[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Fri Sep 15 11:51:24 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT FRI SEP 15, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

Weak offshore flow pushed wildfire smoke from the Cascades into sections of the valley overnight.  Pressure gradients were reversing, to onshore, late this morning, but valley nephelometer readings remain elevated, especially along the Cascade foothills and near Portland.

A weak upper-level ridge will bring sunshine and seasonably warm temperatures today.  The air aloft is still cool enough for good afternoon mixing.  Onshore flow should slightly increase this afternoon, helping to clear wildfire smoke from the valley and possibly creating a limited burning opportunity.  A PIBAL is scheduled for 1 p.m.

TODAY'S FORECAST:

Sunny and warm.  Some wildfire smoke, especially in the Cascade foothills.

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

Relative humidity:  Dropping to 50% by noon and to near 30% by 5 p.m.

Surface winds: N 5-10 mph.

Transport winds: N 8-13 mph.

Mixing height: Rising to 3000 feet about 1 p.m. and to 5000 feet by 5 p.m.

Salem's sunset tonight: 7:22 p.m.

(Salem Airport data for Thursday, September 14th: High 78°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 65)

EXTENDED DISCUSSION:

Tonight through Saturday, offshore flow should transport smoke, from wildfires in the Cascades, back across the Willamette Valley.  Otherwise, skies will be partly cloudy with seasonal temperatures.  A strengthening westerly flow aloft should force a weak surface thermal trough east of the Cascades Saturday night, with a switch to onshore flow improving air quality.

A major change in the weather pattern is expected on Sunday, with the first in a series of fall-like storms coming onshore.  As a cold front approaches the coast, increasing low-level winds will veer to the S-SW Sunday morning.  This may create a midday burning opportunity, before light rain spreads across the valley in the mid-to-late afternoon.  Rainfall amounts near .10" are likely Sunday night.

A stronger weather system will bring cooler temperatures and more significant rain Monday afternoon and night.  Relatively cool and wet weather is expected to extend through most of next week.  Rainfall totals in excess of an inch are likely.  Long-range computer models are indicating a return to a drier weather pattern, beginning next Friday, which may last about a 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.

    5. In accordance with OAR 837-110-0090, all field burning shall be
         banned when any two of the following criteria are present:
         A. Temperature of 95 degrees Fahrenheit or above
         B. Relative humidity of 30 percent or below
         C. Wind speed of 15 miles per hour or higher

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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