[willamette-fcst] SILVERTON HILLS FIELD BURNING FORECAST

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT MON JUL 29, 2013



BURN ADVISORY:



Recommended burn times for agricultural burning are from 12:00pm to 6:30pm.

Prep burning is not allowed.



WEATHER DISCUSSION:



A weak upper level trough resides over the Pacific Northwest this morning.  It has brought some marine low clouds into the Willamette Valley, mainly from Marion County northward.  Surface pressure gradients are fairly light with 1.7 mb onshore gradient from Newport to Salem and a 2.7 mb onshore gradient from Salem to Redmond.  The sounding over Salem this morning showed a weak NW to N flow.



Expect only subtle changes over the state today with the trough strengthening a little over the eastern Pacific which will build minor ridging over the state today and tonight.  Marine layer goes up to around 3500 ft this morning which usually means a burn off time by noon.  Once the marine clouds dissipate, NW wind flow should increase some and make for a possible burn opportunity.



(Salem Airport data for Sunday, July 28th: High 80°F; Rainfall .00")

(Maximum Ventilation Index expected today:  40)

TODAY'S FORECAST:



Morning low clouds with late morning clearing. Near normal temperatures.



Salem's high temperature today will be near 81.

Relative humidity drops to 50% around noon.  Minimum RH 30-35%.

Surface winds: NW to N at 4-7 mph during the morning.  NW to NNW at 5-10 mph during the afternoon.

Transport winds: NW to N at 4-7 mph during the morning. NW to NNW at 6-10 mph during the afternoon.

Maximum mixing height:  4000 ft.

Sunset tonight: 8:42 p.m.



EXTENDED DISCUSSION:



Upper level trough continues to dig southward over the eastern Pacific on Tuesday bringing more ridging over the state.  This should reduce or eliminate the marine low clouds from invading the Valley tomorrow morning as onshore pressure gradients weaken.  Wind flow will be light and variable during the morning but turn onshore during the afternoon as Valley heating draws the flow inland.



On Wednesday the upper trough fully develops bringing southerly flow aloft over the region possibly developing thunderstorms over the Cascades and the foothills.  In addition, as the trough begins to move inland a late day marine push is likely to develop.  Unsure at this time whether this push will be early enough to present a good burn opportunity.



The upper trough moves inland on Thursday bringing southwest flow but also the possibility of some light showers.  The trough will remain overhead with a continued chance of showers into Friday.



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 times

         the transport wind speed 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.



Nick Yonker

ODF Meteorologist

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