[willamette-fcst] Silverton Hills Forecsat

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT WED JUL 11, 2012



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



A typical summer-time weather pattern has set up over the western United States.  A large upper-level ridge is centered over eastern Nevada with a dry and warm southwesterly flow aloft over Oregon.  Very weak onshore flow, at the surface, allowed for only minimal inland penetration of marine air this morning.  Just a few patches of low clouds managed to form in the Willamette valley, mainly in the Portland and Eugene areas.



Skies were sunny at mid-morning in mid Willamette Valley, but there continues to be some haze from wildfires east of the Cascades.  The morning sounding over Salem showed a couple of degrees of warming aloft, compared to yesterday, so expect surface temperatures to also be a little warmer today.



Warm air aloft will suppress mixing again today, with maximum mixing heights likely staying below about 3500 feet this afternoon.  Northerly surface and transport winds will be unfavorable for field-burning.



TODAY'S FORECAST:



Sunny, hazy, and a little warmer.



Salem's high temperature today will be near 88 degrees (normal is 81).

Relative humidity drops to 50% by 1 p.m. and to near 35% by 5 p.m.

Surface winds:  N 5-10 mph this morning; N 8-12 mph this afternoon.

Transport winds: NNE 6-8 mph this morning; N 10-14 mph this afternoon.

Mixing height: Rising to near 3000 feet by 3 p.m.

Salem's sunset tonight: 8:58 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 71       81      87      80

     Relative Humidity:           57%      41%     34%     44%

     Surface Wind Direction:     360      360     360     310

     Surface Wind Speed:           5        8       8       7

     Transport Wind Direction:   010      010     360     330

     Transport Wind Speed:         7       10      12      12

     Mixing Height:             1800     2800    3200    1500

     Ventilation Index:           13       28      38      18



EXTENDED DISCUSSION:



A weak upper-level trough is forecast to increase the southwesterly flow aloft on Thursday.  That should increase in the onshore flow Thursday afternoon and cool temperatures a few degrees.  It may also spread a few clouds across the region, but no precipitation is expected. The weak trough is forecast to remain over Oregon on Friday, with patchy morning clouds giving way to sunshine and seasonably warm temperatures.



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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