[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Fri Sep 8 08:55:39 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT FRI SEP 8, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level trough, centered just off the northern California coast, sent a surge of showers and thundershowers northward into Oregon on Thursday. Strong thunderstorms produced large hail and brief downpours in the northern Cascades but only spotty very light rain in the Willamette Valley.  A switch to weak onshore flow also began to clear wildfire smoke from the valley.

An upper-level trough remains over the region today, but the system off the California coast has drifted southward and will be less of a factor.  Winds aloft have veered from SSE to the SSW, which will stabilize the air mass and inhibit thunderstorm development.  However, the air mass remains very moist, so light showers are possible.

Mostly cloudy skies and increasing onshore flow will hold temperatures in the 70s today.  Elevated wildfire smoke should continue to slowly clear from the region.  High humidity and light winds will limit burning opportunities.

TODAY'S FORECAST:

Mostly cloudy, cooler, and humid.  Not as smoky.  A light shower is possible.

Salem's high temperature today will be near 76°F (average is 79°F).

Relative humidity:  Dropping to 60% about 2 p.m. and to 55% by 5 p.m.

Surface winds: Light this morning; NW 5-10 mph this afternoon.

Transport winds: N 5 mph this morning; becoming NW 10 mph this afternoon.

Mixing height: Rising to 3000 feet by 2 p.m. and to 4000 feet by 5 p.m.

Salem's sunset tonight: 7:35 p.m.

(Salem Airport data for Thursday, September 7th: High 86°F; Rainfall: Trace)

(Maximum Ventilation Index expected today: 40)

EXTENDED DISCUSSION:

An upper-level trough is predicted to move across southern British Columbia on Saturday, dragging a weak cold front across NW Oregon in the afternoon.  Temperatures will cool into the low 70s with a chance of light showers late in the day. Strengthening onshore flow should further improve air quality.

A building upper-level ridge is expected to turn transport winds northerly on Sunday, and offshore on Monday, as temperatures climb back to above normal.  Wildfire smoke may get transported back into the region.  Onshore flow could return as soon as Tuesday, initiating a cooling trend, improving air quality, and possibly creating 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.

     5. In accordance with OAR 837-110-0090, all field burning shall be
         banned when any two of the following criteria are present:
         A. Temperature of 95 degrees Fahrenheit or above
         B. Relative humidity of 30 percent or below
         C. Wind speed of 15 miles per hour or higher

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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