[willamette-fcst] Silverton Hills Forecast - Thu, September 16, 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT THU SEP 16, 2010

BURN ADVISORY:

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

WEATHER DISCUSSION:

An autumn-like cold front dropped up to one-half inch of rain in the northern Willamette Valley overnight with around one-third of an inch falling as far south as Marion County.  Rainfall amounts tapered off to between one-tenth and one-quarter of an inch in Linn County with less than one-tenth of an inch falling in Lane County.

Rain continued to fall this morning along sections of the coast and in the extreme northern Willamette Valley.  After receiving around 1.50 inches of rain overnight, Astoria, on the northern coast, has picked up an additional .40 inches this morning.  Up to one-tenth of an inch of rain has fallen in the northern Willamette Valley in the past 6 hours with trance amounts falling as far south as the central valley.

Skies remain cloudy across western Oregon late this morning due to a very moist southwesterly flow aloft.  The cold front that has been producing the rain is forecast to shift slightly northward this afternoon, so not much addition rainfall is expected as far south as Marion County.  However, cloudy skies, light winds, and high humidity levels will not aid much in the drying of damp fields.

TODAY'S FORECAST:

Cloudy with a chance of rain...mainly in the morning.  Today's high temperature should be near 74 degrees.  The mixing height will climb to near 3000 feet in the mid-afternoon and top out below 4000 feet around 5 p.m.  Relative humidity will likely remain above 60% all day.  The ventilation index will climb to 37 by late this afternoon.

Silverton area sunset tonight: 7:15pm

THREE-HOURLY DATA:

                                 2pm     5pm     8pm
     Temperature:                 73      74      67
     Relative Humidity:           64%     62%     75%
     Surface Wind Direction:     200     230     220
     Surface Wind Speed:           6       6       2
     Transport Wind Direction:   200     210     220
     Transport Wind Speed:        10      10       6
     Estimated Mixing Height:   2500    3700    1000
     Ventilation Index:           25      37       6

EXTENDED DISCUSSION:

Another weather system is slated to come onshore Friday with rain projected to move back into the Willamette valley by early afternoon...making the prospects for burning highly unlikely.  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 are now showing another, weaker, weather system possibly coming onshore Tuesday with more showers...mainly over the northern valley.  The latest guidance is suggesting that a ridge will dry things out Wednesday and Thursday, which could finally allow remaining fields to dry out enough for burning.

EXTENDED FORECAST:

     Friday: Rain likely by the afternoon.  High near 72.
     Saturday: Rain likely.  High near 69.
     Sunday: Showers likely.  High near 68.
     Monday: Mostly cloudy.  Chance of showers.  High near 70.
     Tuesday: Mostly cloudy.  Chance of rain north.  High near 70.
     Wednesday: Partly sunny.  High near 72.
     Thursday: Partly cloudy.  High near 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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