[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Wed Aug 12 11:57:02 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT WED AUG 12, 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 still producing a south-southeasterly flow aloft over Oregon. The air mass is slightly warmer and more stable today, which has decreased, but not completely eliminated, the thundershower threat across the region.  Daytime heating will, once again, destabilize the air mass, so isolated shower and thundershower development, later this afternoon, can't be ruled out.

At the surface, the thermal trough has shifted east of the Cascades.  Increasing onshore flow may provide a burning opportunity this afternoon, once mixing heights reach 3000 feet, after about 2 p.m.  However, SSE winds at the top of the mixing layer may limit today's burning potential.  A PIBAL is scheduled for 2 p.m. to begin closely monitoring transport winds.

TODAY'S FORECAST:

Sunny and very warm.  Slight chance of isolated late-day thundershowers, mainly over the Cascades.

Salem's high temperature today will be near 89°F (average is 83°F).

Relative humidity:  Dropping below 50% by 2 p.m. and to near 40% by 5 p.m.

Surface winds: SW 5-10 mph this afternoon.

Transport winds: SSW 10-15 mph this afternoon.

Mixing height: Rising to 3000 feet about 2 p.m. and to 4500 feet by 5 p.m.

Salem's sunset tonight: 8:23 p.m.

(Salem Airport data for Tuesday, August 11th: High 89°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 68)

EXTENDED DISCUSSION:

The upper-level trough is predicted to swing NE across the region Thursday and Friday.  That will bring a few degrees of cooling on Thursday, which may have another burning opportunity.  Much cooler and showery weather is likely Friday. Temperatures will drop back into the mid-to-upper 70s.  Rainfall totals are generally expected to be near .10". Dry weather is expected over the weekend with temperatures back in the 80s.

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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