[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Wed Aug 26 08:48:53 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT WED AUG 26, 2015

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

Little change is expected in the overall weather pattern today.  A nearly stationary upper-level trough, centered about 800 miles off of the northern Oregon coast, will maintain a dry SW flow aloft and very weak onshore flow at the surface.  That is keeping area wildfire smoke confined mostly to the Cascades eastward.  Air quality remained good overnight in the Willamette Valley, but there is still smoke visible over the Cascades this morning.

Isolated thunderstorm development is possible again this afternoon over the Cascades.  Otherwise, expect fair skies and above average temperatures.  The air aloft has warmed since Tuesday, which will make for lower mixing heights today.  Relative humidity values should drop below 30% again this afternoon, with a continued elevated wildfire risk.

TODAY'S FORECAST:

Partly cloudy and warm.

Salem's high temperature today will be near 86°F (average is 81°F).

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

Surface winds: Var 2-6 mph this morning; W 2-7 mph this afternoon.

Transport winds: Var 5 mph this morning; SW 5-15 mph this afternoon.

Mixing height: Rising to 3000 feet by 3 p.m. and to 4000 feet by 5 p.m.

Salem's sunset tonight: 8:00 p.m.

(Salem Airport data for Tuesday, August 25th: High 85°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 60)

EXTENDED DISCUSSION:

The offshore upper-level trough will move very little through Thursday.  Continued dry SW flow aloft will maintain partly cloudy and warm weather with a chance for isolated afternoon thundershowers over the Cascades. Increasing onshore flow should keep wildfire smoke confined to the Cascades eastward, but the risk for wildfires will remain elevated across western Oregon.

The upper-level trough is expected to advance towards the coast on Friday.  Increasing SW flow aloft and onshore flow at the surface will bring clouds and cooler temperatures.  Wetting rains are expected to move into the region Friday night and Saturday, with over one-half inch possible in the Silverton Hills.  Additional weather systems are expected to move across the region Sunday and Tuesday, bringing more rain and keeping temperatures cool.  That should significantly reduce the wildfire risk across western Oregon.

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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