[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Wed Jul 9 08:49:48 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT WED JUL 9, 2014

BURN ADVISORY:

Recommended times for agricultural burning are from 11 a.m. until 7 p.m.

Prep burning is not allowed.

WEATHER DISCUSSION:

A weak weather system cutting across SW Canada is flattening the upper-level ridge over the Pacific Northwest, with increasing onshore flow into western Oregon this morning.  Low clouds are banked up along the coast, with some penetration into the coastal range gaps.  Some middle and high clouds, associated with a weak upper-level disturbance remain over the Willamette Valley.  Patchy marine clouds may still form this morning but should quickly give way to sunny skies again today.

Weak westerly flow aloft will stabilize the air mass and push the threat of thunderstorm development well east of the Cascades this afternoon.  Minor cooling aloft will make for good afternoon mixing, with temperatures backing off into the upper 80s.  Transport winds should turn more northwesterly, which could provide a burning opportunity, if any fields are ready.

TODAY'S FORECAST:

Partly cloudy this morning.  Sunny but slightly cooler this afternoon.

Salem's high temperature today will be near 88 degrees (average is 81).

Relative humidity:  Dropping below 50% by 11 a.m. and to near 30% by 5 p.m.

Surface winds: NNW 5-10 mph this morning; NW 7-15 mph this afternoon.

Transport winds: NNW 8-15 mph this morning; NW 10-17 mph this afternoon.

Mixing height: Rising to 3000 feet by 11 a.m. and to 5000 feet by 5 p.m.

Salem's sunset tonight: 8:59 p.m.

(Salem Airport data for Tuesday, July 8th: High 94°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 100)

EXTENDED DISCUSSION:

Very little change in the overall weather pattern is expected on Thursday.  The weather system cutting across SW Canada will maintain a dry and stable westerly flow aloft over Oregon.  Weak onshore low-level flow may briefly bring marine clouds inland Thursday morning, but expect another sunny and warm afternoon, with temperatures climbing back into the upper 80s.  NNW transport winds and good afternoon mixing may provide limited burning opportunities, if any fields are ready.

An upper-level ridge is expected to rebuild over the region Friday through Sunday.  Valley temperatures will likely climb back into the 90s.  The flow aloft will back from westerly to more southerly, which will increase the threat of afternoon and evening thundershowers.

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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