Thursday, June 30, 2016

Seasonal Temperature Correlations

Last week reader Gary posed the question of whether there is a connection between this summer's weather in Fairbanks and the extreme heat in the desert southwest of the Lower 48.  Intrigued by the question, I created a set of spatial correlation maps for monthly mean temperatures through the year.  To illustrate how this works, the map below shows the correlation of June mean temperature between Fairbanks airport and every other point on the map.  As expected, the correlation falls off with distance, and the scale of good correlation is quite small at this time of year.  Temperatures in south-central Alaska only have a modest correlation with Fairbanks temperatures, and the correlation is about zero for stations on the eastern North Slope.

Here's a map showing the results for all of North America.  I created a smoothed contour map rather than showing all the points, because there are so many stations in the lower 48 that all the markers overlap.  We see that Fairbanks temperatures do indeed have a modest negative correlation with temperatures in parts of the southwest US at this time of year, and there is a small positive correlation with the northern Plains and upper Midwest.

Below are the maps for each month of the year (click to enlarge).  A surprising result is that the scale of very good correlations (R>0.9) across Alaska and western Canada is greatest in September; I would have expected this to occur in winter.

Sunday, June 26, 2016

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.

Tuesday, June 21, 2016

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.

Friday, June 17, 2016

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.

Wednesday, June 15, 2016

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.

Monday, June 13, 2016

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.

Friday, June 10, 2016

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 temperaturee) between the surface and 500 mb.  The idea here is that a large vertical gradient (decrease) in θe 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 θe 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 θe 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 θe 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 θe 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.