[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT THU JUL 17, 2014



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



The strong upper-level ridge of high pressure over the region is beginning to flatten; yielding to an increasing WNW flow aloft over Oregon.  That forced the surface thermal trough east of the Cascades late Wednesday afternoon; inducing weak onshore flow into the Willamette Valley.



The thermal trough is over eastern Oregon this morning, with Willamette Valley temperatures running about 8-10 degrees cooler than 24 hours ago.  The marine layer is fairly shallow across the interior of NW Oregon, with marine clouds confined to near the Columbia River.  The weak surge of marine air into the valley overnight has resulted in some minor pressure-gradient-stacking this morning, but ample sunshine should reverse that by midday.



Sunny skies will warm temperatures into the mid-80s this afternoon...about 10 degrees cooler than on Wednesday but still above average.  Cooling aloft will significantly improve mixing heights, and NW transport winds should make for favorable burning conditions...perhaps as early as midday.



TODAY'S FORECAST:



Sunny but cooler.



Salem's high temperature today will be near 86°F (average is 83°F).

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

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

Transport winds: NW 3-8 mph this morning; NW 8-15 mph this afternoon.

Mixing height: Rising to 3000 feet by noon and to near 5000 feet by 5 p.m.

Salem's sunset tonight: 8:54 p.m.



(Salem Airport data for Wednesday, July 16th: High 96°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)



EXTENDED DISCUSSION:



The upper-level ridge will continue to flatten and give way to increasing westerly flow aloft on Friday.  Marine clouds will likely penetrate into the Willamette Valley Friday morning, causing more significant pressure-gradient-stacking.  Afternoon sunshine should help to "balance out" the onshore flow, with near average temperatures.  However, transport winds are forecast to turn more northerly, which may inhibit burning chances.



The flow aloft is forecast to turn southwesterly over the weekend.  Expect mostly cloudy skies with temperatures cooling to slightly below average.  A weak weather system may even bring a few light showers to the region.  Unsettled weather is forecast to continue through early next week.



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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