[willamette-fcst] Tue, Aug 24th, 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE AUG 24 2010

BURN ADVISORY:

     **** Fire Marshal Burn Ban Conditions Likely This Afternoon ****

     Recommended burn times for agricultural burning are from 1:00pm to 4:00pm.
     Prep burning is not allowed.
     Propane flaming is not allowed.


WEATHER DISCUSSION:

     An upper level ridge covers the Pacific Northwest this morning. The axis of the
     ridge runs from NE British Columbia, through central Oregon to near Lake Tahoe.
     At the surface, our morning ODF surface analysis shows a thermal trough from the
     central valley of California into NW California, then just offshore along the
     Oregon and Washington coast. There is a strong offshore pressure gradient today.
     The Dalles to North Bend pressure difference is about 10mb and this will produce
     gusty north east winds today.

     The morning Salem sounding showed dramatic warming at all levels. At about 3500
     feet the temperature jumped from 49F yesterday morning to 71F today. Some
     additional warming will occur this afternoon. There was a shallow, surface
     based inversion to about 200 feet that will easily mix out. The sounding also
     showed a more significant, subsidence inversion from about 1300 feet to about
     2200 feet that will not mix out. Maximum mixing heights today will likely stay
     below 2000 feet.

     The air mass is very dry and relative humidities will drop into the teens later
     today.  It is likely that Fire Marshal conditions will be met by early
     afternoon.

     The probability of open field burning today is nil.

     For the mid Willamette Valley today expect sunny skies and gusty northeast
     winds. Salem's high temperature today will be near 95. Relative humidity drops
     to 50% by 9am. Minimum relative humidity this afternoon will be near 15%.

     Sunset tonight: 7:58 pm

THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 76       89      95      87
     Relative Humidity:           33%      20%     15%     22%
     Surface Wind Direction:     360      360     010     340
     Surface Wind Speed:           6    10-15   10-15       6
     Transport Wind Direction:   360      360     350     340
     Transport Wind Speed:        11       11      12       5
     Estimated Mixing Height:    900     1900    1900     500
     Ventilation Index:           10       21      23       2

EXTENDED DISCUSSION:

     The thermal trough will shift inland tomorrow then east of the Cascades late
     tomorrow. Timing of this shift will determine if there is a burn opportunity.
     Computer models suggest that this will be a late afternoon or early
     evening event. Of the three computer models to which we have access two show
     favorable winds beginning about 5pm and one shows a wind shift around 6pm.
     Pibals and careful monitoring of pressure gradients will be necessary tomorrow.

     Thursday will likely see a deep marine layer covering the valley with the
     possibility of some drizzle or light rain. This will be the first day after a
     marine push - days which generally do not produce good burning conditions.

     A deepening trough, now in the Gulf of Alaska will settle over the area late in
     the week and over the weekend. This should result in onshore flow, along with
     increased cloud cover and much cooler temperatures. Shortwaves will periodically
     swing around the base of the trough, helping to dig the trough further south and
     increase the thickness of the marine layer. Models suggest that the potential
     exists for a few rain showers over the weekend, but the timing on these would
     depend on when the shortwaves pass through.


EXTENDED FORECAST:

     Wednesday: Sunny, with a high near 94. E 5-10 mph becoming NW 5-15 late.
     Thursday: A chance of morning drizzle then partly sunny, 75.  NW 5-10  mph.
     Friday: Partly sunny, with a high near 73.
     Friday Night: Mostly cloudy, with a low around 46.
     Saturday: Partly sunny, with a high near 72.
     Sunday: Partly sunny, with a high near 70.
     Monday: Mostly cloudy, with a high near 73.


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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