[willamette-fcst] : Silverton Hills Forecast - Thu, August 19 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT THU AUG 19, 2010

BURN ADVISORY:

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

WEATHER DISCUSSION:

A very weak upper-level impulse was moving onto the northern Oregon Coast late this morning and helping to maintain the onshore flow across western Oregon.  Late-morning satellite imagery showing marine low clouds persisting across most areas west of the Cascade crest and north of Douglas County.  Some drizzle even fell this morning along the north coast.  Skies were sunny, late this morning, over extreme southwestern Oregon and all areas east of the Cascades.

Gradient-stacking has increased, since earlier this morning, with 1.0 mb onshore from Newport to Salem and 4.1 mb onshore from Salem to Redmond.  The western valleys may not get warm enough this afternoon to overcome that much gradient-stacking, so chances for open burning today have lowered since the morning forecast.

The low cloud deck is being more stubborn today than yesterday but beginning to show signs of breaking up over the north coast range.  Look for skies to clear about an hour later this afternoon than they on Wednesday.   Temperatures are running a couple of degrees cooler than 24 hours ago, so the forecast high has been lowered accordingly.

The low clouds should give way to sunshine by around 2 p.m. Ventilation conditions will be marginal for open burning this afternoon.  Even if the gradient-stacking is overcome, which is in doubt, transport winds are forecast to have a significant northerly component.

TODAY'S FORECAST:

Morning clouds, then sunny and mild.  After reaching 79 degrees on Wednesday, Salem's high temperature today will be near 78 degrees.  The mixing height will climb to near 4000 feet around 5 p.m.  An evening sea breeze will drop the mixing height to near 1500 feet shortly before sunset.  Surface and transport winds will be NNW 5-10 mph today. Relative humidity will drop to 50% around 2 p.m. and to near 40% around 5 p.m.  The ventilation index will climb to 32 this afternoon.

Silverton area sunset tonight: 8:06 pm

THREE-HOURLY DATA:

                                  2pm     5pm     8pm
     Temperature:                  70      77      69
     Relative Humidity:            51%     40%     53%
     Surface Wind Direction:      320     330     300
     Surface Wind Speed:            6       8       9
     Transport Wind Direction:    340     340     320
     Transport Wind Speed:          6       8      10
     Estimated Mixing Height:    3000    4000    1500
     Ventilation Index:            18      32      15

EXTENDED DISCUSSION (no changes from the morning forecast):

The flow aloft is forecast to turn slightly more southwesterly Friday with little change in the surface pattern.  Morning clouds should give way to afternoon sunshine with continued north-northwesterly transport winds.  The onshore flow will strengthen over the weekend, as an upper-level trough moves inland.  Temperatures will fall to below normal.  The marine layer will deepen significantly Saturday night with a chance of drizzle or light showers on Sunday.

A flat upper-level ridge is forecast to build over the region next Monday with transport winds turning northerly and mostly sunny skies helping temperatures recover to near normal.  Increasing westerly flow aloft may provide open burning opportunities next Tuesday and/or Wednesday, as transport winds turn back onshore.

EXTENDED FORECAST:

     Friday: Mostly sunny, with a high near 78. Wind: N 5-10 mph.
     Friday Night: Mostly cloudy.  Low near 49. Wind: NW 5-10 mph.
     Saturday: Partly sunny, with a high near 75. Wind: NW 5-15 mph.
     Sunday: Mostly cloudy.  Chance of light showers.  High near 72.
     Monday: Mostly sunny, with a high near 83.
     Tuesday: Mostly sunny, with a high near 83.
     Wednesday: Mostly sunny.  Increasing onshore flow.  High near 81.

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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