[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT FRI AUG 3, 2012



BURN ADVISORY:



...State Fire Marshal Conditions may be met this afternoon (humidity and wind)...



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



The upper-level trough that brought onshore flow the past couple of days is dropping into eastern Montana and western North Dakota this morning. A building offshore ridge of high pressure has turned the flow aloft more northerly over Oregon.  The Salem sounding this morning showed several degrees of warming above 4000 feet, compared to 24 hours ago, and increasing offshore flow above 2000 feet. That is an indication of sinking air aloft, which will further warm and dry the air mass today.



Visible satellite imagery shows low clouds along the coast and over most of the Willamette Valley this morning.  The marine clouds should quickly give way to sunshine, as increasing north to northeasterly winds rapidly dry out the air mass over western Oregon.  Continued warming aloft will suppress mixing this afternoon, which will combine with northeasterly transport winds to make for highly unfavorable burning conditions.



(Salem Airport on Thursday, August 2nd: High 81°F; Rainfall .00")



TODAY'S FORECAST:



Morning clouds, then sunny and warmer.



Salem's high temperature today will be near 87 degrees (normal is 84).

Relative humidity drops to 50% around noon and to near 30% by 5 p.m.

Surface winds:  N 7-12 mph; becoming NNE 10-16 mph this afternoon.

Transport winds: NE 10-15 mph; increasing to 16-22 mph this afternoon.

Mixing height: Rising to 3000 feet by 1 p.m. and to near 4000 feet by 5 p.m.

Salem's sunset tonight: 8:35 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 70       80      86      80

     Relative Humidity:           64%      41%     29%     38%

     Surface Wind Direction:     010      010     020     010

     Surface Wind Speed:           8       10      14      13

     Transport Wind Direction:   020      030     040     030

     Transport Wind Speed:        13       17      20      20

     Mixing Height:             2500     3500    4000    2000

     Ventilation Index:           33       60      80      40



EXTENDED DISCUSSION:



The upper-level ridge is forecast to move directly over Oregon on Saturday with a surface thermal trough pushing northward over the region.  The lack of morning marine clouds will combine with very warm air aloft and offshore flow to produce the warmest temperatures so far this year.  Willamette Valley temperatures should climb into the mid and upper 90s, with State Fire Marshal Conditions likely being met Saturday afternoon.



The upper-level ridge is forecast to shift eastward by Sunday, with increasing southerly flow aloft destabilizing the air mass and introducing middle and high-level monsoonal moisture into the region.  That will begin a cooling process and bring a good chance of thunderstorms, especially Sunday night.  The flow aloft is forecast to turn more southwesterly Monday and Tuesday.  That should stabilize the air mass with increasing onshore flow.  The timing of that transition could create burning opportunities.  Temperatures should return to normal by Tuesday.



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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