[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT TUE AUG 13, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is allowed from 1 p.m. to 3 p.m. with a 50 acre limit.



WEATHER DISCUSSION:



A dry and stable southwesterly flow aloft has weakened the onshore flow today.  Pressure gradients are evenly stacked this morning and have relaxed to just over 2 mb onshore from Newport to Redmond.  Only patchy marine low clouds have formed in the valley this morning and should give way to sunny skies by midday.



The morning sounding over Salem showing several degrees of warming, above 4000 feet, compared to Monday morning.  That will cap maximum mixing heights near 4000 feet, even with surface temperatures warming into the mid 80s this afternoon. NNE transport winds this morning should turn northerly this afternoon.  Winds may turn to the NNW late but not likely enough to allow for significant burning.



TODAY'S FORECAST:



Morning clouds; becoming mostly sunny this afternoon.



Salem's high temperature today will be near 85 degrees (normal is 83).

Relative humidity:  Drops to 50% by noon and to near 30% by 5 p.m.

Surface winds:  NNE 2-6 mph this morning; N 5-10 mph this afternoon.

Transport winds: NNE 5 mph this morning; N 6-12 mph this afternoon.

Mixing height: Rising to 3000 feet by 1 p.m. and to near 4000 feet by 5 p.m.

Salem's sunset tonight: 8:21 p.m.



(Salem Airport data for Monday, Aug. 12th: High 78°F; Rainfall .00")

(Maximum Ventilation Index expected today: 40)



EXTENDED DISCUSSION:



Increasing high clouds are expected on Wednesday, as an approaching upper-level trough increases the southwesterly flow aloft.  That will likely lead to very favorable open burning conditions Wednesday afternoon, as transport winds turn southwesterly.



A weak cold front is forecast to stall as it moves onshore Wednesday night. Some drizzle or light rain is possible along the coast, with minimal inland penetration. More clouds and onshore flow will cool temperatures a few degrees on Thursday, with some burning possible.  Another weak front is forecast to approach the coastline on Friday, which could also create favorable burning conditinos.



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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