[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 THU JUL 31, 2014



BURN ADVISORY:



State Fire Marshal Burn-Ban Conditions may be reached late this afternoon.



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



A strong upper-level ridge is centered over Idaho with a warm SW flow aloft over Oregon.  A surface thermal trough still extends from eastern Washington through SW Oregon with very weak onshore flow into the Willamette Valley.



Weak upper-air disturbances are acting on mid-level monsoonal moisture over the state to trigger clusters of thunderstorms.  Some thunderstorms are still active this morning across central Oregon, and more will develop this afternoon along the thermal trough axis, from SW to NE Oregon.  There is an outside chance that SE winds aloft could develop and steer storms over the Willamette Valley.  Otherwise, expect mostly sunny and dry conditions.



Salem's sounding showed a couple degrees warming aloft compared to 24 hours ago, so expect high temperatures to climb to back into the mid-90s today. NNE transport winds and the potential for State Fire Marshal Burn-Ban conditions do not favor burning today.



TODAY'S FORECAST:



Sunny and hot.  Afternoon thunderstorms over the Cascades.



Salem's high temperature today will be near 95°F (average is 84°F).

Relative humidity:  Dropping below 50% by 11 a.m. and below 30% by 4 p.m.

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

Transport winds: NNE 10-15 mph this morning; NNE 15-20 mph this afternoon.

Mixing height: Rising to 3500 feet by 5 p.m.

Salem's sunset tonight: 8:39 p.m.



(Salem Airport data for Wednesday, July 30th: High 93°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 70)



EXTENDED DISCUSSION:



A strong upper-level ridge of high pressure will remain anchored just east of the state through early next week.  That will maintain a warm south to SW flow aloft over Oregon, with continued above normal temperatures and scattered thunderstorms from the Cascades eastward.  There is a risk of a thunderstorm moving over the Willamette Valley.



The upper-level ridge is forecast to weaken slightly during the second half of next week.  Increasing onshore flow will likely cool temperatures back closer to average and push the thunderstorm threat well east of the region.  Afternoon transport winds may take on more of a westerly component, which would also open the door for burning opportunities.



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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