[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Wed Sep 3 11:41:15 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:40 AM PDT WED SEP 3, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from now until 1:00 p.m. with a 50 acre limit.

WEATHER DISCUSSION:

An upper-level trough extends from western Montana across Oregon, with a dry and brisk northwesterly flow aloft over NW Oregon.  Marine clouds still cover much of NW Oregon but should slowly give way to sunshine, this afternoon, as onshore flow turns northerly.

Cool air aloft will make for high mixing heights today.  However, significant negative gradient-stacking of the onshore flow, from Newport to Redmond, would cause down-mixing of smoke plumes.  An 11 a.m. PIBAL showed light and variable winds in the lowest 3000 feet.  Even if the gradient-stacking gets "balanced" later this afternoon, increasing northerly transport winds will likely inhibit burning chances.  The next PIBAL is scheduled for 1 p.m.

TODAY'S FORECAST:

Slowly clearing skies.

Salem's high temperature today will be near 75°F (average is 80°F).

Relative humidity:  Near 50%; dropping to near 35% by 5 p.m.

Surface winds: Light; becoming N 5-10 mph later this afternoon.

Transport winds: NW 5-10 mph; veering to N 10-15 mph later this afternoon.

Mixing height: Near 3000 feet; rising to near 5000 feet by 2 p.m.

Salem's sunset tonight: 7:45 p.m.

(Salem Airport data for Tuesday, September 2nd: High 80°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

A building upper-level ridge, over the Gulf of Alaska, will bring a dry northerly flow aloft on Thursday and NE flow aloft on Friday.  A building surface thermal trough, along the coast, will turn low-level winds offshore and warm temperatures into the low 90s by Friday.  This warm spell should peak on Saturday, with the thermal trough moving over the interior of western Oregon and valley temperatures climbing into the mid-90s.

Early next week, the ridge is expected to weaken and shift westward.  That would turn the flow aloft northwesterly with increasing onshore flow at the surface.  Although still a dry pattern, temperatures will cool to near average with possible burning opportunities.  The upper-level ridge is predicted to strengthen over Oregon during the second half of next week, for a return to offshore flow and quite warm temperatures.  Longer-range models are maintaining this dry weather pattern through the middle of September.

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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