[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:53 AM PDT WED JUL 27, 2011



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is allowed from now until 2:00 p.m. with a 50 acre limit.

Propane flaming is allowed from now until 5:00 p.m.



WEATHER DISCUSSION:



A broad upper-level trough is finally pushing east of the region, with a drier northwesterly flow aloft developing over western Oregon.  Late-morning visible satellite imagery showed marine clouds breaking up over the Willamette Valley, where temperatures had climbed into the mid to upper 60s.



The surface map shows high pressure pushing into Washington and northwest Oregon and a thermal trough building into southwestern Oregon.  That is increasing the northerly pressure-gradients across the central and southern Willamette Valley.  Late-morning winds were northerly near 10 mph in the Salem area and gusting to more than 20 mph in Eugene.



The air mass is still on the cool side, so high temperatures will top out about 5 degrees below normal this afternoon. Northerly winds will aid in the drying of damp fields.



TODAY'S FORECAST:



Mostly sunny.



Salem's high temperature today will be near 78 degrees (normal is 83).

Relative humidity drops to near 35% by 5 p.m.

Surface winds:  N 10-15 mph.

Transport winds: N 12-15 mph.

Mixing height: Rises to 5500 feet by 5 p.m.

Salem's sunset tonight: 8:44 p.m.





THREE-HOURLY DATA:

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

     Temperature:                 73      77      73

     Relative Humidity:           43%     36%     45%

     Surface Wind Direction:     360     360     340

     Surface Wind Speed:          10      12      10

     Transport Wind Direction:   360     360     340

     Transport Wind Speed:        12      14      15

     Mixing Height:             5000    5500    3000

     Ventilation Index:           60      77      45



EXTENDED DISCUSSION:



Little change in the weather pattern is forecast for Thursday and Friday, with a dry west-northwesterly flow aloft and mostly northerly surface and transport winds.  Very slow warming aloft will help temperatures recover to near normal (low to mid 80s), while slightly suppressing mixing heights.



The flow aloft will turn southwesterly over the weekend, as a weak upper-level trough moves across British Columbia. Temperatures will likely warm a few more degrees with continued mostly sunny skies.  Another weak trough is forecast to approach the coastline early next week.  Although unlikely to cause an precipitation, it should force the surface thermal trough east of the Cascades and induce onshore flow into the Willamette Valley.  That will begin a minor cooling process and could create burning opportunities.



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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