[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 MON AUG 12, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



The upper-level trough that brought scattered thunderstorms to northwestern Oregon over the weekend has moved north of the region and is centered just off the northern Washington coast this morning.  In its wake, strong onshore flow has stabilized the air mass and forced marine low clouds inland across the Willamette Valley.



The morning sounding over Salem this morning was very similar to Sunday morning, with a saturated marine layer extending up to about 4000 feet. Winds are light in the marine layer, with southwesterly winds above it.  Pressure gradients are onshore and evenly "stacked" this morning from Newport to Redmond.  However, skies are clear east of the Cascade crest, so central Oregon will warm faster than western Oregon this morning; initially yielding unfavorable pressure gradients for burning.



The marine clouds should begin breaking up early this afternoon.  As the valley warms, pressure gradients will likely become favorable for open burning.  Low-level winds should remain fairly light and turn northwesterly.  Winds will stay southwesterly near the mixing height, which should rise to near 5000 feet later this afternoon.  If skies clear early this afternoon, as expected, the prospects are good for open burning later in the afternoon.



TODAY'S FORECAST:



Morning clouds; becoming mostly sunny this afternoon.



Salem's high temperature today will be near 80 degrees (normal is 83).

Relative humidity:  Drops to 50% by 11 a.m. and to near 40% by 5 p.m.

Surface winds:  SW 2-6 mph this morning; NW 4-7 mph this afternoon.

Transport winds: W 5 mph this morning; W 6-10 mph this afternoon.

Mixing height: Rising to 4000 feet by 2 p.m. and to near 5000 feet by 5 p.m.

Salem's sunset tonight: 8:22 p.m.



(Salem Airport data for Sunday, Aug. 11th: High 77°F; Rainfall .00")

(Maximum Ventilation index expected today: 50)



EXTENDED DISCUSSION:



A dry and stable southwesterly flow aloft is forecast on Tuesday, with morning clouds giving way more quickly to sunny skies.  Temperatures should warm into the mid 80s with transport winds turning mostly northerly.



Another mostly sunny and warm day is expected on Wednesday, but an approaching upper-level trough will increase the southwesterly flow aloft, which could lead to very favorable open burning conditions Wednesday afternoon.  A weak cold front is expected to come onshore Wednesday night, with more clouds and cooler temperatures on Thursday.



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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