[willamette-fcst] Silverton Hills Forecast - FRI, SEPTEMBER 17, 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT FRI SEP 17, 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 12:00 p.m. to 5:00 p.m.
     Prep burning is not allowed.
     Propane flaming is not allowed.

WEATHER DISCUSSION:

An autumn-like weather pattern has set up across the Pacific Northwest.  A strong upper-level trough is centered about 800 miles off the Oregon Coast with a very moist southwesterly flow aloft continuing to feed surges of rain into the region.

Skies are cloudy across all of western Oregon this morning with radar showing an enhanced region of rain moving inland.  Light rain was already making it into the northern and western Willamette Valley with light to moderate rain along the coast and over the coastal range. This area of rain will be very slow to move eastward today, with moderate to heavy rainfall totals likely from the coast range westward.  Rainfall in the Willamette Valley will increase this afternoon with conditions much too damp for burning.  Rainfall totals, in the Silverton Hills area, will likely range from one-quarter to one-half inch, before the rain tapers off this evening.

TODAY'S FORECAST:

Cloudy with a chance of rain this morning, then rain likely this afternoon.  After a high temperature Thursday of 75 degrees, today's high will only climb to 72, which is 3 degrees below normal.  The mixing height may briefly climb to near 3000 feet in the mid-afternoon, before increasing rain spreads across the valley.  With more rain moving in and with the relative humidity likely remaining above 65% all day, fields will remain too damp for burning.  The ventilation index will only climb to about 20 this afternoon.

Silverton area sunset tonight: 7:13pm

THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 64       69      70      64
     Relative Humidity:           78%      71%     68%     84%
     Surface Wind Direction:     Var      180     190     200
     Surface Wind Speed:           3        3       5       3
     Transport Wind Direction:   040      190     190     200
     Transport Wind Speed:         3        5       8       7
     Estimated Mixing Height:   1500     2500    2000    1000
     Ventilation Index:            5       13      16       7

EXTENDED DISCUSSION:

Rain should be widespread across western Oregon over the weekend, as the parent upper-level trough moves onshore.  Some showers could last into Monday.

Long-range computer models show a weak weather system possibly bringing a few light showers to the north valley and then dry weather for Wednesday.  The dry weather may not last long, with computer models showing a more active pattern late next week.  It is possible that rain could hold off long enough to create a burning opportunity Thursday or Friday, but that is uncertain.

EXTENDED FORECAST:

     Saturday: Rain likely.  High near 72.
     Sunday: Rain turning to showers.  High near 70.
     Monday: Mostly cloudy with some showers.  High near 70.
     Tuesday: Mostly cloudy.  Slight chance of showers north.  High near 72.
     Wednesday: Partly sunny.  High near 75.
     Thursday: Increasing clouds.  Chance of rain late.  High near 75.
     Friday: Chance of rain.  High near 70.

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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