[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT FRI JUL 11, 2014



BURN ADVISORY:



Recommended times for agricultural burning are from 1 p.m. until 7 p.m.



Prep burning is not allowed.



WEATHER DISCUSSION:



An upper-level ridge of high pressure remains over the Pacific Northwest this morning with light southwesterly flow aloft over Oregon.  At the surface, very weak onshore flow forced low clouds onto the coast this morning but will very little penetration east of the coast range.  Winds are light in the Willamette Valley with mainly just some high clouds.  The air mass has warmed since Thursday with valley temperatures running a few degrees higher than 24 hours ago.



An upper-level disturbance off the northern California coast will turn the flow aloft more southerly this afternoon.  Increasing mid and high level moisture will combine with daytime heating to make the air mass more unstable.  Thundershowers will likely spread northward along the Cascades and could make their way over the Willamette Valley by late today.  These storms have the potential to turn both the pressure gradients and transport winds unfavorable for burning.



TODAY'S FORECAST:



Partly cloudy this morning.  Increasing clouds and quite warm this afternoon with a chance of evening thundershowers.



Salem's high temperature today will be near 92 degrees (average is 81).

Relative humidity:  Dropping to near 50% by 11 a.m. to near 30% by 5 p.m.

Surface winds: Light this morning; NW 5 mph this afternoon.

Transport winds: Light and variable this morning; NW 3-7 mph this afternoon.

Mixing height: Rising to 3000 feet by 1 p.m. and to near 4000 feet by 5 p.m.

Salem's sunset tonight: 8:58 p.m.



(Salem Airport data for Thursday, July 10th: High 90°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 28)



EXTENDED DISCUSSION:



An upper-level ridge is expected to strengthen over the region through early next week.  Valley temperatures will likely warm into the mid to upper 90s with a continued threat of thundershowers, especially Sunday afternoon and evening.



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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