[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Tue Sep 16 08:56:52 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT MON SEP 15, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from noon until 1:30 p.m. with a 25 acre limit.

WEATHER DISCUSSION:

The strong upper-level ridge that has been over the Pacific Northwest has shifted eastward, to over the Rockies.  An increasing southwesterly flow aloft finally pushed the surface thermal trough east of the Cascades last night; inducing onshore flow and clearing the dense wildfire smoke (from the "36 Pit" fire east of Estacada) from the Willamette Valley.

A very weak upper-level disturbance brought some sprinkles to the north coast early this morning but just middle and high clouds to the interior of western Oregon.  Some marine low clouds have also penetrated into the Willamette Valley.  The Salem sounding this morning showed considerable cooling below 6000 feet, and low-level winds have turned south-southwesterly.

Transport winds are forecast to veer from SSW to WSW this afternoon, with cooler air aloft likely improving mixing heights enough to allow for open burning.  However, the influx of cooler marine air has created some minor gradient-stacking this morning, which will need to be "balanced-out" by daytime heating.

TODAY'S FORECAST:

Cloudy this morning.  Becoming partly cloudy this afternoon.

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

Relative humidity:  Dropping to below 60% by noon and to near 40% by 5 p.m.

Surface winds: S 4-8 mph; becoming w 4-8 mph late this afternoon.

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

Mixing height: Rising to near 2500 feet by noon and 4000 feet by 5 p.m.

Salem's sunset tonight: 7:21 p.m.

(Salem Airport data for Monday, September 15th: High 88°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 60)

EXTENDED DISCUSSION:

Continued southwesterly flow aloft will push a little stronger weather system onto the southern Oregon coast later Wednesday.  With the system moving up from the SW, low-level winds may be unfavorable for burning ahead of it, Wednesday afternoon.  Some showers are possible in the Willamette Valley Wednesday night and Thursday, as the associated weak upper-level trough comes onshore, mainly across southern Oregon and northern California.

A return to dry and warmer weather is expected Friday through Sunday.  Transport winds will turn northerly on Friday and offshore again on Saturday, which could bring smoke from the "16 Pit" wildfire back into the Willamette Valley.  Onshore flow is expected to return on Sunday, which would clear any wildfire smoke from the valley.  Weak southwesterly flow aloft is predicted for Monday with continued onshore flow at the surface possibly creating a burning opportunity.

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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