[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM PDT WED AUG 14, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



Middle and high clouds continue to stream over Oregon ahead of an autumn-like cold front that stretches from southwestern British Columbia to about 200 miles west of the Oregon coast. The cold front is forecast to weaken as it very slowly approaches the Oregon coast this afternoon and moves onshore this evening.  South-southwesterly flow aloft is also transporting "elevated" smoke over the region from wildfires in SW Oregon and northern California.



As of late this morning, radar showed rainfall just off the Washington and northern Oregon coast.  A few sprinkles had progressed as far south and east as Astoria, on the northern Oregon coast.  Ahead of the storm, temperatures in the Willamette Valley were generally in the mid to upper 70s, which is about 5-8 degrees warmer than 24 hours ago).  The air aloft has also warmed several degrees since yesterday, so mixing heights will not reach 3000 feet until surface temperatures climb into the low 80s this afternoon.



Onshore pressure gradients are already favorable to promote sustained lifting of smoke, so the limiting factors today will be mixing heights and wind direction.  An 11 a.m. PIBAL showed mostly south to southwesterly winds from the surface up to 4000 feet, with mixing heights estimated near 2000 feet.  Both were in-line with computer model guidance.  The forecast is for the winds to slowly turn more southwesterly this afternoon, as mixing heights climb above 3000 feet. That will likely create favorable open burning conditions by mid-afternoon, if not sooner.  Hourly PIBALS are scheduled to closely monitor this predicted shift in transport winds.



Some rain is likely to move onshore this afternoon but should remain west of the Silverton Hills.  It is highly questionable how much rain will eventually make it into the northern Willamette Valley tonight and Thursday, as the frontal system stalls and falls apart.  Some model guidance is suggesting up to one-quarter of an inch of rain may fall in the Salem area tonight through Thursday, but that looks to be overdone.  A trace to a few hundredths of an inch is a more likely target.



TODAY'S FORECAST:



Considerable middle and high clouds.  Chance of light rain late.



Salem's high temperature today will be near 85 degrees (normal is 83).

Relative humidity:  Drops to near 35% by 5 p.m.

Surface winds:  SSW 5-10 mph; Becoming SW 5-10 mph this afternoon.

Transport winds: S 15 mph; Becoming SW 15 mph this afternoon.

Mixing height: Rising to 3000 feet about 1 p.m. and to 4-5000 feet by 5 p.m.

Salem's sunset tonight: 8:19 p.m.



(Salem Airport data for Tuesday, Aug. 13th: High 86°F; Rainfall .00")

(Maximum Ventilation Index expected today: 75)



EXTENDED DISCUSSION:



A decaying cold front will bring mostly cloudy skies, cooler temperatures, and perhaps some light rain to the Willamette Valley tonight and Thursday.  Transport wind directions are forecast to be favorable for burning Thursday afternoon, but conditions may be too damp.



Another weather system is forecast to weaken as it moves onshore Friday.  If the timing is right and conditions remain dry enough, it could create another burning opportunity Friday afternoon.



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



Pete Parsons

ODF Meteorologist
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