[willamette-fcst] Silverton Hills Forecast - Wed, August 4 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT WED AUG 4 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from noon to 6:30pm.
     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.  An upper-level disturbance is triggering thundershowers over the northern Washington Cascades this morning with a more stable west-southwesterly flow aloft over Oregon.  Little change in the upper-level weather pattern is forecast today with shower and thundershower activity possibly sagging southward to near the Washington/Oregon border this afternoon.

Onshore flow brought a fresh surge of cool marine air inland last night.  Visible satellite imagery showed marine low clouds extending from the coast to the crest of the Cascades this morning and extending southward across the entire Willamette Valley and into Douglas County.

The Salem sounding this morning showed a stable atmosphere with the moist marine layer extending from the surface up to about 3500 feet.  Morning low clouds should give way to mostly sunny skies by early afternoon.  The onshore flow is forecast to be slightly weaker today, so even though the morning clouds will burn off later, afternoon temperatures should recover to about where they were on Tuesday.  Once again, transport winds may turn just enough onshore to allow for limited open burning later this afternoon.

TODAY'S FORECAST:

Morning clouds, then sunny and warm.  After reaching 82 degrees on Tuesday, Salem's high temperature today will be near 82 degrees again today.  The mixing height will not climb to 3000 feet until about 2 p.m. and will likely top out at around 3800 feet around 5 p.m.  A cooling sea breeze will drop the mixing height to 2000 feet by shortly before sunset.  Surface and transport winds will be light NW to N this morning, then increase to NNW 5-10 this afternoon and become WNW near 10 mph this evening. Relative humidity will drop to 50% by 2 p.m. and to near 40% by late this afternoon.  The ventilation index will climb to about 30 late this afternoon.

Silverton area sunset tonight: 8:29 pm

THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 67       76      82      76
     Relative Humidity:           63%      48%     40%     48%
     Surface Wind Direction:     360      340     330     280
     Surface Wind Speed:           4        5       7       7
     Transport Wind Direction:   340      350     340     300
     Transport Wind Speed:         4        6       8      10
     Estimated Mixing Height:   2000     2800    3800    2000
     Ventilation Index:            8       17      30      20

EXTENDED DISCUSSION:

Little change to the overall weather pattern is expected through Friday, with a weak upper-level trough remaining over the Pacific Northwest.  Weak onshore flow will maintain the pattern of morning clouds with afternoon sunshine.  Ventilation conditions may allow for limited afternoon open burning each day.

Even cooler temperatures are forecast for this weekend, with a weak cold front likely making it onshore late Saturday and early Sunday.  Moisture with this system is very limited, but it may bring some drizzle to the coast and possibly the Willamette Valley Sunday morning.

EXTENDED FORECAST:

Thursday: Morning clouds, then partly cloudy. Wind: NW 5-15 mph.  55/83
Friday: Morning clouds, then partly cloudy.  Wind: NW 5-15 mph.  55/82
Saturday: Morning clouds, then partly cloudy. 55/77
Sunday: Chance of morning drizzle.  Partly sunny in the afternoon. 54/75

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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