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2017 August 21

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Eclipse Weather Summary

Madras potentially has some of the best weather prospects
for the 2017 USA Total Eclipse of the Sun

See Accuweather USA Satellite Animation for "Eclipse Day" 2016 August 21!
(Clear skies over much of USA)

Eclipse Path
The Eclipse Path Spans the USA Mainland
Map Credit Xavier M. Jubier

(Longest Duration of Totality in W. Kentucky but weather prospects better in the N.W.)

Weather Summary • Madras Climate & Weather
(Click figures to enlarge in separate window)

The path of the 2017 total eclipse spans the entire width of the United States mainland from Oregon through South Carolina, a distance of approximately 2,500 miles (4,000 km). See above map. Besides this favorable circumstance, the point of greatest eclipse also occurs over the USA (western Kentucky). This allows many possible locations for observing totality but climate circumstances significantly favor certain areas.

Climate Types

Fig. 1. Climates Along Eclipse Path. Several climate types populate the eclipse track across the USA.

A Variety of Climate Types The long eclipse track over the breadth of the lower forty eight states crosses several climate types. (See Fig. 1 for examples.) These types range from humid marine west coast weather in the Northwest (with moderate temperatures and usually lesser amounts of rain in summer) through the humid and hot subtropics of the Southeast with often very rainy summer months. Between these areas are found semiarid steppes and highland regions that can bring drier weather to western states and warm wet summers to the Midwest.

Therefore locations with drier summers may have an advantage over eastern areas that often have very rainy summers. Unfortunately, for eastern states, the eclipse occurs not only during summer but also during afternoon hours when thunderstorms are more prevalent.

The Western Advantage In the Northwest, large semipermanent high pressure and low pressure systems and a cool North Pacific (California) Ocean current help moderate temperatures and often produce drier summers. Although some western regions may not have a true dry season, lesser amounts of rain in summer are an advantage for a summer eclipse. In addition, the eclipse occurs during morning hours in the West before warming, rising air may bring clouds.

Climate Graphs

Fig. 2. Climates Graphs for Selected Cities. Monthly precipitation & temperature for nine cities in eclipse path. Red bars show Aug. precipitation. (Cred. U.S. Climate Data)

Oregon However, some western USA regions are still more favorable than others including Oregon with a climate that varies greatly across the state. For example, parts of Oregon can be very dry in summer. Here, mountains help make the climate variable from western to eastern Oregon. Moist Pacific Ocean air moves inland due to prevailing winds and rises over western mountains where the air cools and condenses. This uprising air produces more clouds and rain on the windward sides leaving the leeward eastern mountain flanks drier with less rain and clouds — a "rain shadow." Therefore, the high deserts of Central and Eastern Oregon east of the Cascade Mountain Range are drier than the generally moist though mild western, coastal parts of the state.

Averge August Clouds

Fig. 3. Average August Afternoon Clouds. Map of USA shows path of total eclipse and the average amounts of afternoon clouds in August. Redder colors show higher cloud amount, bluer colors lower cloud amounts. Western states, especially Oregon and Idaho typically have clearer skies in August. Afternoon cloud amounts are important since summer storms tend to build in afternoon hours when the eclipse will occurs for eastern states. Also see Fig. 4. (Cred. Jay Anderson)

Other Locations Meanwhile high latitude semiarid steppe regions to the east through Idaho, Wyoming and into western Nebraska may have some specific good weather locations if one avoids mountainous areas than may have significant summer precipitation. Moving into eastern Nebraska and beyond, humid continental climates can bring wetter summers and many summer thunderstorms. Finally eastern states from Kentucky to the Atlantic coast can suffer from hot, humid weather with frequent summer storms. Tennessee, Georgia and especially South Carolina are thus very poor choices if one expects clear afternoon skies.

Nine locations along the eclipse path (Fig. 2) illustrate this trend in precipitation across the country using monthly precipitation data. In addition, Fig.  3 shows a map displaying the average August afternoon clouds across the USA while Fig. 4 (both courtesy of eclipse meteorologist Jay Anderson) displays a graph showing average August cloud amounts along the eclipse center line.

Both the map and graph are consistent with western states having better weather prospects than more eastern states. Although these data do not guarantee that clear skies will predominate on eclipse day, they represent useful indicators where the probability of good weather is most likely to be found.

Climate Types

Fig. 4. Cloud Amounts. Graph shows average August cloud amounts along the eclipse center line. Duration of totality is illustrated along the top. Unfortunately, the longer durations occur in regions of poorer weather prospects. Also see Fig. 3. (Cred. Jay Anderson)

Madras, Oregon Climate This small North Central Oregon city sits in the rain shadow of the Cascade Range that lies about 35 miles west of the city. Sitting 2,300 feet (700 meters) above sea level, Madras has a semiarid steppe climate characterized by minimum precipitation during summer months. Thus, this location on the eastern, drier flanks of the western mountains produces a low annual rainfall of approximately 12 inches (300 mm) with an 80% frequency of clear skies. (See Madras monthly precipitation graph below for details.)

In Madras a dry season predominates in summer with August, month of the eclipse, typically having the least amount of rainfall (only 0.4  inches or 10 mm). Cool nights are usual (average low 50°F or 10°C) with warm days (86°F or 30°C). Since the eclipse ends before noon, temperatures should be pleasant for eclipse observers.

Indeed, the graph in Fig. 4 shows Madras having one of the lowest average August cloud amounts along the eclipse path.

Climate Types

Fig. 5. Madras Monthly Climate Stats. Table shows monthly temperature and precipitation statistics and annual amounts. August values are shown in green. Also see graphs of Madras Stats listed in the text on right. (Ref. NOAA, National Weather Service Forecast Office.)

Madras Weather Stats Detailed weather statistics for Madras in Fig. 5 give monthly temperature and precipitation (Ref. NOAA, National Weather Service Forecast Office). Typical August days have cool nights, warm days and usually no rain.

A Clear, Eastern Eclipse Sky
Over Madras, Oregon
Madras Eastern Eclipse Sky 2015 Aug. 21

Summary The duration of the eclipse is shorter in the western USA (abt. 2m04s or so depending where you are in Madras) than in eastern states. So, eclipse chasers will need to decide if they want the longest duration (e.g., 2m40s in western Kentucky) or much better weather prospects as in Madras, Oregon.

Other locations may be promising if carefully chosen to avoid summer clouds and rain. And people thinking about observing the eclipse from southeastern states should be especially wary of locating in a region where frequent afternoon storms rule.

For information about the eclipse itself, see Eclipse Summary

Above photo taken 2015 August 21, exactly two years before the "Great American Eclipse." Photo shows Sun (overexposed) at 10:30 a.m. PDT, or about 10 minutes after totality will end over Madras in 2017. Clear, blue skies prevailed on this day in 2015 over Madras, Oregon though previously smoke from the Warm Springs fire to the northwest had filled the sky. (Click photo for enlarged version in new window.)

Disclaimer: Weather prospects depend on expectations based on climate types and history and do not necessarily reflect what will really happen on eclipse day. A well-known caveat, most likely from science fiction writer Robert Heinline, probably best characterizes this: Climate is what we expect, weather is what we get. (″Time Enough for Love,″ 1973.)

Information provided as is without any guarantee of functionality, accuracy or completeness.

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