[willamette-fcst] Silverton Hills AM Forecast_Tom

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT FRI JUL 17, 2015



BURN ADVISORY:



*** State Fire Marshal Burn-Ban Conditions, due to gusty winds and low humidity, may develop late this afternoon. ***



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



An upper-level trough is sliding south across the Canadian border, which will shift northerly winds to weak offshore flow later this afternoon. The surface high offshore Oregon will shift northwest out to sea, helping produce weak offshore flow. Mixing conditions will become well developed in the afternoon hours today, as partly cloudy skies allow rapid heating throughout the morning. Temperatures near seasonal averages will result, but the drying atmosphere and afternoon breezes may produce State Fire Marshal conditions later today.



TODAY'S FORECAST:



Partly Cloudy skies today, and breezy in the afternoon hours.



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

Relative humidity:  Drops to near 50% by 11 a.m. and below 30% by 5 p.m.

Surface winds: N 10-15 mph this morning; N-NE 15-20 mph this afternoon.

Transport winds: N-NE 10-15 mph this morning; NE 20-25 mph this afternoon.

Mixing height: 3000 feet by 12 p.m. and rising to 5000 feet by 5 p.m.

Salem's sunset tonight: 8:54 p.m.



(Salem Airport data for Thursday, July 16th: High 81°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 125)



EXTENDED DISCUSSION:



A Fire Weather Watch will be in effect for the Willamette Valley and western Cascades from 11 p.m. Friday evening through 11 p.m. Saturday. The weekend's weather will feature soaring daytime temperatures, an atmosphere that continues to dry out under offshore flow, and mostly sunny skies with clear nights.



Sunday evening into the early hours of Monday morning, an upper-level low in the Gulf of Alaska and the remnants of Hurricane Dolores, which will be approaching southern California, will weaken the upper-level ridge over the northeastern Pacific Ocean. As it retreats further west, the surface ridge over western Oregon will dissipate and a weak surface frontal system will form out in the Pacific. This will open the gates as onshore flow returns to Oregon, with westerly flow at all levels by Monday afternoon. The dry atmosphere will see clouds move onshore, but limited moisture and no rainfall is expected until late next week. Beginning Monday, open burning opportunities may increase for multiple days based upon these conditions.



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



Tom Jenkins

ODF Meteorologist
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