[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT THU SEP 29, 2011



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 with a warm and dry southwesterly flow aloft over Oregon.  The surface map this morning shows high pressure centered over NE Oregon and a thermal trough centered just off the coast.  That is resulting in strong offshore flow across all of western Oregon.  Surface winds in the Willamette Valley are light, but easterly winds are gusting to around 30 mph at the western end of the Columbia Gorge.  Coastal winds are also light easterly.



The Salem sounding this morning showed very warm air aloft with a surface-based temperature inversion extending up to almost 5000 feet.  Winds were light and variable near the surface with a layer of offshore flow from about 1000 feet to 3000 feet.  Winds shift to the south-southwest above 3000 feet.



The upper-level ridge is forecast to slowly shift eastward today with the surface thermal trough moving over the Willamette Valley.  That will combine with a full day of sunshine to make for unseasonably warm temperatures. Due to very warm air aloft and a lowering autumn sun-angle, mixing heights will struggle to reach 3000 feet today.



As the thermal trough shifts inland this afternoon, offshore transport winds are forecast to weaken and eventually become light south-southwesterly.  That will need to be monitored closely, because it could create favorable enough burning conditions to take care of the few remaining fields late today.

TODAY'S FORECAST:



Sunny and unseasonably warm.



Salem's high temperature today will be near 84 degrees (normal is 72).

Relative humidity drops below 50% by 11 a.m. and to near 30% by 5 p.m.

Surface winds:  N-NE 3-6 mph; becoming variable 3-7 mph this afternoon.

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

Mixing height: Rising to 3000 feet by 3 p.m. and to near 3500 feet by 5 p.m.

Salem's sunset tonight: 6:57 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 66       78      83      68

     Relative Humidity:           49%      33%     30%     52%

     Surface Wind Direction:     020      360     Var     230

     Surface Wind Speed:           3        5       5       5

     Transport Wind Direction:   080      200     220     230

     Transport Wind Speed:         3        6      10       7

     Mixing Height:             1700     2500    3500    1000

     Ventilation Index:            5       15      35       7



EXTENDED DISCUSSION:



The upper-level ridge is forecast to weaken and shift further to the east on Friday.  Increasing southwesterly flow aloft will force the surface thermal trough into eastern Oregon.  Onshore flow will cool temperatures back close to normal with increasing clouds.  There could be an opportunity to burn any remaining fields, if appropriate conditions do not set up today.



A series of weather systems are forecast to move onshore, beginning Saturday.  Stormy weather next week will likely prohibit further burning this season.



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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