[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM PDT TUE AUG 27, 2013



BURN ADVISORY:



Agricultural Burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



An autumn-like upper-level trough remains anchored in the eastern Gulf of Alaska, with south-southwesterly flow aloft holding a surface front in place near the northern Oregon coast.  Satellite imagery shows mostly cloudy skies stretching from western Washington across the NW corner of Oregon, with mostly sunny skies to the south and east.  The Silverton Hills are on the eastern edge of the clouds.  With little movement of the frontal zone expected, skies should remain partly sunny this afternoon.



At the surface, a weak thermal trough is over the Willamette Valley, with light onshore flow from Newport to Salem and light offshore flow from Redmond to Salem.  Daytime heating will need to shift the thermal trough into central Oregon, before any burning can occur this afternoon.



Wind directions near the mixing height are generally southerly and should shift to south-southwesterly later this afternoon.  Low-level winds should also turn southwesterly, as the surface thermal trough shifts east of the region.  Those wind-shifts are not likely to occur before 2 p.m., but winds will be monitored, with PIBALS, beginning at noon (to be sure). The air aloft (up through 10,000 feet above the surface) is still cool enough to support 5000-foot mixing heights, once surface temperatures climb into the 80s.



TODAY'S FORECAST:



Partly sunny.  Not as breezy as it was on Monday.



Salem's high temperature today will be near 83 degrees (normal is 81).

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

Surface winds: Variable 3-6 mph; becoming S 5-10 mph early this afternoon and SW 5-10 mph late this afternoon.

Transport winds: S 10 mph; becoming SW 10-15 mph later this afternoon.

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

Salem's sunset tonight: 7:57 p.m.



(Salem Airport data for Monday, Aug. 26th: High 82°F; Rainfall .03")

(Maximum Ventilation Index expected today: 75)



EXTENDED DISCUSSION:



Little change in the weather pattern is expected on Wednesday, with a stationary frontal zone remaining near the northern Oregon coast. Skies should be partly sunny again in the Silverton Hills with very similar winds and mixing heights compared to today.



Computer models are consistent in showing the offshore upper-level trough moving closer to the coast on Thursday, with a cold front likely spreading wetting rains inland as far as the Willamette Valley.  Drier weather is on tap for Friday and Saturday, before a much stronger upper-level trough is forecast to spread rain back across the region Sunday and Monday (Labor Day).



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 15mph.



     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 and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.



Pete Parsons

ODF Meteorologist
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