[willamette-fcst] Silverton Hills Midday Forecast_Tom

Thu Jul 30 11:38:51 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT THU JUL 30, 2015

***State Fire Marshal Burn-Ban Conditions, due to high temperatures and low humidity, will be observed this afternoon***
***A Red Flag Warning is in effect now through 11 p.m. Friday evening for the Willamette Valley and western Cascades***

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A strengthening upper-level ridge over the Pacific Northwest will continue warming the air aloft and suppressing mixing heights. At the surface, a thermal trough west of the Cascades will strengthen due to intense heating under clear skies, shifting just east of Interstate-5 this afternoon. Newport to Salem is currently light onshore at +0.6mb, but Salem to Redmond is much stronger offshore at -3.4mb, creating north to northeasterly winds in the Willamette Valley. State Fire Marshal conditions are likely to develop this afternoon due to soaring temperatures and low humidity, creating an unfavorable scenario for open burning opportunities. Today will likely be the hottest day for many locations within the past several years, and is poised to shatter local records.

TODAY'S FORECAST:

Clear skies with very hot temperatures.

Salem's high temperature today will be near 105°F (average is 84°F).

Relative humidity:  Dropping near 20% by 5 p.m.

Surface winds: N-NE 10-15 mph this afternoon.

Transport winds: N-NE 15-20 mph this afternoon.

Mixing height: 3000 feet by 3 p.m. and rising to 4000 feet by 5 p.m.

Salem's sunset tonight: 8:41 p.m.

(Salem Airport data for Wednesday, July 29th: High 99°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 80)

EXTENDED DISCUSSION:

The current weather pattern will continue into the weekend, as the upper-level ridge slowly moves across our region. The thermal trough in western Oregon will be a dominant feature, keeping daytime temperatures above seasonal averages with marginal humidity recovery at night. State Fire Marshal conditions will be a distinct possibility for the next several days as a result. This indicates few, if any, opportunities for open burning until early next week. The upper-level ridge will weaken enough to allow cooler, onshore flow to return to Oregon Sunday afternoon. A weak upper-level trough off the coast of California may bring some isolated thunderstorms along the Cascades.

Monday will mark the beginning of a transition period for northwestern Oregon. The upper-level ridge will shift east of Oregon, allowing southwesterly onshore flow to further cool the air aloft. The thermal trough will shift east of the Cascades, allowing afternoon gradients to shift to light onshore flow ahead of a mild sea breeze. Better mixing conditions will result as the atmosphere cools, improving the possibilities for open burning operations. There will be a chance of isolated thunderstorms along the Cascades Monday afternoon.

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

Tom Jenkins

ODF Meteorologist
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