[willamette-fcst] Corrected Silverton Hills AM Forecast.doc

Fri Sep 21 09:00:31 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT FRI SEP 21, 2012

...Corrected "Today's Forecast"...

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.

WEATHER DISCUSSION:

The strong upper-level ridge of high pressure that brought dry and very warm weather to the region for most of the month has shifting eastward, to over the Rockies.   A weak offshore upper-level trough is producing southerly flow aloft over Oregon and enhanced the low-level marine layer overnight.  The Salem sounding this morning and local pilot reports indicate that the marine layer has deepened to about 4000 feet thick.  Areas of drizzle or very light rain are more widespread than yesterday morning, with some spotty reports of measurable rain in the Cascade foothills.

Visible satellite imagery shows marine low clouds blanketing all of northwestern Oregon.  Unlike yesterday, the low clouds are thick enough to cover the coast range.  As is typically the case, the "marine push" has resulted in unfavorable pressure gradient stacking this morning, which is unlikely to reverse this afternoon.  One benefit of the increased onshore flow is that the air quality has improved across northwestern Oregon, although there is still some evidence of residual wildfire smoke in the northern Willamette Valley.

The upper-level ridge will continue to slowly slide eastward today with the approaching upper-level trough maintaining strong onshore flow into western Oregon.  That should keep skies cloudy most of the day, with only some partial clearing possible late this afternoon.  High temperatures will likely stay in the 60s (well below normal).  The chances for open burning are slim.

(Salem Airport data for Thursday, Sept. 20th: High 70°F; Rainfall .00")

TODAY'S FORECAST:

Cloudy with areas of drizzle this morning; partial clearing possible late this afternoon.

Salem's high temperature today will be near 67 degrees (normal is 76).

Relative humidity drops to near 60% by 5 p.m.

Surface winds:  Light and variable; becoming NW 3-8 mph late this afternoon.

Transport winds: Light and variable; Becoming N-NW 5-10 mph this afternoon.

Mixing height: Rising to near 3000 feet by 5 p.m.

Salem's sunset tonight: 7:10 p.m.

THREE-HOURLY DATA:

                               11 a.m.   2 p.m.  5 p.m.  8 p.m.

     Temperature:                 57       62      67      61

     Relative Humidity:           90%      75%     63%     78%

     Surface Wind Dir/Speed:    VAR 3     NW 4    NW 5    NW 5

     Transport Wind Dir/Speed:  VAR 3     NW 4    NW 5    NW 7

     Mixing Height:              1500     2500    3000    2000

     Ventilation Index:             5      10      15      14

EXTENDED DISCUSSION:

A weak upper-level trough is forecast to come onshore tonight and Saturday.  That may squeeze some light showers out of the marine deck over the Willamette Valley, but rainfall totals should be less than one-tenth of an inch.  Ventilation conditions will improve, but transport winds are forecast to be northerly.  Some thunderstorms are likely from the Cascades eastward, with a high potential of starting new wildfires.

The upper-level trough is forecast to stall over the state on Sunday.  That will maintain a deep marine layer over western Oregon.  Skies should remain mostly cloudy with areas of morning drizzle.  Unfavorable gradient stacking should continue with northerly transport winds.

The upper-level trough is forecast to move east of the state on Monday, with weak westerly flow aloft possibly creating more favorable burning conditions.

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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