[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM PDT MON AUG 5, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



A dry and stable westerly flow aloft is bringing sunny skies to the interior of NW Oregon today.  However, weak onshore flow is keeping marine low clouds banked up against the coastline.  At the surface, a weak thermal trough extends from south-central Washington, along the crest of the northern Cascades, into SW Oregon.



Late-morning pressure gradients are 3.6mb onshore from Newport to Salem but only 0.6mb onshore from Salem to Redmond.  The lack of onshore pressure gradients east of Salem will be a limiting factor for burning this afternoon.  Pressure gradients and transport winds will be monitored closely today (PIBALS begin at 1:00 p.m.) for the possibility of enough northwesterly flow to allow for some open burning. However, a significant onshore surge is not expected this evening, as an upper-level ridge begins rebuilding over Oregon.



Smoke from the multiple wildfires in SW Oregon is being transported well up into the atmosphere, where southwesterly winds aloft (above 8000 feet) are transporting it northeastward across central Oregon and into SE Washington.  That, elevated, smoke plume should stay east of the Willamette Valley today.



TODAY'S FORECAST:



Sunny and warm.



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

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

Surface winds:  NW 5-10 mph this afternoon.

Transport winds: 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.  Even though transport winds should remain generally northerly, settling and down-mixing of wildfire smoke may deteriorate air quality in the Willamette Valley.  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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