[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

1140 AM PDT FRI SEP 12, 2014



BURN ADVISORY:



*** State Fire Marshal Burn-Ban Conditions May Be Reached This Afternoon ***



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



An upper-level ridge in the eastern Gulf of Alaska is maintaining a very dry NW flow aloft over Oregon.  Cool air over NE Oregon is still being forced westward into a surface thermal trough along the coast.  Offshore pressure gradients have relaxed since Thursday, so wind-speeds will be considerably lower today.  However, the air mass remains very dry, so sections of the valley may achieve State Fire Marshal Burn-Ban conditions this afternoon.



Due to the elevated risk of wildfires, the National Weather Service has extended the Red Flag Warning for most of NW Oregon, including the Willamette Valley, through 8 p.m. Sunday.



TODAY'S FORECAST:



Sunny, warm and very dry.  Not as breezy.



Salem's high temperature today will be near 88°F (average is 78°F).

Relative humidity:  Dropping to near 15% this afternoon.

Surface winds: NNE 7-15 mph.

Transport winds: NNE 15 mph.

Mixing height: Slowly rising to near 4000 feet later this afternoon.

Salem's sunset tonight: 7:28 p.m.



(Salem Airport data for Thursday, September 11th: High 82°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 60)



EXTENDED DISCUSSION:



The upper-level ridge in the eastern Gulf of Alaska is predicted to slowly progress eastward, to over the western U.S., this weekend.  That will force the coastal surface thermal trough inland, to over western Oregon; further warming valley temperatures but slackening winds.  Afternoon highs will likely climb into the low 90s each day.



As the upper-level ridge continues to progress eastward early next week, to over Idaho and Montana, increasing south to southwesterly flow aloft over Oregon will force the surface thermal trough east of the Cascades; turning low-level winds back onshore and initiating a cooling trend.  The switch to onshore flow could create burning opportunities, but there is also a risk of showers or thunderstorms, especially by the middle of next week.  However, long-range computer models maintain a generally dry summer-like pattern through the next two weeks.



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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