[willamette-fcst] Silverton Hills Forecast - Thu, August 12 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT THU AUG 12 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 1:00pm
     to 6:30pm.
     Prep burning is allowed from 1:00pm to 5:00pm with a 50 acre limit.
     Propane flaming is allowed from 1:00pm to 5:00pm.

WEATHER DISCUSSION:

An upper-level ridge is building just offshore with a dry north-northwesterly flow aloft over western Oregon.  The onshore flow is very weak this morning with satellite imagery showing low clouds only along the coast and in the extreme northern Willamette Valley.  The Salem sounding showed several degrees of warming, since 24 hours ago, from just above the surface up through 10,000 feet, so high temperatures will be much warmer today.  Forecast light northerly transport winds makes open burning highly unlikely this afternoon.

TODAY'S FORECAST:

Sunny and warmer.  After reaching 81 degrees on Wednesday, Salem's high temperature today will be near 87 degrees.  The mixing height will climb to 3000 feet about 2 p.m. and likely top out near 3700 feet around 5 p.m.  An evening sea breeze should drop the mixing height to near 1500 feet by shortly before sunset.  Surface and transport winds will be light northerly this morning, increase to 5-10 mph later this afternoon, then turn NW 5-10 mph this evening. Relative humidity will drop to 50% by noon and to near 30% around 5 p.m.  The ventilation index will climb to only 22 this afternoon.

Silverton area sunset tonight: 8:17 pm

THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 70       81      87      78
     Relative Humidity:           55%      38%     30%     42%
     Surface Wind Direction:     360      350     340     290
     Surface Wind Speed:           2        4       5       6
     Transport Wind Direction:   360      360     360     320
     Transport Wind Speed:         3        5       6       8
     Estimated Mixing Height:   1900     3000    3700    1500
     Ventilation Index:            6       15      22      12

EXTENDED DISCUSSION:

A strong upper-level ridge of high pressure will build over the Pacific Northwest Friday through Monday with mostly clear skies and dry offshore flow boosting high temperatures to near record values. The ridge is forecast to shift east of the region during the middle of next week with the surface flow turning back onshore.  That will cool temperatures back closer to normal by Wednesday and could create open burning opportunities.  A strong enough trough may come onshore by late Wednesday or Thursday to produce showers or thundershowers in the Willamette Valley with temperatures falling to below normal.

EXTENDED FORECAST:

Friday: Sunny and very warm. NE wind 5-15 mph. 55/93
Saturday: Sunny with near-record warmth. NE wind 5-10 mph. 58/97
Sunday: Sunny with near-record warmth. Light wind. 59/98
Monday: Sunny with near-record warmth. Light wind. 61/99
Tuesday: Sunny. A little cooler. Wind becoming W 5-15 mph. 61/94
Wednesday: Partly cloudy and cooler. Wind W 5-15 mph. 56/84
Thursday: Mostly cloudy and cooler. Chance of showers. 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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