[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT MON AUG 24, 2015



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



Substantial wildfire smoke covered virtually the entire state over the weekend, including the Willamette Valley.  Increasing SW flow aloft began the process of clearing smoke from NW Oregon Sunday evening.  By this morning, good air quality had returned to the northern Willamette Valley, about as far south as Salem, with moderate to unhealthy air quality continuing across the central and south valley.  The greatest concentrations of smoke remain in the Cascade foothills, from Silverton to Sweet Home.



Air quality will continue to improve across NW Oregon today, as onshore gradients increase.  SW flow aloft may destabilize the air mass enough for some thunderstorm development over the Cascades this afternoon.  Otherwise, expect mostly sunny skies with slightly cooler temperatures.



TODAY'S FORECAST:



Mostly sunny.  Wildfire smoke clearing, from west to east, across the region.



Salem's high temperature today will be near 84°F (average is 82°F).

Relative humidity:  Dropping below 50% by 11 a.m. and to near 30% by 5 p.m.

Surface winds: N 3-6 mph this morning; NNW 5-10 mph this afternoon.

Transport winds: NNE 5-10 mph this morning; NW 10 mph this afternoon.

Mixing height: Rising to 3000 feet about 1 p.m. and to 5000 feet by 5 p.m.

Salem's sunset tonight: 8:03 p.m.



(Salem Airport data for Sunday, August 23rd: High 88°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 50)



EXTENDED DISCUSSION:



Very little change in the weather pattern is expected on Tuesday, with a mostly dry SW flow aloft continuing over Oregon.  That should keep most of the wildfire smoke east of the valley, but there could be some drainage of smoke into the western Cascade foothills, especially in the overnight hours.



Weak onshore flow is expected to Wednesday afternoon, which will help to clear wildfire smoke from the valley.  However, the flow aloft may turn enough southerly to pull elevated wildfire smoke, from SW Oregon fires, northward across NW Oregon.



On Thursday, SW flow aloft is predicted to strengthen and turn gradients onshore at all levels.  That will begin a cooling trend and should clear any remaining wildfire smoke from NW Oregon.



By Friday, the air mass may moisten enough for a threat of showers across western Oregon, but computer models have been inconsistent that far out.



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 15 mph.



     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 (ft) times

         the transport wind speed (mph) 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 (ODA) and the Oregon Department of Forestry (ODF).  For

information contact ODA at 503-986-4701.



To add/remove your email address from this list, please go to:



http://listsmart.osl.state.or.us/mailman/listinfo/willamette-fcst



Pete Parsons

ODF Meteorologist
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