Friday, July 11, 2014

Southeast Thunderstorm Environment

Reader Mike posed a question the other day about the kind of weather pattern that supports thunderstorms in southeast Alaska.  Of course, thunder is quite rare in that part of the world, as low-level temperatures are rarely warm and humid enough to create sufficient instability for deep convection.  However, thunderstorms do occasionally develop and it's interesting to look at the large-scale weather patterns that accompany these events.

To address this, I examined the hourly weather observations from both Haines and Juneau airports since 1998, when the ASOS platform was introduced.  Remarkably, I was unable to find a single instance of a thunderstorm being reported at Haines; it seems unlikely that this could be correct, but perhaps thunder really is that rare, as the town is surrounded by high mountains on nearly all sides.  In Juneau the situation is better (for this study), with 13 distinct thunderstorm episodes reported since 1998.  Several of these occurred in the cold season and were undoubtedly related to strong lifting associated with vigorous frontal passages.  I'm more interested in the 8 summer events, which are as follows:

June 28, 2000
June 27-28, 2004
July 24, 2004
August 10, 2004
August 18, 2004
June 17, 2013
June 24-25, 2013
July 13, 2013

Interestingly, all but one of these events occurred in 2004 and 2013, which were of course very warm summers in Alaska.  Looking at the sea surface temperature maps from these months, every one shows above-normal temperatures in the northeast Pacific close to the Alaska panhandle; so this appears to be an important factor.

Looking more closely at the individual events, I created maps of the 500 mb height (pressure) anomaly and 850 mb temperature anomaly at 12-hour intervals surrounding the onset time of each thunderstorm event.  These maps are shown below.  Note that two of the episodes included two distinct thunderstorms separated by many hours, so for these I created two sets of maps.  We can (perhaps simplistically) summarize the weather situation in Juneau for each event as follows:

June 28, 2000  -- upper-level low arriving from the Pacific, low-level cold front
June 27-28, 2004  -- very warm low-level air retreating northwestward, upper-level ridging to the north
July 24, 2004  -- upper-level trough arriving from the Pacific, low-level cold front
August 10, 2004  -- cold front arriving from the Pacific
August 18, 2004  -- very warm low-level air, ridging aloft, no obvious trigger
June 17, 2013  -- upper-level low diving down from the Yukon (described by Mike in his comment)
June 24-25, 2013  -- very warm low-level air migrating to the northwest, weak ridging to the north
July 13, 2013  -- upper-level low moving slowly southward over British Columbia.

Overall, we have three events with an obvious cold front arriving from the Pacific; two events with an upper-level low moving southward over Canada; and three events with no obvious upper-level trigger but with very warm air residing over or migrating away from the area.  So we see that there is considerable variety in the specifics of the weather patterns that bring thunderstorms to Juneau; but a common factor to all of the events is the unusually warm ocean water along the coast.  In years with near-normal or below-normal ocean temperatures, it appears that summer thunderstorms are much rarer in Juneau and presumably also in surrounding locations near the coast.


  1. Thank you Richard. I wonder if summer thunderstorm activity in southeast correlates to dry above normal weather in the mainland say Interior Alaska. I'd guess so, as the branch of the North Pacific ridge is more active when there is warmer water in the eastern gulf.

    1. Mike,

      It certainly appears that way based on the limited data set here - 2000, 2004, and 2008 were all dry in Fairbanks in June and July. But more years would be required to say for sure.

  2. Unfortunately many ASOS units, including Haines, do not detect or report thunder (that's what the TSNO at the end of the METEAR remarks means.

    METAR PAHN 161254Z AUTO 12005KT 10SM OVC060 12/09 A3021 RMK AO2 SLP233 T01170094 TSNO=

    I can assure you that Haines gets as many and probably more summer thunderstorms than Juneau. Probably at least one day most summers. Last summer was an overachiever. Not just in the number of days, but in the intensity of the June 24th storms. A cluster of cells moved over the coastal range from northern BC and walloped Haines.and Skagway (also TSNO).The lightning and thunder were incredible and went on for about 6 hours with little intermission. There were plenty of cloud-ground strikes and people reported burning trees on the hillsides (luckily put out by the rain.) Life-long residents could not remember anything like it.

    Thunderstorms also sometimes come down the broad Chilkat Valley (roughly from the west). They don't tend to form in the town area, but usually depend on the better conditions farther inland. One way to look at it might be that coastal areas can become an extension of the interior when conditions allow. For instance strong daytime heating lowers the air density, making what was a stable marine layer impediment an open door to share in the deeper instability of the interior. It is plausible that this can happen easier when the NE Pacific is warmer. Just a theory...what do you all think?

    1. Jim,

      Thanks very much indeed for explaining the mystery regarding Haines thunder! I've seen that TSNO before, but I somehow failed to connect the dots.

      Based on the reanalysis data, the June 24 event last year coincided with the extremely warm low-level air passing right over the area. The instability must have been extreme and sustained to allow for persistent strong storms as you describe. Amazing!

      I think your hypothesis about easier destabilization of the marine layer makes very good sense. It would certainly be fun to run some high-resolution modeling of the area and see the instability pick up when nearby ocean temps are above-normal. One could also calculate a simple measure of instability using surface obs and reanalysis upper-air conditions. A project for another time...

      Thanks again.