[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Thu Aug 21 11:53:59 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT THU AUG 21, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from now until 1 p.m. with a 50 acre limit.

WEATHER DISCUSSION:

Middle and high clouds continue to stream across northern Oregon at midday, in response to an increasing northwesterly flow aloft.  An upper-level disturbance, sliding southward along the British Columbia coastline, will slowly turn the flow aloft more westerly this afternoon; setting up quite favorable burning conditions.

Mixing heights will rise to above 3000 feet, once surface temperatures warm into the low-70s and should rise to near 5000 feet by 2 p.m.  An 11 a.m. PIBAL showed light SW winds below about 3000 feet; abruptly veering to NW above that level.  Winds are expected to turn more westerly at all transport levels this afternoon.  A significant surge of marine air into the valley tonight should be sufficient to evacuate any residual smoke.

TODAY'S FORECAST:

Mostly sunny.

Salem's high temperature today will be near 79°F (average is 82°F).

Relative humidity:  Dropping to near 35% by 5 p.m.

Surface winds: WNW 5-10 mph.

Transport winds: WNW 10-15 mph this afternoon.

Mixing height: Rising above 5000 feet from 2 p.m. through 6 p.m.

Salem's sunset tonight: 8:08 p.m.

(Salem Airport data for Wednesday, August 20th: High 79°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 100)

EXTENDED DISCUSSION:

An upper-level trough will move over the region on Friday.  After areas of morning drizzle, expect partial afternoon clearing with a threat of showers.  High temperatures will likely remain in the 70s.  The upper-level trough is forecast to move east of the state over the weekend with a dry northwesterly flow aloft clearing skies and warming temperatures back into the mid to upper 80s.  The updated long-range guidance is now building a fairly strong upper-level ridge over the region early next week, which would warm valley temperatures back into the 90s with generally north to NE transport winds.

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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