June Anomalies

The month of June was very wet in Fairbanks, with 3.29" of rain, the 4th highest total on record (1930-present).  The monthly mean temperature was slightly below the 1981-2010 normal for the first time since last September.

Interestingly, however, the month was not more cloudy than usual in the Fairbanks area - in fact the mean cloud cover at the airport was lower than any other recent year except 2013 (when June was very hot and sunny).  The CRN site located 11 miles NE of town recorded 18.6 MJ/m² of solar radiation, which is slightly below the 2003-2015 average of 19.3 (standard deviation=2.1), so at that site it was somewhat cloudier.  The high rainfall total combined with only modest cloud cover is consistent with the rainfall being unusually convective; thunder was observed on 12 days, which is the highest number of days in June since 2010.

Compare this pattern to what happened two years ago, when June was the wettest on record.  In that month cloud cover was far above normal and thunder was observed on only 2 days.  The maps below show the 500mb height anomalies in the two months.  Back in 2014 there was a low pressure anomaly over the state, but this year there was a ridge to the southwest and a trough over the Arctic Ocean.  Not all wet Junes are created equal in Fairbanks.

The maps below show the anomalies for June at 16 stations across the state.  Except for Annette Island last month, the below-normal temperatures at Fairbanks and Bettles are the first cool anomalies to show up at any of these stations since December.

Thunderstorm Season Approaching

Fairbanks has yet to observe a thunderstorm this year, according to the ASOS lightning sensor at the airport, but this will soon change if history is any guide.  Two years ago we looked at the climatology of thunder in Fairbanks and saw that the frequency ramps up very quickly in early June (see the chart below, reproduced from that post).  The median date for first thunder is June 6, with "near-normal" ranging from about May 30 to June 15.  Last year the thunderstorm season began on June 7, and it started on June 9 in 2014, but back in 2013 the first thunderstorm didn't occur until June 21, which was remarkable given the hot early summer weather that year.

[Update June 11: and just like clockwork, thunder was reported today at 4:05pm AKDT.  Remarkable.]

The chart below shows the dates of all 408 days with "thunderstorm" reported in the hourly observations at Fairbanks since 1950.  The ASOS platform was introduced in December 1997, so that could explain the lack of outlier dates in recent years, although the ASOS lightning detector is reported to have a higher rate of reporting than human observers (and a very low false alarm rate).

In my post of 2 years ago I looked at seasonal changes in humidity as a possible explanation for the very rapid increase in thunderstorm frequency in early June, but the conclusion was that humidity alone probably isn't enough to explain the phenomenon.

To take this investigation one step further, I recently calculated a measure of vertical instability by taking the difference in equivalent potential temperature (θ_e) between the surface and 500 mb.  The idea here is that a large vertical gradient (decrease) in θ_e indicates greater thermal instability and more potential energy for deep convection (i.e. warm, humid air lying below cold, dry air).  There are many other measures of convective instability that could be used, but the θ_e difference provides a first-order estimate of potential instability.  Note that I used the 3pm AKST Fairbanks sounding data to get the 500 mb conditions, and I restricted myself to thunderstorms observed between 9am and 9pm AKST.

The chart below shows how the frequency of thunderstorms is related to the daily (9am-9pm) maximum in θ_e difference.  As we would expect, thunder is more frequently observed when the atmosphere is more unstable, and thunder is very rare when there's no instability (negative θ_e difference).

Now we can address the question of whether there is a notable early June increase in instability that might explain the arrival of thunderstorms; the chart below shows the frequency of moderate (≥6°C θ_e difference) instability by date.  The answer appears to be a resounding "no", as substantial instability is surprisingly infrequent in early June and doesn't peak until a month later.  It seems that rising instability is not an adequate explanation for the climatological surge of thunderstorm activity in Fairbanks; and so the search for a suitable explanation will have to continue.

One redeeming aspect of the chart is that the gradually decreasing instability in July and August does mirror quite nicely the drop-off in thunderstorm frequency in late summer.  So it seems we can explain the end of the season in terms of the elimination of instability; it's the beginning of the season that is a bit of a mystery.

Thunder Snow in Nome

This morning the National Weather Service noted the occurrence two lightning strikes concurrent with snow in Nome – thunder snow. This is quite the event. Figure 1 shows the text of the Special Weather Statement that was issued by the NWS office in Nome. Since the event was observed by NWS personnel, we assume that the report is valid. Unfortunately the two closest lightning detectors to Nome are both non-operational at the moment so we cannot tell how widespread the event was.

Figure 1. Special Weather Statement issued this morning by the Nome, Alaska, NWS office.

The METARs (see Figure 2) indicate the beginning and ending of the event. Several "special" METARs were issued that note the presence moderate snow and blowing snow concurrent with the lightning. The temperature was noted to be -2°C (28°F) during the thunderstorm.

Figure 2. Screenshot of METARs for Nome, Alaska, between 17:15 Z and 17:31 Z.

Thunder snow requires an unusual set of atmospheric occurrence to come together at just the right time. I am not a forecaster and am not qualified to provide context on the specific atmospheric dynamics at the time. However, for those out there more qualified than I am, here is the 12 Z (3 a.m.) upper air sounding from Nome this morning.

Figure 3. Upper air Skew T/Log P plot from Nome, Alaska, for February 24, 2015 (12 Z).

Just how rare of an occurrence is this? Surprisingly, it is not unprecedented. In fact there are 11 instances of thunderstorms in Nome with a temperature below freezing prior to today's occurrence. Figure 4 shows all of the hourly observations (1960-present) with snow and/or sub-freezing temperatures and a thunderstorm. Only 69 days in Nome have recorded a thunderstorm since 1960; the fact that 11 of those days were winter occurrences is remarkable. Even more amazing is that three instances occurred with a temperature of 2°F or colder!

Figure 4. Hourly observations from Nome, Alaska, that contain a thunderstorm and sub-freezing temperatures.

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.

 

Wet But Not Thundery

With the June rainfall record having been broken this year in Fairbanks, and with July already in the top 10 wettest on record, it is interesting to consider that thunder has been reported only twice this year at Fairbanks airport (June 8 and 23); and these two events contributed zero precipitation to the June total.  This led me to wonder if very wet summer months are typically lacking in thunder in Fairbanks.  If so, this suggests a very non-linear relationship of thunder to total precipitation, because very dry weather would also tend to be associated with a lack of thunder.

To answer the question, below are scatter plots of the total precipitation versus number of days reporting thunder, for June and July.  Indeed we see that in both months, the highest rainfall totals are not associated with a high frequency of thunderstorms.  Instead, the highest thunderstorm frequency tends to be associated with moderately, but not excessively, above-normal precipitation.  In June it is possible to have quite high thunderstorm frequency with well below-normal precipitation, but it seems that in July the very driest months tend to have reduced thunderstorm activity.

In conclusion, the wettest summer extremes in Fairbanks are not associated with much-enhanced thunderstorm activity, but in fact quite the reverse: thunderstorms are less common in the weather patterns that bring the wettest conditions.  This is evident also in the history of heaviest daily rainfall amounts: of the top 20 one-day rainfall events in June and July (for years with hourly weather reports available), only 5 days reported thunder.

Lightning Season Begins

Right on cue, the lightning season has begun in Alaska. Yesterday was the first day that the BLM's lightning detection network recorded more than 10 cloud-to-ground strikes in a day. A total of 198 strikes were recorded on Saturday (5/31).

Figure 1 shows the location of yesterday's cloud-to-ground lightning strikes. All of the strike were between Galena and Eagle and were all north of the Alaska Range. I did not see any thunderstorm reports from a quick look at the METARs but my search was not exhaustive.

Figure 2 shows and a frequency distribution chart of total strikes in Alaska by 10-day interval between 1986 and 2012. ‪It is similar to the charts from last weeks post on thunderstorm and lightning climatology. Since a new lightning detection system was implemented in 2013, the raw values shown in Figure 2 may not be representative any more. However, the temporal pattern is still valid.

Figure 1. All cloud-to-ground lightning strikes in Alaska on May 31, 2014.

Figure 2. Lighting frequency during 10-day periods for all Alaska between 1986 and 2012. (Note: July 31 and August 31 were omitted to keep the interval size consistent.)

Thunderstorms and Dewpoint

In Brian's post the other day, we saw that lightning activity in the Fairbanks area ramps up extremely rapidly in early June, with lightning being very unusual in May but quite frequent by mid-June.  I was struck by this (can't help the pun) because climatological changes in weather phenomena are rarely defined so rigidly by calendar date.  I was curious about the physical explanation for the rapid change and decided to look up the humidity data to see if a rapid rise in average humidity in early June might be a factor.  In other words, is there - as reader Eric speculated - a humidity threshold that must be met before thunderstorms become reasonably common?

The first chart below simply shows the climatological frequency of thunderstorm days, based on the hourly observations from Fairbanks airport since 1981.  This chart is closely akin to Brian's Figure 4, except that Brian showed an even more rapid rise in the total count of lightning strikes.  This makes sense, because as thunderstorms become more frequent they also become stronger on average, with more lightning strikes per storm.  In any case, we still see a very rapid jump in thunderstorm frequency after June 1, while the decline in late July and August is considerably more gradual.

The second chart, below, examines how the thunderstorm frequency depends on the daily (midnight to midnight) maximum in dewpoint temperature.  We see that the majority of thunderstorms in Fairbanks occur when there is rather high humidity, with the daily probability of thunder reaching nearly 30% for daily maximum dewpoints near 15-16 °C (60 °F).  The drop-off in thunderstorm frequency at the highest humidity levels may be an artifact of having too small a sample with such high humidity.  It is interesting that we do NOT see a pronounced threshold that must be satisfied before thunder can occur; the probability rises relatively smoothly with the humidity level.

Finally, I looked at the climatological frequency of days on which the daily maximum of dewpoint temperature is 50 °F or higher, which encompasses the great majority of thunderstorm events.  The chart below shows a rapid increase in early June, as for the thunderstorm frequency, but also shows that the climatological peak is much later than for thunder, as high dewpoints are common well into August.

What can we conclude?  It's very clear that the low-level humidity (dewpoint) is not the only factor determining thunderstorm activity, as the storm frequency drops off markedly even while the humidity is peaking in late July.  Moreover, there does not appear to be a hard dewpoint threshold that clearly defines thunder versus no-thunder days.  However, the rapid rise in both thunderstorm frequency and humidity in early June does suggest that the summer influx of humidity is an important contributor to the onset of summer storms.  A next step in the investigation might be to look at climatological changes in the vertical stability profile to see how the rising low-level humidity acts to destabilize the environment and favor deep convective overturning.

As an aside, the highest dewpoint observed so far this year in Fairbanks is 45 °F; and no thunder has yet been reported at the airport.

Thunderstorm & Lightning Climatology

Thunderstorm season is quickly approaching in Alaska. So when can we expect the lightning to commence and how frequently is it observed at various locations? The Bureau of Land Management (BLM) has lighting detection antennas around the State to monitor potential wildland fire activity. They generously provide the historical data for easy download to the general public as a series of GIS data sets. The data begins in 1986 and runs through 2013 (27 years). However, they implemented a new system for the 2013 season so that data is not necessarily comparable to the previous years and is therefore not utilized in the analysis for this post.

Nearly all lightning strikes occur during the months of May through September and about 85% are due to surface heating. Figure 1 shows the cumulative distribution of strikes by month for the years 2000-2012. As you can see, the overwhelming vast majority of strikes occur in the months of June and July.

Figure 1. Cumulative distribution of lightning strikes recorded by BLM in Alaska from 2000 to 2012.

This provides an interesting perspective on the year-to-year variability but doesn't tell us a lot about the spatial variation. Since the BLM data is geocoded, we can place the data on a map. Figures 2 and 3 show all of the lightning strikes from 1986 to 2012 around Anchorage and Fairbanks respectively. Each map is exactly the same scale and represents an area of 1,120 square miles. Most residents of Alaska are not surprised to find out that more lightning occurs in the Interior than the Southern Coast. However, the disparity is fairly striking (pun intended). The Anchorage area receives 1 lightning strike for every 45.0 square mines annually. The Fairbanks area receives 1 lightning strike for every 2.35 square mines annually. That means lighting is approximately 14 time more common around Fairbanks than Anchorage.

Figure 2. All lightning strikes in the vicinity of Anchorage recorded by BLM in Alaska from 1986 to 2012.

Figure 3. All lightning strikes in the vicinity of Fairbanks recorded by BLM in Alaska from 1986 to 2012.

Looking at just he Fairbanks area, we see that 50,030 lightning strikes have been observed during the 1986-2012 time period for an area within 50 miles of Fairbanks. Half of those strikes occurred between June 16th and July 12th; a 27-day window! Figure 4 shows the distribution of strikes by 10-day time intervals around Fairbanks.

Figure 4. All lightning strikes within 50 miles of Fairbanks from 1986 to 2012 and a chart showing the frequency of strikes during 10-day intervals.

A look at the observations from the Anchorage and Fairbanks International Airports gives a good impression as the proximity of these storms to the respective airports. During that 27-year time period, there were 26 days with thunderstorms at the Anchorage International Airport and 210 days with thunderstorms at the Fairbanks International Airport. That is an average of 1.0 per year in Anchorage and 7.8 per year in Fairbanks. Since the number of lightning strikes around Fairbanks is 14 times more than around Anchorage, and the number of thunderstorm days per year in Fairbanks is 7.8 times more than around Anchorage, this tells us that any given thunderstorm day in Fairbanks is likely to experience twice as many strikes as a thunderstorm day in Anchorage. One factor not accounted for is cloud-to-cloud lightning. I am not sure if the BLM data captures those events or not. Figure 5 shows the annual number of thunderstorm days at the Anchorage and Fairbanks International Airports. 

Figure 5. Number of thunderstorm days per year in Anchorage and Fairbanks (observed at the respective airports) from 1986 to 2012. 

Postscript:

There are several papers that provide maps of thunderstorm frequency across the state. One is linked to in the first paragraph on this post. Others can easily be found using popular search engines.

Low Lightning

Reader Gary noted that there has been very little thunder in Fairbanks-land this summer. Fairbanks Airport has had thunder just ine day, and even here on Keystone Ridge I've heard thunder just once. Here is the plot of lightning strikes from the Alaska Fire Service for the two weeks from July 10 through July 24.

The total of 552 would be lackluster for any one day, much less for two weeks in July. Unfortunately, the strike counts this year (and going forward) are not really comparable to previous years data due to significant changes in the AFS lightning detection system. The new system is much more sensitive that the previous system, going so far as to detect separate branches off of the same leader stroke. Additionally, there were numerous technical glitches earlier this summer that made the data suspect.

The two systems were run in parallel for part of last summer, and while there was considerable day to day variation, the new system is counting something like 1.5 times as many strikes as the old system.

Not Much Thunder Yet

Courtesy of AFS

The Alaska Fire Service as had some issues with the lightning detection network (both under and over reporting strikes), but there is no doubt it's been a slow start to the convective season in Interior Alaska. The plot on the right plots strikes May 25th through Saturday morning. Here on Keystone Ridge, there has been no thunder heard to date. Only once in the past 17 years have we gone past June 9th without any thunder: that was in 1998, when there was no thunder heard here until June 28th. Of course, it is not uncommon for Fairbanks airport to not have reported thunder by June 8th. The fire expansion has slowed with the wetter weather this week. The total acreage burned as of Saturday morning stands at just under 46,000 acres. More than 60% of that total is from the Doestock Creek fire south of Aniak.

Lightning Season Winding Down

Here's a plot of daily lightning strike totals, through July 26th (top) in Alaska Fire Services Tanana Zone (bottom), which covers nearly all of Interior and northern Alaska. The collapse in daily strikes in mid-July is quite common.