[willamette-fcst] Silverton Hills Forecast - Tue, August 3 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE AUG 3 2010

BURN ADVISORY:

     Agricultural burning burning is not recommended.
     Prep burning is not allowed.
     Propane flaming is allowed from 2:00pm to 5:00pm.

WEATHER DISCUSSION:

A broad upper-level trough remains over the Pacific Northwest this morning with smaller scale disturbances rotating through it.  One system moved across western Oregon Monday morning and brought scattered thunderstorms to eastern Oregon Monday afternoon.  In its wake, weak ridging quickly stabilized the atmosphere across western Oregon Monday evening.

Onshore flow increased overnight with visible satellite imagery showing marine low clouds penetrating into most of the Willamette Valley this morning.  The Salem sounding this morning was similar to Monday morning with a mixing cap at about 4800 feet.  The low-level moisture is a bit deeper this morning, but the low clouds should give way to sunshine by midday.

Another weak system is over Vancouver Island, British Columbia this morning and is forecast to drop southward into western Washington later this afternoon.  It may produce a few showers across western Washington but not likely as far south as western Oregon.  Increasing westerly flow aloft will act to increase the onshore flow this afternoon, which may create an open burning opportunity.

TODAY'S FORECAST:

Morning clouds, then sunny and warm.  After reaching 85 degrees on Monday, Salem's high temperature today will be near 84 degrees.  The mixing height will not climb to 3000 feet until about 2 p.m. and likely top out at around 4800 feet around 5 p.m.  A cooling sea breeze will drop the mixing height to 1500 feet by shortly before sunset.  Surface and transport winds will be NW 3-6 this morning and increase to NW 5-12 this afternoon. Relative humidity will drop to 50% by 1 p.m. and to near 36% by late this afternoon.  The ventilation index will climb to about 38 late this afternoon.

Silverton area sunset tonight: 8:30 pm

THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 69       78      84      76
     Relative Humidity:           59%      43%     36%     45%
     Surface Wind Direction:     330      310     300     300
     Surface Wind Speed:           3        5       6       7
     Transport Wind Direction:   300      310     300     300
     Transport Wind Speed:         3        5       8      12
     Estimated Mixing Height:   2200     3000    4800    1500
     Ventilation Index:            7       15      38      18

EXTENDED DISCUSSION:

Little change to the overall weather pattern is expected through Friday, with a weak upper-level trough remaining over the Pacific Northwest.  Varying degrees of onshore flow will maintain at least a chance of open burning each afternoon.  The onshore flow may weaken a bit on Wednesday, and then strengthen again late Thursday and Friday, which could lead to marginal open burning opportunities.

EXTENDED FORECAST:

Wednesday: Morning clouds, then mostly sunny. Wind: NNW 5-12 mph.  55/84
Thursday: Morning clouds, then mostly sunny. Wind: NW 5-15 mph.  56/83
Friday: Morning clouds, then sunny.  55/82

Notes:

     1.  Mixing height, as used here, is the lowest height at which the
         potential temperature exceeds the potential temperature at the
         surface.
         As a practical matter it is the approximate height to which a
         smoke plume will rise assuming good ignition, dry fuels, and
         winds less than about 15mph.
     2.  Transport winds are a layer average through the mixing height,
         weighted slightly toward the winds at the top of the layer.
     3.  Ventilation Index is the height of the mixing layer times
         the transport wind speed divided by 1000.
     4.  Surface wind direction is the general expected wind direction.
         At a specific point surface winds are highly dependent on local
         terrain conditions.

This forecast is provided under an agreement between the Oregon Department of
Agriculture and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.

Pete Parsons
ODF Meteorologist
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