[willamette-fcst] 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
9:00 AM 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 common 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 is continuing to fall this morning along the coast and in the extreme northern Willamette Valley.  Skies are cloudy across all of western Oregon.  The cold front will weaken and shift slightly northward today, as a weak ridge of high pressure builds over Oregon.  Not much addition rainfall is expected as far south as Marion County, but skies should remain generally cloudy with high humidity levels.

TODAY'S FORECAST:

Cloudy with a chance of rain...mainly in the morning.  Today's high temperature should be close to yesterday's high of 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, so damp fields will have a difficult time drying out.  The ventilation index will climb to 37 by late this afternoon.

Silverton area sunset tonight: 7:15pm

THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 66       73      74      67
     Relative Humidity:           81%      64%     62%     75%
     Surface Wind Direction:     160      200     230     220
     Surface Wind Speed:           5        6       6       2
     Transport Wind Direction:   170      200     210     220
     Transport Wind Speed:         4       10      10       6
     Estimated Mixing Height:   1800     2500    3700    1000
     Ventilation Index:            7       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 midday.  That makes the prospect for burning highly unlikely.  Wet weather is forecast this weekend...perhaps lasting into Monday.   Long-range computer models are now showing additional weather systems possibly bringing rain to the valley again Tuesday and Thursday of next week, but confidence in the forecast beyond Monday is low.

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.  High near 68.
     Wednesday: Partly sunny.  Slight chance of showers.  High near 71.
     Thursday: Mostly cloudy.  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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