[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.

Issued: 
     Friday, December 19th, 2008 at 9:00am.

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
     overnight across most of northwest Oregon with some areas
     getting minor accumulations, even on the valley floor.  The
     surface analysis showed a weak low-pressure center moving
     south, near Astoria, with an 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 and temperatures drop into the
     teens and low 20s across western Washington.  Most of Oregon
     remained just south of the Arctic cold front at mid-morning
     with weak onshore flow keeping temperatures near freezing
     west of the Cascades.

     The colder Arctic air was able to seep into Astoria, on the
     extreme north coast, where the temperature had dropped to 28
     degrees 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.  The morning sounding over Salem showed continued
     cooling aloft, since Thursday, with the freezing level at
     the surface.  Weak onshore flow was indicated by light
     west-southwesterly transport winds, which will turn more
     westerly this afternoon.  Cold air aloft and westerly
     transport winds will provide good ventilation for stack
     burning today.

     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 fall this afternoon in the north valley, as
     the Arctic air advances south.  Snow showers will slowly
     taper off this evening and tonight, as colder and drier
     Arctic air pushes south into the region.  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 with valley temperatures falling
     into the upper teens to mid 20s.

Surface Winds:
     SW 5-10 this morning, SW 5-10 this afternoon.
Transport Winds:
     W 12 this morning, 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 ride move onshore,
     into the cold dome of air over westner 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, precipitation should
     also turn to sleet, and eventually to freezing rain, in the
     north valley.  The timing of that transition is difficult to
     forecast, but it will most likely hold off until Saturday
     night, as far north as Portland.  Easterly Gorge winds will
     help to keep the low-level cold air in place longer in the
     north valley.  Snow accumulations will be greatest in the
     Portland area, where the precipitation will remain in the
     form of snow the longest.  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 with precipitation becoming either rain or
     snow...depending on the 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 beynd this weekend is very dependent upon how
     much cold air remains in the Columiba basin...to be pulled
     westward through the Columbia Gorge ahead of approaching
     storms.  That makes all the difference in what form future
     precipitatin will take...especially in the north valley.  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 in now 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