[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:40 AM PDT THU JUL 26, 2012



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is allowed from noon until 2 p.m. with a 50 acre limit.

Propane flaming is allowed from noon until 2 p.m.



WEATHER DISCUSSION:



A dry, warm and stable southwesterly flow aloft is continuing over Oregon.

At the surface, a thermal trough extends from central Washington, through central Oregon, into southwestern Oregon.  Increased onshore flow forced marine low clouds into the northern and southern Willamette Valley this morning, but only patchy clouds formed in Marion County and extreme northern Linn County.  Low clouds were slowly clearing from the north and south valley late this morning, with sunny skies in the Silverton Hills region.



Temperatures are running a few degrees cooler than 24 hours ago, in the central Willamette Valley, with highs expected to reach the mid 80s this afternoon. A very weak upper-level trough is forecast to initiate a significant afternoon sea breeze today, which should turn transport winds westerly and create favorable smoke dispersal conditions for field-burning.  Transport winds will be closely monitored, via pibals, beginning at 1 p.m.



Minor gradient-stacking late this morning (2.0 mb onshore Newport-to-Salem and 2.3 mb onshore Salem-to-Redmond) should reverse early this afternoon.  Mixing heights will likely rise above 3000 feet by 2 p.m., top out near 4500 feet in the late-afternoon, and then rapidly lower in the early evening.



TODAY'S FORECAST:



Sunny and warm.



Salem's high temperature today will be near 85 degrees (normal is 84).

Relative humidity drops to around 40% by 5 p.m.

Surface winds:  Light NW; becoming WNW 5-10 mph this afternoon.

Transport winds: Light NW; becoming W 5-10 mph this afternoon.

Mixing height: Rising to near 4500 feet by 5 p.m.

Salem's sunset tonight: 8:45 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 79      84      74

     Relative Humidity:           47%     39%     54%

     Surface Wind Direction:     290     290     280

     Surface Wind Speed:           4       8       8

     Transport Wind Direction:   270     270     280

     Transport Wind Speed:         6       8      10

     Mixing Height:             3200    4500    1800

     Ventilation Index:           19      36      18



EXTENDED DISCUSSION:



Morning marine clouds will likely penetrate across the entire Willamette Valley Friday morning.  The deeper marine layer will cool temperatures back to below normal, even with afternoon sunshine.  Pressure-gradient-stacking and transport winds will likely be unfavorable for open field-burning.



Little change in the generally dry and stable southwesterly flow aloft is expected through much of next week.  Look for varying degrees of onshore flow, morning clouds, and afternoon sunshine.  That may present additional burning opportunities.  The marine layer could deepen enough for areas of morning drizzle, but that should be the extent of any precipitation.  Onshore flow should hold temperatures at near to slightly below normal.



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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