[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Fri Aug 11 11:51:16 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT FRI AUG 11, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

The strong upper-level ridge that has been anchored over southern British Columbia this week is finally shifting eastward, in response to an approaching trough from the Gulf of Alaska.  SE winds aloft should turn more southerly later today, which will likely keep afternoon thundershower development over the Cascades.

Although the air aloft will be cooling today, it is still quite warm, so mixing heights will be slow to rise, likely staying below 4000 feet.  The surface thermal trough has finally pushed into central Oregon.  Onshore flow will slowly increase today, which should continue to clear the wildfire smoke from the area.

TODAY'S FORECAST:

Partly cloudy and cooler.  Scattered thunderstorms developing over the Cascades later this afternoon.

Salem's high temperature today will be near 86°F (average is 83°F).

Relative humidity:  Dropping to near 40% by 5 p.m.

Surface winds: SW 5 mph; becoming WNW 5-10 mph by late this afternoon.

Transport winds: SW 5 mph; becoming WNW 5-10 mph by late this afternoon...shearing to southerly at the mixing height.

Mixing height: Rising to about 3500 feet by 5 p.m.

Salem's sunset tonight: 8:24 p.m.

(Salem Airport data for Thursday, August 10th: High 92°F; Rainfall: Trace)

(Maximum Ventilation Index expected today: 35)

EXTENDED DISCUSSION:

Increasing SW flow aloft, and onshore flow at the surface, will cool and stabilize the air mass this weekend.  The threat of thunderstorms will end, but a weak cold front will likely bring some light rain to NW Oregon on Sunday...mainly from Salem north.  The latest computer guidance gives Salem about one-tenth of an inch of rain, which would be the first measurable rain since June 16th.  Temperatures will cool from the low-80s on Saturday to the mid-70s on Sunday.  A generally dry and stable onshore flow pattern next week will allow temperatures to slowly warm back to seasonal averages with possible burning opportunities.

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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