Change You Can Believe In

No, not Obama.  This is a science blog.  The change is a meteorological one, with the deep upper-level trough swinging into the western US later this week.  For Utah, it will bring the coldest air of the season thus far.    

Mountain and valley snow?  You betcha.  How much?  On that, the polls are still open.

Vote!

Most of the contiguous US has fair weather today.  Perfect weather in Utah.  Get out and vote if you haven't already.  Especially you young people out there.

Let your voice be heard!

East Coast Nor'easter

The eastern seaboard can't buy a break.  Another cyclone, this one a more traditional nor'easter, is expected to impact the area affected by Sandy on Wednesday and Thursday.

We discussed last week how deep extratropical cyclones often feature a warm-core seclusion, an isolated pocket of warm air near the center of the system.  Indeed, the graphics below show the  development of a warm-core seclusion as the nor'easter intensifies off the eastern seaboard through midweek.

GFS sea level pressure (every 4 mb) and 925-mb temperature (every 2ºC)
forecasts valid 0300 UTC 7 Nov, 1500 UTC 7 Nov, and 0600 UTC 8 Nov 2012

The close-up below better highlights the warm core seclusion, which at its center is about 6ºC warmer than the surrounding airmass.

GFS Sea level pressure (every 4 mb) and 925-mb temperature (every 2ºC)
forecast valid 0600 UTC 8 Nov 2012 

The evolution of the storm above is very reminiscent of the Shapiro-Keyser cyclone model, which is named for atmospheric scientists Mel Shapiro and Dan Keyser.  In their model, the cold front does not "overtake" the warm front and form an occlusion, as it does in many text book descriptions that derive from the so-called Norwegian Cyclone Model.  Instead, it remains perpendicular to the warm front, forming what they call a frontal "T-bone."  The warm front migrates into the airstream behind the low center, which it eventually encircles, forming the warm-core seclusion.  The portion of the warm front that extends behind the low center is called a bent-back front.

The Shapiro-Keyser Model

The nor'easter in the color images above closely mimics this evolution.  The cold front never overtakes the warm front, but instead remains perpendicular to it.  The warm front migrates into the airmass behind the low-center, which it encircles to form the warm-core seclusion.   

The nor'easter will bring another bout of hazardous weather for the vulnerable eastern seaboard.  Let's hope the the latest model forecasts are wrong and the storm shifts eastward and farther out to sea.

Pattern Change

Now there's a forecast ensemble that will warm a skier's heart.  All members calling for a deep trough over the western US.  

Time will tell if this pattern change will bring the goods to the Cottonwoods.

Warm Core Seclusions and Sandy

There is a marvelous example of the formation of what meteorologists call a warm-core seclusion over the Gulf of Alaska today.  Note the isolated pocket of warm air that forms near the low center as the cyclone reaches maturity over the Gulf of Alaska.  A strong frontal zone (indicated by a close packing of the lines of constant temperature), sometimes called a bent-back front, encircles the low center and the warm-core seclusion.

0600 UTC 1 Nov – 1200 UTC 2 Nov GOES-WEST IR
Imagery and GFS sea level pressure (black contours)
and 925 mb temperature (color contours) analyses.

Now, compare the evolution of the storm above, which is a pure extratropical cyclone, with that of "Hurricane" Sandy as she made landfall on the coast of New Jersey.

1500 UTC 29 Oct – 0000 UTC3 Oct GOES-East IR
Imagery, Radar, and Rapid Refresh sea level pressure
(black contours) and 925 mb temperature (color contours) analyses.

Look similar?  Maybe this will help (click to enlarge).

As Sandy moved northward, she acquired frontal characteristics that are commonly found in deep extratropical cyclones, such as the one over the Gulf of Alaska above.  As we have discussed previously, this process is called extratropical transition.  Her tropical warm-core became surrounded by cooler air and an increasingly strong frontal zone.  As she made landfall, her thermal, cloud, and precipitation structure was more reminiscent of an extratropical cyclone than a hurricane.  Her apparent eye (see last frame) may reflect the bent-back front encircling the low center rather than traditional hurricane eye-wall dynamics.  Indeed, deep extratropical cyclones sometimes exhibit eye-like features surrounding the warm-core seclusion.

Drawing a boundary to distinguish between tropical and extratropical cyclones is convenient, but that boundary is ultimately imaginary and arbitrary.  Understanding Sandy requires a broader perspective and consideration of both "tropical" and "extratropical" processes and storm characteristics.

Death to the Ridge!

OK folks, this is getting serious.  We had a crappy ski season last year, and we don't want another.

During 2012, the persistence of mean ridging over our part of the world has been quite remarkable.  The images below show a series of 90-day 500-mb height anomalies (i.e., departures from climatology) going back to the beginning of the year.  The warm colors indicate positive height anomalies and are indicative of ridging.  Although the pattern has shifted around somewhat, Utah has been under the influence of what meteorologists call a "time-mean ridge" for the entire year.

Source: NOAA/ESRL/PSD

So, to paraphrase Jeff Spicoli, "what Steenburgh is saying is that this ridge is bogus, and if we don't get a cool pattern – pronto – this ski season will be bogus too. Get it?"  Death to the ridge!

Note: This is not a forecast of a bad ski season, but merely discussion of the fact that we have had a ridge-dominated pattern for some time and we need that to change.