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

Willamette Valley Field Burning Forecast willamette-fcst at listsmart.osl.state.or.us
Thu Aug 19 09:41:51 PDT 2010


SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT THU AUG 19, 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 west-northwesterly flow aloft increased the onshore flow overnight with satellite imagery showing marine low clouds extending inland to the crest of the Cascades and as far south as Douglas County.  Skies were sunny this morning over extreme southwestern Oregon and all areas east of the Cascades.

There is some gradient-stacking this morning with 0.8 mb onshore from Newport to Salem and 2.9 mb onshore from Salem to Redmond.  The low clouds should give way to sunshine by midday with afternoon heating likely balancing out the onshore flow across western Oregon.  Ventilation conditions will be marginal for open burning this afternoon, with transport winds 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 80 degrees.  The mixing height will climb to 3000 feet about noon and likely top out 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% by 2 p.m. and to near 38% around 5 p.m.  The ventilation index will climb to 32 this afternoon.

Silverton area sunset tonight: 8:06 pm

THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 63       73      80      71
     Relative Humidity:           67%      48%     38%     51%
     Surface Wind Direction:     340      320     330     300
     Surface Wind Speed:           5        6       8       9
     Transport Wind Direction:   330      340     340     320
     Transport Wind Speed:         5        6       8      10
     Estimated Mixing Height:   2500     3200    4000    1500
     Ventilation Index:           12       19      32      15

EXTENDED DISCUSSION:

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, with temperatures falling below normal.  The marine layer will deepen significantly Saturday night with a chance of drizzle or even a light shower 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 a light shower.  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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