[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 WED AUG 05, 2015

***State Fire Marshal Burn-Ban Conditions, due to gusty winds and low humidity, may be observed this afternoon***


BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is allowed from 12 p.m. to 2 p.m. with a 50 acre limit.



WEATHER DISCUSSION:



An upper-level trough is east of the Cascades, with westerly flow aloft cooling the atmosphere. At the surface, a high is forming off the coast of Oregon that will produce onshore flow today. Little to no cloud cover will allow rapid heating, with great mixing conditions as onshore gradients strengthen ahead of a mild afternoon sea breeze. Today appears to offer favorable open burning opportunities once gradients line up, but breezy gusts in the late afternoon hours may produce State Fire Marshal conditions as the sea breeze enters the Willamette Valley.



TODAY'S FORECAST:



Mostly Sunny.



Salem's high temperature today will be near 84°F (average is 84°F).

Relative humidity:  Dropping near 25% by 5 p.m.

Surface winds: N-NW 5-10 mph this morning; NW 10-15 mph this afternoon.

Transport winds: NW 5-10 mph this morning; NW 10-15 mph this afternoon.

Mixing height: 4000 feet by 11 a.m. and rising to 5000 feet by 2 p.m.

Salem's sunset tonight: 8:33 p.m.



(Salem Airport data for Tuesday, August 3rd: High 85°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)



EXTENDED DISCUSSION:



The remainder of the week may present further favorable days for open burning, as the Pacific Northwest stays dry under modest onshore flow. The upper-level trough over Washington will exit east Thursday morning, and an upper-level ridge will approach off the Oregon coast Thursday afternoon. This will build up our surface high near the Oregon coast, increasing onshore flow and gradients, but producing a stronger afternoon sea breeze as a result on Thursday. On Friday, the upper-level ridge will weaken and pass over the Cascades, warming the atmosphere and suppressing mixing heights. Our surface high will weaken as well, and move across the Willamette Valley. This will produce our warmest day of the week, but temperatures and wind speeds will remain below State Fire Marshal conditions. Gradients will be the key factor to monitor Friday afternoon to determine if the environment will be favorable for open burning.



The weather pattern over the weekend will return to cooler and cloudier conditions across the Pacific Northwest. An upper-level trough will approach the Canadian coastline, cooling the atmosphere under more southwesterly flow aloft. Another weak surface high will form off the coast of southern Oregon, producing onshore flow with clouds, but deflecting isolated showers further north into Washington. Temperatures will drop just below seasonal averages, with no rainfall over the Silverton Hills this weekend.



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, AEM

ODF Meteorologist
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