[willamette-fcst] Silverton Hills AM Forecast.doc

Thu Aug 2 08:54:19 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT THU AUG 2, 2012

BURN ADVISORY:

Agricultural burning is not recommended.

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

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

WEATHER DISCUSSION:

An upper-level trough cutting across southwestern British Columbia is maintaining a dry and stable westerly flow aloft and weak onshore flow at the surface across northwestern Oregon.  The Salem sounding this morning showed a very similar temperature profile to Wednesday morning, with continued weak northerly winds below 3000 feet.  Visible satellite imagery shows low clouds along much of the northern and central coast this morning, with patchy low clouds over mainly the northern Willamette Valley.

Today should turn out very similar to yesterday, with brief areas of morning clouds giving way to plenty of sunshine.  There is some minor pressure gradient stacking this morning, but daytime heating may balance that out again this afternoon, much like it did yesterday. Transport winds are forecast to maintain a significant northerly component, so any burning opportunities will likely be limited.

(Salem Airport on Wednesday, August 1st: High 82°F; Rainfall .00")

TODAY'S FORECAST:

Patchy morning clouds; becoming sunny and seasonably warm.

Salem's high temperature today will be near 81 degrees (normal is 84).

Relative humidity drops to 60% by noon and to near 40% by 5 p.m.

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

Transport winds: NNW 8-10 mph; increasing to 10-15 mph this evening.

Mixing height: Rising to 3000 feet around noon and to 4500 feet by 5 p.m.

Salem's sunset tonight: 8:36 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 67       76      80      74

     Relative Humidity:           61%      45%     38%     50%

     Surface Wind Direction:     350      350     350     320

     Surface Wind Speed:           7        7       9      10

     Transport Wind Direction:   350      350     340     340

     Transport Wind Speed:         8        8      10      14

     Mixing Height:             2000     4000    4500    2000

     Ventilation Index:           16       32      45      28

EXTENDED DISCUSSION:

The Canadian upper-level trough is forecast to drop into eastern Montana and western North Dakota on Friday, with a building offshore ridge of high pressure turning the flow aloft more northerly over Oregon.  That should shut off the onshore flow across the Willamette Valley and allow temperatures to climb into the upper 80s.  The ridge is forecast to move directly over Oregon on Saturday with a surface thermal trough pushing northward over the region.  That should produce the warmest temperatures so far this year, with valley highs climbing at least into the mid 90s.

The upper-level ridge is forecast to shift eastward by Sunday, with increasing southerly flow aloft introducing middle and high-level monsoonal moisture into the region.  That will begin a cooling process but also bring in a chance of thunderstorms.  The flow aloft is forecast to turn more southwesterly Monday and Tuesday, which would push the thunderstorm threat east of the region and increase the onshore flow.  That will need to be watched closely, as it could create a significant burning opportunity.

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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