[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT THU JUL 31, 2014



BURN ADVISORY:



State Fire Marshal Burn-Ban Conditions may be reached this afternoon.



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



Southwesterly flow aloft is feeding weak upper-air disturbances across Oregon; triggering clusters of mainly afternoon and evening thunderstorms.  The daily focus of these storms has been along a nearly stationary thermal trough axis, extending from SW Oregon to north-central Oregon, and well east of the Willamette Valley.  However, today's convective activity is beginning further west, across SW Oregon.  It is not impossible that some thundershower activity could make its way over the Willamette Valley this evening.  Otherwise, expect mostly sunny and hot conditions this afternoon.



TODAY'S FORECAST:



Sunny and hot.  Afternoon thunderstorms over the Cascades with a risk of an evening thunderstorm over the valley.



Salem's high temperature today will be near 95°F (average is 84°F).

Relative humidity:  Dropping to below 30% by 4 p.m.

Surface winds: N 10-15 mph.

Transport winds: NNE 15-20 mph.

Mixing height: Rising only to near 3500 feet by 5 p.m.

Salem's sunset tonight: 8:39 p.m.



(Maximum Ventilation Index expected today: 70)



EXTENDED DISCUSSION:



A strong upper-level ridge of high pressure will remain anchored just east of the state through early next week.  That will maintain a warm south to SW flow aloft over Oregon, with continued above normal temperatures and scattered thunderstorms from the Cascades eastward.  There will continue to be a risk of a valley thunderstorm through the weekend.



The upper-level ridge is forecast to weaken slightly during the second half of next week.  Increasing onshore flow will likely cool temperatures back closer to average and push the thunderstorm threat east of the region.  Afternoon transport winds may take on more of a westerly component; leading to possible burning opportunities.



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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