[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT TUE AUG 20, 2013



BURN ADVISORY:



Agricultural Burning:



North of Salem:  Recommended agricultural burn times are noon to 6 p.m.

>From Salem south:  Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



The weak weather system that pushed across southern British Columbia yesterday has moved well east of the region, which is allowing the upper-level ridge to rebuild over Oregon today.  Patchy low clouds in the Willamette Valley this morning should quickly give way to sunshine, with temperatures warming into the mid to upper 80s this afternoon.



At the surface, a thermal trough is building northward along the southern Oregon Coast.  Onshore pressure gradients have relaxed across NW Oregon and are turning northerly.  The Salem sounding this morning showed brisk NE winds from just above the surface through 3000 feet.  Transport winds should remain mostly northeasterly today, which is not favorable for burning.  Warming aloft will also slightly suppress mixing heights.



TODAY'S FORECAST:



Areas of morning low clouds; sunny, warm, and breezy this afternoon.



Salem's high temperature today will be near 87 degrees (normal is 82).

Relative humidity:  Drops to 50% by noon and to near 30% by 5 p.m.

Surface winds:  NNE 5-10 mph this morning; NNE 10-15 mph this afternoon.

Transport winds: NE 10-15 mph this morning; NE 15-20 mph this afternoon.

Mixing height: Rising to 3000 feet at noon and to near 4500 feet by 5 p.m.

Salem's sunset tonight: 8:09 p.m.



(Salem Airport data for Monday, Aug. 19th: High 87°F; Rainfall .00")

(Maximum Ventilation Index expected today: 90)



EXTENDED DISCUSSION:



The flow aloft will turn weakly southwesterly on Wednesday, with continued warming aloft over Oregon.  An upper-level disturbance, over northern California, will trigger afternoon thundershowers across much of southern Oregon, but skies should remain sunny over NW Oregon.  Transport winds are forecast to remain generally northeasterly.



An upper-level disturbance is forecast to lift out of northern California, on Thursday, into south-central Oregon.  That will likely trigger widespread thundershower activity from the Cascades eastward.  Skies should remain mostly sunny across NW Oregon, but increasing onshore flow will initiate a cooling trend and may create an afternoon burning opportunity.



Increasing southwesterly flow aloft will bring dry but cooler conditions on Friday.  An approaching weak weather system may increase the onshore gradients enough to allow for some afternoon burning.



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 15mph.



     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 and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.



Pete Parsons

ODF Meteorologist
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