[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Thu Aug 13 11:59:16 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT THU AUG 13, 2015

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level low-pressure system off the northern California coast is continuing to circulate considerable wildfire smoke northward across the region, via S-SE winds aloft.  Air quality sensors are showing moderately elevated smoke levels extending as far north as the northern Oregon Cascades.

At the surface, a thermal trough extends from central Washington into central Oregon.  Onshore flow across NW Oregon forced marine low clouds into the Willamette Valley this morning.  Cloudy skies are yielding to sunshine at midday, but the later start to heating today should drop maximum temperatures a few degrees compared to yesterday.  The air mass aloft is still quite warm and stable, so there is only a minimal threat of isolated mountain shower or thundershower development later today.

Onshore gradients are fairly evening stacked at midday and should continue to improve with daytime heting.  Predicted westerly transport winds may provide a burning opportunity later this afternoon.  However, SSE winds above the mixing layer will continue to circulate wildfire smoke into the region.  A PIBAL is scheduled for 2 p.m.

TODAY'S FORECAST:

Morning clouds...mostly sunny but hazy this afternoon.  A little cooler.  Increasing chance of showers or thundershowers late today and overnight.

Salem's high temperature today will be near 88°F (average is 83°F).

Relative humidity:  Dropping below 50% by 2 p.m. and to near 35% by 5 p.m.

Surface winds: W 5-10 mph this afternoon.

Transport winds: SW-W 5-10 mph this afternoon.

Mixing height: Rising to 3000 feet about 2 p.m. and to 4000 feet by 5 p.m.

Salem's sunset tonight: 8:22 p.m.

(Salem Airport data for Wednesday, August 12th: High 91°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 40)

EXTENDED DISCUSSION:

The upper-level trough is predicted to swing NE across the region on Friday.  That will bring much cooler and showery weather along with a chance of thundershowers. Temperatures will drop back into the low-to-mid 70s. Computer models remain consistent in predicting about .10" of rain to the Silverton Hills on Friday. Dry weather is expected over the weekend with temperatures recovering back into the 80s.  Another upper-level trough may create favorable burning conditions as soon as Monday afternoon.

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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