[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Tue Aug 18 08:42:24 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:40 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 from 11 a.m. today through 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.  Very warm air aloft will suppress afternoon mixing and allow temperatures to climb to near 100°F.  Air quality is generally good across NW Oregon, but there is some haze, due to area wildfires, mainly near the Cascades.  N-NE winds may bring additional wildfire smoke into the valley this afternoon and tonight.

A building surface thermal trough along the coast has turned pressure gradients offshore, which will keep the air mass very dry and allow for maximum warming of surface temperatures this afternoon.  The combination of heat, low humidity levels, and wind should put the valley into State Fire Marshal Burn-Ban conditions early this afternoon.

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 30% by noon and to near 15% by 5 p.m.

Surface winds: NNE 5-12 mph this morning; N 10-20 mph this afternoon.

Transport winds: NE 10-15 mph this morning; 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 likely occur too late in the day to create a burning opportunity.  Red Flag Warnings are in effect through Wednesday afternoon.

On Thursday, a strengthening NW flow aloft will bring more significant onshore flow and cooling to western Oregon.  High temperatures are expected to drop 15-20 degrees.  A significant burning opportunity is possible Thursday afternoon, 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.  Skies should clear Friday afternoon, but temperatures will hold near 80°F.  Afternoon NW transport winds may provide a burning opportunity, if gradients can become favorable.

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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