[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Fri Jul 18 11:50:41 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT FRI JUL 18, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

Weak northwesterly flow aloft over Oregon brought some morning clouds up the Columbia River and into the extreme northern Willamette Valley.  Otherwise sunny skies will prevail again this afternoon.

Willamette Valley temperatures are running about 2-4 degrees cooler than 24 hours ago.  Much like on Thursday, onshore flow is strongest near the Washington border, with winds turning progressively more northerly further across the Willamette Valley.  Onshore flow has weakened along the coast, where skies have also turned sunny.  That is maintaining poor pressure-gradient-stacking this morning, which may not "balance" until late this afternoon.

Sunny skies will warm temperatures into the mid-80s today.  Cooler air aloft will provide high mixing heights, but transport winds should remain generally northerly.  The air mass is still very dry, so State Fire Marshal Burn-Ban conditions may be reached later this afternoon, due to low humidity and increasing northerly winds.

TODAY'S FORECAST:

Sunny and warm.

Salem's high temperature today will be near 84°F (average is 83°F).

Relative humidity:  Dropping to at or below 30% by 5 p.m.

Surface winds: N 5-15 mph this afternoon.

Transport winds: N 10-20 mph this afternoon.

Mixing height: Rising to at or above 5000 from 2 p.m. through 5 p.m.

Salem's sunset tonight: 8:53 p.m.

(Salem Airport data for Thursday, July 17th: High 88°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 100)

EXTENDED DISCUSSION:

The flow aloft is forecast to turn more westerly on Saturday.  Skies should turn partly cloudy with near average temperatures.  On Sunday, the flow aloft will turn southwesterly, with a weak cold front bringing more clouds, cooler temperatures, and a threat of morning drizzle or very light showers.  However, significant precipitation is not expected.  Southwesterly flow aloft will likely maintain mostly cloudy skies on Monday with onshore flow possibly creating a burning opportunity.  Long-range computer models are showing a stronger upper-level trough moving onshore Tuesday and Wednesday; possibly bringing more significant shower activity into the region.

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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