[willamette-fcst] Silverton Hills Midday Forecast.doc

Fri Sep 30 11:47:54 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 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 was still over the western valleys earlier this.  However, onshore flow is returning to western Oregon, at midday, with the thermal trough shifting eastward into central Oregon.  Winds are light across the Willamette Valley, but southwesterly flow is beginning to penetrate into the Eugene area.

Midday infrared satellite imagery shows a broad band of mainly middle and high clouds, associated with the first in a series of weather systems, advancing across Washington and Oregon.  Some low clouds were also along the coast, with a few cloud build-ups over the interior of northwestern Oregon.  Doppler radar is showing some light rain along the north coast, which is moving into southwestern Washington.  Astoria was seeing some light rain late this morning.  A few sprinkles have advanced as far inland as Hillsboro.

As the upper-level ridge continues to weaken and shift eastward, increasing southwesterly flow aloft will spread considerable middle and high clouds over the region this afternoon.  The surface thermal trough will get forced well east of the Cascades, with increasing low-level onshore flow holding maximum temperatures in the 70s.  There is a slight chance of a sprinkle, but generally dry conditions should prevail.  Slow cooling aloft will yield higher mixing heights this afternoon.  Southwesterly transport winds should provide good smoke dispersal conditions for any remaining field-burning.  A 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 near 40% by 5 p.m.

Surface winds:  WNW 3-7 mph this afternoon.

Transport winds: SSW 10-15 mph this afternoon.

Mixing height: Rising to 3000 feet by 2 p.m. and to near 4000 feet by 5 p.m.

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

THREE-HOURLY DATA:

                                2 p.m.  5 p.m.  8 p.m.

     Temperature:                 74      77      65

     Relative Humidity:           43%     39%     58%

     Surface Wind Direction:     330     300     290

     Surface Wind Speed:           4       4       5

     Transport Wind Direction:   220     220     300

     Transport Wind Speed:        10      15      10

     Mixing Height:             3000    4000    1000

     Ventilation Index:           30      60      10

EXTENDED DISCUSSION:

A slightly stronger weather system is forecast to bring about one-tenth of an inch of rain to the valley on Saturday.  Sunday looks mostly dry, ahead of a much more potent system forecast to come onshore Sunday night.  Much of the valley could pick up more than one-half inch of rain on Monday.  An even stronger system is forecast to bring more rain and some wind 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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