[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT FRI SEP 23, 2011



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is allowed from 1:00 p.m. until 5:00 p.m.



WEATHER DISCUSSION:



A dry and warm southwesterly flow aloft is bringing sunny and unseasonably warm weather to western Oregon for this first official day of autumn.  The only exception is along the immediate coastline, where low clouds have been producing areas of light mist, especially north.



The surface chart shows a broad thermal trough extending from central Washington, through the Willamette Valley, into southwestern Oregon.  Gradients are weakly onshore from Newport to Salem and weakly offshore from Redmond to Salem, with light winds and sunshine across the Willamette Valley.



Satellite imagery shows a broad band of clouds, associated with a strong southwesterly jet stream, extending from northern Vancouver Island, British Columbia to about 450 miles west of the northern Oregon coast.  A wave forming along the front will stall its eastward progress today.



Sunny skies, warm air aloft, and light surface winds will combine to keep valley temperatures more than 10 degrees above normal today.  Warmer surface temperatures, compared to yesterday, should make for slightly higher mixing heights.  South-southwesterly transport winds should creating favorable afternoon burning conditions.  Pibals are scheduled to begin at noon.



TODAY'S FORECAST:



Sunny and unseasonably warm.



Salem's high temperature today will be near 86 degrees (normal is 75).

Relative humidity drops below 50% by 1 p.m. and to near 40% by 5 p.m.

Surface winds:  SSW 4-8 mph this afternoon.

Transport winds: SSW 10-15 mph this afternoon.

Mixing height: Rising to 3000 feet by 1 p.m. and to near 4000 feet by 5 p.m.

Salem's sunset tonight: 7:08 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 82      85      76

     Relative Humidity:           45%     39%     55%

     Surface Wind Direction:     200     200     230

     Surface Wind Speed:           4       5       4

     Transport Wind Direction:   220     220     230

     Transport Wind Speed:        12      12      10

     Mixing Height:             3200    4000    1000

     Ventilation Index:           38      48      10



EXTENDED DISCUSSION:



The flow will become more southerly, on Saturday, as an upper-level trough digs offshore.  Look for increasing high clouds, in the afternoon, with lower mixing heights than today.  A slow-moving frontal system will finally come onshore Saturday night with computer models bringing about one-tenth of an inch of rain into the northern Willamette Valley on Sunday.  A stronger cold front may spread more significant rainfall across western Oregon Monday afternoon through Tuesday morning.  Longer-range models are now more consistent in showing dry weather returning for the second half of next week.



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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