[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT MON AUG 29, 2016



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



An upper-level trough in the eastern Gulf of Alaska is producing a dry SW flow aloft over Oregon.  Just patchy marine clouds made it into the northern Willamette Valley this morning, so onshore pressure gradients are already favorably stacked to keep smoke plume elevated.  An 11:30 a.m. PIBAL showed SSW winds through the lowest 3000 feet.  The air aloft is still fairly warm, so mixing heights are currently only about 2000 feet.  Surface temperatures will need to approach 75°F to obtain 3000-foot mixing heights.  That should occur around 1 p.m.



TODAY'S FORECAST:



Sunny with near-average temperatures.



Salem's high temperature today will be near 81°F (average is 81°F).

Relative humidity:  Dropping to near 35% by 5 p.m.

Surface winds: SW 5-10 mph.

Transport winds: SSW 10-15 mph; veering to WSW by late this afternoon.

Mixing height: Rising to 3000 feet around 1 p.m. and to 4500 feet by 5 p.m.

Salem's sunset tonight: 7:53 p.m.



(Salem Airport data for Sunday, August 28th: High 87°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 68)



EXTENDED DISCUSSION:



Marine low clouds should penetrate across all of western Oregon by early Tuesday, making gradient-stacking a limiting factor for burning.  Pockets of morning drizzle are also possible.  An approaching cold front may help to balance gradients in the afternoon.  Should that happen, cooling aloft will make for quite high mixing heights with SW transport winds expected.



A weakening cold front will likely bring a few showers on Wednesday.  An upper-level trough will maintain cool and showery weather through Saturday.  Rainfall totals may exceed one-quarter of an inch.  Dry weather is expected Sunday and Monday with temperatures recovering to near average.



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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