[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 13, 2012



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



A broad upper-level ridge of high pressure remains over the Pacific Northwest this morning. At the surface, a thermal trough extends northward along the Oregon Coast, with continued offshore flow across western Oregon.  The Salem sounding this morning was not available, but ODF SODAR, located in the central coast range, showed easterly winds from near the surface up through 3500 feet.



Satellite imagery shows mostly clear skies across the state, except for low clouds banked up along the southern coast.  Yesterday's offshore flow transported smoke, from wildfires in the Cascades and central Oregon, into northwestern Oregon. DEQ air quality monitors are showing elevated smoke levels across all of northwestern Oregon this morning.



The axis of the upper-level ridge is forecast to slowly progress eastward, to over Idaho, this afternoon with a weak southwesterly flow aloft over Oregon.  The surface thermal trough will shift from the coast into the Willamette Valley.  That will return onshore flow to the immediate coastline, with winds in the valley becoming light and variable.  The combination of light winds and very warm air aloft will make for poor ventilation conditions today.



(Salem Airport data for Wednesday, Sept. 12th: High 87°F; Rainfall .00")



TODAY'S FORECAST:



Sunny, hazy, and very warm.



Salem's high temperature today will be near 92 degrees (normal is 78).

Relative humidity drops below 30% by noon.

Surface winds:  N-NE near 5 mph.

Transport winds: NE 5 mph; becoming light and variable this afternoon.

Mixing height: Only rising to near 2500 feet this afternoon.

Salem's sunset tonight: 7:25 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 69       84      88      74

     Relative Humidity:           31%      19%     16%     28%

     Surface Wind Dir/Speed:      N 5     NE 5     N 5     N 3

     Transport Wind Dir/Speed:   NE 5    VAR 5   Var 5   Var 3

     Mixing Height:              1200     2000    2500    1000

     Ventilation Index:            6       10      13       3



EXTENDED DISCUSSION:



The upper-level ridge is forecast to weaken on Friday, with the thermal trough shifting into eastern Oregon.  Increasing southwesterly flow aloft and onshore transport winds may create favorable burning conditions Friday afternoon and help to clear wildfire smoke from the valley.  Cooler onshore flow will likely cap high temperatures in the mid 80s.



A very weak and dry upper-level trough is forecast to move across Oregon on Saturday.  Skies should remain mostly sunny, but onshore flow may be just strong enough to bring some morning marine clouds into the valley.  Temperatures should cool back close to normal.  The flow aloft is forecast to turn more northerly on Sunday, with temperatures beginning to warm again.



A strong upper-level ridge is forecast to return next week; bringing another round of offshore flow and very warm weather.  Air quality is likely to deteriorate, as wildfire smoke gets transported back across northwestern Oregon.  The chances for burning opportunities next week look slim.



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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