[willamette-fcst] Mon, Aug 30th, 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT MON AUG 30 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 9:00am to 6:00pm.
     Prep burning is not allowed.
     Propane flaming is not allowed.

WEATHER DISCUSSION:

     Fall-like weather has settled in over the Pacific Northwest for the next few
     days.

     A fairly broad upper level trough extends from the Yukon, across the Pacific
     Northwest and southward to off the northwestern Mexican coast. Impulses rotating
     through this trough will keep clouds and a chance for some light rain in the
     forecast through mid week.

     Satellite pictures this morning show an area of thick clouds moving WNW to ESE
     across the Valley. This cloud mass has produced rain across much of the area
     this morning. Amounts include .11 at the Salem Airport and .15 at Jordan Valley.

     A weak Pacific front will move through western Oregon today followed by drying
     overnight prior to another disturbance which should produce similar amounts of
     rain tomorrow afternoon and evening.

     The morning Salem sounding showed a cool, moist air mass. Maximum mixing heights
     will reach a bit over 3000 feet this afternoon, however wet fields should
     preclude any open field burning today.

     Expect clouds, light rain and cool weather. Temperatures will be about 15
     degrees below average for late August. Salem's high temperature today will be
     near 65. Minimum relative humidity will be about 52%.

     Sunset tonight: 7:47 pm

THREE-HOURLY DATA:
                                 11am     2pm     5pm     8pm
     Temperature:                 59       63      65      62
     Relative Humidity:           69%      56%     52%     62%
     Surface Wind Direction:     030      220     290     270
     Surface Wind Speed:           4        4       6       5
     Transport Wind Direction:   120      150     190     190
     Transport Wind Speed:        10        5       5       3
     Estimated Mixing Height:   2000     2800    3200     500
     Ventilation Index:           20       14      16       2

EXTENDED DISCUSSION:

     A flat upper level ridge will begin building over the area later Wednesday for a
     warming and drying trend. Longer range models show this flat ridge continuing
     Thursday and Friday. Although the ridge will be weak there are indications that
     a slight offshore wind component could develop. This would allow for clearing
     skies and warmer weather, with temperatures back into the 80's. However,
     offshore winds are not favorable for open field burning.

     Computer models suggest that another upper level trough will move over the
     region sometime early in the coming weekend. Thus, any period of warming at
     week's end will likely be short lived.

EXTENDED FORECAST:

     Tuesday:  Mostly cloudy, 65.  Increasing chance of rain late. Wind SW 5-8 mph. A
     Wednesday:  Mostly cloudy, chance of light rain.  High 70.  Wind S 5.
     Thursday Night: Partly cloudy, with a low around 52.
     Friday: Mostly sunny, with a high near 82.
     Saturday:  Mostly cloudy, 78.
     Sunday: Partly sunny, with a high near 77.

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.







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