Monday, August 31, 2015

August Chill

Meteorological summer is ending with a nip in the air across interior Alaska, with hard freezes and some snowfall in certain locations.  The temperature dipped to 20°F this morning at the Norutak Lake RAWS (elev 800'), between Bettles and Ambler; it was 25°F at the Bettles airport.  Closer to Fairbanks, it was 23°F at the Livengood RAWS, and many locations on the north side of Fairbanks dipped just below freezing.  The airport made it to 33°F.  Here is what was reported at the UAF ACRC weather station this morning:

As for snow, light amounts were observed even at some valley locations, such as Delta Junction.  The images below were taken yesterday morning at the Delta Junction FAA webcam; the airport reported light snow for more than 2 hours, with visibility down to 3/4 mile for a time, and a temperature of 32°F.  It's an interesting coincidence that snow fell in both June and August this year in Delta Junction.  However, any accumulation yesterday was probably no more than a trace; the earliest recorded measurable snowfall occurred on September 2, 1983.

Saturday, August 29, 2015

Arctic Ice Update

The Arctic sea ice melt season will be drawing to a close in the next few weeks, and it now appears certain that this year's minimum ice extent will be below last year's minimum.  The following graphic from the Danish Meteorological Institute suggests that a rapid melt-out has been occurring in the past week or two; until then, the ice cover had been holding up quite well.  This analysis now shows 2015 in third place for lowest extent, behind 2011 and 2012.

The University of Washington's PIOMAS estimate of Arctic sea ice volume confirms that the situation is worse than last year, with the volume at the end of July falling below 2014 but still somewhat above 2010-2013.  It's the same picture for average ice thickness - see below.

One interesting aspect of the ice situation this summer has been the relative abundance of ice, including some multi-year ice, in parts of the Beaufort Sea not far north of Alaska.  The latest NWS sea ice analysis (see below) shows some ice coverage very close to the coast around Prudhoe Bay and also includes some multi-year ice not too far to the north.

The excellent commentary from NSIDC discussed the multi-year ice situation on August 5.  The top left panel of their figure, copied below, showed a swath of ice including a large amount of 5+ year-old ice (white pixels) north of Alaska in early July.  The old ice has been swept westward by the Beaufort Gyre, away from its usual residence adjacent to Canada and Greenland.  I'm not an oceanographer, but I would speculate that the farther away old ice is transported from its habitual home to the east, the more rapidly it would be prone to melt; so I wonder if this pattern portends a large loss of multi-year ice this melt season.  I suppose another interpretation would be that multi-year ice coverage has expanded in comparison to recent years; it will be interesting to see if this trend can continue.

Thursday, August 27, 2015

Flooding in Barrow

A strong low pressure system north of Barrow is bringing strong westerly winds to Alaska's Arctic coastline and causing coastal flooding in Barrow.  Here's a webcam image from a few minutes ago (live link here):

The map below shows this morning's surface analysis, courtesy of Environment Canada.  The central pressure of the storm is 984 mb, which is quite strong but certainly not record-breaking for this location and time of year.  The August record for low pressure in the area 65-90°N, 140-180°W is 971 mb, according to the 1948-present global reanalysis; this was observed in 1975.

Wednesday, August 26, 2015

Damp and Cloudy

Measurable rainfall has occurred in Fairbanks on 16 of 25 days in August through yesterday, which is tied for 3rd highest in the 1930-present history.  It's still raining today, and computer forecasts show a 100% chance of rain tomorrow, so that will put August 2015 into 2nd place after 1998.  In that year (1998) it kept on raining and accumulated 22 days of measurable rainfall in the month, so we can't break that record.

Interestingly, however, the total rainfall so far this month is right at the long-term normal, so there has been an unusual number of light rainfall events.  This month has already tied the full-month August record for number of days with rainfall from 0.01 to 0.1 inches.  Not surprisingly, cloud cover has been excessive; this month's average cloud cover has been the highest since 1998.

Looking at the forecast, the potential late-month cold spell that we discussed earlier is now a certainty, with the latest GFS forecast showing 850mb temperatures dipping below -5°C on Friday.  The model also expects precipitation over the weekend, and it looks like being snow on the higher elevations of the interior.

Tuesday, August 25, 2015

El Niño Winter Patterns

A couple of weeks ago a reader requested that we look at the likely effects of El Niño on the upcoming winter's weather pattern in Fairbanks.  This is a big topic that could fill a series of posts, so I'll just scratch the surface here.

First, published work from almost 15 years ago indicated that El Niño winters tend to be significantly warmer than normal in the eastern two-thirds of Alaska, but the signal is much weaker over western regions.  Unusual warmth over much of the state arises from the Aleutian low-pressure zone being more intense than usual, and the associated upper-level flow brings warm air from the southwest into Alaska.  However, the phase of the PDO is also a strong constraint on the winter temperature patterns, as El Niño's warming influence seems to disappear when the PDO is negative.

Some analysis I did here shows basically the same thing, with a rather high frequency of unusual winter warmth across south-central and southeastern Alaska during strong El Niño events.  Fairbanks lies within the zone where above-normal temperatures are favored, although near-normal temperatures are also common.  Again, however, a warm outcome is largely contingent on having a positive PDO phase - which is more likely than not during El Niño, as the two phenomena are inter-dependent and positively correlated.  As far as precipitation is concerned, there is a rather strong dry signal across most of western and interior Alaska in El Niño winters.

Let's look at some maps.  The following sequence of images shows, for each month of winter, the percentage of years that were warmer than normal when El Niño conditions were in the top 10 strongest for that month.  The data come from the NCEP/NCAR global reanalysis, with "normal" defined as the 1951-2010 mean.  It's interesting to see a particularly warm pattern over interior Alaska in December and a much cooler look for January; but February is warm again.  These month-to-month variations could be partly attributable to random chance.






Here's the same map sequence for precipitation, based on a global 1-degree objective analysis from observed data (GPCC analysis, not NCEP reanalysis).  The dry signal that I mentioned earlier shows up strongly in January and February across most of the state and persists in March across the southern interior.






Now let's look at some scatter plots of historical data from Fairbanks.  First, the charts below show the relationship between monthly mean temperature anomaly in Fairbanks and an index of El Niño - Southern Oscillation (ENSO) activity; a more positive ENSO index corresponds to a stronger El Niño, and a negative ENSO index corresponds to La Niña.






There is a lot of scatter, showing that El Niño and La Niña by themselves do not have a particularly strong influence on Fairbanks winter temperatures; the correlation is slightly better for the overall winter mean (see below), but still not great.  Compare the scatter plot for the PDO, also shown below - that's a much better correlation.  Nevertheless, the plots above do show a tendency for unusual warmth during the strongest El Niño events for every winter month except January.  It's interesting that the warmest January's appear to occur when ENSO is relatively close to neutral.

Below are the corresponding results for precipitation, with the long-term median for each month denoted with a dashed line.  It's pretty clear that late winter (January-March) is usually even drier than normal when a strong El Niño is in play.






The dry signal shows up in the overall winter mean, but again the PDO is more highly correlated than ENSO.

I'll mention one other aspect of El Niño's impacts on winter climate.  It has been shown in the literature that high-pressure blocking is less frequent over the Bering Sea during El Niño winters, which is consistent with a stronger Aleutian low.  The lack of blocking to the west of Alaska reduces the frequency of cold northerly flow over the state, thus contributing to the overall warming signal.  I attempted to reproduce this result with the blocking data I've used before; the map below shows the difference in frequency of blocking highs between El Niño and La Niña winters.  There is a slight reduction in cut-off high pressure centers near the western Aleutians, but interestingly the frequency rises over most of Alaska and especially the Chukchi Sea and nearby Arctic Ocean.

The corresponding map for cut-off low pressure centers shows a large increase in frequency over the Aleutians and a large decrease in the Gulf of Alaska.  Taken together, these maps show that El Niño brings more upper-level ridging and less troughing over the Gulf of Alaska and nearly all of Alaska except the Aleutians; but there is a persistent trough over the Aleutians, and this flow configuration creates warm southwesterly flow over the state. 

In summary, if the current El Niño episode continues in the "strong" category throughout the upcoming winter, Fairbanks can expect another warm - but perhaps not excessively warm - winter.  Much will depend on the evolution of the PDO, which is currently still positive but has seen strong fluctuations in recent months.  Snowfall is likely to be less than usual from January through March.

Friday, August 21, 2015

Forecast Difference

Updated Aug 23, see end of post

The end of this month could be shaping up to bring very unusual weather to interior and northern Alaska, judging from recent computer model forecasts; but there is extreme uncertainty as to how it might play out.  In fact I don't recall when I last saw such pronounced disagreement among the leading models, as illustrated below in the 7-10 day 500mb height forecast from the ECMWF, GFS, and Canadian models (click for a larger version):

The ECMWF - widely considered the best model on this time scale - is showing a strong trough and cold anomaly over Alaska, but the Canadian (CMC) model shows a huge high-pressure block.  These are ensemble mean forecasts, so typically when they show a large anomaly at this lead time, it is quite likely to occur - and therefore it's very rare to see such strong disagreement.  The GFS is taking the middle of the road, although recent runs have been flipping back and forth.  The very latest GFS run (more recent than shown above) shows cold air becoming entrenched over the state by the end of the month.

It will be fun to see which model wins out - or if they're all wrong.  In any case, it seems quite likely that there will be an interesting outcome with the potential to break records.  Those with outdoor plans towards the end of the month should pay attention, as the cold scenario would probably bring snow to the hills in many areas.

Update Aug 23: 48 hours later, and it looks like the ECMWF will be nearer to the mark.  No surprise there.  Here's the latest 7-10 day forecast:

Here's a time-height cross-section of temperature above Fairbanks from the latest GFS deterministic (not ensemble) forecast: pretty chilly by next weekend.  It could still be wrong, of course; 5-7 days is a long time in Alaska weather forecasting.

Wednesday, August 19, 2015

Pace of Temperature Records

Last summer we looked at changes in the rate at which temperature records have been set over the last 60 years or so; see here and here.  I've often pondered the same topic since then, and I thought it would be interesting to update the results for 2014 and 2015 to-date, and to look at which stations have warmed the fastest (and slowest).

First, the two charts below show a 19-station average of annual numbers of daily temperature records (scroll down to see the stations listed on another chart below).  The data are taken from 1954-present and records are defined relative to this period only.  The first chart shows occurrences of record warmth, whether for daily maximum (red) or daily minimum (blue) temperature; and the second chart shows occurrences of record cold.  As we noted before, cold records have diminished somewhat more rapidly than warm records have increased.  Among the 4 categories of records, the most rapid changes have occurred for low daily maximum records, followed by low minimum records, then high minimum records, and the least rapid change has occurred for high maximum records.

The 1976 PDO shift is clearly evident on the charts, and the extreme warmth of the past two years is also a striking feature.  2014 saw the highest number of warm records (both maximum and minimum temperatures) and also the least number of low maximum records.  Interestingly the number of low minimum records was slightly lower in 1978.  Remarkably, 2015 looks very likely to outdo 2014, as the number of high minimum records is already almost equal to 2014, and so far there have been just a handful of days with daily cold records at a few stations in 2015.

One minor concern I had with the results above is that there is a higher concentration of stations in southern and southeastern Alaska, so I repeated the analysis after excluding Kodiak, Homer, Talkeetna, Juneau, and Big Delta.  The results are little different, although 2015 to-date looks a little less extreme - see below.

It's interesting to compare the overall pace of change between the different stations.  I attempted to do this by adding the slopes of the linear trends for the warm record counts and subtracting the trend slopes of the cold record counts - see the results below.  Based on this metric, the long-term rate of warming has been greatest at Homer and Talkeetna, but interestingly the relatively nearby stations of Kodiak and Gulkana have seen some of the smallest warming rates.  The difference between Kodiak and Homer is particularly interesting as both stations are heavily influenced by North Pacific sea surface temperatures.  The second chart below shows that there is a long-term trend in the temperature difference between Kodiak and Homer, and 2014 was the first year in which Homer was the warmer of the two locations.  This looks a bit suspicious to me; there might be some station siting issues that have affected the reported temperatures from either or both of these places.

Lastly I'll show the record counts for several individual stations.  The situation at Homer has been very extreme during this year and last:

At Talkeetna the rapid warming pace comes from a very large number of warm records in 2002 and 2003:

The slowest-warming site, Gulkana, still shows a long-term warming trend in all 4 record categories:

Lastly, if we look at Fairbanks with data back to 1930 included, the picture changes slightly, as the high maximum records no longer show a long-term warming trend; however, the long-term decrease in cold records is still very much evident.