[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT TUE SEP 10, 2013



BURN ADVISORY:



Agricultural burning north of Salem: recommended burn times are from noon until 5:30 p.m.



Agricultural burning from Salem south is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



A strengthening upper-level ridge of high pressure will produce dry and stable NE flow aloft over Oregon today.  The morning sounding over Salem showed unseasonably warm air above a relatively cool marine layer extending from the surface up to about 2500 feet.  That will keep maximum mixing heights suppressed to only near 3000 feet this afternoon.



Satellite imagery shows a narrow band of marine low clouds extending down the length of the Willamette valley this morning, mainly along the Cascade foothills.  At the surface, a thermal trough is building northward along the Oregon coast.  A drying NE flow across western Oregon, below about 5000 feet, should help to evaporate the marine clouds by midday, with valley temperatures climbing into the low 90s by late this afternoon.



Increasingly dry NNE winds could put the valley into State Fire Marshal Burn-Ban conditions late this afternoon, but elevated surface dew-point temperatures, due to recent heavy rains, may keep humidity levels above 30%.



TODAY'S FORECAST:



Morning clouds; Sunny and warmer this afternoon.



Salem's high temperature today will be near 92 degrees (normal is 79).

Relative humidity:  Drops below 60% by noon and to near 30% by 5 p.m.

Surface winds: NNE 6-10 this morning; increasing to NNE 7-15 this afternoon.

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

Mixing height: Rising to 2500 feet by noon and to about 3200 feet by 5 p.m.

Salem's sunset tonight: 7:32 p.m.



(Salem Airport data for Monday, Sept. 9th: High 84°F; No Rain)

(Maximum Ventilation Index expected today: 48)



EXTENDED DISCUSSION:



A strong ridge of high pressure will remain over Oregon on Wednesday, with the coastal thermal trough moving over the Willamette Valley.  That will warm valley temperatures into the mid 90s.  North winds will slacken, but the combination of warm temperatures and lowering humidity levels could put the valley into State Fire Marshal Burn-Ban conditions Wednesday afternoon.



The surface thermal trough is forecast to shift into central Oregon on Thursday and eastern Oregon on Friday.  That will initiate a slow cooling trend, with high temperatures backing off into the mid to upper 80s by Friday.  Transport winds may turn enough onshore to allow for the burning of any remaining fields, but details remain uncertain.  A more significant change to an autumn-like onshore flow pattern is advertised for next week.



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 15mph.



     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 and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.



Pete Parsons

ODF Meteorologist
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