[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT THU AUG 17, 2017



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is allowed from noon until 2 p.m. with a 50 acre limit.



WEATHER DISCUSSION:



Onshore flow brought a fresh surge of marine air into NW Oregon overnight with low clouds blanketing most of the Willamette Valley this morning.  A strengthening upper-level ridge will warm the air aloft and clear the marine clouds from the valley this afternoon.  Negatively-stacked onshore pressure gradients will struggle to balance-out this afternoon.  Transport winds will back from NE to N this afternoon and possibly to the NNW late.



TODAY'S FORECAST:



Cloudy this morning; becoming sunny and warm this afternoon.



Salem's high temperature today will be near 85°F (average is 82°F).

Relative humidity:  Dropping to 60% by noon and to near 30% by 5 p.m.

Surface winds: N 5-10 mph.

Transport winds: NNE 10 mph this morning; N 10 mph this afternoon.

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

Salem's sunset tonight: 8:14 p.m.



(Salem Airport data for Wednesday, August 16th: High 87°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 45)



EXTENDED DISCUSSION:



Friday should be mostly sunny with seasonal temperatures.  A weak weather system will flatten the upper-level ridge and slightly increase the onshore flow late in the day.



Expect morning marine clouds with sunny afternoons and seasonal temperatures this weekend.  By Monday, the upper-level ridge will rebuild over the region, with only patchy morning clouds along the immediate coastline.  Valley temperatures should climb back into the upper 80s.



The upper-level ridge will progress eastward by mid-week.  Increasing SW flow aloft will initiate a cooling trend and may create favorable burning conditions.



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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