[willamette-fcst] Silverton Hills AM Forecast_doug.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM PDT FRI. JUL 26, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



The upper level ridge axis is finally edging eastward a bit today, with weak coastline troughing developing. Skies are sunny as dry conditions continue. Pressure gradients are currently weakly favorable with some strengthening in the onshore direction expected later.



Mostly sunny skies and very warm temperatures will continue to dominate the region this afternoon. Transport and surface winds will remain northerly through early afternoon, but a gradual transition towards onshore flow is beginning today. A deep mixed layer is also expected this afternoon. A slight westerly component in the transport winds is likely after 3:00 p.m. or so and we will closely monitor the situation via surface observations, PIBALS and model guidance updates.



(Salem Airport data for Thursday, July 25th: High 94°F; Rainfall .00")

(Maximum Ventilation Index expected today:  50)

TODAY'S FORECAST:



Expect mostly sunny skies with breezy conditions developing this afternoon.



Salem's high temperature today will be near 90.

Relative humidity will be near 30% late this afternoon.

Surface winds: N at 5-15 mph.

Transport winds: N to NNW at 10-20 mph.

Maximum mixing height:  4500 ft.

Sunset tonight: 8:45 p.m.



EXTENDED DISCUSSION:



Troughing will continue to build along the coastline this weekend and slowly extend eastward, with cooler conditions and morning low clouds becoming more prevalent. However extensive, persistent cloudiness is not expected over the Cascade Range.  Some drizzle could develop during cloudy periods, especially for the north and west sections of the valley.



On Monday the onshore pattern is likely to continue with relatively cool low-level conditions and the likelihood of morning low clouds. Persistence is currently expected for Tuesday also, with favorable onshore conditions prevailing. The precise timing of surges and cloud cover for the Monday-Tuesday time period is uncertain but some chances for favorable burning periods are expected.



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.



Doug Wesley

ODF Meteorologist


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