[willamette-fcst] Wed, Aug25th, 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT WED AUG 25 2010

BURN ADVISORY:

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


WEATHER DISCUSSION:

     One more hot day is in store for the Willamette Valley then temperatures cool
     significantly for several days. Satellite pictures this morning show low clouds
     banked up along the coast from northern Washington to Cape Blanco Oregon.
     Otherwise skies are clear except for smoke from wildfires. A trail of smoke from
     the View Lake complex about 50 miles east of Estacada shows up nicely and is
     affecting the northeast portion of the Willamette Valley.

     The morning ODF surface analysis shows a thermal trough from the central valley
     of California northward through the Willamette Valley continuing into the Puget
     Sound. Yesterday that trough was along the coastline and late this afternoon or
     early this evening it will shift east of the Cascades. As the trough shifts,
     cooler marine air will enter the valley. At 8am this morning the Newport to
     Salem gradient was 1.0mb onshore while the Redmond to Salem gradient was 3.5mb
     offshore.

     Also on the weather maps this morning is a weak, summer-time cold front extending
     southwestward from the northern tip of Vancouver Island. This front is
     associated with a fairly strong (for this time of year) upper level low in the
     Gulf of Alaska. This front will sweep across the Willamette Valley overnight
     tonight to further deepen the marine layer and perhaps produce some drizzle or
     light rain by tomorrow morning.

     The morning Salem sounding shows a series of inversions from the surface to
     abuot 2800 feet. At balloon launch time the temperature at ground level was 60F,
     but the balloon reported that the temperature at 2800 feet was a much warmer
     78F. Temperatures were generally 5 to 8 degrees warmer than yesterday moorning
     from the surface all the way to about 10,000 feet.

     With the strong inversion it will be difficult to get very high mixing heights.
     The maximum mixing height will be around 3100 feet at about 5pm this afternoon.

     Computer models indicate that while the thermal trough will move east of the
     valley late this afternoon finding a westerly component to the winds will be
     difficult. The RUC model shows transport winds at 320 to 330 degrees late this
     afternoon, while the NAM model shows transport winds between 330 and 340
     degrees. The Portland WRF model is the most optimistic with respect to wind
     direction showing transport winds coming around to about 310 degrees at 6pm this
     evening.

     Pibals through the afternoon will determine if a burning opportunity develops
     ahead of the transition to cooler weather.

     For the mid Willamette Valley expect sunny and hot weather again today. Salem's
     high temperature will be near 94. Relative humidity drops to 50% by 9am.
     Minimum relative humidity 20%. Sunset tonight: 7:56 pm



THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 80       90      94      81
     Relative Humidity:           35%      25%     20%     34%
     Surface Wind Direction:     010      360     320     270
     Surface Wind Speed:           5        5       7       6
     Transport Wind Direction:   340      340     330     290
     Transport Wind Speed:         2        5       5       5
     Estimated Mixing Height:   1000     2200    3100     500
     Ventilation Index:            2       11      16       2



EXTENDED DISCUSSION:

     The upper level low in the Gulf of Alaska will drop southeastward and settle in
     over the Pacific Northwest through the weekend. This means cooler than average
     temperatures and some unsettled weather. Low level flow will be onshore.
     Disturbances rotating through the bottom of the trough will periodically deepen
     the marine layer over the valley for possible drizzle or showers. Timing of
     these disturbances is in doubt, but right now it looks like Friday will be
     mainly dry, with a better chance of precipitation later Saturday and Sunday.


EXTENDED FORECAST:

     Thursday: Mostly cloudy, a little morning drizzle.  High 79.  Winds NW 7-10 mph.
     Calm wind becoming north northwest between 7 and 10  mph.
     Friday: Partly sunny, 72. NW 5-8.
     Saturday: Mostly cloudy, chance of showers. High 70.
     Sunday: Mostly cloudy, chance of showers, with a high near 70.
     Monday: Mostly cloudy, with a high near 73.
     Tuesday: Partly sunny, with a high near 74.

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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