[willamette-fcst] Corrected Silverton Hills Forecast - FRI, JULY 30 2010

[Willamette Valley Ag/Burning Forecast]

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

...Corrected Wording in Weather Discussion...

BURN ADVISORY:

     Agricultural burning is not recommended.
     Prep burning is not allowed.
     Propane flaming is allowed from 2:00 p.m. until 5:00 p.m.

WEATHER DISCUSSION:

A weak upper-level trough, just offshore, will bring south-southwesterly flow aloft to Oregon again today.  Weak onshore flow brought marine low clouds onto the coastline again this morning, with some penetration into the extreme northern Willamette Valley.  Otherwise, skies were mostly sunny again this morning with light winds.

The Salem sounding this morning was similar to yesterday morning with a very stable atmosphere extending from the surface to about 8000 feet.  That will retard the daily rise in mixing heights again today.  The upper-level trough is forecast to begin moving onshore this afternoon, which will cool the air aloft and increase the onshore flow.  Transport winds may back enough to the northwest to provide an open burning opportunity.  Pressure gradients are forecast to be favorable for keeping smoke elevated, but afternoon thunderstorm development is possible along the Cascades and will need to be closely watched.

TODAY'S FORECAST:

Mostly sunny and warm.  After reaching 85 degrees on Thursday, Salem's high temperature today will be near 84 degrees.  The mixing height will not climb to 3000 feet until around 2 p.m. but will likely jump to nearly 4500 feet, late this afternoon, before a cooling sea breeze quickly drops it to around 1000 feet just before sunset.  Surface and transport winds will be NNW 3-7 this morning and back to NW 6-10 this afternoon. Relative humidity will drop to 50% by noon and to near 32% by late this afternoon.  The ventilation index will be low through midday, before jumping to about 36 late this afternoon.

Silverton area sunset tonight: 8:35 pm

THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 69       78      84      73
     Relative Humidity:           52%      41%     32%     48%
     Surface Wind Direction:     350      320     310     280
     Surface Wind Speed:           4        5       7       7
     Transport Wind Direction:   340      330     320     300
     Transport Wind Speed:         5        6       8       9
     Estimated Mixing Height:   1800     3000    4500    1000
     Ventilation Index:            9       18      36       9

EXTENDED DISCUSSION:

The upper-level trough is forecast to move across Oregon over the weekend with the flow aloft turning northwesterly by Sunday.  That will continue to bring onshore flow and near-normal temperatures to the Willamette Valley.  Another weak upper-level trough may move into the region early next week, with little change in the overall weather conditions.  Western flow aloft and onshore low-level flow may provide multiple burning opportunities next week.

EXTENDED FORECAST:

Saturday: Cloudy through mid morning, then gradual clearing.  High near 73.
Saturday Night: Partly cloudy, with a low around 50.
Sunday: Mostly sunny, with a high near 80.
Monday: Mostly sunny, with a high near 81.
Tuesday: Mostly sunny, with a high near 80.
Wednesday: Mostly sunny, with a high near 83.
Thursday: Mostly sunny, with a high near 82.

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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