[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT MON AUG 5, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



A weak and mostly dry upper-level disturbance, moving across Washington, increased the onshore flow overnight across western Oregon.  Marine low clouds blanket the coastline this morning, with some penetration into the coastal range gaps.  However, skies have remained generally clear over the Willamette Valley.



The upper-level disturbance is forecast to progress eastward, into northern Idaho, today.  The combination of weak westerly flow aloft and onshore flow at the surface may turn transport winds enough northwesterly to allow for some burning today.  However, a significant onshore surge is not expected this evening, as the upper-level ridge begins rebuilding over Oregon.



Satellite imagery shows a plume of elevated wildfire smoke, about 100 miles wide, streaming from the fires over SW Oregon northeastward across central Oregon and into SE Washington.  At least for today, the main elevated smoke plume should stay east of the Willamette Valley.



TODAY'S FORECAST:



Sunny and warm.



Salem's high temperature today will be near 91 degrees (normal is 84).

Relative humidity:  Drops below 60% by 11 a.m. and to near 30% by 5 p.m.

Surface winds:  NNW 3-8 mph this morning; NW 5-10 mph this afternoon.

Transport winds: NNW 5-8 mph this morning; NNW 10-12 mph this afternoon.

Mixing height: Rising to 3000 feet by 2 p.m. and to near 4500 feet by 5 p.m.

Salem's sunset tonight: 8:33 p.m.



(Salem Airport data for Sunday, Aug. 4th: High 92°F; Rainfall .00")

(Maximum Ventilation index expected today: 54)



EXTENDED DISCUSSION:



The upper-level ridge is forecast to rebuild over Oregon on Tuesday.  That will suppress mixing heights and turn transport winds more northerly.  Beginning Wednesday, and continuing through Friday, southerly flow aloft will likely transport elevated wildfire smoke across most of western Oregon.  Transport winds should remain generally northerly.  There will also be an increasing chance of thundershowers each day.



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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