[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Fri Aug 4 08:56:10 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT FRI AUG 4, 2017

BURN ADVISORY:

*** State Fire Marshal Burn-Ban Conditions possible this afternoon. ***

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

The strong upper-level ridge that has been responsible for the record-breaking heat this week has shifting into the eastern Gulf of Alaska.  A dry upper-level trough is dropping into the northern Rockies and has induced onshore flow, at all levels, across western Oregon.  That will cool temperatures about 8-10 degrees today and begin to push the wildfire smoke out of the valley.  The air aloft is still quite warm, so mixing heights will be slow to rise this afternoon.  State Fire Marshal Burn Ban Conditions may be reached late this afternoon, due to warm temperatures and low humidity.

TODAY'S FORECAST:

Sunny and smoky.  Not as hot.  Gradually improving air quality.

Salem's high temperature today will be near 95°F (average is 84°F).

Relative humidity:  Dropping to 40% by noon and to near 25% by 5 p.m.

Surface winds: SW 5-10 mph; becoming W 5-10 mph late afternoon.

Transport winds: SW 10 mph this morning; W 10-15 mph this afternoon.

Mixing height: Rising to 3000 about 3 p.m. and to 4000 feet by 5 p.m.

Salem's sunset tonight: 8:34 p.m.

(Salem Airport data for Thursday, August 3rd: High 103°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

The upper-level ridge is expected to move back over Oregon this weekend through at least early next week.  However, it is not expected to be as strong as it was earlier this week.  Onshore flow will be weak but should hold high temperatures below the century mark and help to keep additional wildfire smoke for entering the airshed.

A weak upper-level disturbance is expected to undercut the ridge and bring a chance of thunderstorms to southern Oregon beginning late this weekend.  Storms are not expected to make it as far north as the Willamette Valley, but that can't be ruled out. Some long-range computer models show the upper-level ridge weakening and progressing east of the region later 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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