[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT MON JUL 18, 2016



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is allowed from now until 1 p.m. with a 50 acre limit.



WEATHER DISCUSSION:



An upper-level trough, centered just off the northern Oregon coast, is producing a cool, moist, and somewhat unstable SW flow aloft over Oregon.  A deep marine layer will keep skies mostly cloudy this afternoon but also allow for excellent mixing with SW transport winds.  The limiting factors for burning will be gradient-stacking and potential shower development.



Cool and damp weather in central Oregon this morning balanced the gradient stacking, but skies are clearing there at midday, which could reverse that trend.  Daytime heating may also trigger shower development, mainly over the Cascades.  A burning opportunity is possible this afternoon.  Transport winds will be monitored with the first PIBAL scheduled for 1 p.m.



TODAY'S FORECAST:



Mostly cloudy.  Slight chance of showers.



Salem's high temperature today will be near 74°F (average is 83°F).

Relative humidity:  Dropping to near 50% by 2 p.m.

Surface winds: SW 5-12 mph this afternoon.

Transport winds: SW 10-15 mph this afternoon.

Mixing height: Rising to 6000 feet by 2 p.m.

Salem's sunset tonight: 8:53 p.m.



(Salem Airport data for Sunday, July 17th: High 73°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 90)



EXTENDED DISCUSSION:



Very little change is expected on Tuesday with continued SW flow aloft and a deep marine layer remaining across the Willamette Valley.  A burning opportunity is possible, with gradient-stacking and the chance of light showers continuing to be the limiting factors.



The upper-level trough will weaken on Wednesday with the air mass warming and stabilizing enough to end the shower activity.  The air aloft will remain cool enough for good daytime mixing, so a burning opportunity remains possible.  Onshore flow should weaken with transport winds veering to the NW.



An upper-level disturbance is predicted to swing onshore Thursday, which may create a burning opportunity but also introduce a chance of light showers.



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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