[willamette-fcst] Silverton Hills AM Forecast_doug.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT FRI. AUG. 2, 2013



BURN ADVISORY:



Agricultural burning is not recommended today.

Prep burning is not allowed.



WEATHER DISCUSSION:



Upper troughing continues over the area today, with widespread low cloud cover and areas of drizzle on the coast and west slopes of the Cascades. Surface winds are light and variable, and the surface pressure gradient pattern in place is not favorable and not expected to improve significantly during the day.



Mostly cloudy skies are generally expected for the area through the morning, with partial clearing beginning late this morning in some areas of the valley. No areas of precipitation more than trace amounts are expected this morning. Surface temperatures will continue to be much cooler than average. Transport winds will become mostly northwesterly later today at speeds of 5 to 15 mph. Surface winds will also likely turn to northwesterly late this afternoon. Mixed layer depth should be improved somewhat over yesterday's depth. If clearing becomes extensive a favorable burn period may exist late this afternoon as the gradient pattern may become more favorable.



(Salem Airport data for Wednesday, July 31st: High 68°F; Rainfall T)

(Maximum Ventilation Index expected today:  35)

TODAY'S FORECAST:



Expect mostly cloudy skies through the morning with areas of drizzle over the west slopes of the Cascades, with some clearing on the south portion. Becoming partly cloudy this afternoon with low-level winds becoming northwesterly.



Salem's high temperature today will be in the low to mid 70s.

Relative humidity will drop to the 50-60% range this afternoon.

Surface winds: NW 5-10 mph.

Transport winds: NW 5-15 mph.

Maximum mixing height:  3500 ft.

Sunset tonight: 8:37 p.m.



EXTENDED DISCUSSION:



Upper troughing is expected to persist just west the area on Saturday, but slowly weaken. Cloudy conditions are expected in the morning, with clearing likely later in the day over the valley and warmer, drier conditions prevailing. Extensive cloudiness is not expected to persist past mid-morning over the west slopes of the Cascades either. Generally northerly transport flow is expected in the afternoon.



On Sunday, the upper trough continues to weaken and is still located off the coast. Upper ridging strengthens east of the trough, over much of Oregon. Generally similar low-level conditions are expected locally, with less morning cloud coverage than Saturday. Mostly sunny skies can be expected for the afternoon. Early next week it appears likely that northerly low-level flow and dry conditions will be dominant.



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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