[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT MON AUG 3, 2015



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



Our recent heat-wave was broken on Sunday, as an upper-level disturbance circulated enough clouds northward over the region to block the sunshine.  That cooled maximum temperatures from the triple-digits down into the low-80s but with very little in the way of rainfall across NW Oregon.



Another upper-level disturbance, over southern Oregon, is continuing to circulate moisture and instability northward over the region this morning.  Skies are generally cloudy across the northern two-thirds of Oregon, but radar is showing little in the way of shower activity making it as far north and west as the Willamette Valley.



Even though the air aloft has cooled since Sunday, cloudy skies will retard surface heating and likely keep mixing heights below 3000 feet until at least 2 p.m.  The moisture feed is expected to shift eastward this afternoon, as a drier and more stable SW flow aloft develops over western Oregon.  That should allow for some clearing, which would help late-afternoon temperatures climb into the upper-80s and lift mixing heights to around 4000 feet.  In addition, increasing onshore flow may turn transport winds northwesterly enough to allow for some burning.



TODAY'S FORECAST:



Slight chance of showers or thundershowers this morning; afternoon clearing.



Salem's high temperature today will be near 87°F (average is 84°F).

Relative humidity:  Dropping below 60% by 11 a.m. and to near 30% by 5 p.m.

Surface winds: Var 2-6 mph this morning; NNW 5-10 mph this afternoon.

Transport winds: NNE 2-6 mph this morning; NNW 5-10 mph this afternoon.

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

Salem's sunset tonight: 8:36 p.m.



(Salem Airport data for Sunday, August 2nd: High 82°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 40)



EXTENDED DISCUSSION:



An upper-level trough, dropping southward along the British Columbia coastline, will turn the flow aloft more westerly Tuesday and Wednesday.  Little to no precipitation is expected, but increasing low-level onshore flow will cool temperatures back to near average.  Morning marine clouds may penetrate into the Willamette Valley with sunshine likely in the afternoons.   Gradient-stacking and north winds could be an issue, but there is potential for burning each day.



A weak upper-level ridge is expected to turn the flow aloft northwesterly Thursday and Friday.  Onshore flow will begin to decrease with temperatures recovering to slightly above average by Friday.



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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