Sunday, August 13, 2017

Total Solar Eclipse Weather Outlook: Will Your View Be Blocked?

Jon Erdman
Published: August 13,2017

Now inside of 10 days away from the first total solar eclipse in the Lower 48 states in over 38 years, meteorologists are getting a look at the overall weather pattern that may be in place and could determine whether you'll get a clear view of this historic event.
(MORE: 8 Mistakes to Avoid to Enjoy the Eclipse)
A swath from Oregon to South Carolina will see the moon completely mask the sun Aug. 21. Nearly all other parts of North America – as well as parts of South America, Africa and Europe – will see at least a partial eclipse.
The eclipse will start mid-morning in the Pacific Northwest, around midday in the nation's heartland and early afternoon in the Southeast, ending in the Lower 48 states after 4 p.m. EDT.
(MORE: 10 Best States to See the Total Solar Eclipse)

First: Thunderstorms and the Eclipse

In summer, we don't typically have large-scale storm systems that can sock in dozens of states with clouds and precipitation as we do in colder months.
That leaves us with, for the most part, thunderstorms.
A single thunderstorm won't last long enough to affect the entire two- to three-hour viewing period of the eclipse in any given area.
However, one of these stray thunderstorms could be so poorly timed it occurs over part of the area seeing an eclipse for the roughly two- to three-minute period of totality.
(MORE: NASA's Eclipse Timing Maps)
Posing more of a threat to block out the eclipse, clusters of thunderstorms known as mesoscale convective systems – MCS, for short – are common in the summer. Their rain or cloud shield can linger over a given location for more than an hour.
However, these clusters are typically more active during the nighttime and early-morning hours – not necessarily during the early-afternoon hours, when the August eclipse will occur in the central and eastern U.S.

Forecast: What We Know Now

It is still too early for us to make a specific forecast for any location for the period the eclipse occurs.
The state of the science is not in a place where we can tell you if a thunderstorm, cluster of storms, or even sufficient cloud cover, will blanket your particular area roughly 8 days from now.
However, the overall pattern in place is giving us some hints at what areas may have a better chance of thunderstorms or clouds.
This is the overall weather pattern that is forecast for August 21, according to various ensemble forecast models. Areas in green show potential rainfall areas in late morning/early afternoon. This forecast is subject to change as we draw nearer to the event.

East Outlook: Iffy

In general, the longer-range ensemble model guidance suggests the jet stream still will tend to take at least a bit of a southward dip in the East.
When this happens in summer, as we've seen repeatedly so far, that increases the chance of afternoon showers and thunderstorms in the Northeast and mid-Atlantic states.
Without suppressing high-pressure aloft overhead, scattered thunderstorms seem a good bet from the Appalachians to the northern Gulf Coast, including the Florida peninsula, as is typical on a summer afternoon.
The eclipse will take place in the early-mid afternoon hours along the East Coast, typically the time during which thunderstorms will flare up.
So, if this pattern holds, you may have to cross your fingers poorly-timed thunderstorms don't block your view.

Central Outlook: Somewhat Iffy

Usually thunderstorms in late August are most numerous in the northern tier of states in late summer, near the jet stream.
If this jet-stream pattern holds, this may mean parts of the Upper Midwest or northern Plains may have to deal with thunderstorm clusters.
However, the timing of the eclipse in the nation's heartland, roughly midday into early afternoon, may allow any morning thunderstorm cluster(s) to die off by the time the eclipse happens.
Farther south in the Plains, the thunderstorm chances are typically less in late summer, and that may hold true on August 21.

West Outlook: Promising

The timing of the eclipse works to the West's advantage.
There should be plenty of afternoon thunderstorms from the Rockies to the Desert Southwest, as is par for the course in August.
However, a late-morning/midday eclipse should avoid most of those pop-up thundershowers.
The only hiccup may be some areas of low clouds near the West Coast in areas where winds blow onshore if they remain stubborn enough to linger into the late-morning hours.

What About a Hurricane or Tropical Storm?

One wildcard is tropical cyclones.
Upper-level winds can spread a canopy of clouds over areas far from the center of a tropical storm or hurricane.
High-resolution satellite image of Tropical Storm Fay along Florida's Atlantic coast on August 20, 2008.
(NASA/Jeff Schmaltz, MODIS Rapid Response Team)
Just the remnant moisture of a former tropical storm or hurricane can either trigger more numerous thunderstorms or block the eclipse with clouds.
This can not only occur in the eastern and southern U.S., but also with remnant eastern Pacific tropical cyclones in the typically sunnier Desert Southwest.
But the chance of a hurricane or tropical storm affecting the U.S. on any particular day during the hurricane season is small.
In the satellite era, since 1966, there have been only four tropical cyclones active very near or over the U.S. on August 21.
  • Tropical Storm Fay (2008): Florida
  • Hurricane Bret (1999): Cloud shield over Texas coast; made landfall the next day
  • Tropical Storm Charley (1998): Again, cloud shield over Texas coast; landfall the next day
  • Tropical Storm Dottie (1976): Had weakened to a tropical depression over South Carolina
For now, we don't see any obvious threats for the U.S. from a tropical storm or hurricane for August 21, but, as with other weather, that scenario can change.
Jonathan Erdman is a senior meteorologist at and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7. Follow him on Facebook and Twitter.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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