[willamette-fcst] Field Burning Forecast - August 23, 2017 (Noon Update)

Wed Aug 23 11:44:16 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
11:45 AM PDT WED AUG 23, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A transition in the weather pattern continues today, as a disturbance approaches the Oregon coast and creates W-SW flow aloft. This feature will cause mixing heights to begin rising rapidly, and remain elevated into the early evening; potential field burning opportunities will may remain open as well. Mid-valley gradients remain positively-stacked, and increasing onshore flow will create afternoon clouds over the Cascades. Mountain rain showers will fall over the Cascades and central Oregon, but the Silverton Hills will remain dry today.

TODAY'S FORECAST:

Mostly Cloudy with Mountain Showers late this afternoon.

Salem's high temperature today will be near 81°F (average is 83°F).
Relative humidity:  Below 60% now and nearing 40% by 5 p.m.
Surface winds: N-NW 5-10 mph now, becoming NW 5-10 mph after 3 p.m.
Transport winds: N-NW 5-10 mph now, becoming W-NW 5-10 mph after 3 p.m.
Mixing height: Near 3000 feet currently and rising above 5000 feet by 5 p.m.
Salem's sunset tonight: 8:04 p.m.

(Salem Airport data for Tuesday, August 22nd: High 89°F; Rainfall: .00")
(Maximum Ventilation Index expected today: 50)

EXTENDED DISCUSSION:

The weather pattern remains unstable on Thursday, as a storm system sweeps across southern Canada and a Cold Front with isolated rain showers drives across Washington in the morning hours. The Willamette Valley will be on the edge of this major feature, but most rainfall will occur north of Marion County. Onshore gradients and favorable mixing will be present all day, but elevated humidity and gusty winds will be limiting factors for field burning Thursday afternoon.

Friday will see W flow aloft prevail in the wake of the storm, keeping onshore gradients favorable and clearing skies as the atmosphere begins to calm ahead of the weekend. Winds will remain W-NW, and lighten up enough to suggest field burning opportunities. The limiting factor will be moisture east of the Cascades, as any thundershowers could pause the gradient flow and slow the evacuation of smoke. Mild, sunny weather will persist for the weekend as an upper-level ridge moves into Oregon. The "Dog Days of Summer" will see temperatures rise above seasonal averages again, thanks to clear skies and declining humidity. Continued W flow aloft and onshore flow with afternoon sea breezes appear likely for the last few days of August, which may offer flexible field burning opportunities before Labor Day Weekend.

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

Tom Jenkins, AEM
ODF Meteorologist
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