[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Tue Aug 18 11:21:20 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:20 AM PDT TUE AUG 18, 2015

BURN ADVISORY:

***State Fire Marshal Burn-Ban conditions are likely this afternoon.***

***A Red Flag Warning is in effect for the Willamette Valley until 6 p.m. Wednesday.  Critical fire weather conditions are expected.***

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level ridge will reach peak strength today with a very dry and warm NW flow aloft over Oregon.  A building surface thermal trough, along the coast, is showing signs of shifting into the Willamette Valley.  That will promote maximum heating this afternoon and allow additional smoke, from wildfires east of the Cascades, to enter the valley.

TODAY'S FORECAST:

Sunny with record heat.  Hazy at times due to wildfire smoke.

Salem's high temperature today will be near 100°F (record: 97°F set in 1967).

Relative humidity:  Dropping below 15% by 5 p.m.

Surface winds: N 10-20 mph this afternoon.

Transport winds: NNE 10-20 mph this afternoon.

Mixing height: Rising to 3000 feet about 3 p.m. and to 3500 feet by 5 p.m.

Salem's sunset tonight: 8:14 p.m.

(Salem Airport data for Monday, August 17th: High 91°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 70)

EXTENDED DISCUSSION:

On Wednesday, the upper-level ridge will weaken with increasing NW flow and minor cooling aloft over Oregon.  The surface thermal trough is expected to shift east of the Cascades in the afternoon...initiating onshore flow into NW Oregon.  That should cap temperatures in the mid-90s but will likely occur too late in the day to create a burning opportunity.

On Thursday, a strengthening NW flow aloft will bring more significant onshore flow and cooling to western Oregon.  Temperatures are expected to drop 15-20 degrees.  Predicted WNW transport winds may provide a significant burning opportunity, depending on the strength and timing of the marine push.

Further deepening of the marine layer should force low clouds into the Willamette Valley Friday morning, but no precipitation is expected as far south as the Silverton Hills.  A building upper-level ridge should clear skies in the afternoon.  Temperatures will top out near 80°F.  A limited burning opportunity is possible, but transport winds may be too northerly.

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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