[willamette-fcst] Forecast - Friday, August 15th, 2008

Fri Aug 15 08:03:11 PDT 2008

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 15th, 2008 at 9:00am.

Burn Advisory:

  ...State Fire Marshal Conditions will likely be met this afternoon...

     Agricultural burning is not recommended.
     Preparatory burning is not allowed.
     Propane flaming is not allowed.
     Stack burning is not allowed.

Weather Discussion:
     A strong upper-level ridge of high pressure brought record
     high temperatures to the interior of Western Oregon
     Thursday.  State Fire Marshal conditions were met by 3pm
     Thursday over most of the valley due to high temperatures
     and low humidities.  Much of the Willamette Valley reached
     100 degrees or better with Portland, Hillsboro, and Eugene
     hitting daily records.  Medford also tied their daily record
     high of 108 degrees, set in 1933.  Salem hit 100 degrees
     Thursday, but that was not a record for the date.  Corvallis
     was the valley cool spot with a maximum of 99 degrees. 

     The following records were tied or broken Thursday in the
     Willamette Valley:

     Location                High       Pervious Record

     Portland (Downtown)     101         97 set in 1942

     Portland (Airport)      102         97 set in 1967

     Hillsboro (Airport)     101        100 set in 1933

     Eugene (Airport)        100 (tied) 100 set in 2002

     The upper-level ridge had shifted over Eastern Washington
     and Eastern Oregon this morning with increasing southerly
     flow aloft feeding middle and high clouds over the western
     half of both states, along with the return of California
     wildfire smoke to much of Western Oregon.  A weak
     upper-level disturbance was rotaing northward offshore and
     Doppler radar was showing shower activity just off the
     coast.  The showers are not expected to make it inland but a
     few sprinkles could clip the coastal region today.

     The ODA surface analysis showed a strong thermal trough over
     the interior valleys of Western Oregon with low pressure
     centers near Portland and Medford.  A high pressure center
     was near North Bend, along the southern coast.  A weak
     influx of marine air was making it though the southern
     coastal gaps into Corvallis and Eugene, where temperatures
     had cooled into the low 60s.  The north valley did not see
     any marine air overnight with minimums in the mid to upper 60s.

     Satellite imagery showed mostly clear skies from the
     Cascades eastward across Oregon and Idaho.  To the west,
     coastal low clouds and fog blanketed the coastline this
     morning, in reponse to onshore flow induced by the thermal
     trough over western valleys.  Some low clouds were
     penetrating into the coastal mountain gaps but none had made
     it into the Willamette Valley.

     The Salem sounding this morning showed very warm air aloft
     with easterly winds from near the surface up to 10,000 feet.
     A strong subsidence inversion had been created by the
     upper-level ridge and will cap mixing heights near 2000 feet
     today.  In addition to poor ventilation conditions, State
     Fire Marshal Conditions will likely be met again this
     afternoon over much of the valley, especially north, due to
     high temperatures and low humidities.

     The upper-level ridge will hold fast today with record warm
     temperatures expected in the western valleys again this
     afternoon...especially from Salem north.  The south valley
     will likely be a few degrees cooler due to the weak influx
     of marine air this morning.  Low clouds and fog will keep
     the immediate coastline cool today with highs there only in
     the 60s.  The onshore flow may turn enough northerly along
     the extreme north coast to clear skies there this
     afternoon...allowing temperatures to climb into the 70s.

     Light southerly flow aloft will continue to feed middle and
     high clouds over the region tonight with temperatures, once
     again, not dipping below 60 across most of the valley.

Surface Winds:
     Var 5-10 north...SW 5-15 south this morning, N 5-10 north...NW 5-15 south this afternoon.
Transport Winds:
     NE 10 north...W 10 south this morning, N 10 north...NW10 south this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 2300 feet.  Ventilation index 23.
High Temperature:
     Salem\'s high temperature today will be near 100.
Humidities:
     Relative humidity drops to 50% by 11am.
     Minimum relative humidity will be near 24%.
Sunrise/Sunset:
     Salem sunset tonight: 8:17pm; sunrise tomorrow: 6:16am.

Extended Outlook:
     An upper-level disturbance is forecast to move close enough
     tothe coastline Saturday to bring a threat of showers or
     thundershowers to the western half of Oregon...especially
     the Southern Cascades.  It will be another scorcher will
     most valley highs over the century mark for the third
     straight day.  There is a pretty good chance of showers or
     thundershowers rolling off the Cascades and over the
     Willamette Valley Saturday evening and/or night.

     The upper-level ridge is forecast to shift to over Western
     Montana Sunday.  Increasing southerly flow aloft will spread
     more mid and high-level moisture across Oregon with the
     chance of showers or thundershowers spreading eastward
     across the state.  The thermal trough is forecast to finally
     shift east of the Cascades which will begin a cooling trend
     for Western Oregon.

     The flow aloft will turn southwesterly by Monday afternoon,
     which will stabilize the air mass and push the thundershower
     threat east of the region.  It will also increase the
     onshore flow and bring a significant cool-down to Western
     Oregon.  A weak cold front may be strong enough to produce
     some light showers along the coast and the Willamette Valley.

     A stronger system is still forecast to move onshore Tuesday
     afternoon or night for another chance of light rain across
     Western Oregon.  Depending on the strength and timing of
     this system, it may create a burning upportunity ahead of
     it.  The valley will have a good chance of showers Wednesday
     with the passing of a stronger upper-level trough.  The flow
     aloft is forecast to turn northwesterly and begin drying out Thursday.

Tomorrow (16 Aug):  Sunny and Hot.  Chance of T-Storms Late.  64/102

Sun (17 Aug):  Mostly Cloudy.  Chance of T-Storms.  62/92

Mon (18 Aug):  Chance of AM T-Storms...Cooler with a Chance of Showers.  58/74

Tue (19 Aug):  Partly Cloudy.  Increasing Clouds Late with a Chance of Light Rain.  53/80

Wed (20 Aug):  Chance of Showers and Cooler.  55/74

Thu (21 Aug):  Mostly Cloudy.  Slight Chance of Showers.  52/74

Fri (22 Aug):  Morning Clouds...Afternoon Clearing.  50/78

ODA Meteorologist
weather at oda.state.or.us