[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Wed Sep 17 11:56:52 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT WED SEP 17, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

South-southwesterly flow aloft is spreading considerable cloudiness across most of Oregon but with only a few sprinkles...mainly along the coast.  A weak surface thermal trough remains is over western Oregon with light and variable winds in the Willamette Valley and light NE winds in the transport layer.

The Salem sounding this morning showed some warming aloft since yesterday, so mixing heights will be slow to climb this afternoon.  However, an approaching upper-level trough may create a favorable burning window late this afternoon, by cooling the air aloft and turning transport winds westerly.  On the flip side, it could also produce sprinkles or light showers.  Transport winds will be monitored closely, beginning with a 2 p.m. PIBAL.

TODAY'S FORECAST:

Cloudy and cooler.  Chance of sprinkles or light showers.

Salem's high temperature today will be near 77°F (average is 77°F).

Relative humidity:  Dropping only to near 50% this afternoon.

Surface winds: Var 5 mph; possibly becoming W 5 mph late this afternoon.

Transport winds: NNE 5 mph; possibly becoming W 5-10 mph late this afternoon.

Mixing height: Rising to 3000 feet by 3 p.m. & perhaps 4000 feet by 5 p.m.

Salem's sunset tonight: 7:19 p.m.

(Salem Airport data for Tuesday, September 16th: High 80°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 25)

EXTENDED DISCUSSION:

A weather system will move onshore tonight and across mainly southern Oregon and northern California on Thursday.  Showers are possible tonight and likely on Thursday, but precipitation totals should be less than .10".  If fields remain dry, continued cooling aloft and westerly transport winds could make for favorable burning conditions Thursday afternoon.

A return to dry and warmer weather is expected Friday through Monday.  Transport winds will turn northerly on Friday.  Offshore winds on Saturday may transport smoke, from the "16-Pit" wildfire, back across the valley.  That should be a short-lived event, with onshore flow quickly returning on Sunday; clearing any smoke from the region.

Increasing southwesterly flow aloft and onshore flow near the surface could create favorable burning conditions Monday afternoon. A more substantial weather system may bring some light rain to the region next Tuesday.

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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