[willamette-fcst] Silverton Hills AM Forecast_Tom

Wed Sep 23 08:28:16 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT WED SEP 23, 2015

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level trough is out over the Pacific Ocean, and has slowed its advance as it deepened overnight. Southwesterly onshore flow aloft will cool the atmosphere as thin clouds drift over Oregon. Down at the surface, southerly flow will become more southwesterly as a surface frontal system begins to develop off the coast of Oregon late this afternoon. Mixing is currently marginal, but will steadily improve as heating occurs today. Light offshore gradients will be present first thing this morning, but are expected to improve quickly as the Sun passes overhead; this will be the key factor to monitor for favorable open burning Wednesday afternoon.

TODAY'S FORECAST:

Partly Cloudy skies throughout the day.

Salem's high temperature today will be near 75°F (average is 75°F).

Relative humidity:  Dropping near 40% by 5 p.m.

Surface winds: S-SW 5-10 mph this morning; NW 5-10 mph this afternoon.

Transport winds: S-SW 10-15 mph this morning; SW 10-15 mph this afternoon.

Mixing height: Above 3000 feet after 10 a.m. and 5000 feet after 3 p.m.

Salem's sunset tonight: 7:08 p.m.

(Salem Airport data for Tuesday, September 22nd: High 71°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

The weather pattern across western Oregon and the Pacific Northwest at large will become increasingly unsettled for Thursday and Friday. The upper-level trough near the Oregon coast will slowly approach our area, making the atmosphere unstable. A surface frontal system will stretch out Thursday and produce favorable onshore flow with lift, while rainfall is directed further north into Washington. This suggests a dry Cold Front passing through, and bringing in marine air behind it for Friday afternoon. Convection could still occur beneath the upper-level trough, creating problematic gusty winds and conditions that favor down-mixing of smoke emissions.

The upper-level trough will weaken and split as it near the Oregon coast Friday evening, with fragments of energy pushed into both northern Washington and back out to sea in the Pacific. An area of higher pressure over Nevada will keep skies mostly clear over Oregon. This will promote light offshore flow as temperatures climb just above seasonal averages for the last weekend of September. The "lost" upper-level trough out over the Pacific will have regained strength by Monday afternoon as it heads due east towards California. A potential storm system at the surface from this feature provides concern through mid-week, as historical evidence shows "Sou'westers" often produce offshore flow and isolated thundershowers for western Oregon.

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 15 mph.

     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 (ODA) and the Oregon Department of Forestry (ODF).  For

information contact ODA at 503-986-4701.

To add/remove your email address from this list, please go to:

http://listsmart.osl.state.or.us/mailman/listinfo/willamette-fcst

Tom Jenkins, AEM

ODF Meteorologist
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