[willamette-fcst] Silverton Hills AM Forecast_Tom

Fri Sep 11 08:32:10 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT FRI SEP 11, 2015

***State Fire Marshal Burn-Ban conditions, for high temperatures and low humidity, are likely this afternoon.***

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level ridge is currently over western Oregon, and it will slowly enter the Cascades later today. At the surface, a thermal trough is positioned in the Willamette Valley, with higher pressure in central Oregon creating a strong offshore gradient of -5.3mb from Redmond to Salem. These factors will cause temperatures to soar well above seasonal averages as humidity plummets, possibly developing State Fire Marshal conditions this afternoon. Mixing of the atmosphere will be significantly restricted, remaining below 3000 feet all day.

TODAY'S FORECAST:

Mostly Sunny becoming hazy and hot this afternoon.

Salem's high temperature today will be near 95°F (average is 78°F).

Relative humidity:  Dropping near 20% by 5 p.m.

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

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

Mixing height: Below 3000 feet all day.

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

(Salem Airport data for Thursday, September 10th: High 88°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 41)

EXTENDED DISCUSSION:

Early Saturday morning, a weak upper-level trough will approach the southern coast of Oregon and switch the flow aloft to southeasterly. This will allow clouds to slowly move in from the south, restricting the amount of heating over the weekend. Temperatures will begin to drop closer to seasonal norms through Sunday, though western Oregon will remain warmer than usual for September. The upper-level trough will fill and be pushed back south into California Sunday evening.

Monday will see onshore flow with increasing clouds throughout the day, as a second upper-level trough approaches the Pacific Northwest from the Gulf of Alaska. An area of lower pressure will form off the coast of Washington. A cold front sweeps across the Silverton Hills Monday evening with isolated showers. A second storm system mid-week will bring scattered showers with wetting rains to the Silverton Hills. Isolated thunderstorms are possible in higher elevations Wednesday afternoon, with spotty showers tapering off by Friday morning.

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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