[willamette-fcst] Silverton Hills Forecast - Mon, August 2 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT MON AUG 2 2010

BURN ADVISORY:

     Agricultural burning burning is not recommended.
     Prep burning is allowed from 12:00pm to 2:00pm with a 50 acre limit.
     Propane flaming is allowed from 12:00pm to 5:00pm.

WEATHER DISCUSSION:

A weak upper-level trough remains over the Pacific Northwest this morning. Visible satellite imagery shows a band of what appears to be elevated wild fire smoke extending from eastern and central Washington back across northwestern Oregon, in association with an upper-level disturbance moving through the region.  Marine low clouds were blanketing the coast, but inland penetration was limited to the extreme northern Willamette Valley, near Portland.  There were also patches of light fog in the extreme south valley, near Eugene.  Skies were mostly sunny over the remainder of the state.

The Salem sounding showed several degrees of warming aloft, since Sunday morning.  The marine air was very shallow with quite dry conditions above 2500 feet.  The low-level temperature profile is stable, with a temperature inversion at about 4700 feet that should cap maximum mixing heights this afternoon.

The weak upper-level disturbance that is over northwestern Oregon this morning is forecast to continue eastward into central Oregon this afternoon. Northwesterly flow aloft should promote an afternoon sea breeze.  Transport winds and pressure gradients will need to be monitored for the possibility of limited open burning later this afternoon.

TODAY'S FORECAST:

Smoke and haze aloft this morning, otherwise mostly sunny.  After reaching 80 degrees on Sunday, Salem's high temperature today will be near 84 degrees.  The mixing height will climb to 3000 feet about noon and likely top out at around 4700 feet later this afternoon.  A cooling sea breeze will drop the mixing height to 1500 feet by shortly before sunset.  Surface and transport winds will be N 3-6 this morning and back to NW 6-10 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 36 late this afternoon.

Silverton area sunset tonight: 8:32 pm

THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 71       79      84      75
     Relative Humidity:           57%      44%     36%     50%
     Surface Wind Direction:     350      320     310     280
     Surface Wind Speed:           4        5       7       7
     Transport Wind Direction:   360      310     330     290
     Transport Wind Speed:         5        6       8      15
     Estimated Mixing Height:   2400     3800    4700    1500
     Ventilation Index:           12       23      38      22

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, as the upper-level flow turns southwesterly, and then strengthen again late Thursday and Friday.  The best chances for open burning opportunities appear to be Tuesday and Thursday afternoons, but that is certainly subject to change.

EXTENDED FORECAST:

Tuesday: Patchy morning clouds, then mostly sunny. Wind: NW 5-12 mph.  55/84
Wednesday: Mostly Sunny. Wind: NNW 5-12 mph.  55/86
Thursday: Mostly sunny. Wind: NW 5-15 mph.  56/84
Friday: Morning clouds, then mostly sunny.  55/79

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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