[willamette-fcst] Forecast - Thursday, December 18th, 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 18th, 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 warm front brought a wintry mix of precipitation to
     northwestern Oregon and southwestern Washington Wednesday. 
     The precipitation started generally as snow but eventually
     turned to rain below about 1000 feet, as brisk southerly
     winds warmed the lowest levels of the atmosphere above
     freezing.  Some of the higher locations in the north valley
     received several inches of snow, before the changeover to
     light rain showers and fog late in the day.  Western Oregon
     stayed in the warm-sector of this storm overnight with
     onshore flow keeping surface temperatures mostly above
     freezing below 1000 feet.  Snow showers continued to fall
     above 1000 feet.

     The Salem sounding this morning showed very cold air aloft
     with the freezing level measured at just 2200 feet. 
     Transport winds were southwesterly, which was keeping
     relatively warmer air near the surface.  That will make for
     good ventilation conditions today with mixing heights rising
     to near 5000 feet this afternoon.  Mid-morning valley
     temperatures were in the mid 30s and should climb no higher
     than the upper 30s this afternoon.

     The mid-morning surface analysis showed a very complx
     weather pattern over Washington and Oregon.  A low-pressure
     center was over extreme northwest Washington with a cold
     front extending southward to the northern Oregon Coast.  In
     addition, another cold front snaked across the
     Washington/Oregon border, separting Arcitc air over
     Washington from more mild Pacific air over Oregon.  The
     Arctic cold front is forecast to reamin nearly stationary
     through this afternoon, before sagging south across northern
     Oregon tonight and Friday.  The cold front along the coast
     is forecast to move inland today with frequent showers
     across western Oregon.  Snow levels will drop to near the
     valley floors this afternoon with local accumulations of wet
     snow possible.

     The air aloft is continuing to cool with time, as a cold
     upper-level trough moves onshore.  By tonight, showers will
     be mostly of snow across the entire Willamette Valley with
     additional accumultions of an inch or two possible.  At the
     same time, the cold Arcitc frontal boundary will be slowly
     moving south, into the northern Willamette Valley. 
     Temperatures will fall below freezing in valley overnight. 
     The combination of snow showers and colder air moving into
     the valley will likely make for difficult travel conditions,
     as wet roadways turn icy.


Surface Winds:
     S 10-15 this morning, S 10-15 this afternoon.
Transport Winds:
     SW 15 this morning, WSW 15 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 5000 feet.  Ventilation index 75.
High Temperature:
     Salem\'s high temperature today will be near 38.
Humidities:
     Minimum relative humidity will be near 82%.
Sunrise/Sunset:
     Salem sunset tonight: 4:32pm; sunrise tomorrow: 7:46am.

Extended Outlook:
     Snow showers will continue into early Friday, with valley
     accumulations on the order of 1-2 inches likely.  Arctic air
     will seep, from north to south, across the Willamette Valley
     Friday afternoon and night. High temperatures will not make
     it out of the 20s in the north valley with near 30 in the
     south valley.

     Clouds will increase Saturday from the next weather system
     with increasing cold easterly winds pouring out of the
     Columbia Gorge into the north valley.  Temperatures will
     stay below freezing across the valley with a chance of snow
     by Saturday evening.  Western Washington and Oregon are in
     for a wintry mix of precipitation Saturday night and Sunday
     with easterly Gorge winds helping to keep the low-level cold
     air in place, especially in the north valley.

     Some models show a warm-up early next week, while others
     bring a weak reinforcing surge of Arctic air back into the
     Willamette Valley.  It is possble that the north valley will
     stay in the cold air, near Portland, while the central and
     south valley transition to a more mild air mass.  It appears
     that warmer weather will take over by Christmas.

Tomorrow (19 Dec):  Snow Showers Ending...Becoming Partly Cloudy. Continued Cold.  18/27

Sat (20 Dec):  Increasing Clouds, Cold, and Windy.  15/25

Sun (21 Dec):  Snow and Windy...Wintry Mix South...Spreading North. 23/32

Mon (22 Dec):  Decreasing Rain or Snow Showers.  28/33

Tue (23 Dec):  Chance of Rain or Snow.  25/35

Wed (24 Dec):  Chance of Rain or Snow.  31/37

Thu (25 Dec):  Chance of Rain.  33/45

ODA Meteorologist
weather at oda.state.or.us