[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Tue Jul 29 11:45:34 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT TUE JUL 29, 2014

BURN ADVISORY:

...State Fire Marshal Burn-Ban Conditions may be reached late this afternoon due to high temperatures and low humidity...

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A strong upper-level ridge is centered over the Rockies with a warm SW flow aloft over Oregon.  A surface thermal trough extends from eastern Washington through central and SW Oregon.  Weak onshore flow is bringing marine clouds onto virtually the entire coastal strip but with little inland penetration.

Monsoonal moisture is flowing northward over mainly eastern Oregon, but a very weak upper-level disturbance was continuing to generate isolated thunderstorms over the central Cascades and north-central Oregon at midday.  These storms will continue to develop and slowly migrate northward this afternoon.  There is a risk that SE mid-level winds could develop and steer storms over the Willamette Valley this evening and/or tonight.

Onshore pressure gradients are weak (Newport to Salem 3.4 mb), especially from Salem to Redmond (1.2 mb).  Transport winds are forecast to remain northerly this afternoon, but an increase in onshore flow could turn winds slightly northwesterly, which would be more favorable for burning.  A PIBAL is scheduled for 3 p.m.

There are several limiting factors for burning today.  Warm air just above the surface will make for reducing mixing depths, thundershower development over the Cascades is problematic for good smoke evacuation, and hot surface temperatures may put the valley into State Fire Marshal Burn-Ban Conditions.

TODAY'S FORECAST:

Sunny and warm.  Scattered thunderstorms over the Cascades.  Risk of a thunderstorm over the valley late.

Salem's high temperature today will be near 95°F (average is 84°F).

Relative humidity:  Dropping to below 30% by 5 p.m.

Surface winds: N 8-12 mph this afternoon.

Transport winds: N 10-15 mph this afternoon.

Mixing height: Rising to 3000 feet by 2 p.m. and to 4000 feet by 5 p.m.

Salem's sunset tonight: 8:42 p.m.

(Maximum Ventilation Index expected today: 60)

EXTENDED DISCUSSION:

Very little change in the overall weather pattern is predicted this week.  A strong upper-level ridge of high pressure will stay anchored over the Rockies with a south to SW flow aloft over Oregon.  That will keep skies mostly sunny with well above normal temperatures.  It will also maintain a threat of afternoon and evening thunderstorms...mainly over the Cascades.  Very weak onshore flow could provide limited burning opportunities.  A change to more favorable patterns for burning is forecast for next week, as the ridge weakens and allows for slightly more onshore flow.

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 15 mph.

     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 (ODA) and the Oregon Department of Forestry (ODF).  For

information contact ODA at 503-986-4701.

To add/remove your email address from this list, please go to:

http://listsmart.osl.state.or.us/mailman/listinfo/willamette-fcst

Pete Parsons

ODF Meteorologist
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20140729/5d065665/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Silverton Hills Midday Forecast_Pete.doc
Type: application/msword
Size: 35328 bytes
Desc: Silverton Hills Midday Forecast_Pete.doc
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20140729/5d065665/attachment.doc>