[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Tue Sep 20 11:57:34 PDT 2016

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT TUE SEP 20, 2016

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level trough, dropping southward just offshore, is producing a WSW flow aloft over Oregon.  A band of middle and high clouds, associated with the jet stream, has pushed mostly east of the Willamette Valley, allowing for more sunshine today.  That will aid in the drying of damp fields.

Cool air aloft and surface heating will provide excellent mixing today but is already causing cumulus cloud development.  A shower can't be ruled out, especially over higher terrain, but the bulk of the shower activity will remain offshore today.  Northerly low-level winds should veer to the NW later today, which may create a burning opportunity, if fields can dry out enough.

TODAY'S FORECAST:

Partly cloudy.  Risk of a shower, mainly over higher terrain.

Salem's high temperature today will be near 71°F (average is 76°F).

Relative humidity:  Dropping to 50% around 2 p.m. and to near 40% by 5 p.m.

Surface winds: N 5-10 mph; becoming NNW 5-10 mph later this afternoon.

Transport winds: N 10 mph; becoming NNW 10-15 mph later this afternoon.

Mixing height: Rising to 3000 feet around 1 p.m. and to 5000 feet by 5 p.m.

Salem's sunset tonight: 7:12 p.m.

(Salem Airport data for Monday, September 19th: High 68°F; Rainfall: 0.00")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

The upper-level trough will onshore on Wednesday.  Cooling aloft will continue to provide excellent mixing, but the chance of showers will increase.  NNW winds may provide a burning opportunity for dry fields.

The upper-level trough will push SE of the region on Thursday with a drier and more stable NW flow aloft ending the threat of showers.  Increasing WNW transport winds may produce favorable burning conditions for dry fields.

A weakening frontal system is expected to turn transport winds to the SSW on Friday, possibly creating another burning opportunity.  This system may also bring rain to the region as soon as Friday afternoon, but rainfall totals are expected to be only around one-tenth of an inch or less.

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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