[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Tue Sep 16 11:55:10 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT TUE SEP 16, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from noon until 1:30 p.m. with a 25 acre limit.

WEATHER DISCUSSION:

A strong upper-level ridge has moved east, to over the Rockies.  A switch to onshore flow overnight cleared the dense wildfire smoke from the Willamette Valley.  Southwesterly flow aloft is spreading some middle and high clouds across NW Oregon at midday, along with a few residual marine low clouds.

The Salem sounding this morning showed considerable cooling below 6000 feet, and low-level south-southwesterly winds.  Mixing heights should hit 3000 feet once surface temperatures warm to 74°F, which could be as early at 1 p.m.  Transport winds are forecast to veer from SSW to WSW this afternoon.  The influx of marine air overnight created some minor gradient-stacking this morning, but it appears that will be "balanced-out" soon by daytime heating.

TODAY'S FORECAST:

Mostly sunny...some high clouds.

Salem's high temperature today will be near 80°F (average is 77°F).

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

Surface winds: S 4-8 mph; becoming w 4-8 mph late this afternoon.

Transport winds: SW 10-15 mph.

Mixing height: Rising to 3000 feet by 2 p.m. and to 4000 feet by 5 p.m.

Salem's sunset tonight: 7:21 p.m.

(Salem Airport data for Monday, September 15th: High 88°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 60)

EXTENDED DISCUSSION:

Little change in the weather pattern is expected on Wednesday.  Southwesterly flow aloft and weak onshore at the surface may creating favorable burning conditions in the afternoon.  A little stronger weather system will approach the southern Oregon coast late Wednesday.  Some showers are possible Wednesday night and Thursday, as the associated weak upper-level trough comes onshore.  The main impact of this system will be across southern Oregon and northern California.  Rainfall totals should be less than .10".

A return to dry and warmer weather is expected Friday through Monday.  Transport winds will turn northerly on Friday.  Offshore winds on Monday may transport smoke, from the "16-Pit" wildfire, back across the valley.  That should be a short-lived event, with onshore flow quickly returning on Sunday; clearing any smoke from the region.  Weak southwesterly flow aloft is predicted for Monday with continued onshore flow at the surface possibly creating another burning opportunity.

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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