[willamette-fcst] Corrected Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:08 PM PDT THU SEP 29, 2011



Corrected Discussion wording: "south-southeasterly" to "south-southwesterly"



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is allowed from 2:00 p.m. until 5:00 p.m.



WEATHER DISCUSSION:



A strong upper-level ridge of high pressure remains centered over the Rockies with a warm and dry southwesterly flow aloft over Oregon.  The surface thermal trough was still centered just west of the coastline late this morning.  Offshore pressure-gradients were slowly weakening, but easterly winds were still gusting to around 30 mph at the western end of the Columbia Gorge, with light easterly winds making it all the way to the coast.



Offshore flow has significantly warmed the low-level air mass since yesterday.  5000-foot temperatures over Salem early this morning were in the mid 60s.  That will suppress mixing heights today.  ODF Sodar, located in the central coast range, was showing offshore flow from near the surface up to 3500 feet late this morning, with south-southwesterly flow above 3500 feet.



The upper-level ridge is forecast to shift only slightly eastward today.  A reversal of the low-level offshore flow, to onshore, is looking less likely this afternoon, with the thermal trough staying over western Oregon.  Winds at the top of the mixing layer are forecast to be south-southwesterly, but persistent low-level offshore flow would decrease the chances for open field-burning this afternoon.  Pibals are scheduled to begin at 2 p.m.

TODAY'S FORECAST:



Sunny and unseasonably warm.



Salem's high temperature today will be near 84 degrees (normal is 72).

Relative humidity drops below to near 30% by 5 p.m.

Surface winds:  Variable 3-7 mph this afternoon.

Transport winds: ESE 3-6 mph; becoming SSW 6-10 mph this afternoon.

Mixing height: Rising to 3000 feet by 3 p.m. and to near 3500 feet by 5 p.m.

Salem's sunset tonight: 6:57 p.m.



THREE-HOURLY DATA:

                                2 p.m.  5 p.m.  8 p.m.

     Temperature:                 78      83      68

     Relative Humidity:           33%     30%     52%

     Surface Wind Direction:     360     Var     230

     Surface Wind Speed:           5       5       5

     Transport Wind Direction:   200     210     180

     Transport Wind Speed:         6      10       7

     Mixing Height:             2500    3500    1000

     Ventilation Index:           15      35       7



EXTENDED DISCUSSION:



The upper-level ridge is forecast to weaken and shift eastward on Friday.  Increasing southwesterly flow aloft will force the surface thermal trough into eastern Oregon.  Onshore flow will cool temperatures back close to normal with increasing clouds.  Dry conditions should prevail, with southwesterly transport winds likely allowing for the burning of any remaining fields.



Computer models show the first in a series of weather systems coming onshore Saturday.  This first system should weaken, as it moves inland, but still bring around one-tenth of an inch of rain.  Sunday looks like a mostly dry day, ahead of a much more potent system forecast to come onshore Sunday night.  That storm could dump more than one-half inch of rain by Monday evening.  An even stronger system is forecast to come onshore late 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



-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20110929/9328c515/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Silverton Hills Midday Forecast.doc
Type: application/msword
Size: 33792 bytes
Desc: Silverton Hills Midday Forecast.doc
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20110929/9328c515/attachment.doc>