Wednesday, August 16, 2017

Potentially Harsh Realities of Eclipse Chasing and Cloud Forecasting

A lot of assumptions, many quite poor, are being made concerning sky cover for Monday's eclipse.

Meteorologists who have to produce very detailed cloud forecasts, particularly those in which even a thin layer of clouds can make or break military mission success, alternative energy projections, or civilian aviation operations, know just how difficult it is to predict the existence of a thin cloud layers.  I once met the director of Air Force Weather and asked him what is number one forecast problem was.  He said instantly and without hesitation, "clouds."

Which brings us to the eclipse.

If you are desperate to observe totality, you have a window of less than 3 minutes and one thin patch of cloud could totally ruin your experience.  It doesn't take much to generate a thin patch of cloud.  There are many days in which the sky is partly cloudy and there's a patch veil of altostratus or cirrostratus.  Many.  You don't think about the weather on these days, because the weather is just fine for regular activities, but you would if you were in the Air Force or the solar industry.

Today provides a great example of a fair weather day that might just ruin your eclipse experience.  Let's imagine that the eclipse was today in Salt Lake City.  The 500-mb pattern is fairly innocuous, but there is a weak trough over Nevada and California.

The National Weather Service forecast calls for mostly sunny skies and a high of 88ºF.  A beautiful summer day.  You'd probably be excited if that was the forecast for the eclipse.

Not so fast.  A look at the satellite imagery shows patch altostratus over portions of Nevada and northern Utah.  On a meteorological scale of 1 to 10, these rate about a 0, but on an eclipse risk scale of 1 to 10, they could be a 10 if you happen to be in the wrong spot for that 2 minute period of totality.  

Sound crazy?  What if the eclipse was happening when I walked out of the Student Life Center?  Sorry, you're partially obscured.

Well, that's not so far from the cloud edge, you could just drive a bit down the road, if it wasn't clogged with traffic.

Or maybe the eclipse happens a bit later?  Hmmmm....

Here's something for you to ponder.  You wake up Monday morning and there's altostratus around.  Those are benign clouds for garden variety weather, but a disaster for eclipse viewing.  Do you move elsewhere or wait it out and hope for the best?  Alternatively, it's later in the morning.  The moon is sliding in front of the sun.  Traffic is everywhere because a million people have converged on the totality strip.  There is a patch of altostratus between you and the sun.  Totality is 30 minutes away.  What do you do?  Do you try to move?  Do you sit it out and hope for the best?

I have dealt with these sorts of problems during field campaigns, especially those with mobile radars. We can't precisely predict where storms will develop and you're not always in the right spot.  It can be an agonizing decision about whether or not to stay put, hoping the storm shifts, or move.  Moving has consequences.  It takes time.  You can miss out while on the road (at least with the eclipse, you know exactly when it's going to happen).  Computer models offer no help under such a scenario.  It's just you and the radar and your best nowcast.

Such a scenario is not out of the realm of possibility for the eclipse, where it could be just you, your eyes, and the clouds.  Forecasts for Monday include solutions with weak flow and mid-level moisture over western Wyoming and southeast Idaho, prime territory for Salt Lake eclipse viewers.  The GFS forecast shows the potential for mid level cloudiness over eastern Idaho and Wyoming Monday morning (eclipse time is around 1730-1740 UTC over Idaho and western Wyoming).  Time height sections (not shown) show some mid-level moisture in the region.

I can't tell you if there will be midlevel clouds because we're still dealing with a 5+ day forecast with a lot of uncertainty.  I hope the morning dawns "severe clear" for you without a cloud in the sky.  But what if there are some clouds around?  What would you do?  Could a shift of a few miles do the job?  Would you need to go farther?  In times like that, as Dwight Eisenhower said, "plans are nothing, planning is everything." 

The "Official" 2017/18 Ski Season Outlook

Released yesterday with a tweet of Trumpian proportions, we now share via conventional blog channels the Official Wasatch Weather Weenies 2017/18 Ski Season Outlook.

The outlook serves as a friendly reminder of the limited value of extended outlooks for doing any real planning for your ski season, vacation, or adventures.  The skill of such outlooks is generally low and there's really little to help guide us this coming season since it is likely to be a "No Niño" winter (see No Niño Winter Likely Ahead).  There may be a slight loading of the dice for a warmer winter and below average snowfall in warmer, lower elevation regions, but that's about it.  There are equal chances of above average, average, or below average snowfall wherever you are planning to ski.

That also means that if you are looking for powder this winter in the contiguous U.S., the best odds are found, as usual, in the Cottonwoods, because of their highly favorable climatology.  Globally, the best odds for powder are found in snowy regions of Japan's Honshu and Hokkaido Islands in January  (see my book Secrets of the Greatest Snow on Earth).  The challenge there is often not finding deep, but finding steep (and clear, at least in January).  It's there though if you know where to look and are willing to earn your turns.

Professor Powder enjoying a rare January bluebird day in the Hida Mountains, Japan

Tuesday, August 15, 2017

Meteorological Changes During Next Week's Eclipse

Unless you live in a cave, you know that all of North America will experience an eclipse of the sun this coming Monday (Aug 21), with the path of totality passing just to our north across central Idaho and Wyoming.

Source: NASA
Source: NASA
Salt Lake will see around 90% coverage and southern Utah around 75%.

Meteorological changes accompanying the eclipse are likely to be substantial and greatest near the path of totality in areas that are cloud free and prone to large diurnal cycles.  Studies of previous solar eclipses have documented dramatic declines in incoming solar radiation and temperature followed by recovery.  In some cases, surface-based inversions form, as one would expect under clear skies overnight.  For example, observations of the 10 May 1994 total eclipse show near-surface temperature falls of as much as 6ºC (about 11ºF) and the development of a surface-based inversion for approximately one hour.
(a) Solar irradiance (dark line) during the 10 May 1994 eclipse at Springfield, IL. I marks the beginning of the eclipse, II the beginning of the annular phase, III the end of the annular phase and IV the end of the eclipse.  Light curves estimate expected irradiance on a clear day.  (b) Ground and air temperature at several heights.  i and f mark beginning and end of eclipse-initiated surface inversion.  Source: Segal et al. (1996).
One of my former students, Jay Shafer, uncovered a study of the total eclipse of 31 August, 1932.  I am a sucker for these old studies as they required painstaking work on the part of the scientists just to collect, integrate, and analyze the data (in contrast to today where you can do it instantly on your phone).  The path of totality traced across New England where temperatures fell an average of 3ºC (6ºF, analysis below appears to be in ºF) and as much as 9ºF (5ºC).

Source: Brooks et al. (circa 1932)
Under relatively weak large-scale pressure gradients, the cooling associated with an eclipse can also affect winds.  Given that the cooling is localized, one might expect a weak area of higher pressure to develop along the eclipse path, resulting in a weak anticyclonic wind perturbation.  One might also expect a disruption of daytime upslope and upvalley winds, with possible reversal to downslope and downvalley as typically occurs near or after sunset.  This was documented recently over Switzerland during penumbral (partial) shading of an eclipse that reached totality across the UK and Russia (e.g., Eugster et al. 2017).

If you love these sorts of weather impacts, a great aspect of Monday's eclipse is that it is tracking across Jackson, WY and Stanley, ID, to locations know for large diurnal cycles.  If skies are clear, there should be a very dramatic response in temperature and thermally forced flows in those areas.  Northern Utah has a shot as well.  The Peter Sinks, for example, known for remarkably cold temperatures, might see a very dramatic drop in temperature, and canyons like Red Butte, Emigration, and Parleys, the development of a down valley exit jet.

Much will ultimately depend on weather as these effects will be most dramatic if skies are clear and relative humidity low.  Note that the Earth Systems Research Lab has been testing code that integrates eclipse effects on solar radiation into their experimental version of the High Resolution Rapid Refresh (HRRRX) model and real-time experimental HRRR ensemble.  If all goes well, experimental HRRR forecasts will include eclipse effects beginning at 0000 UTC 20 August (Saturday evening) in the HRRRX, which runs out 48 hours.  Those forecasts are available here.

There is one prediction that is probably relatively easy.  Attendance on the first day of classes at the University of Utah, which is also Monday, is likely to be light!

Monday, August 14, 2017

Signs of Change

Signs of change were quite apparent when I exited the house to head to campus this morning.  Lake stink, smoky air, and a light northwest wind were all evidence of an overnight airmass change.

MesoWest observations indeed show that a surface trough pushed through northern Utah and at 0730 MDT was draped across the southern Salt Lake Valley.  Strong northerly winds were evident over the Great Salt Lake and Desert.
Source: MesoWest
There's not much of a temperature contrast across the trough, but it does mark the leading edge of cooler air over the northwest.

The 1200 UTC HRRR calls for the front to linger around southern Salt Lake County and northern Utah County through about 1800 UTC (1200 MDT).

After that, it finally pushes southward as the main upper-level wave migrates eastward.

And speaking of that wave, it's producing about the most organized "s-shaped" cirrostratus deck we've seen around here in months.  Check it out to our west this morning, accompanied by a band of precipitation.

That feature may give us some scattered showers and thunderstorms tonight.  No guarantees of precipitation at your place, but some might get it.

Then there's the forecast for tomorrow.  Post trough, tomorrow dawns with 700-mb temperatures around +7ºC, probably the coldest airmass we've seen around here since mid June.

Bring a jacket tomorrow if you're riding the crest trail.

Sunday, August 13, 2017

Salt Lake County "Gets the Shaft"

When it comes to thunderstorms, it's often better to be lucky than good.

Driving back from a quick hike up the 'bird early this afternoon, a well-defined shaft of precipitation was moving across the northwest Salt Lake Valley.

Radar imagery showed some very localized and isolated cells around that time over the Salt Lake Valley and the Oquirrh Mountains.

Source: NCAR/RAL
 Those cells eventually merged, with high radar reflectivities over the central Salt Lake Valley.

Source: NCAR/RAL
 Eventually, the covered a good chunk of southwest Salt Lake County from Point of the Mountain to Parley's Canyon.

Source: NCAR/RAL
You'l notice that there wasn't much else going on west of the Wasatch Mountains as most of the action this afternoon was to our east and north.

Sometimes when it comes to convection, it's better to be lucky than good.  Of course, that assumes the storms didn't rain (and hail) on your parade.  For me, I was glad we got the "shaft" as it cooled things down at my place, even though we only got a trace of rain.

Models are suggesting some sort of a trough/frontal passage tomorrow.  The forecast high from the NWS for Tuesday is 82, which would be the lowest maximum we've seen around here since, gasp, June 17 (when it hit 82).  For lower than 82, you have to go back a few more days to June 14.

Friday, August 11, 2017

Author of Snow Bible Retires

Nolan Doesken.  Source: CSU Water Center.
I learned today that after a 40 year career, including more than a decade as Colorado's State Climatologist, Nolan Doesken has retired from Colorado State University.

Nolan has had an impressive career, but there are two of significant interest to readers of this blog.  First, along with Art Judson, he authored The Snow Booklet: A Guide to the Science, Climatology, and Measurement of Snow in the United States, which serves as the "snow bible" in the United States.

Published in the 1990s, it is now freely available online from Colorado State University, so click and have a look.  It is written in very accessible language and a good read for all snow lovers and Wasatch Weather Weenies.

Second, Nolan is the founder of the Community Collaborative Rain, Hail, and Snow Network, better known as CoCORaHS.  A great example of citizen science, CoCoRaHS is a network of volunteer observers who measure rain, snow, and hail using inexpensive gauges and techniques.  There are a remarkable number of observes (check the web site), with observations used for everything from operational weather forecasting to meteorological research.  Observer density in the Intermountain West is relatively low compared to other regions, so get involved if you are interested.  There's also this article that appeared in Weatherwise in 2010.