[willamette-fcst] Forecast - Sunday, September 21st, 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: 
     Sunday, September 21st, 2008 at 12:00pm.

Burn Advisory:
     Agricultural burning is not recommended.
     Preparatory burning is not allowed.
     Propane flaming is not allowed.
     Stack burning is not allowed.

Weather Discussion:
     Westerly flow aloft began clearing the wildfire smoke from
     Western Oregon Saturday with some very light showers falling
     late Saturday through early this morning. Rainfall amounts
     were generally less than .05 inches across the Willamette
     Valley.  The greatest amounts were in the north valley near
     the cascade Foothills.  There were a couple of reports of
     just less than one-tenth of an inch near Stayton. 

     Westerly flow aloft will continue today with a cool
     upper-level trough over Washington and Oregon.  Doppler
     radar showed another area of light showers moving east of
     the I5 corridor in Northern Marion and Southwestern
     Clackamas Counties.  There were also scattered very light
     showers showing up in Eastern Linn County.  There appeared
     to be a significant break in the shower activity from I5
     westward to offshore, with spotty breaks in the cloud-cover
     showing up on satellite imagery and surface reports.  That
     may allow time for fields to dry early this afternoon and
     for surface temperatures to warm into the mid 60s.

     The Salem sounding this morning showed cooling below 4000
     feet, compared wtih Saturday, so mixing heights should climb
     above 3000 feet, when surface temperatures reach about 62
     degrees. Transport winds were southwesterly this morning and
     are forecast to slowly veer to westerly early this
     afternoon...eventually becoming northwesterly this evening. 
     The ODA surface analysis showed high pressure centered over
     the Southern Oregon Coast with onshore flow across most of
     Oregon...although weaker than on Saturday.  There is some
     gradient-stacking that would need to be compensated for,
     with daytime heating, to insure good ventilation conditions
     for burning.

     If we do get a break from the shower activity, and some
     surface warming, there may be a brief burning opportunity,
     for dry fields, in the mid-afternoon period.  The best
     chance for that will be in the south valley, where only a
     couple hundredths of an inch of rain fell.  Daytime heating
     may promote an increase in the shower activity late this
     afternoon, and that will need to be closely monitored.  Open
     burning is not advised near showers, due upredictable
     transport winds and potential down-mixing of smoke.  There
     is a chance of showers through Monday morning, mainly in the
     north valley, as a weak upper-level disturbance moves across
     Western Washington.

Surface Winds:
     S 5-10 at midday...becoming W 5-10, then NW 5-15 late this afternoon.
Transport Winds:
     SW 10 at midday...becoming W 10 this afternoon and NW 10 this evening.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 3500 feet.  Ventilation index 35.
High Temperature:
     Salem\'s high temperature today will be near 65.
Humidities:
     Minimum relative humidity will be near 54%.
Sunrise/Sunset:
     Salem sunset tonight: 7:10pm; sunrise tomorrow: 7:00am.

Extended Outlook:
     High pressure is forecast to build over the region Monday
     afternoon, with the flow aloft becoming northwesterly and
     the surface flow turning more northerly.  Transport winds
     may stay northwesterly, in the north valley, but fields may
     be too damp for any burning.  A trasitory ridge of high
     pressure is forecast for Tuesday wtih the next weather
     system being delayed until Wednesday afternoon.  There may
     be a burning opportunity ahead of that system Tuesday and/or
     Wednesday.

     The valley could pick up one-tenth of an inch of rain
     Wednesday afternoon through Thursday morning...especially in
     the north.  Westerly flow aloft is forecast to continue
     Thursday with onshore flow at the surface keeping lots of
     marine clouds over the valley, but rainfall should taper off
     after the early morning.

     The long-range models differ on the strength and timing of a
     weak system that may move onshore Friday.  That could be
     another burning opportunity but is way too far out to call. 
     A ridge is now forecast to build over the region for the
     weekend, but that is getting beyond where I trust the
     models, which have been flip-flopping around like a
     fresh-caught trout.

Tomorrow (22 Sep):  Chance of Showers Early...Mainly North.  Becoming Partly Sunny.  46/65

Tue (23 Sep):  Mostly Sunny.  43/73

Wed (24 Sep):  Cloudy and Cooler.  Increasing Chance of Rain.  50/66

Thu (25 Sep):  Morning Clouds/Drizzle.  Partly Sunny in the Afternoon.  50/73

Fri (26 Sep):  Partly Sunny.  Increasing Clouds late.  47/74

Sat (27 Sep):  Becoming Partly Sunny.  48/74

Sun (28 Sep):  Partly Cloudy.  46/75

ODA Meteorologist
weather at oda.state.or.us