[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Thu Aug 10 08:54:23 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT THU AUG 10, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A strong upper-level ridge, centered over southern British Columbia, is being undercut by a moist and unstable upper-level trough moving into northern California.  With the exception of the immediate coastline, northern Oregon has stayed sunny and hot, with highly-elevated nephelometer readings due to wildfire smoke.  In addition to the wildfire smoke, southern Oregon has also been getting significant thundershower activity all week.

Today will be another sunny, hot, and smoky day for NW Oregon.  However, moisture and instability from the California system will push further northward, inducing thundershower development over the central and possibly northern Cascades.  There is also a risk of thundershowers moving off the Cascades and over the Willamette Valley late today and tonight.  These would be slow-moving storms with potentially wetting rains.

TODAY'S FORECAST:

Sunny and warm.  Continued smoky.  Chance of thundershowers late.

Salem's high temperature today will be near 95°F (average is 83°F).

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

Surface winds: Light; becoming N 5-10 mph this afternoon.

Transport winds: NE 5 mph; becoming N 10 mph this afternoon.

Mixing height: Rising to 2000 by noon and to 3000 feet by 5 p.m.

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

(Salem Airport data for Wednesday, August 9th: High 96°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 35)

EXTENDED DISCUSSION:

Southerly flow aloft will keep skies at least partly cloudy on Friday, and the threat of thundershowers can't be ruled out.  Storms could produce rainfall in excess of .10".  Increasing onshore flow will begin a cooling trend and may bring some morning marine clouds into the valley.

An approaching upper-level trough will turn the flow aloft southwesterly over the weekend, which should further increase the onshore flow and stabilize the air mass.  Although the threat of thunderstorms will decrease, a weak cold front may bring drizzle or a few light showers on Sunday.  Rainfall amounts should be generally less than .10".  Temperatures should cool into the 80s Friday and Saturday and the 70s on Sunday.  A generally dry and stable onshore flow pattern may bring favorable burning opportunities next week.

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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