[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 WED JUL 16, 2014



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



A strong upper-level ridge of high pressure, centered directly over Oregon this morning, is beginning to flatten in response to a strengthening northwesterly jet stream over the northern Gulf of Alaska.  A gradual cool-down will commence today, as the surface thermal trough, which was over the Willamette Valley on Tuesday, shifts into eastern Oregon.  That will allow increasing amounts of marine air to seep into western Oregon, over the next few days, cooling temperatures back to near average by Friday.



Pressure gradients this morning are weakly onshore from Newport to Salem but flat from Salem to Redmond, which means that the surface thermal trough is slowly shifting east of the Willamette Valley.  Marine low clouds are banked up along the entire coastline with some penetration into the coastal mountain passes and up the Columbia River.  Skies are sunny in the Willamette Valley with temperatures running a few degrees warmer than 24 hours ago.  As the marine air begins filtering into the valley this afternoon, it should keep temperatures from getting quite as warm as yesterday.



The air aloft is still very warm, so mixing heights will be slow to climb this afternoon. However, if the surface thermal trough shifts far enough eastward to allow for evacuation of smoke over the Cascades, and transport winds turn enough to the NW, some burning of fields may be possible.



TODAY'S FORECAST:



Sunny and warm.



Salem's high temperature today will be near 94°F (average is 83°F).

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

Surface winds: Light this morning; NNW 5-10 mph this afternoon.

Transport winds: NNW 5 mph this morning; NW 7-12 mph this afternoon.

Mixing height: Rising to 3000 feet by 2 p.m. and to near 3500 feet by 5 p.m.

Salem's sunset tonight: 8:55 p.m.



(Salem Airport data for Tuesday, July 15th: High 96°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 42)



EXTENDED DISCUSSION:



The upper-level ridge will continue to flatten and give way to increasing westerly flow aloft Thursday and Friday.  Marine clouds will likely to penetrate into the Willamette Valley each morning with afternoon sunshine.  Temperatures should cool into the mid 80s Thursday and low 80s Friday.  Cooling aloft will improve afternoon mixing, with NW transport winds likely making for favorable burning conditions.  One limiting factor will be the amount of gradient-stacking that will need to be overcome each afternoon.



The flow aloft is forecast to turn southwesterly over the weekend, with a weak weather system possibly bringing light showers to the region and Saturday.  Look for temperatures to cool to slightly below average under mostly cloudy skies.



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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