[willamette-fcst] Silverton Hills Field Burning Forecast - Noon Update

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM PDT THU AUG 22, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



An upper low is to the south of the region rotating clouds, showers and thundershowers toward the Willamette Valley.  Radar echoes show showers to the NE of Salem in the Cascades and echoes moving north through the Roseburg area.  The surface thermal trough is still in the Willamette Valley this morning but moving eastward. Newport to Salem pressure gradient is 3.0 mb with a pressure gradient of -1.2 mb from Salem to Redmond.



The upper low will continue to move up from the south today bringing the chance of showers or thundershowers over the region but mainly over the Cascades.  Clouds will be variable throughout the day.  The pressure gradient will slowly turn more onshore later this afternoon but mixing will remain quite poor - under 3000 ft.



(Salem Airport data for Wednesday, August 21st: High 91°F; Rainfall .00")

(Maximum Ventilation Index expected today:  20)

TODAY'S FORECAST:



Variable clouds with chance of showers or thundershowers.



Salem's high temperature today will be near 83.

Relative humidity drops to 50% around 1 p.m..  Minimum RH near 40%.

Surface winds: Light and variable during the morning to early afternoon.  SW to W at 3-7 mph during the afternoon.

Transport winds: Light and variable during the morning. S to W at 4-8 mph during the afternoon.

Maximum mixing height:  2700 ft.

Sunset tonight: 8:06 p.m.



EXTENDED DISCUSSION:



Upper level low kicks out to the northeast overnight and into Friday leaving the region under dry southwest flow aloft with a trough off the coast.  Onshore flow will bring marine low clouds into the region overnight tonight and into Friday morning.  Expect some clearing tomorrow afternoon.  If there's enough clearing there should be a burn opportunity with mixing heights rising to 4000-5000 ft and wind flow SW'erly.



Over the weekend and into early next week the trough off the coast moves in closer and strengthens.  Expect a pretty deep marine layer with a chance of drizzle or light showers.  Expect cooler conditions with high temperatures potentially staying in the upper 60s.  Wind flow will be light during the early morning becoming NW'erly during the late morning and afternoon.



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 times

         the transport wind speed 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.



Nick Yonker

ODF Meteorologist

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