[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Mon Aug 10 11:53:08 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT MON AUG 10, 2015

***  The Portland National Weather Service has issued a Red Flag Warning for the western slopes of the northern and central Oregon Cascades, valid from noon today until 11 p.m. Tuesday.

Scattered thunderstorms are expected to develop over the Cascades and move to the north and NW.  The combination of gusty winds, abundant lightning, and very dry fuels poses a significant wildfire risk.  ***

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An offshore upper-level trough is producing a strong and unstable southerly flow aloft over western Oregon.  Daytime heating will further destabilize the air mass, likely leading to shower and thundershower development over the Cascades, and possibly the Willamette Valley, this afternoon and evening.

Southerly flow aloft typically does not produce very strong onshore flow, and late-morning gradients were only 1.6mb onshore from Newport to Salem and weakly offshore (-0.4mb) from Redmond to Salem.  Light southerly surface and transport winds may turn to the SW this afternoon, but winds at the mixing height will remain generally southerly.

Weather conditions will need to be closely monitored this afternoon.  A PIBAL is scheduled for 2 p.m.  Any thunderstorm development over the region would be unfavorable for open burning, due to erratic winds and the potential for smoke down-mixing.  In addition, the atmosphere could become unstable enough for the heat from field-burns to spawn thunderstorm development.

TODAY'S FORECAST:

Partly cloudy.  Chance of afternoon and evening showers or thundershowers.

Salem's high temperature today will be near 86°F (average is 83°F).

Relative humidity:  Dropping to near 50% by 11 a.m. and to near 35% by 5 p.m.

Surface winds: S 3-6 mph; possibly becoming SW 3-8 mph this afternoon.

Transport winds: SSW 5-10 mph; possibly becoming SW 5-10 mph this afternoon.

Mixing height: 3000 feet early this afternoon; rising to 5000 feet by 5 p.m.

Salem's sunset tonight: 8:26 p.m.

(Salem Airport data for Sunday, August 9th: High 86°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 50)

EXTENDED DISCUSSION:

The offshore upper-level trough will slowly drift south, to off the northern California coastline, Tuesday and Wednesday.  S-SE winds aloft will maintain a threat of mainly afternoon and evening showers and thundershowers.  Temperatures should warm into the upper-80s both days.

The upper-level trough is predicted to come onshore and move across the region Thursday and Friday.  That will bring much cooler onshore flow along with ample marine clouds and a chance of showers.  Temperatures should drop back into the mid-to-upper 70s.  Rainfall totals are generally expected to be less than .10".

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/20150810/0a94473d/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Silverton Hills Midday Forecast_Pete.doc
Type: application/msword
Size: 36352 bytes
Desc: Silverton Hills Midday Forecast_Pete.doc
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20150810/0a94473d/attachment.doc>