[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Fri Jul 18 08:35:54 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:30 AM PDT FRI JUL 18, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

The weather pattern has changed very little since Thursday.  The strong upper-level ridge of high pressure that has been anchored over the region virtually all of July has flattened slightly, yielding to light NW winds aloft and a cooler onshore surface flow into western Oregon.

Willamette Valley temperatures are running about 2-6 degrees cooler than 24 hours ago.  Much like on Thursday, onshore flow is strongest near the Washington border, with winds turning progressively more northerly further south across the Willamette Valley.  Marine clouds made their way up the Columbia River into the extreme north valley this morning, but otherwise skies remain clear.  The weak surge of marine air into the valley overnight has resulted in some minor pressure-gradient-stacking again this morning, but rapid daytime heating should "balance" the gradients by early this afternoon.

Sunny skies will warm temperatures into the mid-80s this afternoon...only a couple of degrees cooler than on Thursday.  Cooler air aloft will provide high mixing heights, but mostly northerly transport winds will limit burning opportunities.  The air mass remains very dry, so State Fire Marshal Burn-Ban conditions may be reached late this afternoon, much like what happened on Thursday, due to low humidity and increasing wind.

TODAY'S FORECAST:

Sunny and warm.

Salem's high temperature today will be near 84°F (average is 83°F).

Relative humidity:  Dropping to 50% by 11 a.m. to near 30% by 5 p.m.

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

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

Mixing height: Rising to 3000 feet by 11 a.m. and to near 5000 feet by 2 p.m.

Salem's sunset tonight: 8:53 p.m.

(Salem Airport data for Thursday, July 17th: High 88°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 100)

EXTENDED DISCUSSION:

The flow aloft is forecast to turn westerly on Saturday and southwesterly on Sunday.  Skies should remain mostly sunny on Saturday, but a weak cold front should spread clouds and cooler conditions across the region Sunday, with a chance of sprinkles or light showers.  However, significant precipitation is not expected.  Southwesterly flow aloft will likely maintain mostly cloudy skies on Monday with onshore flow possibly creating a burning opportunity.  Long-range computer models are showing a stronger upper-level trough moving onshore Tuesday, with the threat of showers across the region.

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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