[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Tue Aug 11 11:58:08 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT TUE AUG 11, 2015

***  The Portland National Weather Service has issued a Red Flag Warning for the western slopes of the northern and central Oregon Cascades, valid today until 11 p.m.

Scattered thunderstorms are expected to develop over the Cascades this afternoon and generally move to the NW.  These storms have the potential to produce gusty winds, abundant lightning, and hail.  Any lightning, combined with near-record dry fuels, poses a significant wildfire risk.  ***

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level low-pressure system is centered about 350 miles west of the northern California coast.  The counterclockwise circulation around it is feeding mid-level moisture and instability northward across Oregon, which produced some isolated shower and thundershowers on Monday.  The air aloft warmed and stabilized a bit overnight, leaving only a veil of high mostly high clouds draped across the region at midday.

Daytime heating will, once again, destabilize the air mass, likely leading to at least isolated shower and thundershower development over the Cascades this afternoon.  SE flow aloft may direct these storms over the western foothills of the Cascades and the Willamette Valley later this through tonight.

At the surface, the thermal trough is expanding westward into the Willamette Valley, with onshore flow from Newport to Salem but offshore flow from Redmond to Salem.  Transport winds may turn weakly NNW this afternoon, but winds at the mixing height will remain southeasterly.  That is not a favorable wind pattern for evacuating smoke from the valley.  In addition, any thunderstorm development over the region would create erratic and gusty winds and increase the potential for down-mixing of smoke.

TODAY'S FORECAST:

Partly sunny.  Slight chance of showers or thundershowers through tonight.

Salem's high temperature today will be near 87°F (average is 83°F).

Relative humidity:  Dropping to near 35% by 5 p.m.

Surface winds: NW 5-10 mph this afternoon.

Transport winds: NNW 5-10 mph this afternoon.

Mixing height: Rising to 5000 feet by 5 p.m.

Salem's sunset tonight: 8:25 p.m.

(Salem Airport data for Monday, August 10th: High 86°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 50)

EXTENDED DISCUSSION:

The upper-level low-pressure area off the northern California coast will move very little on Wednesday.  Expect partly cloudy skies with a marginally unstable S-SE flow aloft maintaining a threat of showers and thundershowers, mainly over the Cascades.  Onshore flow may increase late in the day, creating a burning opportunity, but that is far from certain.  Temperatures will likely hold in the upper-80s.

The upper-level trough is predicted to swing NE across the region Thursday and Friday.  That will bring much cooler onshore flow along with scattered showers and a chance of thundershowers...mainly late Thursday through Friday.  Another burning opportunity is possible, just ahead of this transition to damp weather, Thursday afternoon.  Temperatures will drop back into the mid-to-upper 70s.  Rainfall totals are generally expected to be less than .10". Dry weather is predicted to return over the weekend with temperatures warming back into the 80s.

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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