Thursday, April 21, 2011

Nighttime Thunderstorms

Scattered thunderstorms moved across northern Utah not only yesterday afternoon, but also overnight.  The Salt Lake City Airport, for instance, reported lightning to the east at 0653 UTC (12:53 AM MST).

A radar loop covering that time shows several strong cells moving across the Great Salt Lake and a line of strong convection late in the loop near the Utah-Idaho border.

Many people are mystified by the development of nighttime thunderstorms, especially in Utah.   Are nighttime thunderstorms unusual?

In a seminal paper on the diurnal variation of summer thunderstorm frequency, Wallace (1975) showed that in midwest of the United States, summer thunderstorms are more common at night than during the day.  

The diurnal cycle of thunderstorm frequency in the summer.   The barbs
indicate the normalized amplitude of the diurnal cycle (half barb=5%; full barb=10%;
flag=50%).  Phase indicated by orientation.  From the north indicates midnight
(0000 LST) maximum, east 0600 LST, south 1200 LST, and west 1800 LST.
Source: Wallace (1975).

On the other hand, in the Intermountain West, the maximum is in the late afternoon and early evening.  So, it is not unusual for thunderstorms to develop at night, but in Utah, it is somewhat less common.

Last night, however, differential temperature (and moisture) advection with respect to height helped to generate instability.  Note how the wind in yesterday afternoon's KSLC sounding veers (turns clockwise) with height in the low levels (from the surface to about 700 mb) and then backs (turns counterclockwise) with height from 700 mb to 500 mb.  This implies (based on our dynamical understanding of atmospheric flows) low-level warm advection and mid-level cold advection, which helps to generate thermodynamic instability.

Overnight, moisture was also on the increase at low levels.  In fact, by 6 AM MDT this morning (1200 UTC), we have a positively juicy sounding to play with.  There's a stable layer at low levels, but a parcel lifted from the top of this stable layer (831 mb, dashed line) has a deep layer positive area and 877 J/kg of CAPE to play with.  

So, last night we destabilized the atmosphere through differential temperature and moisture advection.  No solar heating needed!  It is pretty quiet at the moment, but instability and moisture are present and things should get interesting later today as a trough approaches from the west and provides the lift needed to trigger convection and thunderstorms once again.

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