[willamette-fcst] Silverton Hills AM Forecast.doc

Thu Oct 11 08:57:08 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT THU OCT 11, 2012

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.

WEATHER DISCUSSION:

An upper-level ridge remains over Oregon this morning.  At the surface, a broad thermal trough covers most of western Oregon with weak pressure gradients and calm winds across the Willamette Valley.  A blanket of fog formed across the Willamette Valley overnight, and visibilities this morning are locally less than one-quarter of a mile.  Air quality monitoring equipment is also continuing to show some haze across the valley.

The Salem sounding and local pilot reports indicate that the top of the fog layer is no higher than 2000 feet, so it should burn off by mid-afternoon.  Much like yesterday, mixing heights will likely top out between 2000 and 2500 feet, once surface temperatures are able to climb into the upper 60s.  Transport winds are forecast to remain light today, but may turn lightly onshore late this afternoon, as the upper-level ridge begins to weaken and shift to the east.

There may be a late-afternoon burning opportunity today, very similar to yesterday, if the valley can get enough sunshine.  Transport winds will likely remain light, but an approaching cold front will bring increasing onshore flow tonight, which will aid in smoke evacuation.  This will likely be the last burning opportunity of the season (see extended discussion).

(Salem Airport data for Wednesday, Oct. 10th: High 72°F; Rainfall .00")

TODAY'S FORECAST:

Morning fog; becoming sunny but a little hazy in the afternoon.

Salem's high temperature today will be near 70 degrees (normal is 66).

Relative humidity:  Dropping to near 50% by 2 p.m. and to 45% by 5 p.m.

Surface winds:  Light and variable.

Transport winds: Light and variable.

Mixing height: Rising to near 2500 feet this afternoon.

Salem's sunset tonight: 6:33 p.m.

THREE-HOURLY DATA:

                               11 a.m.   2 p.m.  5 p.m.  8 p.m.

     Temperature:                 53       65      69      57

     Relative Humidity:           77%      50%     44%     67%

     Surface Wind Dir/Speed:     Calm    Var 3   Var 3    SW 3

     Transport Wind Dir/Speed:   SE 3     NW 3    NW 3    SW 4

     Mixing Height:              1200     2200    2500    1000

     Ventilation Index:            4        7       8       4

EXTENDED DISCUSSION:

The upper-level ridge that has been over the Pacific Northwest since July is finally going to give way to a strengthening westerly jet stream on Friday.  The first in a series of progressively stronger weather systems will come onshore Friday morning.  The latest computer guidance is showing rain moving into the northern Willamette Valley around midday, so a burning opportunity ahead of this system appears unlikely.  Rain will likely spread southward across the valley Friday afternoon.

Look for mostly cloudy skies over the weekend with a chance of light rain at times, as the main storm track takes aim at Washington.  The jet stream is forecast to sag southward across Oregon late Sunday and Monday, with significant rain likely.

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 15mph.

     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 times

         the transport wind speed 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 and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.

Pete Parsons

ODF Meteorologist
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