[willamette-fcst] Silverton Hills AM Forecast.doc

Fri Sep 30 08:39:15 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:38 AM PDT FRI SEP 30, 2011

THIS IS THE FINAL FORECAST FOR THE 2011 FIELD-BURNING SEASON

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 strong upper-level ridge of high pressure remains centered over the Rockies but is shifting eastward, with increasing southwesterly flow aloft over Oregon.  The surface thermal trough moved onshore late Thursday and is over the western valleys this morning.  Onshore flow has returned to the immediate coastline, but pressure-gradients remain offshore from central Oregon to the Willamette Valley. Surface winds in the valley are light, and the easterly outflow from the Columbia Gorge has stopped.

Infrared satellite imagery this morning shows a broad band of mainly middle and high clouds, associated with a weakening frontal system, extending southward from British Columbia across much of Washington and Oregon.  Doppler radar was not showing any precipitation from this system.

As the upper-level ridge continues to weaken and shift eastward, increasing southwesterly flow aloft will force the surface thermal trough into eastern Oregon this afternoon.  Onshore flow will cool temperatures back close to normal.  Dry conditions should prevail, with considerable middle and high clouds.  Slow cooling aloft should provide better mixing this afternoon with southwesterly transport winds.  Today would be a good day to take care of any remaining fields, since a pronounced change to cooler and damp conditions is forecast for next week.

TODAY'S FORECAST:

Partly sunny and cooler.

Salem's high temperature today will be near 77 degrees (normal is 72).

Relative humidity drops to 50% by 11 a.m. and to near 40% by 5 p.m.

Surface winds:  Light; becoming WNW 3-7 mph this afternoon.

Transport winds: SSW 3-6 mph this morning; SSW 10-15 mph this afternoon.

Mixing height: Rising to 3000 feet by 3 p.m. and to near 4000 feet by 5 p.m.

Salem's sunset tonight: 6:55 p.m.

THREE-HOURLY DATA:

                               11 a.m.   2 p.m.  5 p.m.  8 p.m.

     Temperature:                 67       74      77      65

     Relative Humidity:           50%      43%     39%     58%

     Surface Wind Direction:     Var      330     300     290

     Surface Wind Speed:           3        4       4       5

     Transport Wind Direction:   220      220     220     300

     Transport Wind Speed:         5       10      15      10

     Mixing Height:             1500     3000    4000    1000

     Ventilation Index:            8       30      60      10

EXTENDED DISCUSSION:

A major shift to autumn-like weather begins this weekend.  Computer models show a slightly stronger weather system coming onshore Saturday.  It will also weaken but should bring about one-tenth of an inch of rain to the region.  Sunday looks mostly dry, ahead of a much more potent system forecast to come onshore Sunday night.  The Willamette Valley could pick up more than one-half inch of rain on Monday.  An even stronger system is forecast to come onshore late Tuesday.

The National Weather Service's digital forecast is available at:

http://forecast.weather.gov/MapClick.php?lat=44.90549606158295&lon=-122.8106689453125&site=pqr&unit=0&lg=en&FcstType=text

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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