Weak Monsoon Surge Today

A tip of the hat to the astronomers for getting yesterday's forecast right.  It was a great day for science.  Much thanks from the weather guessers.

And, speaking of the weather, keep your fingers crossed that we see some hydrometeors today.  We have a weak monsoon surge pushing in from the south, with moisture on the increase.  The NAM analysis and forecast loop below shows cloud cover and precipitable water values on the increase overnight and today for northern Utah.

The HRRR forecast for later today shows scattered showers and thunderstorms across northern Utah and Nevada.

The chaotic nature of atmospheric convection prevents us from precisely predicting the location and intensity of those showers, but let's hope they deliver.  A bit of rain later today and evening would be greatly welcomed.

Scenes from the Eclipse

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

Before.

Source: CIRA

 After.

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.  

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!

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." 

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!