[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT MON SEP 26, 2016



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



A strong and broad upper-level ridge, centered over Idaho, is slowly sifting eastward.  A surface thermal trough was along the coast this morning and is shifting over the Willamette Valley at midday.  Pressure gradients are flat from Newport to Salem and remain offshore from Redmond to Salem.  Today will likely be apex of this warm spell and the warmest day until next spring.



Warm air aloft will likely keep mixing heights from climbing much above 3000 feet today.  The surface thermal trough is expected to shift into central Oregon later this afternoon, which may turn transport winds northwesterly and create a late-day burning opportunity.  A PIBAL is scheduled for 2 p.m.



TODAY'S FORECAST:



Sunny and unseasonably warm.



Salem's high temperature today will be near 87°F (average is 74°F).

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

Surface winds: Var 5 mph; becoming NW 5-10 mph late this afternoon.

Transport winds: Var 5 mph; becoming NW 10 mph late this afternoon.

Mixing height: Rising to 3000 feet around 3 p.m.

Salem's sunset tonight: 7:01 p.m.



(Salem Airport data for Sunday, September 25th: High 84°F; Rainfall: 0.00")

(Maximum Ventilation Index expected today: 35)



EXTENDED DISCUSSION:



A weak cold front will push across SW British Columbia tonight with the trailing edge bringing some clouds and a marine push into NW Oregon.  Tuesday will be about 10 degrees cooler than today with morning clouds and afternoon sunshine.  Gradient-stacking will likely be unfavorable for burning with northerly transport winds.



SW flow aloft will continue through Thursday with onshore cooling temperatures back close to average.  Transport winds will be mostly northerly but could turn enough NNW in the afternoons for limited burning.  Friday could be a burning opportunity ahead of a cold front.



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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