[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:15 AM PDT SUN SEP 20, 2015



***  Supplemental Weekend Forecast. ***



***  An Updated Forecast Will Be Issued Around 11:45 a.m.  ***



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



The upper-level ridge that brought sunny and warm weather to western Oregon on Saturday is beginning to weaken.  Increasing westerly flow aloft has turned low-level winds onshore and forced extensive low clouds across western Washington and the extreme NW corner of Oregon.  Skies are mostly sunny this morning from Marion County southward across the interior of western Oregon.



A cold front is expected to push inland across western Washington this afternoon with increasing clouds across NW Oregon.  Cooling aloft should combine with daytime heating to push mixing heights above 3000 feet by early this afternoon.  Westerly transport winds should make for favorable burning conditions for dry and/or fluffed fields.  PIBALS are scheduled to begin around noon, to closely monitor transport winds for the possibility of burning this afternoon.



TODAY'S FORECAST:



Mostly sunny with near-average temperatures.



Salem's high temperature today will be near 78°F (average is 76°F).

Relative humidity:  Dropping below 60% by 11 a.m. and to near 40% by 5 p.m.

Surface winds: S 2-6 mph this morning; SW 3-8 mph this afternoon.

Transport winds: SW 5-8 mph this morning; becoming W 7-12 mph this afternoon.

Mixing height: Rising to 3000 feet by 1 p.m. and to near 4500 feet by 5 p.m.

Salem's sunset tonight: 7:14 p.m.



(Salem Airport data for Saturday, September 19th: High 81°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 55)



EXTENDED DISCUSSION:



A weak cold front should bring a significant marine push overnight along the areas of very light rain.  Rainfall totals are expected to be less than one-tenth of an inch.  Look for just some afternoon clearing on Monday with temperatures struggling to reach the low-70s.  Unfavorable burning conditions are expected due to gradient-stacking and northerly winds.



Very little change is in store for Tuesday with morning clouds and afternoon clearing.  Temperatures will remain slightly below average.  Unfavorable gradients and mostly northerly winds will make burning unlikely.



The flow aloft should turn more southwesterly by Wednesday, ahead of a developing weather system.  That may create more favorable burning conditions Wednesday afternoon, with transport winds turning to the WSW.  There may be another burning opportunity Thursday afternoon, depending on the timing of an approaching cold front.  Wetting rains are expected Thursday night with showers lasting into Friday.



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 15 mph.



     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 (ft) times

         the transport wind speed (mph) 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 (ODA) and the Oregon Department of Forestry (ODF).  For

information contact ODA at 503-986-4701.



To add/remove your email address from this list, please go to:



http://listsmart.osl.state.or.us/mailman/listinfo/willamette-fcst



Pete Parsons

ODF Meteorologist
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