[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT THU SEP 20, 2012



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



Late-morning visible satellite imagery showed marine low clouds still covering most areas west of the Cascade crest.  Some breaks in the clouds were showing up over the coast range.  Pressure gradients are weakly onshore with appropriate stacking to maintain lift.  However, very warm air aloft  will keep mixing heights below 3000 feet until surface temperatures warm into the mid 70s.  That is not likely until the late afternoon.  Smoke levels remain somewhat elevated across northwestern Oregon from area wildfires.



The upper-level ridge will continue to slide eastward today with increasing southerly flow aloft.  The surface thermal trough will likely progress no farther east than central Oregon this afternoon.  Even though the low-level flow is onshore, winds at the top of the mixing layer are forecast to be southerly this afternoon, which is not a favorable direction for open burning.  In addition, there may not be enough mid-level onshore flow to evacuate existing wildfire smoke from the valley and the Cascades.



TODAY'S FORECAST:



Slowly clearing skies this afternoon.



Salem's high temperature today will be near 76 degrees (normal is 76).

Relative humidity drops to near 50% from 2 p.m. until 5 p.m.

Surface winds:  Light and variable; turning WNW 5-10 mph late this afternoon.

Transport winds: Light and variable; Becoming S 7-12 mph this afternoon.

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

Salem's sunset tonight: 7:12 p.m.



THREE-HOURLY DATA:

                                2 p.m.  5 p.m.  8 p.m.

     Temperature:                 72      75      67

     Relative Humidity:           51%     49%     63%

     Surface Wind Dir/Speed:    VAR 5    NW 5    NW 8

     Transport Wind Dir/Speed:    S 7     S 9   NW 10

     Mixing Height:              2500    3000    1500

     Ventilation Index:           18      27      15



EXTENDED DISCUSSION:



A weak upper-level trough is forecast to approach the Oregon Coast on Friday; further increasing the southerly flow aloft.  That will introduce a chance of afternoon and evening showers or thunderstorms over the Cascades and possibly over the Willamette Valley.  Low-level winds will be from the NNW, but winds at the top of the mixing layer are forecast to remain southerly.



The upper-level trough is forecast to come onshore Saturday and stall over the state on Sunday; deepening the marine layer over western Oregon for further cooling.  There is also a chance of morning drizzle or light showers.



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.



Pete Parsons

ODF Meteorologist
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