Monday, August 21, 2017

Scenes from the Eclipse

Note: Post has been updated to correct location of site with 10ºF temperature fall.


Source: CIRA
Source: CIRA
Incoming solar radiation (Salt Lake area sites).

Source: Alex Jacques, MesoWest, University of Utah
Temperature (Salt Lake area sites).

Source: Alex Jacques, MesoWest, University of Utah
Temperature at Peters Sinks (notorious cold spot in Bear River Range) site in the Bear River Range.  Fall of about 10ºF, although it's unclear if we're at the bottom yet (probably close).

Source: Utah State University, MesoWest

Morning Eclipse Nowcast

Morning has broken and things are looking good along the path of totality over the western U.S.  It's early for a visible loop, but as can be seen below, a band of mid and upper-level clouds is slowly but surely exiting the totality path area of far western Wyoming and Idaho.  

Here's a look at the GOES-16 geocolor imagery from CIRA at 13:32 UTC (7:32 AM) and it looks pretty good too.  There are some thin patchy clouds over portions of the Idaho panhandle and western Montana moving southward that are just barely discernible in the imagery.   

Source: CIRA
You couldn't ask for a better forecast from the operational HRRR than the one below, valid about a half hour after totality.  No clouds predicted along the entire track from Wyoming to the Pacific Coast. 

Source: ESRL
That forecast might be a bit optimistic as there may be a few high clouds around over Idaho and the remnants of the cloud band evident that is over southern Idaho and western Wyoming this morning could linger near and along the totality track over central Wyoming, as indicated by the NAM forecast below.  

Nevertheless, conditions look quite good for the totality track from Jackson to the Willamette Valley with just the threat of some thin clouds spilling down from the north.  You can see these clouds, for example, in the Montana Snowbowl web cam image below.  

Good viewing to those of you along the path.

Addendum @ 8:25 AM MDT

Now that the sun is a little higher, visible satellite imagery shows quite a bit of smoke in the valleys of the central Idaho Mountains.

Based on web cams, I don't think this smoke will obscure the sun, although it may redden it.  Hopefully it won't spoil any views.  

Sunday, August 20, 2017

Eclipse Forecast Update at T-1 Day

"Eclipse viewing is about synoptic possibilities, 
but succeeding or failing based on unresolved cloud processes" 
Slight paraphrasing of quote about storm chasing from meteorologist Chuck Doswell

Model forecasts have trended in the "right" direction since our post on outlooks for the eclipse on Friday.  At that time, the models, such as the NAM below, alled for a short-wave trough to be over Idaho at eclipse time, with at least partial cloud cover over portions of Idaho and Wyoming.

As I write this, the latest (0600 UTC) NAM forecast calls for that short-wave trough to be both further east and weaker, with the Pacific coast ridge also shifted further east.  Much of Oregon is still in a prime spot for lack of cloud cover, but the situation, at least synoptically, is better for Idaho and the Jackson area than one would have anticipated from the forecasts on Friday.

If we look at time-height sections, however, there remains one concern to give me a little heartburn, and that is a sliver of upper-level moisture coming through around the time of the eclipse, as evident in both the Sun Valley and Jackson time-height sections.  Note, in particular, the area of higher relative humidity air in both sections [note that time increases to the left and that the eclipse occurs just prior (to the right) of 18Z Mon].

Because of that sliver, one can't rule out the possibility of some high clouds being around.  The odds are low, but non-zero, an perhaps higher in Jackson than the central Idaho mountains.

There's also some smoke around, as evident in satellite imagery from 13:55 UTC this morning. 

Source: CIRA
The webcam from Stanley looks pretty good this morning, however, so perhaps it is not dense enough to be a concern.  

By and large, the synoptic odds for eclipse viewing look good.  The primary issue now is whether or not unresolved cloud processes throw a low probability monkey wrench over your location at the time of the eclipse.    

Friday, August 18, 2017

Forecast Outlooks and Products for Eclipse Planning

As I write this, we are now about 3 days from the 2017 Eclipse and the ultimate test of transportation and communications infrastructure in rural areas of Idaho and Wyoming, where the vast majority of Utahns and University of Utah students hope to view the eclipse (Question: Will anybody attend class on the first day of the semester, which is also Monday?).

The large-scale forecast for Monday seems to be stabilizing, but I still consider highly specific cloud forecasts to be difficult given the weak large-scale forcing.  A high-amplitude ridge parked over our area would be a godsend for forecasters, but that's not what we're looking at for Monday.

Instead, the GFS calls for a weak upper-level shortwave trough to sweep across Idaho and Wyoming from 1200–1800 UTC (0600–1200 MDT), and be accompanied by some mid- and high-level clouds.  A short-wave ridge further west builds along the Pacific Northwest coast.

Eclipse time is approximately 1721 UTC (1121 MDT) in Redmond, OR, 1730 UTC (1130 MDT) in Stanley, ID, 1736 UTC (1136 MDT in Jackson, WY, and 1741 UTC (1141 MDT) in Riverton WY.  That's just before the bottom image above, which given the GFS forecast would yield the lowest cloud cover odds and fractions during the eclipse over eastern Oregon and increasing cloud cover odds and fractions as one moves eastward to western Wyoming.  

The NAM agrees with the basic synopsis being advertised by the GFS, but note that the shortwave trough orientation is more from SW to NE (positively tilted in meteorological vernacular, and that the structure and characteristics of the clouds varies when one examines the gory details.  

That variation in the structure and characteristics of the clouds represents the dilemma for forecasts along the path of totality over Idaho and Wyoming.  This is a weak shortwave trough, so a routine "public" forecast would be pretty straightforward.  Probably mostly sunny given the fact that some mid and high level clouds aren't going to be a big deal.  

However, exactly where and when clouds will be at the time of eclipse is difficult to ascertain.  Will one have a clear view of the sun in Jackson, but have an untimely patch in Driggs?  Impossible to say.  In part, this reflects the unpredictability of such cloud cover at such long lead times, but also the fact that present day forecast models do not explicitly resolve cloud processes, adding to the forecast uncertainty.  Timing will also matter.  For example, if you just happen to be underneath a local area of clouds at the time of eclipse, that's a bummer.  

Based on current forecasts, the greatest likelihood of clear skies over the interior mountain west eclipse path is eastern Oregon.  The potential for some mid or high clouds exists as one moves eastward, especially over eastern Idaho and Wyoming.  The timing, location, and extensiveness of that cloud cover remains uncertain.  

For your planning purposes, here are a few products for your consideration:

1. NWS Digital Forecasts. You'll need to use the drop-down menu to request "Sky Cover (%)" and select the appropriate time.  12 PM is the closest available.  One disadvantage of these forecasts is that they are "deterministic" and don't show the full range of possibilities.  Numbers represent percent of cloud cover.  

Source: NWS
2. Experimental High Resolution Rapid Refresh (HRRRX). HRRRX is an experimental version of the HRRR that is being developed and tested by the NOAA Earth Systems Research Laboratory for future operational implementation.  Forecasts are available out to varying lead times (probably dependent on computer time availability), including cloud products.  At 3-km grid spacing, this is the model to go to for short-range cloud-cover guidance.  In addition, they have added sun-obscuration mods to account for reductions in solar radiation during the eclipse (details here).  I've been told that a more "crash proof" web access to the HRRRX is available here.

Keep in mind that even at short time scales, errors in cloud cover are to be expected.  Use the HRRRX (and other model forecasts) as guidance, but not absolute truth.

3. GOES-16 Imagery.  I'm a huge fan of these high-frequency, geocolor loops from CIRA.  Use for "eclipse chasing" the morning of the event and fine tuning during the event, if traffic permits.

Hopefully, the forecast verifies with minimal clouds and not an unfortunate veil of thick cirrus.  Good luck!

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.

Thursday, August 10, 2017

"No Niño" Winter Likely Ahead

The Climate Prediction Center (CPC) issued its El Niño/Southern Oscillation (ENSO) Diagnostic Discussion today, which suggests that ENSO-neutral conditions are favored (55%) this winter with just a small chance that El Niño (15-20%) or La Niña (25-30%) develop during the winter.  These odds reflect the fact that there's "nothing exciting" going on in terms of winds, sea surface temperatures, thermocline depths, and ocean heat content in the tropical Pacific.  Most models also lean toward ENSO-Neutral conditions during the winter.

So, the dice are loaded toward No Niño this winter and that means that nobody really has any idea how much precipitation will fall this winter over the western U.S.  As a result, the CPC is giving equal chances of above average, average, and below average precipitation.

Source: CPC
Of course the odds are tilted a bit toward above average temperatures being more likely than below average.  Betting on warmth isn't a guarantee, but the dice are loaded in a warming world.

Bottom line: Keep calm, carry on, and ignore any long-term forecasts for this coming winter.  Of course, I say that every year.

Tuesday, August 8, 2017

Long-Range Eclipse Cloud Forecasts Are Pointless

Sometimes a thin layer of clouds is enough to ruin your day
Now that we're within t-minus 2 weeks, I'm already seeing weather forecasts for the eclipse on August 21st.

Don't waste your time reading them.

Even at relatively short ranges of a couple of days, predicting cloud cover is quite difficult, just ask anyone in Air Force Weather who has to provide detailed cloud cover forecasts for military operations.

Long ranges are even worse.   First, present-day computer models have very little skill at predicting the large-scale upper-level circulation at long lead times.  One common metric used to evaluate model skill is the 500-mb anomaly correlation, which assesses the accuracy of upper-level forecasts.  Based on experience, 60% is considered a minimum threshold for a "useful" forecast.  Although anomaly correlations have been steadily increasing over the past few decades, they remain well below 60% for a 10 day forecast (and are even worse at longer lead times).  An example is provided below for the ECMWF forecast system.

Source: ECMWF
Second, the ensemble spread is so large at these lead times that essentially you can't extract numbers for cloud-cover odds that are any better than climatology.  Below is the GEFS ensemble forecast for the morning of 21 August.  Some members have troughs near the west coast, but differ in amplitude and position.  One instead has a short-wave trough near Jackson Wyoming (lower right, GASP!).  Much different solutions.

Source: Penn State E-wall
Third, people like to infer cloud cover from a large-scale forecast (like those provided above), but this is actually quite difficult to do.  The odds of cloud cover might be somewhat higher downstream of an upper level trough and somewhat lower downstream of a ridge, but dry slots, topographic effects, convection, and other quasi-chaotic phenomenon are going to play a role, in some cases for the better (e.g., clearer) or for the worse (e.g., cloudier).

Finally, there is no medium range forecast model today that has sufficient resolution to explicitly predict cloud-scale processes.  At best, they resolve the processes contributing to layered clouds forced by large-scale processes.  However, during summer over the western U.S., we're dealing primarily with clouds driven by much smaller scale processes [stratus and stratocumulus (west coast), altostratus and altocumulus, cumulus and cumulonimbus, cirrus].   Imagine trying to forecast a west coast marine push or a monsoon thunderstorm with 2-week lead time and you see my point.  And don't assume that the eclipse path is too far north for monsoon influence.  Monsoon surges into the northern half of the western US happen.

Bottom line.  Keep calm and carry on.

Monday, August 7, 2017

Summer Doldrums

For a synoptic meteorologist, the weather pattern right now over Utah is about as boring as it gets.  Weak flow, slow moving large scale weather systems, and just some scattered thunderstorms about.

Throw in some smoke and it's downright depressing.

Requests for controversial topics for discussion are now being accepted!

Saturday, August 5, 2017

Super Smoky British Columbia

Under ordinary circumstances, western British Columbia is one of the most spectacular places on the planet, but currently it is shrouded in a haze of wildfire smoke.

We got a first-hand view on our flight from Ketchikan to Seattle today.  This is remarkably deep, thick smoke given the height of those peaks.

Seattle has had some relief, as might be inferred from the satellite image above, but we could spot neither Mt. Baker or Glacier Peak and only got a slight glimpse of Mt. Rainier.

Gonna miss the clear air of Alaska!