[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Thu Jul 20 11:54:53 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT THU JUL 20, 2017

BURN ADVISORY:

Recommended times for agricultural burning are from now until 7 p.m.

Prep burning is not allowed.

WEATHER DISCUSSION:

Low clouds continue to blanket NW Oregon, at midday, in the wake of last night's marine push.  An upper-level trough will promote good daytime mixing and maintain the strong onshore flow today.  Skies should slowly clear this afternoon, with temperatures struggling to reach the upper 70s.

Mixing heights and transport winds are favorable for burning today.  However, negatively gradient-stacking would cause smoke plumes to down-mix.  Gradients are unlikely to balance-out this afternoon but will be closely monitored.

TODAY'S FORECAST:

Gradually clearing skies this afternoon.  Cooler.

Salem's high temperature today will be near 78°F (average is 83°F).

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

Surface winds: SW 5-10 mph; becoming NW 5-10 mph in the late-afternoon.

Transport winds: SW 10 mph; becoming W 10 mph in the late-afternoon.

Mixing height: Rising to 6000 feet by 5 p.m.

Salem's sunset tonight: 8:51 p.m.

(Salem Airport data for Wednesday, July 19th: High 83°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 60)

EXTENDED DISCUSSION:

The air mass will begin warming on Friday, with weak westerly flow aloft and decreasing onshore flow.  Morning clouds will be less extensive and give way sooner to sunshine, allowing valley temperatures to climb back into the 80s.  Westerly transport winds and good mixing may create a burning opportunity, if negative gradient-stacking can be balanced by daytime heating.

Dry and warmer weather is expected over the weekend, with high temperatures climbing to around 90°F. Increasing SSW flow aloft will introduce a chance of thundershowers across SW Oregon by Sunday night and Monday.  It is possible that these storms could make it as far north as the Willamette Valley.  If not, increasing onshore flow may create a burning opportunity.

A marine push will initiate a cooling trend Tuesday afternoon and possibly create a significant burning opportunity.  A moderate onshore flow pattern is expected to produce morning clouds with afternoon sunshine 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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