[willamette-fcst] Field Burning Forecast - July 25, 2017

Tue Jul 25 08:46:33 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
8:45 AM PDT TUE JUL 25, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level disturbance south of the Willamette Valley has shifted further inland and weakened, allowing surface onshore flow to recover earlier than previously expected. Gradients are already positively stacked, and will be the limiting factor to closely monitor today for potential field burning opportunities. Mixing conditions should develop easily, and if onshore flow increases enough, will shift N winds to NW this afternoon. This pivot in wind direction will slow wind speeds as surface and transport winds move out of alignment, simultaneously removing the threat of State Fire Marshal Burn-Ban criteria being triggered again by lower humidity and breezy conditions.

TODAY'S FORECAST:

Mostly Sunny becoming Partly Cloudy this afternoon.

Salem's high temperature today will be near 92°F (average is 84°F).
Relative humidity:  Currently below 60% and dropping near 30% by 5 p.m.
Surface winds: N 3-7 mph this morning; shifting NW 7-12 mph this afternoon.
Transport winds: N 10 mph this morning; NW 10-15 mph this afternoon.
Mixing height: Rising to 3000 feet around 10 a.m. and over 4000 feet by 5 p.m.
Salem's sunset tonight: 8:46 p.m.

(Salem Airport data for Monday, July 24th: High 91°F; Rainfall: .00")
(Maximum Ventilation Index expected today: 65)

EXTENDED DISCUSSION:

The upper-level disturbance south of the Willamette Valley continues to weaken tonight and shift into SE Oregon, allowing onshore surface flow to continue building for the next several days. Marine clouds intrude Wednesday morning over the southern valley, but merely raise the humidity over the Silverton Hills. Onshore gradients are expected to become positive by mid-day, with NW surface winds becoming SW flow aloft ahead of a stronger marine push Wednesday evening. Marine clouds linger longer on Thursday, keeping humidity elevated and returning temperatures closer to seasonal averages. An approaching disturbance Thursday evening will form clouds over the Cascades, and may prevent positive onshore gradients from developing on Friday. Timing of this feature's arrival will be continuously evaluated, as an earlier arrival would help scour out stubborn marine air and may provide field burning opportunities ahead of the 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.

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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