[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Fri Aug 8 11:55:14 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT FRI AUG 8, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

Some marine clouds extended from the north coast into the extreme northern Willamette Valley, at midday, with sunny skies elsewhere across western Oregon.  Onshore pressure gradients are still stronger from Salem to Redmond than they are from Newport to Salem, but continued heating should be sufficient to balance the onshore flow this afternoon.  Mixing heights will be high again today, so the limiting factor for burning will be wind direction.  Low-level winds are currently northeasterly and will not likely take on a westerly component today.  A PIBAL is scheduled for 2 p.m.

TODAY'S FORECAST:

Sunny and seasonably warm.

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

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

Surface winds: NE 7-12 mph; becoming N 8-14 mph.

Transport winds: N-NNE 10-15 mph.

Mixing height: Rising to near 5000 feet by 5 p.m.

Salem's sunset tonight: 8:29 p.m.

(Salem Airport data for Thursday, August 7th: High 86°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

An upper-level trough is forecast to approach the northern California coast on Saturday, turning the flow aloft southeasterly over Oregon.  That will slacken the onshore flow and warm temperature back to above average.  On Sunday, increasing SE flow aloft will further warm temperatures and circulate moisture and instability northward over the state.  Scattered thundershowers will likely development over SW Oregon Sunday afternoon and may advance into NW Oregon by Sunday evening.  SE flow aloft will continue on Monday, with more clouds likely preventing further warming.  Daytime heating should be sufficient to produce at least scattered thundershowers statewide.  These storms will produce locally wetting rains.

An approaching trough, from the Gulf of Alaska, will turn the flow aloft southwesterly on Tuesday, pushing the thundershowers east of the region.  Increasing onshore flow will cool temperatures back to near normal and could create a significant burning opportunity, if fields are not too wet.  Dry and warmer weather is predicted for the second half of next week, with transport winds turning mostly northerly.

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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