[willamette-fcst] fbs0819p.docx

Wed Aug 19 11:45:06 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT WED AUG 19, 2015

***State Fire Marshal Burn-Ban Conditions, due to winds, temperatures & humidity, are likely to be observed this afternoon***
***A Red Flag Warning has been issued for the Willamette Valley and western Cascade foothills until 6 p.m.***

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level ridge over the region is beginning to weaken, and will shift southeast of the Willamette Valley later today. This will keep elevated wildfire smoke over the region, with hazy conditions at the surface. Calm winds have remained over the southern Willamette Valley as a thermal trough has approached the I-5 corridor. Onshore gradients from Newport to Salem are +2.6mb, but offshore gradients from Redmond to Salem persist at -0.8mb. State Fire Marshal Burn-Ban conditions are likely and critical fire weather conditions are not expected to provide favorable open burning opportunities today.

TODAY'S FORECAST:

Hazy with very warm temperatures.

Salem's high temperature today will be near 95°F (average is 82°F).

Relative humidity:  Dropping near 25% by 5 p.m.

Surface winds: NW 5-10 mph this afternoon.

Transport winds: NW 10-15 mph this afternoon.

Mixing height: 3000 feet by 2 p.m. and rising to 4000 feet by 5 p.m.

Salem's sunset tonight: 8:12 p.m.

(Salem Airport data for Tuesday, August 18th: High 97°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 60)

EXTENDED DISCUSSION:

The atmosphere will continue to change for the remainder of the week, as cooler onshore flow returns to the Pacific Northwest. An upper-level trough will move south along the Canadian coastline Thursday, cooling the air aloft and improving mixing conditions across Oregon. This will help clear out remnant smoke aloft and onshore flow will build ahead of a mild sea breeze, suggesting favorable opportunities for open burning.

By Friday morning, the deeper marine layer will require several hours of heating to lift and scatter clouds. Onshore gradients may improve enough to provide favorable burning opportunities again with better mixing conditions in the afternoon. This points to a late afternoon window for open burning ahead of the weekend. Winds are expected to be more northerly, and the strength & timing of the sea breeze will determine the overall possibility for open burning. Wildfire smoke is expected to draw back into the Willamette Valley as the weekend begins Friday evening.

The weather pattern over the weekend will see a return to warm and drier conditions, as a weak upper-level ridge forms off the coast of Oregon. No rainfall is forecast for the Silverton Hills. The warming air aloft will suppress mixing conditions, and smoke from regional wildfires may create a hazy afternoon Saturday as temperatures remain above seasonal averages. A weak sea breeze Sunday afternoon with partly cloudy skies will return onshore flow to the region, and the early part of next week may see favorable open burning opportunities as an upper-level trough approaches the Oregon coast.

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

Tom Jenkins, AEM

ODF Meteorologist
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