[willamette-fcst] Forecast - Friday, August 29th, 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, August 29th, 2008 at 9:00am.

Burn Advisory:
     Agricultural burning is not recommended.
     Preparatory burning is allowed from 12:00pm until 2:00pm with a 100 acre limit.
     Propane flaming is allowed from 12:00pm until 5:00pm.
     Stack burning is not allowed.

Weather Discussion:
     Light westerly flow aloft prevailed over Oregon this
     morning, with a flat ridge of high pressure sliding across
     the state.  The area of clouds and light rain that was over
     Western Washington most of Thursday had retreated northward
     with rainfall confined to the northern Washington Coast and
     near the Candian border across the interior of Western
     Washington.

     Skies were fair overnight in the interior of Western Oregon.
     Dew-point temperatures are quite high (low 60s) this
     morning across Western Oregon, most likely aided by
     evaporation from the early-week rainfall.  Visible satellite
     imagery and surface reports show areas of low clouds and fog
     forming across the Willamette Valley.  With daytime heating
     and a lack of onshore flow across the valley, the low clouds
     and fog should evaporate by late this morning.

     The ODA surface analysis showed a weak thermal trough of low
     pressure centered near Salem with weak pressure gradients
     over the Willamette Valley.  The coastline had very light
     onshore flow with low clouds and fog.  Infrared satellite
     imagery showed a band of clouds, associated with a cold
     front, stretching from Vancouver Island, British Columbia
     southwestward to about 300 miles off the northern Oregon Coast.

     An upper-level trough is forecast to approach the coastline
     this afternoon with the flow aloft becoming southwesterly. 
     Valley temperatures should start off warmer than on Thursday
     but get capped in the low to mid 80s by increasing onshore
     flow in the afternoon.  The increasing onshore flow should
     back the transport winds from north-northeasterly to
     northwesterly early this afternoon in the north valley and
     by late this afternoon in the south valley.   Cooling aloft
     will help mixing heights approach 5000 feet later this
     afternoon.

     An open burning opportunity is likely for the north valley
     this afternoon, and there is a good chance that transport
     winds will turn enough northwesterly, later in the
     afternoon, for open burning in the south valley.  Increasing
     low-level onshore winds may act to inhibit smoke plume
     development by late in the afternoon.  The cold front will
     likely spread light rain onto the northern Oregon Coast by
     around 5pm with a chance of light rain in the Willamette
     Valley this evening.  The cold front should pass through the
     valley by midnight with the threat of rain decreasind, but
     skies should remain mostly cloudy with strong onshore flow.

Surface Winds:
     N 5 this morning, NW 5-15 this afternoon.
Transport Winds:
     NNE 12 this morning, NW 12 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 4900 feet.  Ventilation index 59.
High Temperature:
     Salem\'s high temperature today will be near 82.
Humidities:
     Relative humidity drops to 50% by 3pm.
     Minimum relative humidity will be near 46%.
Sunrise/Sunset:
     Salem sunset tonight: 7:53pm; sunrise tomorrow: 6:33am.

Extended Outlook:
     A cold but fairly dry upper-level trough is forecast to move
     over the region Saturday and Sunday with increasing onshore
     flow cooling valley temperatures back into the 70s.  Some
     drizzle is possible...especially along the coast.  This
     system will bring a lot oc clouds to the region but not a
     lot of rainfall.  It will be unseasonably cold in the
     mountains with the snow level possibly dropping as low as
     5000 feet by Sunday morning.  There will be scattered
     showers over the mountains this weekend but only a chance of
     showers over the Willamette Valley.

     The trough is forecast to push east of the region Monday
     with the flow aloft turning northwesterly That will begin to
     clear skies and warm temperatures, with transport winds
     turning northerly.  A weak weather system, in the
     northwesterly flow aloft, could create a burning opportunity
     Tuesday...especially for the north valley.  There is only a
     slight chance of showers for the north valley associated
     with this system.

     An upper-level ridge of high pressure is forecast to begin
     building over the region Wednesday and will likely last for
     the remainder of next week.  Transport winds may turn
     strongly offshore by Thursday, which would allow for the
     burning of remaining fields on the west side of the valley.

Sat (30 Aug):  Mostly Cloudy and Much Cooler.  Chance of Drizzle or Light Showers.  55/69

Sun (31 Aug):  Mostly Cloudy and Cool.  Chance of Drizzle or Light Showers.  46/69

Mon (01 Sep):  Becoming Mostly Sunny.  A Little Warmer after a Cool Start.  45/74

Tue (02 Sep):  Partly Sunny.  Slight Chance of a Light Shower North.  49/75

Wed (03 Sep):  Sunny.  North Winds.  50/78

Thu (04 Sep):  Sunny and Warmer.  NE Winds.  50/80

Fri (05 Sep):  Sunny. NE Winds.  51/82

ODA Meteorologist
weather at oda.state.or.us