[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Mon Sep 22 08:52:17 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT MON SEP 22, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

The weather pattern is changing in-sync with the season today.  Autumn officially begins at 7:29 p.m., and our persistent summer-time upper-level ridge is concurrently giving way to our first set of fall storms.

Increasing southwesterly flow aloft has pushed the surface thermal trough into eastern Oregon.  The switch to onshore flow began on Sunday and has already produced significant cooling below 12,000 feet.  The first in a series of weather systems has spread clouds across most of the state, but it will weaken as it moves onshore today.  This system may bring some sprinkles or light showers to the coast, and over the Cascades, but the threat of precipitation is reduced across the Willamette Valley.

If fields remain dry and humidity levels are low enough, favorable burning conditions are likely this afternoon. Surface and transport winds will be southwesterly with mixing heights between 3000 and 4000 feet.  This may be the last burning opportunity, before fields get wet for an extended period of time.  Light rain is possible tonight and Tuesday, with the passage of this first weather system.  Rainfall amounts should be less than .10".

TODAY'S FORECAST:

Mostly cloudy and cooler.

Salem's high temperature today will be near 77°F (average is 75°F).

Relative humidity:  Dropping to 65% by 11 a.m. and to near 50% by 5 p.m.

Surface winds: S 5 mph this morning; SW 5-10 5 mph this afternoon.

Transport winds: SW 10 mph this morning; SW 10-15 mph this afternoon.

Mixing height: Rising to 3000 feet at 11 a.m. and to near 4000 feet by 5 p.m.

Salem's sunset tonight: 7:09 p.m.

(Salem Airport data for Sunday, September 21st: High 86°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 60)

EXTENDED DISCUSSION:

After a chance of light rain on Tuesday, a strengthening southwesterly flow aloft is forecast to drive a much more potent weather system onshore Wednesday.  That system should dump more than .50" of rain across the Willamette Valley.  An upper-level trough will maintain cool and showery weather Thursday and Friday.  A transitory upper-level ridge is expected to bring drier and warmer weather this weekend, with temperatures recovering to near average.  It is uncertain as to whether the dry conditions will extend into next week.

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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