[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 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 was driving a dry cold front across southeastern Washington late this morning.  Weak low-level onshore flow brought marine low clouds onto western Washington and the northwestern corner of Oregon this morning.  Skies remained clear across the remainder of the state.



Late-morning satellite imagery showed some residual marine clouds over extreme northwestern Oregon, along and near the Columbia River.  Skies were clear over the remainder of the state, 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 northeastern Oregon.  Northerly pressure-gradients were increasing across western Oregon, in response to a thermal trough building into southwestern Oregon.  The ODF Sodar, located in the central coastal range, was showing NE winds from the surface up to 3000 feet.



What's left of the patchy morning low clouds should quickly give way to sunshine this afternoon, as an upper-level ridge of high pressure builds onshore.  Transport winds are forecast to remain northeasterly and increase this afternoon.  Slightly warmer air aloft will keep maximum mixing heights at or below 4000 feet, with high temperatures climbing into the low 80s.



TODAY'S FORECAST:



Sunny with near-normal temperatures. Increasing northerly winds.



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

Relative humidity drops to near 30% by 5 p.m.

Surface winds:  NNE 10-15 mph this afternoon.

Transport winds: NE 14-18 mph this afternoon.

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

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



THREE-HOURLY DATA:

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

     Temperature:                 75      80      73

     Relative Humidity:           39%     32%     41%

     Surface Wind Direction:     005     010     015

     Surface Wind Speed:          10      12       8

     Transport Wind Direction:   030     030     030

     Transport Wind Speed:        15      17      15

     Mixing Height:             3000    4000    1500

     Ventilation Index:           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.  Some smoke, from wildfires east of the Cascades, will likely get transported over western Oregon, so skies should turn 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 any 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.  Transport winds are forecast to turn back offshore next Wednesday through Friday, which could, once again, transport wildfire smoke into the region.



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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