[willamette-fcst] Silverton Hills Midday Forecast_doug.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM 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, slowly edging northeastward. Deep easterly flow aloft is in place over western OR as the closed low slowly edges to the northeast. Surface winds are continuing light and variable, and the surface pressure gradient pattern is weakly favorable as of 11:00 a.m. Partly to mostly cloudy conditions are in place.



Cloud cover will increase this afternoon with scattered shower activity moving into the region from the east by early evening, accompanied by possible thunderstorms. Surface temperatures will be a bit cooler than yesterday, mixing heights will be moderate, and transport winds will be mostly northerly later today at speeds of 5 to 15 mph. Surface winds will also become N-NE this afternoon and then NNW this evening.



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

(Maximum Ventilation Index expected today:  30)

TODAY'S FORECAST:



Expect skies to become mostly cloudy late this afternoon with scattered showers and a chance of thunderstorms by evening, some of which could produce significant rain tonight, especially at higher elevations. Low-level winds and transport winds becoming N to NE but remaining weak.  Winds could get erratic this evening in the vicinity of thunderstorms. Weakly favorable gradients are expected to continue. NNW surface flow is likely this evening.



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 becoming NNW 5-15 mph this evening.

Transport winds: N 5-15 mph.

Maximum mixing height:  2500-3000 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 over the region with scattered showers late in the day. Rainfall tonight and late Saturday will likely reduce the potential for burning in many areas. Weak transport flows are also likely to continue, but there is again 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 clearing skies possible. 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, especially on Tuesday.



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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