[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT FRI SEP 5, 2014



BURN ADVISORY:



***  State Fire Marshal Burn-Ban Conditions Are Likely All Afternoon ***



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



A strong upper-level ridge of high pressure covers much of the eastern Gulf of Alaska.  A warm and very dry NE flow aloft is producing sunny skies across most of Oregon.  The lone exception is along the extreme southern coast, where low-level onshore flow is forcing coastal stratus against the beaches.



At the surface, a thermal trough extends northward along, and just off of, the coast, with high pressure centered over NE Oregon.  The resultant offshore flow is bringing very warm and dry conditions all the way to the northern and central Oregon beaches.  Increasing NE winds and low relative humidity levels have already put much of NW Oregon into State Fire Marshal Burn-Ban Conditions.  Willamette Valley temperatures are running about 10 degrees warmer than 24 hours ago, so highs should easily climb into the 90s.



TODAY'S FORECAST:



Sunny, breezy, and hot.  Very low humidity.



Salem's high temperature today will be near 93°F (average is 80°F).

Relative humidity:  Below 30%; dropping below 20% by 5 p.m.

Surface winds: NE 15-25 mph this afternoon.

Transport winds: NE 20-25 mph this afternoon.

Mixing height: Rising to 3000 feet around 2 p.m. and to 4500 feet by 5 p.m.

Salem's sunset tonight: 7:41 p.m.



(Salem Airport data for Thursday, September 4th: High 86°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 110)



EXTENDED DISCUSSION:



Weak SW flow aloft will push the thermal trough over the interior of NW Oregon on Saturday.  That should be the peak of this warm spell, with Willamette Valley temperatures topping out in the mid-to-upper-90s but with less wind.  Onshore flow should begin a cooling trend on Sunday, with temperatures dropping back into the upper-80s.



Early next week, increasing NW flow aloft will push the thermal trough into eastern Oregon.  Stronger onshore flow will provide more significant cooling but with little or no precipitation.  Temperatures will cool to near average by Tuesday, with possible burning opportunities Monday and Tuesday.



An upper-level ridge is predicted to strengthen over Oregon again later next week, bringing a return of offshore flow and warmer temperatures.  Mostly dry weather, with above average temperatures, is expected through mid-September.



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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