[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT THU JUL 28, 2011



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



The Pacific Northwest is under a dry and stable west-northwesterly flow aloft today.  Salem's sounding this morning showed several degrees of warming, compared to 24 hours ago, from the surface to about 10,000 feet with north to northeast winds throughout the layer. Visible satellite imagery showed some patchy marine clouds along the north coast, and in the extreme northern Willamette Valley, with clear skies across the remainder of the state.



The surface map shows high pressure pushing into Washington and a weak thermal trough building northward into southwestern Oregon.  That will maintain dry north to northeasterly winds over the Willamette Valley today, which should aid in the drying of damp fields.  Warmer air aloft will lead to slightly lower mixing heights today, as temperatures return to near normal.



TODAY'S FORECAST:



Sunny and warmer.



Salem's high temperature today will be near 84 degrees (normal is 83).

Relative humidity drops to 50% about 11 a.m. and to near 35% by 2 p.m.

Surface winds:  N 5-10 mph; N 10-15 mph this afternoon.

Transport winds: NNE 10-15 mph this morning; NNE 15-20 mph this afternoon.

Mixing height: Rises to 3000 feet about noon and to 4000 feet by 2 p.m.

Salem's sunset tonight: 8:43 p.m.





THREE-HOURLY DATA:

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

     Temperature:                 70       80      83      77

     Relative Humidity:           51%      37%     33%     42%

     Surface Wind Direction:     360      360     010     360

     Surface Wind Speed:           7       10      13      12

     Transport Wind Direction:   015      010     015     010

     Transport Wind Speed:        13       16      18      20

     Mixing Height:             2800     4000    4300    2800

     Ventilation Index:           36       64      77      56



EXTENDED DISCUSSION:



A weak system cutting across British Columbia will turn the flow aloft slightly more westerly on Friday, but surface and transport winds should remain generally northerly with little change in temperatures or mixing heights.



The flow aloft will turn southwesterly on Saturday, as a stronger upper-level trough moves across British Columbia. Temperatures will likely warm a few degrees with continued mostly sunny skies and northerly low-level winds.  By Sunday, an offshore trough will increase the southwesterly flow aloft.  Temperatures should be seasonal with patchy morning marine clouds and afternoon sunshine.  That general pattern is forecast to continue through much of next week, with afternoon northwesterly transport winds possibly creating favorable burning conditions at times.



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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