[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Wed Jul 16 11:51:50 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT WED JUL 16, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A stubborn upper-level ridge of high pressure will slowly weaken this afternoon.  However, the associated surface thermal trough remains anchored near the Cascade crest at midday.  Pressure gradients are onshore from Newport to Salem but are still flat from Salem to Redmond.  As a result, the air mass over the Willamette Valley is currently a little warmer than 24 hours ago.  The surface thermal trough is forecast to shift into central Oregon this afternoon.  The resultant minor influx of cooler marine air may clip a couple of degrees off of Tuesday's highs.

The air aloft is still very warm, so mixing heights will be slow to climb this afternoon...likely not getting much above 3000 feet. Unless marine air begins filtering into the valley this afternoon, State Fire Marshal Burn-Ban Conditions will be approached due to high temperatures and low humidity levels.  However, if the surface thermal trough shifts far enough eastward to allow marine air to penetrate into the valley, some field-burning may be possible.  The first PIBAL will be at 1 p.m.

TODAY'S FORECAST:

Sunny and warm.

Salem's high temperature today will be near 94°F (average is 83°F).

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

Surface winds: NW 5-10 mph this afternoon.

Transport winds: NW 5-10 mph this afternoon.

Mixing height: Rising to 2500-3500 feet from 2 p.m. and 5 p.m.

Salem's sunset tonight: 8:55 p.m.

(Salem Airport data for Tuesday, July 15th: High 96°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 35)

EXTENDED DISCUSSION:

The upper-level ridge will continue to flatten and give way to increasing westerly flow aloft Thursday and Friday.  Some marine clouds will likely penetrate into the Willamette Valley each morning with afternoon sunshine.  Temperatures should cool into the mid 80s Thursday and low 80s Friday.  Cooling aloft will improve afternoon mixing, with NW transport winds likely making for favorable burning conditions.  One limiting factor will be the amount of gradient-stacking that must be overcome, before burning can begin, each afternoon.

The flow aloft is forecast to turn southwesterly over the weekend.  Expect mostly cloudy skies and below average temperatures, with a weak weather system possibly bringing light showers to the region.

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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