[willamette-fcst] Silverton Hills AM Forecast.doc

Fri Sep 2 08:58:18 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT FRI SEP 2, 2011

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.

WEATHER DISCUSSION:

Northwesterly flow aloft is driving a dry cold front across eastern Washington, this morning.  That turned pressure-gradients enough onshore to bring marine low clouds onto western Washington and the northwestern corner of Oregon.  Some low clouds made it up the Columbia River into the northern Willamette Valley.  Otherwise, skies were clear this morning across Oregon, with the exception of areas of wildfire smoke east of the Cascades.

The surface map this morning shows high pressure building into western Washington with a weak trough extending from eastern Washington into north-central Oregon.  Pressure-gradients are mostly northerly across western Oregon, in response to a thermal trough building into southwestern Oregon.  The Salem sounding this morning showed a few degrees of warming throughout the lower atmosphere.  Winds were northeasterly below 3000 feet; backing to northwesterly aloft.

Patchy morning low clouds should quickly give way to sunshine today, as an upper-level ridge of high pressure builds onshore.  Surface and transport winds are forecast to be north to northeasterly and increase this afternoon.  Slightly warmer air aloft will generally keep maximum mixing heights at or below 4000 feet, with high temperatures climbing into the low 80s.

TODAY'S FORECAST:

Patchy morning clouds, then sunny with near-normal temperatures.

Salem's high temperature today will be near 81 degrees (normal is 80).

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

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

Transport winds: NE 10-15 mph this morning; NE 15-18 mph this afternoon.

Mixing height: Rises to 3000 feet by 2 p.m. and to near 4000 feet by 5 p.m.

Salem's sunset tonight: 7:47 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 67       75      80      73

     Relative Humidity:           49%      39%     32%     41%

     Surface Wind Direction:     360      010     010     010

     Surface Wind Speed:           6       10      12       8

     Transport Wind Direction:   030      020     020     020

     Transport Wind Speed:        12       15      17      15

     Mixing Height:             2500     3000    4000    1500

     Ventilation Index:           30       45      68      23

EXTENDED DISCUSSION:

An upper-level ridge is forecast to build over Oregon Saturday.  At the surface, a thermal trough will build northward along the coast.  Transport winds will turn strongly offshore, which will warm valley temperatures to near 90 degrees.  Smoke from wildfires, east of the Cascades, may get transported over western Oregon, so don't be surprised if skies become hazy.

As the upper-level ridge slides east of the state, increasing southerly flow aloft may bring isolated thunderstorms to the Cascades by late Sunday.  Transport winds are forecast to turn weakly onshore in the afternoon, which may help to clear potential wildfire smoke from the region.  Further warming aloft should boost valley temperatures into the low 90s.

Weak onshore flow may bring minor cooling Monday and Tuesday, but the overall weather pattern forecast for next week is not very favorable for open burning.  A broad ridge of high pressure will likely rebuild over the Pacific Northwest.   Expect a continuation of dry and quite warm conditions.  If the ridge sets up far enough to the east, then afternoon sea breezes could provide limited burning opportunities.

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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