[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Wed Jul 30 08:49:39 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT WED JUL 30, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

There has been little change in the weather pattern since Tuesday.  A strong upper-level ridge remains centered over the Rockies with a warm SW flow aloft over Oregon.  A surface thermal trough still extends from eastern Washington through SW Oregon.  Very weak onshore has forced marine clouds onto the coast, but skies are mostly sunny across the interior of NW Oregon.

Weak upper-air disturbances are acting on the mid-level monsoonal moisture over the state to trigger clusters of thunderstorms.  The system that brought thunderstorms to the central Cascades on Tuesday is exiting the NE corner of Oregon this morning.  However, another upper-air disturbance, approaching the southern Oregon coast this morning, will likely trigger thunderstorm development over the Cascades again this afternoon.  There is an outside chance that SE flow aloft could develop enough to steer storms over the Willamette Valley, but they are generally expected to stay over the Cascades.

Salem's upper-air sounding today showed a few degrees of cooling in the layer below about 12,000 feet, so high temperatures should back off to around 90 degrees this afternoon.  NE transport winds this morning should veer to more northerly this afternoon, with burning opportunities unlikely.

TODAY'S FORECAST:

Sunny and warm.  Scattered thunderstorms over the Cascades.

Salem's high temperature today will be near 90°F (average is 84°F).

Relative humidity:  Dropping below 50% by noon & to near 30% by 5 p.m.

Surface winds: N 5-10 mph this morning; N 8-12 mph this afternoon.

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

Mixing height: Rising to 3000 feet by 2 p.m. and to 4500 feet by 5 p.m.

Salem's sunset tonight: 8:41 p.m.

(Salem Airport data for Tuesday, July 29th: High 94°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 68)

EXTENDED DISCUSSION:

Very little change in the overall weather pattern is predicted this week.  A strong upper-level ridge of high pressure will stay anchored over the Rockies with a south to SW flow aloft over Oregon.  That will keep skies mostly sunny with well above normal temperatures.  It will also maintain a threat of afternoon and evening thunderstorms...mainly over the Cascades.  Very weak onshore flow could provide limited burning opportunities.  A change to more favorable patterns for burning is forecast for next week, as the ridge weakens and allows for slightly more onshore 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

Pete Parsons

ODF Meteorologist
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