[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT THU JUL 21, 2011



BURN ADVISORY:



Recommended burn times for agricultural burning are 12:00 p.m. to 7:00 p.m.

Prep burning is allowed from 12:00 p.m. until 2:00 p.m. with a 50 acre limit.

Propane flaming is allowed from 11:00 a.m. until 5:00 p.m.



WEATHER DISCUSSION:



The Pacific Northwest is under the influence of an unseasonably strong upper-level trough.  After a brief warm-up yesterday, a weak weather system moved onshore early this morning and deepened the marine layer across western Oregon.  Visible satellite imagery showed marine low clouds covering all of western Washington and extending south across most of western Oregon. Areas of light drizzle were falling in the northern and central Willamette Valley.



The morning surface analysis showed weak southerly pressure-gradients across the Willamette Valley where surface reports indicated mostly southerly winds at 5-10 mph.  The Salem sounding this morning had a similar temperature profile to yesterday, but there was a marked shift in the transport winds from northwesterly to southwesterly.



Low-level southwesterly flow will slowly veer to westerly, by late this afternoon, as a second weak weather system swings onshore.  Skies should stay mostly cloudy with an ongoing threat of light showers.  However, even slight clearing will allow high temperatures to climb into the low 70s, since the sun angle is so high this time of the year.



TODAY'S FORECAST:



Mostly cloudy & cooler.  Chance of drizzle and/or light showers.



Salem's high temperature today will be near 72 degrees (normal is 83).

Relative humidity drops to 60% about 12 p.m. and to near 50% by 5 p.m.

Surface winds:  S 5-10 mph this morning; W 5-10 mph later this afternoon.

Transport winds: SW 10 mph this morning; W 10-15 mph later this afternoon.

Mixing height: Rises to 3000 feet about 11 a.m. and to 4500 feet by 5 p.m.

Salem's sunset tonight: 8:51 p.m.





THREE-HOURLY DATA:

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

     Temperature:                 66       69      72      67

     Relative Humidity:           65%      57%     51%     61%

     Surface Wind Direction:     190      220     270     300

    Surface Wind Speed:           5        6       8      10

     Transport Wind Direction:   210      240     270     310

     Transport Wind Speed:        10       10      13      15

     Mixing Height:             3000     4000    4500    2500

     Ventilation Index:           30       40      59      38



EXTENDED DISCUSSION:



A drier northwesterly flow aloft is forecast for Friday, as the upper-level trough shifts east of the region.  Morning marine clouds should give way to mostly sunny skies in the afternoon with transport winds turning northerly and temperatures recovering into the upper 70s.  A flat ridge will move over Oregon Saturday for continued warming and drying of the air mass.



The flow aloft will turn southwesterly on Sunday, as the next upper-level trough approaches the coastline. High temperatures may approach 90 degrees with transport winds turning southwesterly in the afternoon.  As the upper-level trough comes onshore Monday, a significant marine push will cool temperatures back to normal.  A broad upper-lever trough is forecast to remain over the region for the duration of next week.  It looks generally dry, but onshore flow will likely keep temperatures below normal.



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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