[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Thu Sep 11 11:52:00 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT THU SEP 11, 2014

BURN ADVISORY:

*** State Fire Marshal Burn-Ban Conditions Have Been Reached ***

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A strong offshore flow pattern has set up across NW Oregon.  An amplifying upper-level ridge, over the eastern Gulf of Alaska, is driving an early autumn-like weather system into the northern Rockies.  Cool air over NE Oregon is being accelerated westward into a surface thermal trough along the coast; resulting in brisk dry east to NE winds across NW Oregon.  The strongest winds will be near the Columbia Gorge and in the Portland area.

State Fire Marshal Burn-Ban Conditions exist throughout the Willamette Valley and will likely continue through this evening.  Due to the elevated risk of wildfires, the National Weather Service has issued a Red Flag Warning for most of NW Oregon, including the Willamette Valley, through 6 p.m. Friday.

TODAY'S FORECAST:

Sunny, breezy, and warmer with very low relative humidity.

Salem's high temperature today will be near 85°F (average is 78°F).

Relative humidity:  Dropping below 15% by 5 p.m.

Surface winds: N-NE 15-25 mph.

Transport winds: NE 20-25 mph.

Mixing height: Slowly rising to near 4000 feet by late this afternoon.

Salem's sunset tonight: 7:30 p.m.

(Salem Airport data for Wednesday, September 10th: High 81°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 80)

EXTENDED DISCUSSION:

Little change in the weather pattern is expected on Friday, except for some reduction of the offshore flow.  A coastal thermal trough will maintain the dry NE transport winds, but with gradually decreasing wind speeds, from south to north, across NW Oregon.  Valley temperatures will likely climb into the upper-80s, with State Fire Marshal Burn-Ban Conditions likely.

The thermal trough is forecast to move inland, over western Oregon, by Sunday.  That will further relax the winds, but valley temperatures should warm into the low-90s.

South to southwesterly flow aloft will increase early next week; turning the low-level winds back onshore and initiating a cooling trend.  The switch to onshore flow could create burning opportunities. Although the threat of a thunderstorm can't be ruled out, no significant precipitation is predicted through at least the middle of 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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