Thursday, January 12, 2017

Ice Fog and Surface Temperatures

Rick T, here. Just a quick follow-up on the question of ice fog and possible impacts on surface temperatures. Ice fog and its impact on surface temperature is a tough nut to crack to due many confounds: societal changes, e.g. people leaving vehicles running, urbanization changes, airport siting and activity, ASOS vs. human observed visibility, etc.

My sense is that, physically, the long wave radiational depth of the ice fog (analogous to optical depth) is what matters for surface temperatures, which I don't think has much correlation to reported (horizontal) visibility. I think the the temperature manifestation of this can be seen in the RAOBs with very low surface temperatures when lapse rates become neutral or negative (i.e temperatures are steady or following with height). 
Here are two very cold soundings, one from January 1969, the pre-pipeline era, and one from 1989, with a significantly expanded urban environment and many more automobiles operating.  The 1969 sounding looks like a myriad of other winter soundings in Fairbanks: the temperature at the airport was about 15C lower than 400 meters up. The 1989 sounding is much different and quite unusual, with slight cooling in the lowest 400 meters. No two situations are of course the same, but part of the reason for the dramatic difference is, I think, the depth of the ice fog. In spite of very low visiblities in both cases, I'd suggest that in the 1969 case the ice fog was very shallow, while in 1989 it was thick enough that the effective radiative surface was the ice fog top (~600), with weak mixing within the ice fog.
In any case, no hard science here, just my sense of some of the issues involved based on my years of trying to forecast in these conditions.

Arctic Update

Here's a brief update on temperatures at climate observing sites around the Arctic Ocean in the month of December.  According to monthly data from a set of 19 stations, December was nearly but not quite the warmest in the period of record I've used here (1971-present), with an average departure from normal of +3.3°C.  In this data set, December 2005 was slightly warmer than December 2016.

The relatively high temperatures in December continued the pattern of persistent warmth that was observed all year in 2016.  According to data from these particular stations, the average temperature was at least 1°C above the 1981-2010 normal for all 12 months of the year; it's the first time that has happened in this data set.  Perhaps more strikingly, the 19-station mean temperature was above normal for all but 5 days of the year.

If we look at the annual number of days with a mean daily temperature below normal (see below), there was a sudden drop-off in 2005, but for the past decade the number of below-normal days has been relatively stable; the 2005-2015 average was about 75 days or 20% of the year.  However, 2016 was quite different, with below-normal days becoming rare for the Arctic coastline as a whole, and even more so in the second half of the year; the last below-normal day was August 5, 2016 - more than 5 months ago.

The near-complete absence of below-normal temperatures in 2016 is entirely consistent with ECMWF model estimates of high Arctic (80-90°N) air temperatures, as calculated by the Danish Meteorological Institute; their 2016 chart (see below) shows no below-normal days at all outside the summer season.  The charts provided for earlier years on the DMI website show nothing comparable for sustained anomalous warmth in the reanalysis history back to 1958.

Wednesday, January 11, 2017

Historic Fairbanks Cold Snaps

Rick T. here with a short post for future reference. Here's an annotated plot of the most extreme cold snaps during the Weather Bureau/NWS era (since late 1929) in Fairbanks, from downtown, Weeks Field, International Airport, wherever the observations were being made. Here we're looking at all the cold snaps with a 7-day average temperature of 40 below or lower. Note: average temperature, not average low temperature. The dramatic dropoff in frequency and intensity of cold snaps post-1975 is partly PDO shift, partly trend and partly urban growth and the attendant ice fog, which serves as something of a "blanket" for the valley floor.

For those in Fairbanks in 1989: yes, the lowest weekly mean temperature reported from the Airport was -43.0F, and I agree, I think there was a problem with the temperature sensor on at least a few days. But based on the Ft. Wainwright observations, such as there were, it was not greatly lower than that.

Sunday, January 8, 2017

Severe Cold Spell Possible

For those hardy residents of Alaska's interior who enjoy the occasional spell of intensely cold winter weather, and who have felt that the warmth of recent years has been over the top, the latest forecast indications may be welcome.  Others will feel less enthusiastic.  Recent forecasts have been coming into agreement with expectations for a period of very cold weather later this month, and there's a chance the cold may be extreme and prolonged.  Here are the 500mb height anomaly forecasts from 3 leading models for 7-10 days from now (click to enlarge): notice the extremely pronounced trough over Alaska and the good agreement between the models.

Here are the CPC's latest 6-10 and 8-14 day forecasts, emphasizing the likelihood of unusual cold over southwestern portions of the interior in particular:

And here are a few words from Rick Thoman (note this is personal opinion, not official NWS guidance):

"Models having been pointing this way for some days, and now we have increasing agreement on the potential for a period of prolonged deep cold Jan 15-21 or so. The basic forecast pattern has similarities to portions of the 1989 and 1999 cold events. The devil is in the details (of course), especially east of 150W, where there is potential for clouds and snow to wrap around from the GoA, and so keep surface temps higher (just as we saw in the 1999 event).

North of the Alaska Range there is a plausible potential for multiple days with 850mb temps lower than -35C and surface temps lower than -60F, which in the past has resulted in some of the commuter airlines (and Everett's) shutting down service to rural communities."

For reference, here's a chart of the mean 500mb height anomaly during January 20-30, 1989, when extraordinarily cold conditions affected the interior.  It's hard to imagine, but the temperature in Bettles dropped below -60°F on 9 out of the 11 days in this period, including -69°F on January 26.  Fairbanks reached "only" -51°F but had many days that stayed at or below -40°.

Saturday, January 7, 2017

Colder Again

The development of a strong high pressure system (1055mb+) across northern and interior Alaska sent temperatures into a dive in Fairbanks after snow ended on Thursday, with a drop from +22°F to -28°F in 24 hours.   Today the temperature struggled to get above -20°F at the airport.  It seems as if temperatures have been on quite a roller-coaster so far this winter, as seen in the chart below, but in fact the variability is not at all unusual.  Since November 1, the standard deviation of daily temperature anomalies has been 12.6°F, compared to the 1981-2010 normal of 13.3°F for the same period on the calendar.  Perhaps the volatility seems unusual to me because last winter was one of the least variable winters on record as far as temperature in Fairbanks is concerned.

It's interesting to look at how the winter-time temperature variance depends on the phases of ENSO, the PDO, and the NPM.  El Niño winters (like last winter) tend to bring reduced temperature variability to much of Alaska because of the persistent flow patterns associated with low pressure to the southwest of Alaska.  La Niña winters tend to be more variable as high-pressure blocking episodes develop and then dissipate in the vicinity of the Bering Sea.

The chart below shows a box-and-whisker representation of Fairbanks November-March temperature variability for different phases of ENSO.  For reference, last winter's standard deviation of daily temperature anomalies was only 10.8°F, the second lowest on record.

A parallel analysis for the PDO phase shows a similar relationship but with a more notable enhancement of variance on the "cold" side, i.e. negative PDO.

Finally, the NPM chart shows a surprisingly strong relationship, given that the NPM has little relationship with mean winter temperature in Fairbanks; negative NPM winters tend to have lower temperature variance.  Of course the fact that the NPM is uncorrelated with mean temperature does not at all mean that the circulation patterns are similar; the negative NPM flow pattern is more El Niño-like in the vicinity of Alaska, whereas the flow more resembles a La Niña pattern when the NPM is positive.

Given that this winter has so far been characterized by a modestly positive PDO and a strongly negative NPM, we would expect the temperature variance to be lower than normal in Fairbanks.  However, as we discussed with respect to the heavy December snowfall, this winter is not quite conforming to expectations.

Monday, January 2, 2017

Extremely Warm Day in Arctic Alaska

In the wake of the recent Bering Sea storm, a deep and very strong southerly flow has transported extremely warm air into northern Alaska, leading to exceptional warmth across the North Slope.  Remarkably, temperatures rose above freezing everywhere on the North Slope yesterday (or at least everywhere with a thermometer reporting in real-time), and today isn't much cooler.  The map below shows the maximum temperatures observed in the 24-hour period ending 5pm AKST yesterday.

Here is yesterday afternoon's 500mb analysis, showing the torrent of warm air flowing northward over the Bering Sea and far western Alaska.

Looking at the climate observing sites with long term histories, Utqiaġvik (Barrow) and Deadhorse both had a high temperature of 36°F yesterday, and Kotzebue hit 38°F for the midnight-to-midnight period.  This is the first time in the common period of record (December 1968-present) that all three stations have exceeded 35°F on the same day in winter; this has never before been observed between October 25 and April 13.  The chart below attempts to illustrate this in graphical form.

Saturday, December 31, 2016

December Snowfall

Here's a quick update on yesterday's post: snowfall for December is now up to 32.9" in Fairbanks, which is the 4th highest on record (1930-present).  Nearly half of that fell in the last three days.  The airport recorded wind gusts as high as 52 mph early this morning, and sustained winds of 33 mph were measured; this sustained wind speed is the highest since February 2011.  It's also the highest for early-mid winter (pre-February) since 1970.

This morning's sounding showed an extraordinarily strong westerly flow aloft, with 850mb winds of 75 knots (86 mph).  This ties the all-time record for strongest 850mb wind speed in Fairbanks (1948-present); this has been an extreme and historic winter storm.

Returning to the topic of the relationship between snowfall and North Pacific sea surface temperature patterns, the following chart illustrates the unusual nature of this month's heavy snowfall.  The horizontal axis shows the difference between the PDO and NPM index values (PDO minus NPM), and December's snowfall is shown on the vertical axis.  When the PDO is positive and the NPM is negative, as they are now, it is unusual to have significantly above-normal snowfall.  Note that I calculated the linear trend line after transforming the snowfall data into an approximately normal distribution, because ordinary least-squares regression is not suitable for a skewed distribution like we have here (it weights the outliers too heavily).

Friday, December 30, 2016

Major Storm

A sprawling storm system stretching from far eastern Russia to interior and northern Alaska brought heavy snow to the Fairbanks area last night and is now producing strong winds (gusting to 50mph at the airport in the past couple of hours).  Here are preliminary snowfall totals from the NWS as of this morning:

Public Information Statement
National Weather Service Fairbanks AK
822 AM AKST Fri Dec 30 2016

...Snowfall Reports...

Location                     Amount    Time/Date       Lat/Lon              

...Middle Tanana Valley...
3 S Fox                      9.4 in    0500 AM 12/30   64.92N/147.62W       
20 NW Fox                    9.0 in    0500 AM 12/30   65.18N/148.07W       
1 NW Fairbanks               8.0 in    1100 PM 12/29   64.85N/147.69W       
2 NW College                 8.0 in    1000 PM 12/29   64.89N/147.88W       
2 N College                  8.0 in    0900 PM 12/29   64.90N/147.82W       
3 SSW Fox                    8.0 in    1000 AM 12/29   64.92N/147.65W       
4 NNW Fairbanks              7.6 in    1000 AM 12/29   64.89N/147.68W       
3 N Fairbanks                5.9 in    0830 PM 12/29   64.88N/147.65W       
Fox                          5.8 in    0845 PM 12/29   64.96N/147.63W       
2 W Two Rivers               5.5 in    1000 PM 12/29   64.87N/147.17W       
Two Rivers                   5.5 in    1000 PM 12/29   64.87N/147.09W       
North Pole                   4.0 in    0800 PM 12/29   64.75N/147.36W       
4 SSW College                12.9 in   0400 AM 12/30   64.82N/147.88W       
3 SW College                 11.8 in   0530 AM 12/30   64.84N/147.90W       
3 SW College                 11.8 in   0200 AM 12/30   64.84N/147.90W       
2 NNW College                11.7 in   0617 AM 12/30   64.89N/147.85W       
12 SW Ester                  11.0 in   0300 AM 12/30   64.74N/148.32W       
3 N Fairbanks                10.4 in   0530 AM 12/30   64.88N/147.65W       
1 E College                  10.0 in   0123 AM 12/30   64.87N/147.79W       

...Lower Koyukuk and Middle Yukon Valleys...
Kaltag                       9.0 in    0600 PM 12/29   64.34N/158.69W       

...Central Interior...
Nenana                       10.0 in   0300 AM 12/30   64.54N/149.09W

The surface and 500mb charts show the powerful southwesterly flow that is typical of heavy snow events in Fairbanks.  Here are the maps from 3pm AKST yesterday (click to enlarge):

and from 3am today:

The intensity of the flow into the interior is very pronounced and explains both the abundant moisture (snowfall) and windy conditions.  This afternoon's sounding from Fairbanks airport measured a wind speed of 47mph at 925 mb (~2000 feet above ground), which is the strongest in more than 6 years during the winter season.  The sounding (see below) also shows a remarkably strong lapse rate in the lower troposphere, meaning that the temperature profile is highly favorable for vertical mixing and downward transport of momentum.  It's very rare to see this kind of temperature profile in mid-winter: only one December day in Fairbanks history had a more unstable sounding from the surface to 700mb; that day was December 12, 1972, when 13.9" of snow fell over a 3-day period.

December 2016 is now in 5th place for total snowfall (1930-present) and will probably reach 3rd place by tomorrow.