[willamette-fcst] (no subject)

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT MON JUL 18, 2011



BURN ADVISORY:



Recommended burn times for agricultural burning are from 12:00pm to 7:00pm.

Prep burning is not allowed.

Propane flaming is allowed from 1:00pm to 5:00pm.



WEATHER DISCUSSION:



Persistent upper low and frontal systems brought significant rainfall to the Willamette Valley this weekend with totals ranging from around a half to one inch.  This is an event rarely seen in the region during the middle of July.



Upper low circulating off the northern California coast will continue to give a threat of showers to the area through this afternoon before weakening and shifting slowly eastward on Tuesday.  Expect a little more sunshine today but still generally cloudy skies.  Temperatures will warm over yesterday's cool 60s temperatures to get into the 70s this afternoon.



Morning upper air sounding over Salem generally showed a moist and stable atmosphere with low level winds northerly at 5-10 mph shifting to easterly between 3000-4000 ft, then to southerly by 7000-8000 ft.  This variable wind pattern is quite common with upper level lows near the region.  As this low weakens, wind flow should consolidate to a more northwesterly flow up through the lower atmosphere.  Surface pressure gradients are quite weak and from the north this morning but afternoon heating will slowly turn the gradient more onshore.



TODAY'S FORECAST:



Fog and low clouds early, becoming mostly cloudy with a chance of showers this afternoon.



Salem's high temperature today will be near 76.

Relative humidity drops to 50% by 4pm.  Minimum relative humidity 48%.

Surface winds:  Light northerly winds turning NW'erly during the afternoon.

Transport winds: NNW to N at 3-7 mph, turning NW'erly at 6-10 mph during the afternoon and evening.

Maximum mixing height:  Mixing below 2000 ft before noon, rising to 4500 ft by late afternoon.

Sunset tonight: 8:53 pm





THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm

     Temperature:                 66       73      76      69

     Relative Humidity:           70%      53%     48%     61%

     Surface Wind Direction:     360      340     310     290

     Surface Wind Speed:           5        5       7       7

     Transport Wind Direction:   360      330     320     310

     Transport Wind Speed:         5        5       7      10

     Estimated Mixing Height:   1500     3500    4500    2000

     Ventilation Index:            8       18      32      20



EXTENDED DISCUSSION:



Even though today's upper low gradually weakens and moves through on Tuesday, the upper level pattern will remain generally trough-like through the week.  Expect continued cloudy skies, especially during the morning under onshore flow.  Clouds will break up more in the afternoon but temperatures will warm at best to close to normal, but more likely below normal.  Another impulse moves through the trough pattern early Thursday, increasing the onshore flow and bringing another chance of light showers.



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.



Nick Yonker

ODF Meteorologist



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