[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 WED. AUG. 7, 2013



BURN ADVISORY:



Agricultural burning is not recommended today.

Prep burning is not allowed.



WEATHER DISCUSSION:



An upper-level closed low is spinning off the northern CA coastline today, and the associated cloudiness and even some light showers are spreading northward into the south end of the valley. Deep southerly to southeasterly flow aloft (above 10000 feet AGL) is developing as the closed low slowly moves northeast. Surface winds are light and variable, and the surface pressure gradient pattern in place is weakly favorable but not expected to improve significantly during the afternoon. At 11:00 a.m. the gradient was 3.6 mb Newport-Salem and 1.6 mb Salem-Redmond.



Mostly cloudy skies are generally expected for the area through the rest of the day, with some light shower activity reaching the ground over south portions. Surface temperatures will be about 10 degrees cooler than highs yesterday due to the cloud cover. Transport winds will be mostly north-northeasterly later today at speeds of 5 to 15 mph. Any changeover to NNW would be this evening. Surface winds will also likely become N late this afternoon, with a possible switch to NNW after 6pm. Mixed layer depth will be decreased somewhat over yesterday's depth.



Hazy skies are also likely to continue today as some wildfire smoke continues to be transported north from far southwest OR.



(Salem Airport data for Tues., August 6: High 95°F; Rainfall 0)

(Maximum Ventilation Index expected today:  30)

TODAY'S FORECAST:



Expect mostly cloudy skies this afternoon with low-level winds becoming generally N but remaining weak.  Some light showers will be scattered though the area, most numerous over the south side. Winds could get erratic this afternoon in the vicinity of showers.



Salem's high temperature today will be in the low to mid 80s.

Relative humidity will drop to the 35-40% range this afternoon.

Surface winds: N 5-10 mph.

Transport winds: N 5-15 mph.

Maximum mixing height:  3500 ft.

Sunset tonight: 8:30 p.m.



EXTENDED DISCUSSION:



Upper troughing is expected to continue on Thursday, with partly to mostly cloudy skies and the continued chance of light showers. N transport flows are also likely to continue.



On Friday, general persistence is expected as the upper trough is likely to be centered just south of the valley and cloud cover will be significant. Shower activity will likely increase, and an onshore wind pattern is not expected. The trough is very slow to move to the north and on Saturday no significant changes are expected at this time.



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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