[willamette-fcst] Silverton Hills Midday Forecast.doc

Thu Sep 27 11:56:47 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT THU SEP 27, 2012

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.

WEATHER DISCUSSION:

Little change in the dry and stagnant weather pattern is forecast for today, with a broad upper-level ridge of high pressure remaining parked over the Pacific Northwest.  A very weak upper-level disturbance is producing some middle and high clouds over mainly central and eastern Oregon.  Skies are mostly sunny across western Oregon at midday, with the exception of low clouds and fog along the extreme south coast.

Warm air aloft will keep mixing heights suppressed today.  A surface thermal trough remains over western Oregon, at midday, with weak pressure gradients across the Willamette Valley.  The trough is forecast to move only slightly east this afternoon.  Light northerly transport winds will continue to be unfavorable for burning. Temperatures should be at least 5 degrees warmer than on Wednesday, with valley highs in the low 80s.

TODAY'S FORECAST:

Mostly sunny and warm.

Salem's high temperature today will be near 81 degrees (normal is 73).

Relative humidity:  Dropping to near 30% by 5 p.m.

Surface winds:  N 3-8 mph.

Transport winds: N to NE 4-8 mph.

Mixing height: Rising to near 2200 feet by 2 p.m. and 2700 feet by 5 p.m.

Salem's sunset tonight: 6:59 p.m.

THREE-HOURLY DATA:

                                2 p.m.  5 p.m.  8 p.m.

     Temperature:                 76      80      68

     Relative Humidity:           36%     29%     49%

     Surface Wind Dir/Speed:      N 4     N 6     N 4

     Transport Wind Dir/Speed:    N 5     N 8     N 6

     Mixing Height:              2200    2700    1000

     Ventilation Index:           11      22       6

EXTENDED DISCUSSION:

On Friday, the upper-level ridge is forecast to move over the Rockies with a weak westerly flow aloft over Oregon.  A decaying cold front will likely bring some increasing middle and high clouds and force the thermal trough into eastern Oregon.  Transport winds may turn enough northwesterly to allow for some burning.  However, ventilation conditions will be marginal, at best, due to continued warm air aloft and slow wind speeds.

The weak cold front will pass mainly north of the state Friday night, with no precipitation expected across western Oregon.  High pressure is forecast to quickly rebuild across the region this weekend.  Northerly winds on Saturday will likely turn offshore on Sunday, with the potential to transport wildfire smoke back over the Willamette Valley.  The upper-level ridge will stay anchored over the Pacific Northwest through at least Tuesday.  Some long-range models are showing a breakdown of the ridge late next week, but that is not certain.

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 15mph.

     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 times

         the transport wind speed 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 and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.

Pete Parsons

ODF Meteorologist
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