[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. 9, 2013



BURN ADVISORY:



Agricultural burning is not recommended today.

Prep burning is not allowed.



WEATHER DISCUSSION:



An upper-level closed low continues to spin just to our southwest off the coast today. Deep easterly flow aloft is in place over western OR as the closed low slowly edges to the northeast. Surface winds are light and variable, and the surface pressure gradient pattern is weakly favorable at the moment. Cloudy conditions are widespread with light showers moving westward over the southern portions of the Silverton Hills.



Deep moisture will dominate the weather today with scattered shower activity likely to continue, with possible thunderstorms late in the day and this evening. Surface temperatures will be a bit cooler than yesterday, mixing heights will be lower as a result, and transport winds will be mostly northerly later today at speeds of 5 to 15 mph. Surface winds will also become N-NE late this afternoon.



(Salem Airport data for Thurs., August 8: High 85°F; Rainfall 0)

(Maximum Ventilation Index expected today:  25)

TODAY'S FORECAST:



Expect skies to remain mostly cloudy this afternoon with scattered showers and a chance of thunderstorms, some of which could produce significant rain especially at higher elevations. Cooler than average temperatures are expected, with low-level winds and transport winds becoming N to NE but remaining weak.  Winds could get erratic late today in the vicinity of thunderstorms. Weakly favorable gradients are expected to continue.



Salem's high temperature today will be around 80.

Relative humidity will drop to the 40-45% range this afternoon.

Surface winds: N to NE 5-10 mph.

Transport winds: N to NE 5-15 mph.

Maximum mixing height:  2500 ft.

Sunset tonight: 8:27 p.m.



EXTENDED DISCUSSION:



The center of the upper trough gradually will move northward on Saturday, and mostly cloudy skies are expected to continue in the region with scattered showers. Rainfall tonight and Saturday will likely reduce the potential for burning in many areas. Weak transport flows are also likely to continue, but there is a chance of some onshore gradients developing late in the day on Saturday.



On Sunday, the upper low is likely to progress further northward into western WA. Expect extensive cloudiness and remnant shower activity in the morning over the Silverton Hills, but a drying trend is probable in the afternoon with some possible clearing skies. Low clouds could persist through the day in the valley. Cool low-level conditions will continue.



The current outlook for early next week is generally more favorable for some onshore flow as the upper flow trends towards southwesterly.



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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