100 feet.  Ventilation index 25.
High Temperature:
     Salem\'s high temperature today will be near 36.
Humidities:
     Minimum relative humidity will be near 67%.
Sunrise/Sunset:
     Salem sunset tonight: 4:33pm; sunrise tomorrow: 7:47am.

Extended Outlook:
     A moist and warmer Pacific storm will move onshore, into the
     cold dome of air over western Oregon, Saturday.  Falling
     pressures offshore will increase the cold easterly outflow,
     from the Columbia Gorge, into the north valley. 
     Temperatures will stay below freezing across the valley with
     snow developing by midday...possibly changing to sleet and
     then to freezing rain in the central and south valley Saturday
     evening, as the intensity of the precipitation increases.

     As the air aloft continues to warm, snow levels will jump to
     near 6000 feet.  Precipitation in the western valleys and
     the mountain passes should turn to sleet, and eventually to
     freezing rain, where cold low-level persists.  The timing of
     that transition is difficult to forecast, but it will most
     likely hold off until Saturday night, as far north as
     Portland.  Sub-freezing air will continue to pour out of the
     Columbia Gorge, into the Portland area.  For that reason,
     snow and/or ice accumulations will be greatest in the
     Portland area.  Valley snowfall amounts will likely have a
     wide range, with as little as one inch in the south valley
     to as much as 6 inches near the Columbia Gorge.  Some
     locations may also get a significant ice accumulation, due
     to freezing rain.

     By Sunday morning, it is likely that the south valley will
     have changed to plain rain with temperatures well above
     freezing.  The north valley, however, may still have enough
     cold low-level air for sleet and freezing rain...especially
     near the Columbia Gorge.  Colder air aloft will move over
     the region Sunday afternoon and night, with snow levels
     dropping to below 3000 feet.  That should end the sleet and
     freezing rain across western Oregon, with precipitation
     becoming either rain or snow...depending on elevation and
     exposure to cold Gorge easterly winds.  Precipitation will
     likely remain as rain in the south valley but may change to
     snow in the Portland area...especially near the Columbia
     Gorge.  Heavy snow is likely in the mountains, with the snow
     level dropping to 2000 feet Sunday night.

     The forecast beyond this weekend is very dependent upon how
     much cold air remains in the Columbia basin...to be pulled
     westward through the Columbia Gorge ahead of approaching
     storms.  If that air is cold enough, then there is still a
     chance for low-elevation snowfall in the Willamette
     Valley...especially in the Portland area.  A cold upper-level
     trough will move directly over Oregon Monday with the snow
     level dropping to about 1500 feet.  Showers will taper off by
     Monday night and Tuesday.  Cold outflow from the Columbia Gorge
     my keep the snow level locally near the valley floor in the
     Portland area, but central and south valley temperatures will
     likely be too warm for valley floor snow.

     Another cold system is forecast to drop into the region,
     from the Gulf of Alaska, Christmas Eve.  Snow levels will be
     around 1000 feet, but may drop to the valley floors, if there
     is still enough low-level cold air available, via the Columbia
     Gorge.  Showers will taper off late on Christmas, with snow
     levels remaining very low.  A white Christmas is possible for
     much of the state, especially for areas above 1000 feet.

Sat (20 Dec):  Snow Developing...Possibly Changing to Sleet and Then Freezing Rain.  25/30

Sun (21 Dec):  Rain South...Wintry Mix North. Rain or Snow North...Rain South PM.  30/37

Mon (22 Dec):  Rain or Snow Showers North. Rain Showers South. Snow Level 1500 Feet. 32/38

Tue (23 Dec):  Chance of Rain or Snow Showers.  Snow Level near 1000 Feet. 25/35

Wed (24 Dec):  Chance of Rain or Snow. Snow Level Near or below 1000 Feet. 31/37

Thu (25 Dec):  Chance of Rain or Snow Showers.  Snow Level Near or Below 1000 Feet.  32/38

Fri (26 Dec):  Partly Cloudy and Cool.  30/39

ODA Meteorologist
weather at oda.state.or.us