[willamette-fcst] Silverton Hills Forecast - SAT, SEPTEMBER 25, 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT SAT SEP 25, 2010

BURN ADVISORY:

     Agricultural burning burning is not recommended.
     Prep burning is not allowed.
     Propane flaming is not allowed.

WEATHER DISCUSSION:

An upper-level ridge has shifted east, to over Idaho, with increasing southwesterly flow aloft over Oregon this morning.  Satellite imagery shows only patchy fog and low clouds in the Willamette Valley, which should quickly give way to sunny skies later this morning.

The Salem sounding showed a surface-based temperature inversion that extended upward to about 1800 feet.  That will not likely get mixed out until temperatures climb into the mid 70s early this afternoon.  The air aloft is quite warm, so look for valley temperatures to climb at least into the low 80s by late this afternoon.

The ODA surface analysis showed weak southeasterly pressure gradients this morning over western Oregon with valley winds generally 5 mph or less.  As a weakening offshore cold front moves closer to the coastline today, southerly surface and transport winds are forecast to turn more southwesterly, which may provide adequate ventilation for limited open burning later this afternoon.

TODAY'S FORECAST:

Patchy morning fog, then sunny and warmer.  After reaching 76 degrees on Friday, today's high will top out near 83.  The mixing height will rise to 3000 feet around 2 p.m., top out near 4000 feet around 5 p.m., then quickly drop to 1500 feet by 8 p.m.

Surface winds S 3-8 mph this morning, becoming SW 5-10 mph this afternoon.
Transport winds S 5 mph this morning, veering to SW 12-18 mph this afternoon.

Relative humidity will drop to 50% around noon and bottom out near 35% around 5 p.m.  The ventilation index will climb to near 70 late this afternoon.

Silverton area sunset tonight: 6:58 p.m.

THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 70       80      82      71
     Relative Humidity:           55%      38%     34%     53%
     Surface Wind Direction:     170      220     230     250
     Surface Wind Speed:           7        5       6       6
     Transport Wind Direction:   180      200     230     240
     Transport Wind Speed:         5       12      18      15
     Estimated Mixing Height:   1500     2800    4000    1500
     Ventilation Index:            8       34      72      22

EXTENDED DISCUSSION:

A weak cold front is forecast to move onshore Sunday morning, with light rain likely along the coast and in the northern Willamette Valley.  There is a good chance of rain as far south as the Silverton Hills region.

The upper-level ridge is forecast to rebuild over the region Monday, with sunshine and warm temperatures helping to dry any damp fields.  Another weak system may turn transport winds onshore and create an open burning opportunity next Tuesday.  The ridge is then forecast to strongly build over the state for the second half of next week, with above normal temperatures.  The next burning opportunity appears to be next Friday.

EXTENDED FORECAST:

     Sunday: Mostly cloudy. Chance of light rain. High near 74. Wind SW 5-12.
     Monday: Areas of morning fog...mostly sunny. High near 80. Light wind.
     Tuesday: Increasing clouds.  Chance of light rain late.  High near 75.
     Wednesday: Partly sunny.  High near 78.
     Thursday: Mostly sunny and warm.  High near 80.
     Friday: Increasing clouds. High near 78.

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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