[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT THU JUL 14, 2011



BURN ADVISORY:



Recommended burn times for agricultural burning are 9:00 a.m. to 7:00 p.m.

Prep burning is allowed from 12:00 p.m. to 2:00 p.m. with a 50 acre limit.

Propane flaming is allowed from 12:00 p.m. to 5:00 p.m.



WEATHER DISCUSSION:



An upper-level trough was centered over northwestern Washington this morning.  To the south of the circulation center, a moist west-southwesterly flow was producing cloudy skies, with areas of light rain, across western Washington.  Considerable clouds extended southward into northwestern Oregon, with some showers along the north coast and over the northern coast range.  The Silverton Hills region lies on the southern edge of the more solid cloud-cover this morning, with increasing amounts of clearing in the central and southern Willamette Valley.



The Salem sounding this morning was almost identical to yesterday morning's sounding, showing an unseasonably cool and moist air mass over the region.  The surface map also showed little change from yesterday, with weak southerly pressure gradients across the Willamette Valley.



The upper-level trough is forecast to weaken slightly today, as it slowly moves eastward today.  It will extend from north-central Washington to northwestern Oregon late this afternoon.  The air aloft remains cool, so daytime heating will, once again, destabilize the air mass and lead to a few afternoon showers, mainly from the Silverton Hills region northward.  Surface and transport winds will veer to more westerly late this afternoon.  Skies should clear significantly this evening, as the upper-level trough moves east of the region.  Temperatures will continue to be 5-10 degrees below normal.



TODAY'S FORECAST:



Partly sunny.  Slight chance of a shower.  Clearing skies this evening.



Surface winds:  S 5-8 mph this morning; becoming WSW 6-10 mph this afternoon

Transport winds: SSW 10 mph this morning; becoming WSW 10 mph this afternoon

Salem's forecast high temperature:  74 degrees

Maximum mixing height:  6000 feet

Relative humidity:  Drops to 60% by noon and to near 43% by 5 p.m.

Salem sunset tonight: 8:56pm



THREE-HOURLY DATA:



                                 11am     2pm     5pm     8pm

     Temperature:                 63       69      73      69

     Relative Humidity:           66%      53%     43%     48%

     Surface Wind Direction:     190      210     260     290

     Surface Wind Speed:           7        8       8      10

     Transport Wind Direction:   200      210     260     270

     Transport Wind Speed:        10       10      10      12

     Estimated Mixing Height:   3000     5000    6000    3500

     Ventilation Index:           30       50      60      42



EXTENDED DISCUSSION:



A drier and slightly warmer westerly flow aloft is forecast on Friday. Low-level winds will remain onshore, so expect some morning marine low clouds to form in the northern Willamette Valley.  With partly cloudy skies and some warming aloft, afternoon temperatures should reach the mid to upper 70s.  Normal high temperatures are in the low 80s.  High afternoon mixing heights and southwesterly transport winds may provide favorable burning conditions.



Another upper-level trough is forecast set up camp just offshore this weekend.  A weak cold front will swing inland early Saturday, bringing a renewed threat of light showers and slightly cooler temperatures.  The flow aloft will turn southwesterly Sunday and Monday with the upper-level trough moving inland on Tuesday.  That will maintain at least a treat of showers and keep temperatures below normal.



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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