[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Thu Aug 10 11:55:40 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT THU AUG 10, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A strong upper-level ridge will bring another sunny and warm afternoon to NW Oregon, but along with considerable wildfire smoke.  A weather system over northern California will push enough moisture and instability northward for thundershower development over the Cascades this afternoon.  Light SE winds aloft could push some storms over the Willamette Valley, from this evening through tonight, mainly south of Salem.

TODAY'S FORECAST:

Sunny and warm.  Continued smoky.  Risk of a thundershower late.

Salem's high temperature today will be near 95°F (average is 83°F).

Relative humidity:  Dropping to 30% by 5 p.m.

Surface winds: Light; becoming N 5-10 mph.

Transport winds: NE 5 mph; becoming N 10 mph.

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

Salem's sunset tonight: 8:25 p.m.

(Salem Airport data for Wednesday, August 9th: High 96°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 35)

EXTENDED DISCUSSION:

Southerly flow aloft will keep skies at least partly cloudy on Friday, and maintain a threat of a thundershower.  Increasing onshore flow will begin a cooling trend and may bring some morning marine clouds into the valley.

Increasing SW flow aloft, and onshore flow at the surface, will cool and stabilize the air mass this weekend.  The threat of thunderstorms will decrease, but a weak cold front may bring drizzle or light showers on Sunday.  Rainfall amounts should be less than .10".  Temperatures will cool from the mid-80s on Saturday to the mid-70s on Sunday.  A generally dry and stable onshore flow pattern may bring favorable burning opportunities next week.

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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