[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT FRI JUL 13, 2012



BURN ADVISORY:



Recommended burn times for agricultural burning are from noon to 7:00 p.m.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



A very weak upper-level trough remains centered over western Washington, with a dry southwesterly flow aloft over western Oregon.  Marine low clouds continue to blanket most of western Washington and extend southward, to near Cape Blanco, along the Oregon coast. The low clouds that managed to form in the northern Willamette Valley earlier this morning have evaporated, except for along the Columbia River.



Midday temperatures are running a couple of degrees cooler than 24 hours ago, in the northern Willamette Valley, but ample sunshine should help readings recover to about where they were Thursday afternoon.  Also like yesterday, afternoon mixing heights should climb into the 3-4000 foot range.  Pressure gradients are weaker then yesterday, so NNW winds will likely remain less than 10 mph.  A cooling sea breeze will quickly lower evening mixing heights.



TODAY'S FORECAST:



Sunny and seasonably warm.



Salem's high temperature today will be near 84 degrees (normal is 82).

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

Surface winds:  NNW 5-10 mph; becoming WNW 7-12 mph this evening.

Transport winds: NNW 6-12 mph; becoming WNW 8-14 mph this evening.

Mixing height: Rising to near 3500 feet by late this afternoon.

Salem's sunset tonight: 8:56 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 78      83      75

     Relative Humidity:           42%     33%     45%

     Surface Wind Direction:     350     340     300

     Surface Wind Speed:           7       6       8

     Transport Wind Direction:   340     330     300

     Transport Wind Speed:         8      10      12

     Mixing Height:             3000    3500    1500

     Ventilation Index:           24      35      18



EXTENDED DISCUSSION:



A very weak upper-level trough is forecast to slowly drop southward, to over northwestern Oregon, on Saturday.  That should maintain enough onshore flow to bring back morning marine clouds into the Willamette Valley.  Expect partly cloudy skies, by the afternoon, with seasonably warm temperatures.  However, some cooling aloft will combine with daytime heating to promote a slight chance of afternoon showers or thunderstorms.

A northwesterly flow aloft should stabilize the air mass Saturday night and Sunday morning, but another weak upper-level trough is forecast to drop into the region late Sunday and Monday.  That will strengthen the onshore flow, cool temperatures to slightly below normal, and maintain at least a chance of light showers or morning drizzle.  The trough is forecast to weaken and move over southwestern Oregon by Tuesday.  That will turn the flow aloft more southerly and reintroduce a slight chance of thunderstorms.



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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