[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 MON SEP 8, 2014



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



A weak upper-level trough has initiated a cooling trend across western Oregon.  The surface trough is shifting into central Oregon with increasing onshore flow from Newport to Redmond.  Low clouds remain banked up along the coast with middle and high clouds now covering much of NW Oregon.  Temperatures are similar to 24 hours ago but should top out a few degrees cooler than yesterday, due to increasing cloud-cover and onshore flow.



The air aloft is still quite warm, so mixing heights will initially be slow to climb this afternoon.  However, continued cooling aloft and daytime heating should make for good mixing later this afternoon.  Pressure gradients are already favorable to keep smoke elevated, so wind direction will be the main factor limiting burning today.  The latest computer guidance is suggesting that transport winds will back from northerly to northwesterly, which would allow for some burning.  The first PIBAL is scheduled for 1 p.m.



TODAY'S FORECAST:



Considerable middle and high clouds.  Cooler.



Salem's high temperature today will be near 80°F (average is 79°F).

Relative humidity:  Dropping to near 35% by 5 p.m.

Surface winds: N 3-6 mph; becoming NW 5-10 mph later this afternoon.

Transport winds: N 5 mph; becoming NW 10-15 mph later this afternoon.

Mixing height: Rising to 3000 feet by 2 p.m. and to 5000 feet by 5 p.m.

Salem's sunset tonight: 7:36 p.m.



(Salem Airport data for Sunday, September 7th: High 87°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 60)



EXTENDED DISCUSSION:



In the wake of today's upper-level trough, more marine air will pour into western Oregon tonight.  Expect some low clouds across the Willamette Valley Tuesday morning, with areas of drizzle possible.  A dry northwesterly flow aloft should clear skies Tuesday afternoon.  Cool air aloft will maintain good afternoon mixing, but poor gradient-stacking and predicted northerly transport winds will limit burning opportunities.



An upper-level ridge is expecgted to strengthen over Oregon again later this week, bringing a return of offshore flow and warmer temperatures.  Long-range models are maintaining a generally dry weather pattern, with above average temperatures, through the middle of the month.



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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