[willamette-fcst] Silverton Hills Midday Forecast_Tom

Thu Aug 27 11:42:54 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT THU AUG 27, 2015

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level trough is approaching the coast of Oregon, bringing abundant clouds under southwesterly flow aloft. Down at the surface, an area of low pressure is forming frontal boundaries with isolated showers approaching southwestern Oregon and far northwestern Washington. Onshore flow will increase over the next 24 hours, as the atmosphere continues to mix and push regional wildfire smoke further northeast. Isolated thunderstorms are possible over the Cascades this afternoon. Rainfall is projected to reach the entire Oregon coast just before midnight this evening.

TODAY'S FORECAST:

Mostly cloudy skies today; becoming overcast this evening.

Salem's high temperature today will be near 86°F (average is 81°F).

Relative humidity:  Dropping near 30% by 5 p.m.

Surface winds: SW 5-10 mph this afternoon.

Transport winds: SW 5-10 mph this afternoon.

Mixing height: 3000 feet by 12 p.m. and rising to 5000 feet by 5 p.m.

Salem's sunset tonight: 7:58 p.m.

(Salem Airport data for Wednesday, August 26th: High 89°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 50)

EXTENDED DISCUSSION:

An unstable weather pattern begins on Friday, as the first in a series of disturbances moves through the Pacific Northwest. The upper-level trough will shift east throughout the day, bringing in much cooler air aloft from the Gulf of Alaska. The surface frontal system will produce wetting rains, as vigorous mixing of the atmosphere occurs throughout the day. Just less than 0.10 of an inch of rain is forecast for the Silverton Hills. Onshore flow will become more northwesterly as a cold front passes the Silverton Hills Friday afternoon with heavy showers and isolated thunderstorms.

Foul weather sets in for the weekend, as a second upper-level trough off the coast of California taps into tropical moisture over the Pacific Ocean. A fast-moving surface frontal system will again bring scattered showers and isolated thunderstorms to western Oregon, with heavier showers in higher elevations. Showery weather will persist overnight into Sunday, with rainfall slowly tapering off before sunset. At this time, ~1.25 inches of rainfall are projected for the Silverton Hills over the weekend.

Westerly flow aloft sets up for Monday, with an area of higher pressure forming off the coast of Oregon. This will maintain onshore flow under mostly cloudy skies, allowing the atmosphere to dry out as temperatures increase closer to seasonal averages. Another upper-level disturbance will be developing in the Gulf of Alaska, and will generate isolated late-summer showers ahead of the Labor Day weekend. These showers will become more widespread into the holiday weekend as remnant moisture from Tropical Storms Ignacio and Jimena gets incorporated into the westerly flow.

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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