[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Wed Sep 3 08:55:52 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT WED SEP 3, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from 11:30 a.m. until 1:00 p.m. with a 50 acre limit.

WEATHER DISCUSSION:

An upper-level trough is centered over NW Montana, with a dry and brisk northwesterly flow aloft over Oregon.  Marine clouds over much of western Washington and NW Oregon this morning will slowly give way to sunshine this afternoon, as the flow aloft turns more northerly and continues to dry out.

Cool air aloft will make for high mixing heights today, but northwesterly transport winds are expected to veer to northerly this afternoon, which will limit burning opportunities.  The first PIBAL is scheduled for 11 a.m., to closely monitor the shifting transport winds.

TODAY'S FORECAST:

Partly cloudy.

Salem's high temperature today will be near 77°F (average is 80°F).

Relative humidity:  Dropping to 60% by 11:30 a.m. and to near 35% by 5 p.m.

Surface winds: Var 0-5 mph this morning; becoming N 5-10 mph this afternoon.

Transport winds: NW 5-10 mph; veering to N 10-15 mph later this afternoon.

Mixing height: Rising to 3000 feet by 11:30 a.m. and to 5000 feet by 2 p.m.

Salem's sunset tonight: 7:45 p.m.

(Salem Airport data for Tuesday, September 2nd: High 80°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

A building upper-level ridge, over the Gulf of Alaska, will bring a dry northerly flow aloft on Thursday and northeasterly flow aloft on Friday.  A corresponding surface thermal trough will build northward along the Oregon coast, turning low-level winds offshore and warming valley temperatures back into the low 90s by Friday.

This warm spell should peak on Saturday, with valley temperatures possibly topping out in the mid-90s.  The ridge is expected to shift westward early next week, which would turn the flow aloft northwesterly.  Little or no precipitation is expected, but increasing onshore flow will cool temperatures back to near average and may create burning opportunities.  Long-range weather models are showing generally dry weather with above average temperatures through the first half of September.

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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