[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Wed Aug 27 08:57:29 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT WED AUG 27, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

The strong upper-level ridge over Oregon is beginning to flatten, as weak weather systems move across SW Canada.  That has turned the winds above 8000 feet from northeasterly to westerly.  However, below 8000 feet, this morning's sounding over Salem looks almost identical to Tuesday morning.  Valley temperatures are also very similar to 24 hours ago.  A surface thermal trough is over western Oregon this morning, with onshore flow from Newport to Salem and offshore flow from Redmond to Salem.

Like yesterday, warm air aloft will keep mixing heights under 3000 feet until surface temperatures reach about 87°F (around 2 p.m.).  One possible change today is that the thermal trough may shift east of the Cascades this afternoon; allowing some marine air to seep into the Willamette Valley.  If that happens, it would cap high temperatures in the low 90s (a few degrees cooler than on Tuesday) and could turn transport winds northwesterly.

TODAY'S FORECAST:

Sunny and warm.

Salem's high temperature today will be near 91°F (average is 81°F).

Relative humidity:  Dropping below 50% by 11 a.m. and to near 25% by 5 p.m.

Surface winds: N 3-6 mph this morning; N-NW 4-10 mph this afternoon.

Transport winds: NNE 4-8 mph this morning; N-NW 5-10 mph this afternoon.

Mixing height: Rising to near 3000 feet at 2 p.m.; to 4000 feet by 5 p.m.

Salem's sunset tonight: 7:58 p.m.

(Salem Airport data for Tuesday, August 26th: High 95°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 40)

EXTENDED DISCUSSION:

The upper-level ridge will further flatten on Thursday with increasing westerly flow above about 6000 feet.  That should force the surface thermal trough into eastern Oregon with increasing onshore flow cooling Willamette Valley temperatures back into the mid-80s.  Transport winds may turn northwesterly enough to provide an afternoon burning opportunity.

A strengthening west-southwesterly flow aloft will significantly increase the onshore flow on Friday, with morning marine clouds likely and valley temperatures cooling into the upper 70s.  Cooling aloft will improve mixing.  If morning marine clouds are not too persistent, creating poor gradient-stacking conditions, onshore transport winds will be favorable for burning.

An upper-level trough is forecast to move across Oregon Friday night with a cool NW flow aloft on Saturday.  This system looks mostly dry, but some morning drizzle or a few light showers are possible, with temperatures cooling to the mid-70s.  A mostly dry northwesterly flow aloft is forecast to continue through early next week, with varying amounts of morning clouds, afternoon sunshine, and slightly below average temperatures.

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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