Category Archives: Cyclones

A Remarkable Forecast of a Beast of a Storm

Intense frontal cyclones don't come much prettier than the nor'easter rampaging up the east coast today.  Satellite imagery and radar for 1200 UTC shows the beautifully wrapped up system with the low center just a shade ESE of Virginia Beach.  Precipitation was heaviest near or just offshore (radar imagery well offshore is nonexistent). 

If we slap on the RAP 925-mb analysis (roughly 750 m above sea level and a good level for seeing the frontal structure) we see the classic structure of an intense frontal cyclone predicted by yesterday's NAM (see prior post) with the cold and occluded/warm fronts oriented at right angles, weakening of the temperature contrast associated with the cold front near the occluded/warm fronts (known as the frontal fracture), the frontal temperature contrast associated with the occluded front maximizing near and west of the low center, and the occluded front extending through the low center as a back-bent occlusion.  One can also see a near cutoff pocket of warm air near the low center (a.k.a. the warm-core seclusion) as cold air encircles the system.  The area in purple shows an intense low-level jet with winds in excess of 40 m/s (80 knots, light purple) wrapping cyclonically from the west to south of the low center, culminating in a maximum in excess of 45 m/s (90 knots, dark purple) known as the poisonous tail of the back-bent occlusion.  In this case, the wind maximum likely represents a sting jet, a local wind maximum near the tip of the comma cloud head that is produced by the descent of strong winds from aloft.  For more on this subject, see What is a Sting Jet?

The 30 hour NAM forecasts that we presented yesterday were quite remarkable and I've reproduced them below for comparison with the RAP analyses above.  The frontal structure, low center position, and low center intensity are very well captured.  The NAM forecast central pressure of 961 mb is a bit overdone compared to the RAP analysis 967 mb, but analysts at the National Weather Service Weather Prediction center put it at 960 mb, so the NAM forecast is certainly within the uncertainty. 

Such a forecast is a remarkable scientific achievement.  We shouldn't take it for granted.  Through the mid 1980s, operational numerical modeling systems frequently failed to predict intense frontal cyclone development of this type.  Scientific papers describe errors in central pressure forecasts of as large as 55 mb.  You read that right.  55 mb.  Basically, a complete and total failure to predict the cyclone development. 

Today, it's almost impossible to believe numerical forecasts could be that bad, but they were, and many on the high seas paid with their lives.   It is only through advances in understanding, observing systems, computing infrastructure, and numerical modeling techniques that we knew a storm like the one above was coming (in fact, many days in advance).  Surely there will be some issues with details of the local forecasts that require further research and model improvements, but forecasts of intense frontal cyclones have come a long long ways. 

A Remarkable Nor’easter

Let me provide a couple of observations that will blow your mind this morning.

At 6 AM MST, it was 31ºF on Alta's Mt. Baldy (11,066 ft.).

At the same time, it was 30ºF in Tallahassee, Florida, with ice pellets and snow.

That's right.  SNOW.  In Florida.

A remarkable weather event is unfolding as a nor'easter develops along the eastern seaboard.  It is a textbook event, of the type I watched for as a kid viewing the evening news.  The loop below shows the classic high-amplitude wave setup with a ridge over western North America and a deep trough over eastern North America.  In addition, there is a short wave trough moving through the long wave pattern.  You can see it moving from near Wyoming to the Golf coast in the loop below.  

In the loop above, you can see how the large-scale ascent associated with the upper-level trough is generating clouds and precipiation as it moves over the southeast United States.  However, that trough will also spark cyclogenesis as it rounds the long-wave trough and is able to move over warm, unstable, water-vapor-laden air over the Atlantic Ocean and along the Gulf Stream.  

The end result is a very powerful nor'easter.  The NAM forecast initialized at 0600 UTC deepens the system from a weak, inverted trough along the Florida coast with a minimum sea level pressure of about 1013 mb at 1200 UTC this morning to a 961 mb low off the coast of North Carolina by 1200 UTC 4 Jan and then 953 mb by 0000 UTC 5 Jan over the Gulf of Maine.  

The GFS isn't quite as deep (not shown), but the 0000 UTC had similar numbers to the NAM with central pressures of 961 mb and 948 mb at 1200 UTC 4 Jan and 0000 UTC 5 Jan respectively.  

Using numbers from the NAM, that is a fall in central pressure of 52 mb in 24 hours and 60 mb in 36 hours, easily meeting the criteria for what is known as explosive cyclogenesis, or rapid cyclone development.  

The structure of the NAM-forecast storm off the North Carolina coast is consistent with an intense marine cyclone.  The cold and warm/occluded fronts are oriented at right angles, forming a frontal T-bone.  The temperature gradient along the cold front weakens near the warm/occluded, a characteristic known as the frontal fracture (there is some uncertainty in the exact cold front position in this forecast and one might place it further west than I have).  The 925-mb temperature contours (black) become quite concentrated along the occluded front near the low center and its back-bent extension south of the low, where the strongest 925 mb winds are found (color fill).  This area of strong winds is referred to as the poisonous tail of the back-bent occlusion by Norwegian meteorologists.  Cold air has encircled the low center, cutting off a pocket of warm air known as a warm-core seclusion.  

The severity of ompacts along the eastern seaboard will depend strongly on storm track as a slight shift to the west or east will make a difference in the amount of precipitation, which  and severity of winds along the coast.  One need only look at the watches, warnings, and advisories map from the Boston National Weather Service Office to realize what a mess this storm could bring.  

If you are in the area, monitor official forecasts.  

Überströmungs Cyclogenesis!

A classic example of Alpine lee cyclogenesis of the "Überströmungs-type" is bringing heavy snowfall to portions of the Alps and southern Europe.

Alpine lee cyclogenesis is the birth of a cyclone in the lee (downstream side) of the Alps.  Überströmungs-type Alpine lee cyclones form in northwesterly large-scale flow as cold air is blocked and flows cyclonically to the west of the Alps, while the upper-level trough continues to move downstream, inciting cyclone development in the western Mediterranean region.  Terrain impacts are especially pronounced in cyclones that form in this fashion.

The satellite and sea level pressure loop below shows the situation from 1200 UTC 11 November through 1200 UTC 13 November.  The system begins as a surface trough that plunges southeastward across the British Isles, France, and Germany.  The cold air plunges southward through the low elevation region between the Alps and Pyrenees (note faint rope cloud in the loop), and strong cyclogenesis occurs over northern Italy.

@severe-weather.EU is the feed to monitor this morning as they are tweeting out some wonderful images from across the region.  Here are a few.

 Enjoy the look at the fresh snow.  Let's hope we can join the party later this week.

Impacts of Post-Tropical Ophelia on Ireland

Following up on yesterday's post, here's a few tweets from Ireland showing the impacts of Ophelia.

Irish Eyes Aren’t Smiling

Hurricane Ophelia has had an unusual life cycle in the northeastern Atlantic Ocean and as of 11 AM AST this morning, was still a category 1 hurricane off the coast of Portugal. 
Source: National Hurricane Center
Over the next 24 hours, Ophelia is expected to undergo what is known as extratropical transition, the transformation from a tropical cyclone into an extratropical cyclone.  It is expected to track northeastward and bring strong winds to Ireland and Scotland on Monday. 

The GFS sea-level pressure and wind speed (meters per second) forecast from the GFS is below and it shows the storm strengthening and broadening just southeast of Ireland, before weakening just a bit prior to landfall. 

Nevertheless, the system is quite strong at landfall.  The areas in yellow feature sustained winds of 28 m/s (56 knots) and orange around 35 m/s (70 knots), the latter are hurricane force.  These areas are found over water.  Winds are weaker over land, but still quite strong. 

The Irish Meteorological Service, Met Éireann, has issued a National Weather Warning and is expecting sustained winds of 80 km/h (43 knots) and gusts in excess of 130 km/h (70 knots) in the southern half of the country.

Source: Met Éireann, 9:30 MDT 15 Oct 2017
Much will depend on the precise track of the storm, but it looks like this will be a strong windstorm for Ireland, and perhaps Scotland and other portions of the UK. 

Getting Real along the Pacific Coast

For the Pacific Coast of the northwest U.S. and southwest Canada, the doldrums of summer are officially over.  Two major cyclones are expected to bring strong winds to the area, along with a litany of other impacts, although the precise locations of the most intense winds with each cyclone remains unclear.

The first windstorm is associated with a midlatitude cyclone expected to develop off the coast of Oregon and Washington tomorrow and track northeastward toward Vancouver Island or the Olympic Peninsula.  The 0600 UTC NAM placed the ~970 mb low center very near the tip of the Olympic Peninsula at 0900 UTC Friday, a near optimal location for strong winds over western Washington.

The GFS solution was not quite as deep and has the low center making landfall along the central coast of Vancouver Island.  In terms of the severity and location of the strongest winds, much will depend on the precise track of the system.

As if that's not enough, a second cyclone follows over the weekend.  Forecasts at present push it further north, limiting wind severity over the Pacific Northwest, but it is a storm worth watching and its history is quite fascinating.

Just a couple of days ago on 10 October 2016, a decaying Typhoon Songda was minding its own business over the western Pacific Ocean just east of Japan (identified by red box below).  That's a good place for Typhoons, well off the coast and away from densely populated southeast Asia and Japan.

However, as is prone to happen in the fall, Songda meandered slowly northward and, as shown in the loop below, is forecast to be swept up by the midlatitude westerlies.  As it sweeps around the Aleutian low, it is expected to undergo extratropical transition, redevelopment as a extratropical cyclone, before crashing into the Pacific Coast.  

Storms of this type can produce powerful windstorms along the Pacific coast.  For example, the infamous 1962 Columbus Day Storm, was produced by the extratropical transition of Typhoon Freda. 

I've always wanted to be on the west coast of Vancouver Island or the Olympic Peninsula for a storm series like this.  It is fall break, but I have plans for southern Utah, so I think I'll stick with those.

For more on these storms, see the Cliff Mass Blog.

Alpine Lee Cyclone Wreaking Havoc on Tour de France

Spoiler Alert: If you don't want to know what happened in today's (Wed 13 July) stage 11, read no further.

Strong winds wreaked havoc on the Tour de France today, with the Peter Sagan (stage win) and Chris Froome (putting time on rivals) taking advantage.

The weather is also expected to wreak havoc tomorrow and it has already been announced that the stage up Mt. Ventoux will stop at Chalet Reynard, about 500 vertical meters below the summit.

The culprit is a strong Alpine lee cyclone that has developed on the south side of the Alps.  The 6-hour GFS forecast valid at 1200 UTC today (around the time of Stage 11) shows the classic signature that one sees when a cold front impinges on the Alps, stalls on the north slopes due to topographic blocking, and a variety of processes contribute to cyclogenesis to the lee near or over the Gulf of Genoa.

The strong pressure gradient drives the strong Mistral (northwest) winds that affected Stage 11.

For tomorrows stage 12, the main surface low has moved off into northern Europe, but the trailing cold front remains stalled over the north slopes of the Alps and an intense lee trough remains on the south side.

Mt. Ventoux will surely be buffeted by strong winds.  It may just south enough to avoid precipitation, but I wouldn't be surprised if there was some rain.  Expect the unexpected.

It's a shame that the climb will be shortened, but it could be an interesting stage nonetheless.

Why Utah Meteorologists Love Tax Day

If you are a meteorologist in Utah, there are few days on the calendar as fun as tax day.

It seems like something exciting happens every year on or about April 15, and there are good reasons for it.  April 15 lies at the beginning of the Intermountain cyclone season, which spans from April to early June and frequently features the development of strong troughs or low pressure systems over the Great Basin downstream of the Sierra Nevada.  Such systems frequently bring strong winds and blowing dust.  April 15 also lies in the heart of the wettest month climatologically in Salt Lake, with April averaging a whopping 2.10 inches of precipitation, good for big storms.  Finally, April 15 is in the middle of the transition from winter to summer and we can get some extremely large temperature swings in April and everything from deluges to dumps.  If you don't like the weather in April, wait 5 minutes.

Perhaps the biggest and baddest example of extreme weather on April 15 is the 2002 Tax Day storm, which produced the second lowest sea level pressure observed in Utah since observational records began in 2002 and the strongest cold-front passage observed in recent decades.  With frontal passage in Salt Lake City, temperatures fell 13ºF in 10 seconds and 34ºF in 2 hours.  Blowing pre-frontal dust produced by the storm forced the closure of US-6 and left a chocolatey-brown coat of dust on the mountain snowpack for the remainder of the snowmelt season.

The storm advertised for later this week isn't as exciting, but will probably make things more meteorologically interesting.  As predicted by this morning's 1200 UTC NAM, a surface cyclone and frontal precipitation band are predicted to develop over the Great Basin by Wednesday afternoon (0000 UTC 14 April).

The surface trough and frontal band linger to our northwest for some time, but the NAM eventually brings them into the Salt Lake Valley in earnest on Thursday morning.

Yes, I know that's 14 April, not Tax Day, but that's close enough for Rock 'n' Roll.

As things stand now, the event looks like a modest spring storm for northern Utah, certainly not of the caliber of the 2002 Tax Day Storm and many other strong spring events in Salt Lake City.  If the NAM were to verify, we would, however, see some significant changes in weather including some bench snow on Thursday, and perhaps some decent powder skiing in the mountains on Friday.

However, I've done this gig long enough to know not to read too much into the small-scale details of an Intermountain front event that is still about 3 days away.  There's a lot of sensitivity of these events to the track, speed, and structure of the primary upper-level trough and its interaction with the Sierra Nevada.

Thus, don't bet the farm on this morning's NAM forecast.  A lot could change the next couple of days.   Let's see how things look on Wednesday.  

Threesome Thursday

Although it's going to be a beautiful bluebird day, I'm working and there's three topics to discuss this Thursday.

1. 2015 Was Utah's Third Warmest

As noted in the previous post, 2015 was easily the warmest year on record globally, and by a wide margin.  For Utah, 2015 comes in as the 3rd warmest on record, behind 1934 and 2012.  It also extends our run of years at or above the 20th century average temperature to 22.  The last year cooler than the 20th century average temperature was 1993, with 2011 right on the average.

Source: NCEI
With regards to temperature, two highlights stand out in my mind from 2015.  The first is the remarkable warmth of the first 3 months of the year, which collectively rank as the warmest Jan-Mar period on record.  The only years with anything even close to what we experienced in Jan-Mar of 2015 are 1934 and 1986.  If it seemed crazy warm last winter, it was.

On the other hand, July was simply splendid, and came in below the 20th centuray average, the first July to do so since 1997.

If you aren't a snow lover, that's a great combination.  A mild winter followed by a mild summer.  For snow lovers, last winter left much to be desired, but I suspect we all appreciated the wonderful summer, which was one of the nicest I can remember in my 20 years in the Salt Lake Valley.

2. Blog Post Reflections

My post last Friday (The Hits Just Keep Coming) was one of the more optimistic that I've issued over the past couple of years, called for a series of quick-hitting storms, and concluded:
We'll have to see if the storms farther out in the forecast evolve as advertised, but by and large this looks like a great weather pattern for backcountry skiing.   There are some avy concerns out there, the wind might do some damage at times, and we might go a bit upside down with the storm late Saturday and Saturday night, but by and large, we should be happy.
The storms materialized, as well as some good skiing, but the wind proved far more problematic than I anticipated during the period, both for avy concerns and snow quality, with the former further complicated by a very heterogeneous snowpack with multiple persistent weak layers.

3. Eastern Storm

The eastern U.S. will be in the crosshairs of a major winter storm through Sunday with significant accumulations in some areas.  I don't pay much attention to the weather east of the divide, but did put together a loop of the storm from the latest (0600 UTC) GFS, just for entertainment purposes.

This NAM Forecast Is a Work of Art

We rarely see such a beautiful comma cloud and precipitation pattern over the Intermountain West as forecast by the NAM for 0900 UTC (0300 MDT) Saturday.  Simply spectacular!

0600 UTC 14 May 2015 NAM forecast of 500-mb wind (vectors), temperature (red contours every 1ºC)
3-h accumulated precipitation, and outgoing long-wave radiation valid 0900 UTC 16 May 2015
Patterns like this are associated with well developed upper-level troughs, but often such troughs have cloud and precipitation patterns that are broken up by the topography of the west. That's not the case in the NAM forecast above.

The beautiful comma cloud and hook-shaped precipitation pattern is associated with two major airstreams.  The first is sometimes called the TROWAL (TRough Of Warm air ALoft) airstream.  The TROWAL airstream is associated with warm advection and ascends along a tongue of warm air that wraps round the back side of the upper level low, forming the comma.  

The second airstream descends on the back side of the upper-level trough, wraps around the forward side of the upper-level trough, and forms the dry slot.  At upper-levels, this airstream is typically dry because it has descended from the upper-troposphere.  The interlacing of these two three-dimensional airstreams leads to the beautiful comma cloud.

The term dry slot can give the false impression that this is an area of benign weather.  Au contraire in some situations.  Although the dry slot may be cloud free and feature low relative humidity air at upper-levels, at low levels, the air beneath the dryslot can vapor laden.  The airmass beneath the dry slot often features large Convective Available Potential Energy (CAPE), which is jacked up further by solar heating during the day if the dry slot is cloud free.  Being ahead of the upper-level trough, the forward portion of the dry slot is also an area of rising motion.  As a result, convection frequently breaks out within or at the leading edge of the dry slot.  The vertical wind shear in the dry slot region is also sometimes favorable for the development of severe convection.  Even the low-resolution NAM is generating some dry-slot convection in the forecast above.