[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Wed Jul 30 11:54:38 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT WED JUL 30, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A warm southwesterly flow aloft is periodically sending weak upper-air disturbances across the state; triggering clusters of thunderstorms.  One system brought numerous thunderstorms to the central Cascades on Tuesday, but the next system, currently over SW Oregon, does not have as much moisture associated with it and has yet to trigger any storms today.

Skies remain generally clear across western Oregon at midday, with the exception of patchy low clouds along the coastal strip. There is very weak onshore flow into the Willamette Valley, where temperatures are running about 2-4 degrees cooler than 24 hours ago.  Highs this afternoon should top out near 90 degrees.  A PIBAL is scheduled for 3 p.m. to monitor the transport winds, which are expected to be predominantly northerly this afternoon.

TODAY'S FORECAST:

Sunny and warm.  A few thunderstorms developing over the Cascades.

Salem's high temperature today will be near 90°F (average is 84°F).

Relative humidity:  Dropping to near 30% by 5 p.m.

Surface winds: N 8-12 mph.

Transport winds: N 15 mph.

Mixing height: Rising to 3000 feet by 2 p.m. and to 4500 feet by 5 p.m.

Salem's sunset tonight: 8:41 p.m.

(Maximum Ventilation Index expected today: 68)

EXTENDED DISCUSSION:

A strong upper-level ridge of high pressure will stay anchored over the Rockies with a south to SW flow aloft over Oregon.  That will keep skies mostly sunny with well above normal temperatures.  It will also maintain a threat of afternoon and evening thunderstorms...mainly over the Cascades.  There is a chance that the flow aloft will turn enough southeasterly to push storms over the Willamette Valley.  Very weak onshore flow could provide limited burning opportunities.

A change to more favorable patterns for burning is forecast for next week, as the ridge weakens and allows for slightly more onshore flow.

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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