[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:30 AM PDT THU JUL 20, 2017



BURN ADVISORY:



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



Prep burning is not allowed.



WEATHER DISCUSSION:



A weak upper-level trough induced a marine push last night.  Low clouds blanket the coast and all of NW Oregon this morning.  Drizzle has been falling along much of the coast and in areas of the Willamette Valley.  The marine layer is about 6000 feet thick, so clouds will be slow to clear this afternoon.  As a result, temperatures will struggle to reach the upper 70s.



Although mixing heights and transport winds will be favorable for burning today, negative gradient-stacking should persist, which would promote down-mixing of smoke.



TODAY'S FORECAST:



Cloudy this morning with areas of drizzle.  Clearing this afternoon.



Salem's high temperature today will be near 78°F (average is 83°F).

Relative humidity:  Dropping to 60% by 11 a.m. and to near 40% by 5 p.m.

Surface winds: SW 5-10 mph; becoming NW 5-10 mph in the late-afternoon.

Transport winds: SW 10 mph; becoming W 10 mph in the late-afternoon.

Mixing height: Rising to 3000 feet around 11 a.m. and to 6000 feet by 5 p.m.

Salem's sunset tonight: 8:51 p.m.



(Salem Airport data for Wednesday, July 19th: High 83°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 60)



EXTENDED DISCUSSION:



The air mass will begin warming on Friday, with weak westerly flow aloft and decreasing onshore flow.  Morning clouds will be less extensive and give way to more sunshine, allowing valley temperatures to climb back into the 80s.  Westerly transport winds and good mixing may create a burning opportunity, if negative gradient-stacking can be overcome by daytime heating.



Dry and warmer weather is expected over the weekend, with high temperatures climbing to around 90°F. Increasing SW flow aloft will introduce a chance of thundershowers across SW Oregon by Sunday night and Monday.  It is possible that these storms could make it as far north as the Willamette Valley.  A marine push may create a burning opportunity Tuesday afternoon, with a generally dry onshore flow pattern expected to bring moderate temperatures later 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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