[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Wed Sep 10 08:48:03 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT WED SEP 10, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

Oregon lies between a building upper-level ridge, in the eastern Gulf of Alaska, and a cool "autumn-like" weather system diving southward into Montana.  That is producing a dry northwesterly flow aloft today with weakening onshore low-level winds.  Some marine low clouds have penetrated into the northern Willamette Valley this morning but should quickly give way to sunshine today.

The sounding over Salem this morning showed considerable warming of the layer between 2000 feet and 8000 feet, so mixing heights will struggle to reach 3000 feet this afternoon.  As a thermal trough builds northward along the coast, dry north to NE low-level winds will increase this afternoon, possibly putting sections of the valley into State Fire Marshal Burn-Ban Conditions by late this afternoon.

TODAY'S FORECAST:

Patchy morning clouds...otherwise sunny with near-average temperatures.

Salem's high temperature today will be near 81°F (average is 79°F).

Relative humidity:  Dropping to 50% around 11 a.m. and to near 30% by 5 p.m.

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

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

Mixing height: Rising to 3500 feet by 5 p.m.

Salem's sunset tonight: 7:32 p.m.

(Salem Airport data for Tuesday, September 9th: High 76°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 55)

EXTENDED DISCUSSION:

The building upper-level ridge in the eastern Gulf of Alaska is forecast to push closer to the coast Thursday and Friday.  A dry northwesterly flow aloft will continue with a general warming of the air mass over Oregon.  A coastal thermal trough will turn low-level winds northeasterly; warming valley temperatures into the mid-80s Thursday and upper-80s on Friday.

The combination of NE winds and low humidity will raise wildfire concerns and may put much of the valley into State Fire Marshal Burn-Ban Conditions both Thursday and Friday afternoon.  The thermal trough is forecast to move inland over western Oregon this weekend, which will decrease winds but spike temperatures in the low-90s.

Southwesterly flow aloft will turn low-level winds back onshore and initiate a cool-down early next week, with the potential for burning opportunities.  No significant precipitation is predicted through the middle of next week.

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

Pete Parsons

ODF Meteorologist

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