[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Fri Aug 1 11:47:58 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT FRI AUG 1, 2014

BURN ADVISORY:

State Fire Marshal Burn-Ban Conditions may be reached this afternoon.

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

The flow aloft backed from southwesterly to south-southeasterly overnight, sending an upper-level disturbance northward across western Oregon.  Widespread thundershowers, from the coast range to the Cascades, produced gusty winds and locally around one-tenth of an inch of rain up the length of the Willamette Valley.  That weather system has moved into Washington, with clearing skies at midday across NW Oregon.  The flow aloft has reverted back to southwesterly, temporarily stabilizing the air mass over NW Oregon.

However, the surface thermal trough, which has been over central Oregon, also migrated westward and remains over the western Cascade foothills.  As daytime heating destabilizes the lower levels of the air mass, new thundershowers are most likely to develop along that trough axis this afternoon and overnight, posing a threat to the Willamette Valley.

In the meantime, valley temperatures are already near 80° F and should climb into the low to mid 90s again this afternoon.  With humidity levels likely dropping below 30% today, increasing surface winds and temperatures threaten to put the valley into State Fire Marshal Burn-Ban Conditions again this afternoon.  With the thermal trough positioned west of the Cascade crest, pressure gradients remain offshore from Redmond to Salem, which is also unfavorable for burning.  Transport winds should remain NE to N today and will be confirmed via a 2 p.m. PIBAL.

TODAY'S FORECAST:

Partly cloudy and hot with a chance of thunderstorms returning late.

Salem's high temperature today will be near 94°F (average is 84°F).

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

Surface winds: N 8-15 mph.

Transport winds: N 15 mph.

Mixing height: Rising to near 4000 feet by 5 p.m.

Salem's sunset tonight: 8:38 p.m.

(Salem Airport rainfall so far today: .15")

(Maximum Ventilation Index expected today: 60)

EXTENDED DISCUSSION:

A strong upper-level ridge of high pressure will remain anchored just east of the state through early next week.  That will maintain a warm south to SW flow aloft over Oregon, with continued above normal temperatures and clusters of mainly afternoon and evening thunderstorms.  There will continue to be an elevated chance of thunderstorms over the Willamette Valley on Saturday, with the storm focus shifting to mainly from the Cascades eastward on Sunday.

The upper-level ridge is forecast to weaken slightly by Tuesday.  Increasing onshore flow will likely shift the thundershower threat well east of the region and cool temperatures back into the upper 80s.  Transport winds may turn enough onshore to allow for some burning.  Additional weakening of the ridge may further increase the onshore flow by Friday, creating another potential 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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