[willamette-fcst] Forecast - Thursday, May 15th, 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, May 15th, 2008 at 9:00am.

Burn Advisory:
     Agricultural burning is allowed.  Suggested burn times are from 12:00pm until 6:30pm.
     Stack burning is not allowed.

Weather Discussion:
     A building upper-level ridge of high pressure, just off the
     west coast, turned the flow aloft from westerly to northerly
     over Washington and Oregon on Wednesday.  Skies slowly
     cleared, from west to east, across Oregon Wednesday
     afternoon and evening.  Very warm air aloft acted like a
     lid...trapping low-level cooler and moist air near the
     surface in the Willamette Valley overnight.  That lead to
     the formation of extensive low clouds over the valley early
     this morning.  The Salem sounding this morning showed a
     strong temperature inversion from near the surface up to
     about 3000 feet with relatively cool and very moist air
     trapped below 2000 feet.  Winds aloft were northeasterly, in
     the 15-20 mph range, from just above the surface to more
     than 10,000 feet.

     Visible satellite imagery this morning showed extensive low
     clouds covering much of Western Washington and the
     Willamette Valley in Oregon, with generally clear skies over
     the remainder of both states.  Areas of early morning fog
     and low clouds had even cleared from the Oregon Coast.  It
     was a mild night across the region. Morning minimums were in
     the low to mid 50s across Western Oregon with 40s and 50s
     east of the Cascades.  It was a balmy night in the mountains
     with Timberline Lodge recording a low temperature this
     morning of 49 degrees.

     Daytime heating will start the process of mixing the air
     mass over the Willamette Valley with the much warmer and dry
     air mass aloft.  That should act to evaporting the low
     clouds within a couple of hours with skies becoming sunny by
     midday.  A building surface thermal trough, just off the
     Southern Oregon Coastline, will combine with high pressure
     over Washington and NE Oregon to turn the surface winds
     slightly offshore this afternoon across Western Oregon.  The
     offshore flow will combine with ample sunshine and very warm
     air aloft to send Western Oregon temperatures into the 80s
     this afternoon...even along the beaches.  The southwestern
     valleys will likely see the mid to upper 90s this afternoon.
     Highs in the Willamette Valley could easily reach the mid
     80s (15 degrees above normal).

     Very warm air aloft will cap mixing heights at about 3000
     feet this afternoon, but that will provide enough
     ventilation for agricultural burning.  However,
     northeasterly transport winds are not favorable for
     evacuation of smoke from the valley from stack burns, so
     they are not allowed today.  It will be another mild night
     with fair skies and valley minimums in the mid to upper 50s.

Surface Winds:
     N 5-15 (G20 south valley) this morning, NNE 10-20 (G25 south valley) this afternoon.
Transport Winds:
     NE 18 this morning, NE 22 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 3000 feet.  Ventilation index 66.
High Temperature:
     Salem\'s high temperature today will be near 86.
Humidities:
     Minimum relative humidity will be near 37%.
Sunrise/Sunset:
     Salem sunset tonight: 8:35pm; sunrise tomorrow: 5:42am.

Extended Outlook:
     The upper-level ridge is forecast to continue to build over
     the Pacific Northwest Friday with the surface thermal trough
     remaining along the coastline for offshore flow across
     Western Oregon.  That will push Willamette Valley high
     temperatures to daily record values...in the low to mid 90s.
     Onshore flow will likely kick in along the southern and
     central coast Friday afternoon, as the center of the thermal
     low-pressure area moves up the coast.  

     It appears that Friday will be the warmest day west of the
     Cascades.  the upper-level ridge is forecast to shift east
     of the Cascades Saturday, along with the associated surface
     thermal trough.  That should bring cooler onshore winds into
     the Willamette Valley Saturday afternooon, in time to cap
     tempertures in the mid 80s.  That will shift the record hot
     weather into Central and Eastern Oregon Saturday afternoon. 
     Cooler ocean air begin filtering into Central and much of
     Eastern Oregon Sunday, with increasing onshore flow west of
     the Cascades cooling temperatures back into the upper 70s
     and perhaps low 80s.

     A increasing southwesterly flow aloft will eventually bring
     even cooler temperatures and a chance of rain back to
     western Oregon next week.

Tomorrow (16 May):  Sunny with Record High Temperatures.  55/94

Sat (17 May):  Sunny and Very Warm.  57/87

Sun (18 May):  Partly Cloudy and Cooler.  52/77

Mon (19 May):  Mostly Cloudy.  Chance of Rain Late.  50/70

Tue (20 May):  Chance of Rain and Cooler.  46/65

Wed (21 May):  Partly Sunny.  42/69

Thu (22 May):  Partly Sunny.  43/72

ODA Meteorologist
weather at oda.state.or.us