Friday, October 23, 2020

Freezing Drizzle

The Fairbanks scene finally gained a more wintry decoration on Monday, with a couple of inches of snow produced by a weak upper-level disturbance.  But by Wednesday, as strong high pressure developed overhead, a different kind of wintry precipitation developed: freezing drizzle.  According to the airport instrument, drizzle occurred for about 7 hours, with temperatures in the mid 20s.

Here's a previous post on the topic of freezing drizzle.  As noted there, the phenomenon tends to occur more often in early winter than mid-late winter in Fairbanks, and this is consistent with the fact that the saturated lower part of the atmosphere needs to be entirely above about -10°C.  If any part of the cloud is colder, then ice is usually present, and solid hydrometeors always grow preferentially over liquid ones in a mixed phase cloud.

The soundings from Fairbanks at 4am and 4pm Wednesday confirm that the low-level cloud layer was shallow, with temperatures no lower than -10°C, and with dry air above the cloud.

Below are the surface and 500mb analyses from 4pm Wednesday, courtesy of Environment Canada.

With cold air aloft (-34°C at 500mb) and a strong anticyclone in place, one might have expected colder conditions at valley level, but all the moisture kept temperatures relatively high.  This is typical of freezing drizzle situations: in 34 days with freezing drizzle at Fairbanks since 1998, the daily mean temperature was above normal in 32 of 34 cases.

Here's a chart showing the number of hours each winter with freezing drizzle reported by the ASOS instrument at the top of the hour.  This is now the 8th consecutive winter with at least one occurrence.

Saturday, October 17, 2020

A Chill in the Air

What a difference a week makes.  Interior Alaska temperatures quickly dropped off from a high temperature of 51°F last Saturday, and thermometers marched steadily down through the week, with each day colder than the last until today.  Yesterday was remarkably cold in some spots; here's a round-up of low temperatures yesterday morning (click to enlarge).

The -11°F (actually -12°F) at Smith Lake on the UAF campus is quite impressive for the date, although the sensor is in a serious cold spot.  Fairbanks airport only made it down to +3°F.

Here's a look at the seasonal plunge over the last 3 weeks as registered at the Smith Lake site.


The Salcha RAWS measured -9F, and not surprisingly the river there is already iced up.  Here's a beautiful photo from this morning, courtesy of Twitter user NateoftheNorth.  You really should click to enlarge this one.


Despite the cold, measurable snow still hasn't arrived in Fairbanks, with just a trace reported on Wednesday and Thursday.  It's interesting to consider the coldest conditions that have ever occurred prior to measurable snow: back in 1941, the temperature dropped to -6°F on October 14, and snow wasn't measured until 4 days later.  This year's +3°F ties with 1969 for second coldest prior to having snow on the ground.

If we look at accumulated cold as measured by total freezing degree days, then this year is so far in 3rd place for cold prior to any snowpack.  2016 also had a decent freeze prior to the October 20 first snowfall, but most years have little or no freeze-up prior to snow; the median FDDs is zero.

Saturday, October 10, 2020

Warmth Will End

Exceptionally warm weather has continued across interior and northern Alaska, with Fairbanks currently sitting at the 3rd warmest start to October on record (1930-present); only 1969 and 2003 were warmer.  As of today, the normal daily high temperature is 36°F, but the coldest day so far this autumn was only 42°F back on September 24.  The average high temperature so far in October has been 56°F.

Fairbanks airport also hasn't seen even a flake of snow, and we're two and a half weeks past the date when that normally happens.  The median date for first snow is September 22, and about 85% of years receive measurable snow on the ground by October 10.  However, it looks like the white stuff will arrive next week, so the record for latest first flakes (October 20 in 2018) won't be threatened.

One might be tempted to think that such a warm spell in late autumn would portend a mild winter, but in fact there's a slight inverse correlation between September and subsequent November-March temperatures in the interior.  And as I mentioned a couple of weeks ago, with La Niña under way in the tropical Pacific, a cold winter is more likely than a warm one in interior, southern, and southeast Alaska.

Speaking of La Niña, the latest guidance from long-range forecast models has become considerably more bullish, and it's looking increasingly likely that this La Niña will end up as one of the stronger events of recent decades.  The models have also come into dramatically better alignment on expected cold for much of Alaska; here's the latest multi-model ensemble mean anomaly for January-March, which has the coldest 3-month period in the forecast.



The inter-model agreement is unusually good, as all of the models are now being heavily influenced by the La Niña forcing (see below).  One comment on this: confidence in the forecast is not as high as this agreement would suggest, because much depends on the evolution of the Arctic Oscillation and other high-latitude patterns, and these are much less predictable than the standard La Niña influence.  The models are good at showing the overall influence of La Niña, so they inevitably look similar in these situations, but other aspects of the winter circulation pattern can easily modify the outcome at middle and high latitudes.

It's really very interesting that the forecast for the upcoming late winter period (January through March) looks very similar indeed to the outcome last year across most of the extratropical Northern Hemisphere - compare the two maps below.  There's remarkable similarity across the North Pacific domain in particular, but of course the tropical Pacific is much different: last winter wasn't a La Niña at all, as we noted at the time (see here), but it looked a lot like a La Niña outcome in Alaska.

For reference, here's the January-March temperature anomaly in 10 strong La Niña events of the past.

Finally, the maps below show the monthly progression of NMME forecast maps.  Note that the cold signal doesn't come into play at all until December, and it peaks in February.  But as noted above, this is just the model signal, influenced mainly by the La Niña forcing; don't put too much faith in it, as there will inevitably be much more variability from month to month.

Sunday, October 4, 2020

Much Bigger Chinook

Following on the heels of last weekend's Brooks Range chinook, a much more significant warm event has unfolded across much of the state in the past few days, with deep and strong flow from the south rather than low-level flow from the north.  Here's the 500mb setup at 4pm AKDT on Friday; the big western Canada ridge and Bering Sea trough are classic for a warm surge into mainland Alaska.


The result has been one of the most notable warm episodes on record for the time of year; Fairbanks has exceeded 60°F for the past 4 days, with 68°F on Thursday and 66°F on Friday.  This is only the 4th year since 1930 with 65°F or higher in Fairbanks in October.  It's also the first time since the 1930s that Fairbanks has seen a daily low temperature above 45°F after September (the low was 47°F on Friday).

In terms of departure from normal, Friday's mean temperature anomaly of +22°F was one of the largest daily departures from normal on record prior to mid-October.  The greatest, of course, occurred in the extreme chinook of September 1995, when a low temperature of 65°F occurred on the 20th of the month, followed by a high of 78°F the next day.

Readers may recall that I mentioned the 1995 event in this post back in July, when I noted that 1995, like this year, saw its highest temperature of the year in May.  It's interesting that 2020, like 1995, has produced highly unusual warmth in both May and in mid-autumn, while some of the major global climate drivers are similar (developing La Niña during summer, a very warm North Atlantic, and a very active Atlantic hurricane season).

Here's a look at high temperatures on Thursday the 1st (click to enlarge).  That's a lot of 60's, including at Chicken in the southeast interior; the co-op site (not shown here) had 61°F, which is only the second time with 60s in October (previously 64°F on Oct 2, 2003).

And the following map gives a sense of the scale of the departures from normal, based on gridded model data:

Finally, what if the similarity with 1995 persists?  Then we would expect a cold start to winter in the interior; the winter of 1995-96 was colder than normal through January, particularly in eastern and southeastern Alaska.  January was particularly cold in the southeast, with Northway seeing a monthly mean temperature of almost -30°F: about the same as last year, in fact.  Juneau also had their coldest January of recent decades in 1996.

Monday, September 28, 2020

Brooks Range Chinook

Yesterday afternoon the temperature reports across northern Alaska caught my eye, with a striking temperature contrast across the Brooks Range.  Mid-elevation stations on the north side of the divide, such as Anaktuvuk Pass and Howard Pass, saw temperatures around 20°F - nothing unusual for the time of year - but downsloping produced dramatic warming on the south side of the mountains.  Bettles reached 54°F, and the Norutak Lake RAWS hit 56°F, which is very warm for so late in September.

Here's a map of temperature reports at 5pm AKDT - click to enlarge.

A very strong Gulf of Alaska storm explains this situation, as there was a pronounced north-south pressure gradient across the state at 4pm yesterday - see below.  As a result, strong cross-barrier flow occurred over the Brooks Range, and impressive chinook warming was able to develop even though the elevation changes are more subtle than in southern Alaska (Anaktuvuk Pass and Norutak Lake differ by only 1300').  Such is the power of elevation for modifying weather and climate.

Sunday, September 27, 2020

La Niña Winter On Tap

Even casual observers of weather and climate are probably aware that a La Niña episode is currently under way in the tropical Pacific Ocean, and this will affect the weather patterns that unfold in the higher latitudes as we go into Northern Hemisphere winter.  In particular, Alaska tends to see quite reliable winter impacts from La Niña: namely, colder than normal, particularly in the south, and often drier than normal in the south and southeast.

Here are a couple of overview maps based on strong La Niña conditions since 1950, showing the percent of winters with above-normal temperature and precipitation respectively.  The temperature data is obtained from the NCEP/NCAR global reanalysis, and we're not taking into account long-term trends: the "normal" baseline here is 1951-2010.



The cold in southern Alaska is very clearly connected to cold across the northeastern Pacific, which reflects a negative phase of the PDO (Pacific Decadal Oscillation); all of the top La Niña winters also have a negative PDO.  Currently the PDO is negative, but that's not because the northeastern Pacific is cool; it's just less extremely warm than the waters to the north of Hawaii, and that makes for a negative PDO index.  So given that current SSTs are well above normal, not below normal, in the Gulf of Alaska, we would expect this to take the edge off the La Niña cold in southern Alaska - at least until the SST pattern shifts towards a more canonical negative PDO state.


A more focused view of what we might expect this winter is shown in the figures below.  Here I've selected past years with La Niña conditions in autumn and then looked ahead to winter; this avoids making the assumption that La Niña will be strong throughout winter.  I've also restricted myself to La Niña years since 1979, I'm using the ERA5 reanalysis, and I'm showing the average departure from normal rather than the percent of years above normal.

The temperature signal is quite substantial in this average of 8 similar years, with a negative departure from normal of 1.5-2°C from the western interior to the northern Alaska Peninsula.  Compare this to the climatological (interannual) standard deviation of a little over 2°C for the 5-month period of November through March.  The average precipitation anomalies are relatively less dramatic, with south-central Alaska seeing a seasonal deficit of a little over 10% on average.  The top precipitation map above shows that the dry signal is quite reliable near the south coast, but there is a discrepancy between the two precipitation maps over western Alaska.

What's the cause of these patterns?  The maps below show that years with La Niña during autumn tend to see an upper-level trough extending from the Arctic Ocean north of Alaska to southeastern Alaska in the subsequent winter, and sea-level pressure is usually higher than normal over the Bering Sea.  This configuration draws cold air down from the north more often than normal, and the relative absence of storminess in the Bering Sea and Aleutians translates into reduced precipitation over southern Alaska.  Another way to think of it is that the jet stream spends more time to the south of Alaska rather than bringing Pacific storm systems up into western and southern parts of the mainland.

How about wind speed?  The first map below shows the average departure from normal in similar past years, and it points to reduced wind on the North Slope and enhanced wind in the southern Alaska Peninsula and eastern Aleutians.  To highlight the signal over the interior, where average winds are mostly light, I added a second map to show the percent departure from normal.  The magnitude of the signal is very small, but a reduction in wind in the western interior is consistent with reduced storminess and - perhaps more significantly - higher pressure that tends to produce stronger inversions with calm, clear, and cold conditions.


Finally, snow depth - see below.  This is interesting: even though liquid-equivalent precipitation tends to be lower than normal around Bristol Bay, snow depth tends to be higher because of the cold (i.e. there's much less rain).  The same is true in parts of southeast Alaska.  But in much of the west, interior, and south-central Alaska, the ERA5 data suggests that below-normal snow depth is more common.


Much could be said about the extent to which historical station data support these results from the ERA5 product, but I'll leave that for another time.  To conclude, here's a hemispheric view of these climate variables, plus a bonus map for solar radiation at the end.  It's particularly striking to see that western Alaska has the most notable cold signal in the hemisphere, while the La Niña "hot spot" is in central Arctic Russia (where - interestingly - it has been excessively warm throughout this year so far).

Saturday, September 19, 2020

Sea Ice Minimum

Arctic sea ice has reached its seasonal minimum, and after a summer of exceptional warmth near the Russian coast, this year's minimum ice extent is the second lowest on record.  For only the second time in modern history, Arctic sea ice extent dropped below 4 million km2, but 2012 still holds the record for most open water in the Arctic Ocean.

Comparing the ice concentration images from 2012 and 2020, the situation is pretty similar, although more ice remains in the Beaufort Sea this year, and there's a larger area of reasonably high ice concentration in the central Arctic Ocean.  However, melt-out has advanced closer to the pole on the Russian side than in 2012; there is wide open water north of 85°N at 60°E, i.e. only about 300 miles from the pole.


It's interesting to consider how summer weather patterns affect the magnitude of ice loss.  Using the annual difference in ice extent between May and September to measure summer ice loss, the map below shows the correlation with June-August mean 500mb height over the last 20 years.  This reveals quite clearly that a negative phase of the Arctic Oscillation, involving higher heights (pressure) over Greenland and the high Arctic, is a pattern that favors more ice loss than the inverse (positive) AO phase.

A negative AO also brings low pressure to northwestern Europe, and so it turns out that a wet summer in the British Isles is significantly associated with greater ice loss.  Here's the correlation map for precipitation (click to enlarge).

Did this summer fit the pattern for enhanced ice loss?  In a very broad sense yes, as there was a very notable ridge over the Arctic Ocean, but it wasn't aligned with the typical Greenland epicenter of the Arctic Oscillation, and the AO index was only slightly negative.  However, the trough over southern Alaska was quite characteristic of low ice years, and it was indeed quite a wet summer in parts of the British Isles - see below.

Below are the correlation maps and the corresponding 2020 anomaly for a few other variables.

Mean sea level pressure shows the Arctic Oscillation pattern very clearly, with a ring of low pressure around the Arctic margins surrounding the Greenland high pressure anomaly in a typical low-ice year.  The rather strong inverse correlation with MSLP over most of North America is interesting; I'm not sure what would explain such a widespread and apparently significant connection.

For temperature, eastern North America and of course most of the Arctic tend to be warmer when ice loss is large, but southern Alaska and western and northern Europe more often have a cool summer in these years.  This year didn't match these patterns particularly well, although July was exceptionally cool in northern Europe.

Finally, and perhaps surprisingly, solar radiation tends to be lower than normal across most of the Arctic except the Beaufort Sea in low sea ice years - at least based on these ERA5 estimates for the last 20 years - and 2020 certainly fit the bill in this regard.

If this is correct, it strongly suggests that clear skies and increased sunshine are not responsible when summer melt-out is large.  Rather, it seems to me that increased cloudiness would provide greater surface warming via increased longwave radiation, and this effect appears to be particularly important in the Eastern Hemisphere, where sea ice is more vulnerable with less thick multi-year ice.  The result also suggests that the ice albedo feedback may not be a significant factor in high melt years, although I'll admit that I'm straying outside my area of expertise here.

One final comment to make is that causation goes both ways when it comes to the typical weather patterns accompanying higher or lower sea ice loss.  Obviously, surface water temperatures can rise well above freezing once ice is gone, and near-surface air temperatures are no longer held down to the same extent.  Similarly, evaporation is much higher over open water than over ice, and this could explain greater cloud cover in low ice years.  So while I introduced this discussion as "how summer weather patterns affect the magnitude of ice loss", it's undoubtedly true that the weather in turn responds to the ice situation - and not just on seasonal time scales, but also in terms of longer-term trends.