[willamette-fcst] Silverton Hills AM Forecast_doug.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT WED JUL 10, 2013



BURN ADVISORY:



Recommended burn times for agricultural burning from 11:00 a.m. until 7:00 p.m.

Prep burning is not allowed.



WEATHER DISCUSSION:



An upper-level ridge remains centered over the southwestern US with dry southwesterly flow aloft over Oregon. Upstream troughing aloft over western Oregon and the eastern Pacific has increased over the past 24 hours, but low-level onshore flow is weak and disorganized at this point with some increased coastal low-cloud intrusions in the lower elevations.



Marine clouds are confined mostly to west of the coastal range, but some cooling aloft over western Oregon will limit valley maximum temperatures to around 80 degrees this afternoon.  Transport winds should remain mostly northerly, with a weak westerly component developing late in the day.



TODAY'S FORECAST:



Mostly sunny and cooler.



Salem's high temperature today will be in the low 80s (average is 80).

Relative humidity:  Will drop below 60% by noon and to near 40% by 5 p.m.

Surface winds:  N 5-10 mph this morning through the afternoon; NNW 7-15 mph this evening.

Transport winds: NNE 5-10 mph this morning with a bit of an increase this afternoon.

Mixing height: Rising to near 5000 feet this afternoon.

Salem's sunset tonight: 8:59 p.m.



(Salem Airport data for Tuesday, July 9th: High 92°F; Rainfall .00")

(Maximum Ventilation index expected today: 50)



EXTENDED DISCUSSION:



The approaching upper-level trough will continue to strengthen and thus increase the southwesterly flow aloft over the area on Thursday, with a wave within the trough progressing eastward into Montana and Idaho.  Increasing onshore flow should bring some marine clouds into the Willamette Valley particularly early in the day and then again in the evening.  Otherwise, expect sunshine and continued cooler temperatures with maximums in the mid-70s.  Transport winds are forecast to turn northwesterly late in the day with cooling aloft improving mixing.



Upper-level troughing will persist just west of Oregon Friday and Saturday, with good onshore flow aloft and during the late afternoons/nighttime at low-levels.  No showers are expected, but strong onshore flow should blanket the Willamette Valley with morning low clouds. Partly cloudy conditions are likely in the afternoons.  Maximum temperatures will continue in the mid 70s to around 80. Some warming on Sunday is expected as the western Rockies upper ridge is expected to begin to retrograde westward.



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 (ft) times

         the transport wind speed (mph) 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.



Doug Wesley

ODF Meteorologist
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