Weather Prospects for the 2017 Eclipse
eclipse arrives at a propitious time: the summer thunderstorm season
is winding down and retreating southward; the Arizona monsoon is
breaking; and the storm-carrying jet stream has not yet begun its
journey southward from Canada. The dry and generally sunny fall
season is about to begin. After a 38-year eclipse drought, this
one arrives to the open arms of a friendly August climatology. The
best weather prospects are found in the west and midwest, but weather
forecasting has now reached a level of accuracy that movement to
a favourable area can be planned several days or a week in advance.
westerly winds that bring weather systems onto the North American
continent first have to cross the mountain barriers that border
the Pacific coast. The moisture-laden air is forced to rise up the
windward slopes, giving seaward-facing Washington and Oregon unfortunate
reputations for cloud and precipitation. It's an undeserved bad
rep, as the cloud is confined mostly to the coast and only a short
move inland will bring some of the best weather of the track. To
top it off, August is not really a very wet and cloudy month to
begin with. The diminutive Coast Range is sufficient to erase the
cloud from the sky, giving the Willamette Valley and the city of
Salem some very fine eclipse weather.
2: Eclipse-track map showing the average afternoon August
cloudiness derived from 22 years of satellite observations.
The 50 and 75 percent contours are outlined. Best weather
prospects are found in northern Oregon, Idaho, central Wyoming,
and western Nebraska. The resolution in this figure is 1°
x 1° in the original dataset. Data: CIMSS/NOAA/UW-Madison.
of Salem, the eclipse track crosses the Cascade Mountains and the
cloudiness bumps up a touch - not enough to detract from the appeal
of watching an eclipse from the majesty of a beautiful range of
mountains. Once over the Cascades, the eclipse path moves onto the
Columbia River Basin, where it finds the very best weather conditions
anywhere along the track. Even though the summer months are already
notable for their abundant sunshine, the Cascade mountains manage
to extract the little moisture remaining in the Pacific westerlies,
giving Madras, Oregon, and its surroundings along the Deschutes
River a very low average cloud cover, as seen in both the satellite
cloud observations in Graph 1 and the surface-based observations
at Redmond in Table 1. In the recent past, few eclipse tracks have
been able to offer such a low average cloudiness.
of Madras, the cloud cover rises across the central and eastern
portions of the Columbia Basin. Part of this is because of the terrain,
which rises into a rougher series of small mountains, but the area
is also the breeding ground for summer thunderstorms when enough
moisture creeps into the plateau. Over the eastern Plateau, midsummer
precipitation is about 70 percent more than on the leeward slopes
of the Cascades near Madras, but both are noted for their dry desert-like
Columbia Basin is interrupted along the Idaho state line by the
Snake River Valley. Once again, the westerly Pacific winds must
drop to lower altitude, drying adiabatically as they descend and
creating another region of very promising cloud cover climatology
near Huntington, Oregon, and later in the Payete River Valley, at
Cascade, Idaho. Given the uncertainties in the measurement of cloud
cover by both satellites and humans, it is impossible to say that
the Oregon-Idaho boundary has less cloud than the Deschutes River
Valley, as the difference is no more than a few percentage points.
the Snake River Valley, cloudiness follows the terrain as the Moon's
path rattles across the Rockies. Each ascent is accompanied by an
increase in cloudiness; each descent brings a Chinook-like drying
and an increase in sunshine and eclipse prospects. The general trend
in cloudiness is upward until the track finally descends onto the
Great Plains for good at Casper, Wyoming. Thunderstorms
are the main cloud-making culprit here, as the upper slopes of the
Rockies, particularly Idaho and western Montana, are a storm nursery
- storms that later head out onto the plains if moisture and wind
conditions are right. They also start forest fires and eclipse chasers
will have to monitor and move to avoid any smoke that may make its
way onto the eclipse track, possibly from as far south as Arizona
and New Mexico if upper winds are out of the south. Smoke tends
to have a northeastward trajectory under the usual summer conditions,
so points in Oregon and western Idaho are less likely to be obscured.
If smoke is present in any amount, the eclipse may adopt an unbecoming
pink colour, even though the Sun will be relatively high.
it lies on the east side of the Rockies and should have a dry climate,
Casper is shown to have a relatively high cloud amount in the satellite
statistics, but a very much smaller one - comparable to Madras -
in the surface climatology. In this case, the surface record is
preferred. Casper lies north of the Laramie Mountains, a spur of
the Rockies that runs southeast-northwest rather than north-south,
the general trend of the mountain chains. Satellite measurements
have a footprint of 1 degree (about 100 km) on a side and the value
at the location of Casper is capturing much of the cloud on the
spur. The Laramie Mountains, covered with dark forests, have a tendency
to form thunderstorms because of their low albedo, which, by absorbing
sunlight, warms the slopes more strongly than the non-forested lowlands.
1: Average cloud cover along the eclipse centreline, extracted from
20 years of satellite imagery. Source: Patmos-X: CIMMS/ SSEC.
ups and downs of mountain cloudiness fades away as the eclipse track
moves out onto the Great Plains east of Casper and is replaced by
a steady rise in cloudiness toward the Mississippi River. This trend
is a reflection of the northward flow of moisture from
the Gulf of Mexico that feeds
the Plains thunderstorms in the warm months. From Nebraska to Missouri,
average cloud cover as measured by satellite increases from about
50 percent to 65 percent. In the surface record, the increase is
from around 25 percent to 45 percent. This dichotomy likely results
from the sensitivity of the satellite sensors to thin high-level
cloud, but human observers are also relatively insensitive to the
same sky condition, tending to observe less cloud. In both measures,
the lowest cloud amount along this part of the centreline is found
to the west of North Platte, Nebraska.
1: Weather statistics collected from surface stations located along
and close to the eclipse track. Note that all weather statistics,
including those from satellites, have biases and errors. Use them
for the comparison of one site with another, not the absolute probability
of seeing the eclipse.
the shadow track crosses through Missouri, it moves into rougher
terrain of the Ozark Plateau south of St. Louis and cloudiness climbs
more steeply, reaching a maximum at Festus, just before crossing
the Mississippi River. The Ozarks are not particularly high, and
so the cloud cover maximum is probably related to both the albedo
of the forests and the air flow that must rise over the terrain.
Cloudiness falls briefly over the flat landscape of southern Illinois,
but begins a steady climb to a maximum on moving into Tennessee.
After passing Nashville, the lunar shadow will begin the slow climb
to the crest of the Appalachians, which is reached along the Tennessee-North
Carolina border. Average cloudiness rises through the whole of this
part of the track, peaking as high as 80 percent by satellite or
just short of 60 percent in the surface data. The two datasets are
in poor agreement in this part of the lunar path; the greatest cloud
amount recorded by observers on the ground comes at Nashville, Tennessee,
while the satellite record suggests a peak just short of the Atlantic
Ocean, near Columbia. Of all of the states along the track, Tennessee
may have the most complicated weather, as the terrain climbs from
the Gulf Coastal Plain in western parts, to the Highland Rim in
middle Tennessee and to the Cumberland Plateau with the Blue Ridge
Mountains and all of its tributaries in the east. To complicate
the terrain, the Nashville Basin provides a low-level haven in the
Highland Rim, while the Tennessee River Valley does the same in
the Blue Ridge Mountains.
Appalachians are well known for their cloudiness at all seasons:
on the western slopes, Gulf moisture is forced to rise; on the eastern
slopes, Atlantic moisture is attracted inland and upslope. There
is no substantial lee slope drying on either side of the eastern
mountains, but individual valleys to benefit from the descent of
air whatever the source, and so there is considerable up-and-down
in the cloud averages, particularly in the surface-based observations.
21, 2011 GOES East satellite image for 1800 UTC
21, 2012 GOES East satellite image for 1800 UTC
final descent to the Atlantic Ocean brings a small
7- or 8-percent improvement before the track heads out over the
water. Some of this will be due to the suppression of convection
by the cool breezes from the ocean.
eclipe observers will have to be conscious of the hurricane season,
which is well underway in late August. Fortunately, they are relatively
uncommon on any single day, and so the likelihood of storm intervention
is low. In their favour - skies tend to clear in the wake of a hurricane.
Ship-board observers are likely to be more affected than those on
land, as movement away from an approaching hurricane on the water
will take place days in advance, and return to the track may be
thoughts on Satellite- and Human-based Weather Observations
don't observe clouds, they observe visible and infrared radiances
and at night, only infrared radiances. Depending on the algorithm
used to extract cloud information, they may be biased to over- or
under-report high cloud or low cloud, or perhaps both. There are
at least a half-dozen global cloud datasets, and they all give different
estimates for cloudiness, sometimes by a very large amount. The
dataset used to construct Graph 1 is the one that most resembles
the surface observations.
are far from perfect as well. In the table above, observers at the
Spirit of St. Louis Airport and Lambert-St. Louis International
Airport have observed very different cloud climatologies over the
decades, though the two sites are only 26 km apart and observe much
of the same sky. Humans tend to record too much cumulus cloud and
probably discount thin cirrus, but the explanation for the differences
at St. Louis defies complete explanation.
these data for comparative purposes, rather than absolute probabilities
of seeing the eclipse. The percent of possible sunshine statistics
are more reliable, but pertain to the whole day rather than the
hours near maximum eclipse and are only available for a few sites.
In general, human observations are 20 to 30 percent lower than satellite-based
United States has a sophisticated private forecast industry and
a great resource in the National Weather Service. Reliable weather
forecasts will be available for a week or more before the eclipse,
and with a little mobility, no one who wants to travel to see this
event should be disappointed.