[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT TUE AUG 27, 2013



BURN ADVISORY:



Agricultural Burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



A decaying frontal system stalled near the northern Oregon coast last night; dumping about one-quarter of an inch of rain across the extreme northern Willamette Valley.  However, rain totals tapered off rapidly to the south, with only a few hundredths of an inch falling in the Silverton Hills.



An autumn-like upper-level trough remains anchored in the eastern Gulf of Alaska this morning, with southwesterly flow aloft holding the surface front in place near the northern Oregon coast.  With little movement of the frontal zone expected today, skies should remain partly sunny across the northern Willamette Valley today.  That should allow for continued drying of fields.



Winds were too strong for burning on Monday.  Weaker wind speeds are expected today.  Wind directions near the mixing height are forecast to stay mainly southerly, but low-level winds are forecast to veer from southeasterly this morning to more westerly by late this afternoon.  The air aloft is still cool enough to support 5000-foot mixing heights.



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 50% by noon and to near 35% by 5 p.m.

Surface winds: SE 4-8 mph this morning; becoming S 5-10 mph early this afternoon and W 5-10 mph late this afternoon.

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

Mixing height: Rising to 3000 feet by 11 a.m. and 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.



A cold front will push onshore Thursday and should be strong enough to bring rain into the Willamette Valley.  Things dry out again on Friday, with continued southwesterly flow aloft.



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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