[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:40 AM PDT FRI JUL 22, 2011



BURN ADVISORY:



Recommended burn times for agricultural burning are 11:00 a.m. to 7:00 p.m.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



An upper-level trough moved east of the Cascades overnight and was pushing into northern Idaho and western Montana this morning.  A drier northwesterly flow aloft has set up over western Oregon.  Near the surface, onshore flow is decreasing and turning northerly, especially in the northern Willamette Valley.  Mid-morning visible satellite imagery showed considerable marine low clouds covering mainly the central and southern Willamette Valley with clearing skies in the north valley.



This morning's sounding over Salem revealed a much drier air mass than 24 hours ago.  There was still enough moisture to form marine clouds from 3000-4500 feet.  However, winds below 4500 feet had already turned mostly offshore, so look for morning clouds to quickly give way to sunny skies.  The air mass aloft is stable but still cooler than normal.  Afternoon temperatures will warm into the upper 70s.



TODAY'S FORECAST:



Morning clouds, then sunny and warmer.  Increasing northerly winds.



Salem's high temperature today will be near 79 degrees (normal is 83).

Relative humidity drops to near 50% about 11 a.m. and to near 30% by 5 p.m.

Surface winds:  N 5-10 mph this morning; N 10-15 mph this afternoon.

Transport winds: N 10 mph this morning; NNE 12-15 mph this afternoon.

Mixing height: Rises to 3000 feet by 11 a.m. and to 5500 feet by 5 p.m.

Salem's sunset tonight: 8:50 p.m.





THREE-HOURLY DATA:

                               11 a.m.   2 p.m.  5 p.m.  8 p.m.

     Temperature:                 68       75      78      73

     Relative Humidity:           51%      37%     31%     41%

     Surface Wind Direction:     360      360     005     010

     Surface Wind Speed:           8       10      12      12

     Transport Wind Direction:   010      010     015     020

     Transport Wind Speed:        10       12      13      15

     Mixing Height:             3200     5000    5500    3000

     Ventilation Index:           32       60      72      45



EXTENDED DISCUSSION:



A flat ridge will move over Oregon Saturday for continued warming and drying of the air mass.  High temperatures will likely climb into the mid 80s.  The flow aloft will turn southwesterly on Sunday, as the next upper-level trough approaches the coastline. High temperatures may approach 90 degrees.

As the upper-level trough comes onshore Monday, a mostly dry marine push will cool temperatures back to normal.  A broad upper-lever trough is forecast to remain over the region through next week.  Temperatures will cool to below normal Tuesday and Wednesday but may recover to near or slightly above normal by Friday.



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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