Here is my new weekly weather quiz question. What is a “Back-building thunderstorm?” Please remember to post your answer as a comment by clicking on “no comments /comments” in the upper right hand portion of this page under the caption, then add your comment. The first person to post the correct answer will win a week of free personalized weather forecasts tailored to your needs. The answer has to be a comment to win. An email answer will not count. Good luck.

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An upper level low (Shaded “U” shape with blue arrows) that has been controlling our weather will be replaced by high pressure (Shaded “Arch” shape with blue arrows) and that means the rain goes away again, but not quite yet. The low will move southward through Oregon bring the slight chance of showers and thunderstorms Friday, then the high pressure takes over.

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Air Quality Index:

Eugene-Springfield: The Air Quality Index is 28 in the Good category measuring Particulate Matter.

Oakridge: The Air Quality Index is 8 in the Good category measuring Particulate Matter.

Cottage Grove: The Air Quality Index is 27 in the Good category measuring Particulate Matter.

Data courtesy of Lane Regional Air Protection Agency (LRAPA).

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Advisories:

• A HEALTH ADVISORY CONTINUES FOR SUNSET BAY STATE PARK BEACH LOCATED IN COOS COUNTY FOR HIGHER-THAN-NORMAL FECAL BACTERIA WHICH CAN RESULT IN DIARRHEA, STOMACH CRAMPS, SKIN RASHES, UPPER RESPIRATORY INFECTIONS AND OTHER ILLNESSES. DIRECT CONTACT WITH THE WATER – ESPECIALLY BY CHILDREN AND THE ELDERLY, WHO MAY BE VULNERABLE TO WATERBORNE BACTERIA – SHOULD BE AVOIDED IN THIS AREA UNTIL THE ADVISORY IS LIFTED. VISITORS SHOULD AVOID WADING IN NEARBY CREEKS, POOLS OF WATER ON THE BEACH, OR IN DISCOLORED WATER AND STAY CLEAR OF WATER RUNOFF FLOWING INTO THE OCEAN.
• A HEALTH ADVISORY CONTINUES FOR UPPER KLAMATH LAKE AND AGENCY LAKE DOWN TO KENO DAM, LOCATED JUST NORTH OF KLAMATH FALLS ALONG HIGHWAY 97 IN KLAMATH COUNTY. WATER MONITORING BY THE USGS HAS CONFIRMED THE PRESENCE OF BLUE-GREEN ALGAE TOXINS WHICH CAN BE HARMFUL TO HUMANS AND ANIMALS. SWALLOWING, INHALING OR SKIN CONTACT WITH WATER DROPLETS SHOULD BE AVOIDED.

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Forecast for the Southern and lower Mid Willamette Valley including Eugene-Springfield and Albany-Corvallis:

• Partly cloudy today and tonight,
• A mix of clouds and sun and a bit cooler with a slight (20%) chance of showers and thunderstorms Friday afternoon, partly cloudy with a slight (20%) chance of showers and thunderstorms in the evening, partly cloudy Friday night and Saturday AM, mostly sunny in the afternoon, then partly cloudy Saturday night.
• highs 72-70 warming to 75 Saturday lows 45-48.
• Partly cloudy Sunday and Sunday night,
• Partly cloudy in the AM, mostly sunny and a bit warmer Monday afternoon (Labor Day), partly cloudy Monday night through Wednesday AM, then mostly sunny in the afternoon.
• highs 76-80 lows 48-50. (seasonal averages high 80 low 49)

Forecast for the Umpqua Basin including Roseburg:

• Partly cloudy today, mostly clear this evening, mostly cloudy late tonight,
• Mostly cloudy Friday AM, partly cloudy with a (30%) chance of showers and a slight (20%) chance of afternoon and evening thunderstorms, just partly cloudy Friday night and Saturday (warmer), then mostly clear Saturday night.
• highs 72-76 lows 48-50 cooling back to 48 Saturday.
• Mostly sunny and  warmer Sunday and Monday (Labor Day), mostly clear both nights, then mostly sunny and a bit warmer Tuesday and Wednesday and mostly clear at night.
• highs 80-85 lows 56-52. (seasonal averages high 82 low 54)

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Forecast for the South Oregon Coast including Coos Bay and North Bend:

• Partly cloudy today through this evening, mostly cloudy late tonight,
• Mostly cloudy Friday AM, partly cloudy and warmer in the afternoon and evening with a slight (20%) chance of inland showers, just partly cloudy at night and Saturday, then mostly clear Saturday night.
• highs 57-67 lows 52-54 cooling back to 52 Saturday night.
• Mostly sunny and a bit warmer Sunday and Monday (Labor Day), mostly clear both nights,
• Then partly cloudy Tuesday through Wednesday.
• highs near 65 lows 56-53. (seasonal averages high 66 low 52).

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Forecast for the Cascades in Lane County:

• Mostly sunny this afternoon, partly cloudy tonight,
• Mostly cloudy with a slight (20%) chance of AM showers, a good (50%) chance of showers and a slight (20%) chance of thunderstorms Friday afternoon and evening, partly cloudy with a slight (20%) chance of showers late Friday night,
• Mostly cloudy with a slight (20%) chance of showers Saturday AM, just partly cloudy and a bit warmer Saturday afternoon, mostly clear Saturday night.
• free air freezing level 7,000 ft. rising to 7,500 ft. this afternoon, 8,000 ft. tonight, snow level 6,500 ft. Friday through Saturday afternoon, then free air freezing level 9,000 ft. Saturday night.
• highs 55-53 warming to 58 Saturday lows 34-40.
• Mostly sunny Sunday AM, partly cloudy in the afternoon and at night,
• Mostly sunny Monday (Labor Day), partly cloudy Monday night and Tuesday AM, mostly sunny in the afternoon, partly cloudy at night and Wednesday AM, then mostly sunny Wednesday afternoon.
• free air freezing level 10,000 ft. Sunday, 11,000 ft. Sunday night through Monday night, 12,000 ft. Tuesday and Tuesday night, then 12,000 ft. Wednesday.
• highs 63-66 cooling to 63 Wednesday lows 42-45 cooling back to 42 Tuesday night.

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**Because weather forecasting is a combination of science, intuition, and timing there can be no absolute guarantees that individual forecasts will be 100% accurate. Nature is in a constant state of flux and sudden unexpected weather events can happen.

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Get your local Eugene-Springfield and Lane County news on-line at eugenedailynews.com.

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