[willamette-fcst] Mon, 13 Sep 10

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT MON SEP 13 2010

BURN ADVISORY:

     Agricultural burning burning is not recommended.
     Prep burning is allowed from 12:30pm to 5:00pm with a 50 acre limit.
     Propane flaming is not allowed.


WEATHER DISCUSSION:

     Upper air charts show a weak ridge aloft with a minor embedded disturbance off
     the southwest Oregon coast. The low clouds in the Willamette Valley have been
     quite persistent through the morning hours. Satellite pictures show a narrow
     band of low clouds stretching from portland to about Roseburg. The base of the
     cloud deck is at about 1300 feet and pilot reports indicate tops are at about
     2100 feet. We still should see the clouds burn off this afternoon. As of 11:00am
     this morning areas under the clouds were running 2-5 degrees cooler than
     yesterday. Cloud free areas were 2-5 degrees warmer. The clouds are thin enough
     that we still expect sunshine to break through this afternoon.

     The ODF late morning surface analysis shows higher pressure to the north and a
     thermal trough in the central Valley of California, extending northward into
     northern California. This is giving a northerly gradient to the Willamette
     Valley that will likely produce north or north-northeast transport winds
     throughout the day. This is not favorable for open field burning.

     The morning Salem sounding showed a slightly warmer airmass this morning from
     about 2300 feet to about 8500 feet. There is a small inversion between 1800 and
     2500 feet. This inversion will easily mix out with some afternoon sunshine and
     mixng heights today will climb to 3500 feet. The morning Salem sounding also
     showed those north-northeast transport winds.

     Wind direction will likely be the major factor in preventing open field burning
     this afternoon.

     The upper disturbance off the SW Coast will rotate inland and could touch off a
     few showers or thunderstorms over the southern and central Oregon Cascades this
     afternoon.

     For the mid Willamette Valley expect skies to turn sunny this
     afternoon with light northerly winds. Salem's high temperature today will be
     near 78. Minimum relative humidity 42%.

     Sunset tonight: 7:21 pm


THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 69       74      78      72
     Relative Humidity:           61%      48%     42%     51%
     Surface Wind Direction:     010      010     010     310
     Surface Wind Speed:           6        5       6       5
     Transport Wind Direction:   360      360     360     360
     Transport Wind Speed:        08       12      13      05
     Estimated Mixing Height:   2200     3100    3500     500
     Ventilation Index:           18       37      46       2



EXTENDED DISCUSSION:

     An upper level low will develop off the coast of British Columbia later today
     and tomorrow and drift southward to off the Oregon Coast. By later Wednesday
     this will edge closer to the coastline and could produce some light rain by
     Wednesday night. As this moves over the region the chances of rain will increase
     and remain elevated through the weekend.

     Models indicate a more westerly transport flow tomorrow. However, they also
     indicate gradient stacking to the east and pressure patterns may not turn
     favorable until early evening tomorrow. Even though conditions look only
     marginal, tomorrow looks to be the day with the best chance for open field
     burning this week, with Wednesday also a slight possibility.


EXTENDED FORECAST:

     Tuesday: Mostly sunny, with a high near 80. NW 3-6.
     Wednesday: Mostly cloudy, 78.  Chance of rain Wednesday evening.  SW 6.
     Thursday: Chance of rain early then partly sunny, 75.
     Friday: A chance of rain. Partly sunny, with a high near 73.
     Saturday: A chance of rain. Mostly cloudy, with a high near 70.
     Sunday: A chance of rain. Mostly cloudy, with a high near 71.

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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