Strong Thunderstorms

This is just a quick post to document the strong storms occurring in the vicinity of Fairbanks today, as noted by reader Gary.  The sequence of radar images below, at one-hour intervals, shows the widespread development of thunderstorm cells that showed a tendency to merge into larger thunderstorm clusters.

The radar-estimated total precipitation is above 2" in a number of places and even exceeds 4" west of Fairbanks, which would be enough to cause serious local flooding problems.

The 4pm Fairbanks sounding shows a very high 61°F dewpoint at the surface (see below), although the airport ASOS didn't report anything higher than a 57°F dewpoint - but even so, that's quite a lot of moisture.

Wet Summer So Far

The scene is wet and breezy in Fairbanks today, as showers move quickly through the area on a brisk westerly flow.  This is the 14th day with measurable rainfall so far in June, which is the most since 1994; only 7 days have remained dry this month.  The month-to-date rainfall of 2.25" (not including today) is the 3rd highest on record, behind only 1955 and 1977.

The chart below shows that year-to-date precipitation is now about an inch above normal, which is a huge turn-around from the record dry conditions at the beginning of the year.  The difference is that we no longer have El Niño-related low pressure in the North Pacific, keeping dry southerly flow over Alaska; this month so far we have seen quite the reverse, with above-normal 500 mb heights south of the Alaska Peninsula and low pressure over the Beaufort Sea (see the map below).  Unsurprisingly Barrow is also seeing a wet summer so far, with a record 10 days of measurable precipitation and near-record 0.61" of total precipitation already this month.

Here is today's 3am AKST 500 mb analysis from Environment Canada; note the strong shortwave disturbance over the North Slope.

This year is now wetter to-date than any of the past 5 years in Fairbanks (see below), but the record summer rains of 2014 got under way at about this date, so it's highly unlikely that this year will remain ahead of that year for much longer.

With respect to temperatures, the excessive warmth of spring has been replaced by some modestly negative anomalies at times in recent weeks.  Only 16 days so far this year have reached 70°F, which is the lowest number since 2008; but this is also equal to the 1981-2010 normal, so the coolness could hardly be called unusual except in comparison to recent years.

May Arctic Ice Revisited

At the end of May we noted a severe shortfall of Arctic sea ice, but the analysis was subject to some uncertainty because of an emergency transition between satellite instruments.  As of a few days ago the NSIDC has declared that the data from the new instrument is being properly calibrated, and the May sea ice data has been released.

As expected, the mean Arctic sea ice extent for the month of May was extremely low - the lowest on record for May, and nearly 5% below the previous May record.  In fact, it was the lowest monthly anomaly in the entire satellite record in terms of standard deviations below the 1981-2010 mean.  The chart below shows the monthly standardized anomaly since 1979; last month narrowly beat out August 2012, which was 3.39 SD below the mean.  It may be fair to say that Arctic sea ice has never been in worse shape during the satellite era.

In last month's post I speculated that a very low rate of melting would be needed this summer to avoid a record low ice extent in September; but it turns out that this is not quite right.  To match the 2012 record minimum, we would need a decrease of 8.4 million km² in mean monthly ice extent from May to September, but only 3 years have seen that much melt in the past.  If we take the average May-September ice loss from the last 10 years, then this September's mean extent will be 3.90 million km²; this would be above the 2012 record minimum but below the 2nd place year, 2007.

The chart below shows the May and September ice extent for each year back to 1979; notice the big jump in summer melt rate in 2007.  Assuming that this year is likely to be similar to recent years, I would estimate a chance of 25-50% that 2016 brings a new record minimum in September.  The latest daily update (also see below) from NSIDC indicates that ice extent is now equivalent to this time in 2012 - which is a big improvement from last month.

Midsummer Frost

Clear skies and light winds under an approaching ridge aloft allowed temperatures to fall below freezing in some locations last night, despite the date being within a week of the summer solstice.  Here are some of the minimum temperatures reported:

26F   Marguerite Creek RAWS near Healy 13NE (1735' elevation)
27F   McKinley River RAWS (863')
27F   Denali Visitor Center (1650')
28F   Chalkyitsik RAWS
28F   Salcha RAWS
29F   Angel Creek RAWS (1100')
29F   Goldstream Creek CWOP
30F   Goldstream Creek COOP
32F   Circle Hot Springs COOP
32F   Chicken COOP

Here's the 3am AKST 500mb analysis.

Record Rainfall

Saturday's thunderstorm in Fairbanks ended up becoming a prolonged and significant rainfall event, with continuous rain and (later) drizzle from about 6:30pm Saturday through 4:30pm Sunday.  The calendar day rainfall total of 0.95" on Saturday was a record for the date, and it is the earliest in the year that such a large rainfall amount has been observed in a single day (1930-present).  About 75% of such events occur in July and August, and the latest date for such a rainfall event is September 27.

As reader Gary noted in comments, the initial round of rainfall was convective in nature with storms forming to the southeast and drifting west-northwest.  A steady rain then followed for many hours.  Here are simple radar animations for two consecutive 90-minute periods.

We've noted before that easterly winds aloft are relatively more favorable for precipitation in summer than in other seasons, although a westerly wind regime is still the most favorable.  In the recent event the 700mb wind direction was close to easterly but not particularly strong; there was a weak pressure gradient between a low to the southwest and a weak ridge to the northeast.  Here's the 500mb analysis from 3am AKST on Sunday morning.

Update June 15: here's Saturday's 4pm AKDT sounding from Fairbanks.  It appears the balloon was actually released at 3:04pm, which was exactly one hour before the ASOS first reported lightning.  The sounding shows very little instability, but forced lifting during the subsequent hour could well have transformed the temperature profile into something more unstable.  Another possibility is that higher low-level moisture was available nearby but was not sampled by the balloon.  Given sufficient moistening at low levels, the sounding could become very unstable.

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.

Snowing in Umiat

This is just a quick note to observe that Umiat has been seeing wet snow today.  It's not unusual for coastal Arctic Alaska to see snow at this time of year, but Umiat is inland and warmer at this season.  In the 34 years of climate observations between 1946 and 2000 (with a break in the middle), measurable snow was observed in June in only 5 years.  I'm not sure if today's snowfall would count as measurable, as it has been melting on the ground.  Here's a webcam photo.

Gridded Historical Data

Last year I wrote a post on the accuracy of the NCEP global reanalysis temperatures and precipitation for Fairbanks, and it turns out that a paper has recently been published by UAF researchers on a similar topic.  Lader et al (2016) looked at 5 different reanalysis products and attempted to assess their accuracy for statewide historical temperature and precipitation.  The general conclusion is that there is no one reanalysis that is universally best, as each one has different biases, but for specific purposes it is possible to identify which products are superior; for example, NASA’s MERRA reanalysis appears to be the best for wildfire concerns (late spring and summer interior temperature and precipitation).

An aspect of the paper that caught my eye was that they used an independent high-resolution gridded analysis of Alaska temperature and precipitation as the “ground truth” for measuring the error in the various global products.  (They also separately looked at station observations, so the overall assessment was based on both sources of verification data.)  I had not previously come across the high-resolution gridded data, so I downloaded it from NOAA (source) and took a quick look.  The development of this data was described in a paper last year by Hill et al.

The maps below show the 1981-2009 average temperature for January and July according to the high-resolution analysis.  (The data extends from 1961 to 2009 at 2 km resolution.)  The January map appears to show some kind of discontinuity along the Alaska-Yukon border, which is troubling; I can’t imagine what would cause this.  Other than that, however, the analysis looks reasonable and of course it shows lots of nice spatial detail; it is fundamentally based on the PRISM model, which is a state-of-the-art statistical technique for estimating localized climate variations, particularly in regions of complex terrain (see e.g. here and here).

The January and July normals for the Fairbanks area also look OK – see below.

To look more closely at the local accuracy of the gridded analysis, I pulled out the closest grid points for Fairbanks airport and for the Keystone Ridge co-op site.  There is more than 1100' elevation difference between these two sites, so it is interesting to see if the analysis adequately represents the temperature differences.  The charts below show the January and July mean temperatures at the two sites for the common periods of record in both gridded and observed data.

For Fairbanks airport, the correspondence between modeled and observed temperatures is very good in January, with almost no bias and a high correlation, but the analysis is much less good in July, with a correlation coefficient of only 0.64 - and the gridded analysis is more than 1.5°F too cold on average.  At Keystone Ridge there is a January warm bias of nearly 4°F over the short period of record, but the correlation is virtually perfect in both January and July.  It's worth noting too that the mean temperature difference between the two sites is only about 0.5°F different from reality, both in January and July - so the vertical gradient of temperature is correctly represented.

The correlation between gridded and observed monthly temperatures for the two locations is shown below.  We see that July is the worst performing month at Fairbanks airport (matching what we saw for the global reanalysis), but the correlation is almost perfect for Keystone Ridge throughout the year.  The latter result is interesting, because the starting point for the 2-km analysis was the PRISM 1961-1990 normals, which would not have included any data from Keystone Ridge.  However, Hill et al (2015) added observed monthly anomalies to the PRISM normals, so they obviously included the Keystone Ridge data; and it would seem that no other stations were close enough to dilute or modify the Keystone Ridge signal - otherwise the analysis would not show a perfect fit at that location.

Presumably the reason that Fairbanks AP fails to show a perfect fit is that there are many other nearby valley-level stations, all reporting slightly different anomalies from each other month by month, and when many of them are combined in an objective analysis scheme, the result is less than perfect at any one of the stations.

A better test of the gridded data would be to compare results for a station whose data did not make it into Hill's analysis, but we would need to see his list of stations in order to know where to start.

In conclusion, a first look at this high-resolution gridded historical data leaves me impressed and eager to see this kind of product undergo further development.  It would be nice to see the Alaska PRISM normals updated for a more modern era (rather than 1961-1990) by taking advantage of the wealth of RAWS data that is now available, and it would also be great if a near-realtime update could be achieved.  Anyone looking for a graduate thesis project?

Non-Threatening Wildfire Outlook

The National Interagency Fire Center in Boise released an updated national wildfire outlook today, and it contains a modest dose of good news for Alaska: the fire potential remains "normal" for the summer months throughout the entire state.  Last month's forecast had an elevated risk of fire in May for the south-central region, but no departure from normal is predicted for the rest of the season.

So far only 5700 acres have burned in Alaska this year, but of course significant fire activity doesn't usually kick in until June.  Last year the fire acreage was only 6400 acres on May 29, but the end of June and beginning of July were awful.  The record fire year of 2004 had even lower (almost zero) fire acreage at the beginning of June.  In contrast, 2014 was bad in May (almost 200,000 acres) but the rest of the season was quiet.

Here's the link to today's report:

http://www.predictiveservices.nifc.gov/outlooks/monthly_seasonal_outlook.pdf

The absence of an above-normal risk forecast seems a little surprising in view of the extremely warm spring experienced across Alaska together with the CPC forecast for continued abnormal warmth statewide this summer.  Perhaps the forecasters have taken note of the shift to a wetter pattern in May and, in the past couple of weeks, relatively cooler conditions as well.

Here are the temperature and precipitation anomalies for the April-May period at 16 long-term observing sites.  The area of each circle denotes the magnitude of the anomaly relative to the 1981-2010 distribution, in terms of standard deviations for temperature and percentile rank for precipitation.  If the colored circle is missing (as in some locations for precipitation), this indicates that conditions were close to normal.

Arctic Ice Update

There's been considerable news coverage of the woeful state of Arctic sea ice this year, but it's worth posting an update on this anyway.  First off, a caveat: a critical U.S. satellite sensor failed last month, forcing a hasty, unplanned transition to a newer satellite to maintain the U.S. sea ice monitoring program.  Read about it here.  The NSIDC is showing current data from the new satellite, but it has not undergone a careful calibration process and is considered provisional.  That said, here's the latest time series of total Arctic sea ice extent; the sensor transition occurred on April 1, but there's no obvious discontinuity at that time.

If the new satellite data are to be trusted, sea ice has retreated rapidly this month and now sits far below the 1981-2010 normal; it looks like the anomaly is between 3 and 4 standard deviations, which is huge.  The previous record lowest extent was observed in (September) 2012 and this year's ice is far less extensive than 2012 at this date.  This doesn't mean 2016 is guaranteed to see a new record minimum, but I'd guess the seasonal rate of melt would have to be remarkably slow from here on out to avoid a new record.

Here's a similar figure from the Danish Meteorological Institute; they are using the same satellite data as NSIDC, so it's not surprising the charts look similar.

The latest map of Arctic sea ice shows big deficits compared to normal in the Barents Sea and along the east coast of Greenland, and of course sea ice is still well below normal in the Bering Sea.  Remarkably there is a large open region in the Beaufort Sea between Banks Island and Alaska, and another open area is evident in the northern part of Baffin Bay.

The PIOMAS model of Arctic sea ice shows that estimated ice volume grew uncharacteristically slowly in the first 4 months of the year and was far below normal at the end of April.

There won't be any prizes for guessing why ice growth was so anemic this winter; it was undoubtedly related to the extreme warmth around the Arctic basin.  To illustrate this, I created maps of monthly temperature anomaly from November through April, using both the station data and the gridded reanalysis (model) data - see below, and note that the color scales are the same (click to enlarge the maps).  I like to think of the left-hand maps as the "ground truth", as they're based on station measurements, but the reanalysis gives a generally credible picture of conditions over the Arctic ocean itself. 


Note the amazingly strong and persistent warm anomaly in the northern Barents Sea, a direct result of the abnormal lack of sea ice.  This anomaly might be a bit overdone by the reanalysis model, but perhaps not by much.  There is an observatory on one of the islands in Franz Josef Land, and their mean November-March temperature was 12.4°F, compared to an estimated 1981-2010 normal of -6.6°F, for a 5-month mean anomaly of +19°F.  (This station doesn't show up on the maps because I required 28 years of data to calculate the normals, but this station only had 24.)

Finally here's the ECMWF model-estimated 2m temperature north of 80°N so far this year, courtesy of the Danish Meteorological Institute.  It would appear that not a single day has been cooler than the 1958-2002 normal.