[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:35 AM PDT THU JUL 27, 2017



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is allowed from noon until 2 p.m. with a 50 acre limit.



WEATHER DISCUSSION:



The upper-level trough that brought thundershowers to central Oregon on Wednesday pushed into Idaho overnight.  Increasing WSW flow aloft has stabilized the air mass but is inducing a marine push into NW Oregon this morning.  Low clouds blanket the northern and central Willamette Valley with areas of mist being reported in the Salem area.



The deep marine layer will provide good mixing today.  However, gradient-stacking has turned negative and should become even more negatively-stacked through this morning.  As skies clear this afternoon, gradients will improve but may not balance out.  Transport winds may also become too northerly to allow for burning.



TODAY'S FORECAST:



Cloudy this morning with afternoon clearing.  Cooler.



Salem's high temperature today will be near 81°F (average is 84°F).

Relative humidity:  Dropping to 60% by noon and to near 40% by 5 p.m.

Surface winds: NW 5-10 mph this morning; NNW 5-10 mph this afternoon.

Transport winds: NW 10 mph this morning; NNW 10-15 mph this afternoon.

Mixing height: Rising to 3000 feet around noon and to 5000 feet by 5 p.m.

Salem's sunset tonight: 8:44 p.m.



(Salem Airport data for Wednesday, July 26th: High 91°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)



EXTENDED DISCUSSION:



Onshore flow will weaken on Friday.  Patchy morning clouds should give way to sunshine and seasonal temperatures.  Although gradient-stacking will likely become more balanced, northerly transport winds may not allow for burning.



Sunny and warmer weather is expected this weekend through the first half of next week, with temperatures climbing back into the lower 90s by Monday.  There is hope for a burning opportunity around the middle of next week, if a weak weather system can produce enough onshore flow at the right time of day.



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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