[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT MON SEP 8, 2014



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



The strong upper-level ridge of high pressure responsible for the recent spell of hot and dry weather is giving way to a very weak upper-level trough this morning.  The surface thermal trough has shifted east of the Cascades with increasing onshore flow across NW Oregon.  Marine low clouds blanket the coast but do not extend east of the coast range.



Bands of middle and high clouds, associated with the approaching upper-level trough, cover much of Washington and extend across the NW corner of Oregon.  These cloud-bands will sweep across northern Oregon today, but no precipitation is expected.  Temperatures will cool back to near average.



The morning sounding over Salem still showed NE winds below 3000 feet but a switch to NW winds from 3000 feet through 8000 feet, along with significant cooling.  Because the air temperatures are still quite warm near 3000 feet, it may take until about 2 p.m. for mixing heights to reach that level.  Continued cooling aloft should lift mixing heights to near 5000 feet late this afternoon.  Transport winds may turn northwesterly enough to create a burning opportunity and will need to be closely monitored.



TODAY'S FORECAST:



Increasing middle and high clouds.  Cooler.



Salem's high temperature today will be near 80°F (average is 79°F).

Relative humidity:  Dropping to 50% by noon and to near 35% by 5 p.m.

Surface winds: N 3-6 mph this morning; NW 5-10 mph this afternoon.

Transport winds: N 5 mph this morning; NW 10-15 mph this afternoon.

Mixing height: Rising to 3000 feet around 2 p.m. and to 5000 feet by 5 p.m.

Salem's sunset tonight: 7:36 p.m.



(Salem Airport data for Sunday, September 7th: High 87°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 60)



EXTENDED DISCUSSION:



In the wake of today's upper-level trough, a stronger surge of marine air will likely force low clouds into the Willamette Valley overnight, with local drizzle possible early Tuesday.  A dry northwesterly flow aloft should clear skies Tuesday afternoon.  Cool air aloft will maintain goo afternoon mixing, but mostly northerly transport winds will limit burning opportunities.



An upper-level ridge is predicted to strengthen over Oregon again later this week, bringing a return of offshore flow and warmer temperatures.  Long-range models are maintaining a generally dry weather pattern, with above average temperatures, through the middle of the month.



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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