Storm Follow-Up

As if to add insult to injury, freezing drizzle is being reported around the Fairbanks area today, following a modest snowfall earlier in the day.  A vertical profile from this morning's GFS model run, valid at 9am, shows a cloud-top temperature of about -14°C, which is a bit colder than you would expect for freezing drizzle, but presumably in reality the cloud top is a bit lower and warmer than that.  See here for an old post on freezing drizzle.

A big portion of the interior is under a Winter Storm Warning again, and the latest available GEFS forecast shows the potential for an inch or more of additional liquid-equivalent precipitation in total with this latest event.

Then it will get cold.  The lowest 850mb temperature in Fairbanks back in November was -28°C, and it appears that this weekend may rival that.  There's certainly the potential for -40° again at valley level.

Meanwhile, extremely warm - and in some places absurdly warm - conditions are persisting in southwestern Alaska, as the powerful North Pacific ridge continues to pump warm air up from the south.  This is the 12th consecutive day that Bethel has seen 32°F or higher, which seems like a prolonged warm spell, but the record is 26 consecutive days in - no surprise - January 1937.

The turn-around from the November cold is really striking:

I can't fail to mention also the ridiculous warm record that was broken in Kodiak on Sunday: 65°F at the airport (the official climate site) and 67°F at the harbor tide gauge.  This is a new record for the entire state of Alaska in the month of December.  Astonishingly, a temperature of 65°F or higher has not previously been seen in Kodiak between early October and late April, i.e. more than 6 months of the year.  On that basis, it would be the equivalent of Fairbanks reaching nearly 70°F at this time of year. (Note that there is Kodiak data from January 1937, but the best Kodiak could do then was 50°F).  

What could explain such a dramatic shift from the cold of November to the extreme warmth this month?  Obviously it's tied to the position of the North Pacific ridge, but why did it shift east?

To illustrate the shift, here's a time-longitude diagram showing the recent and forecast evolution of 500mb heights in the 45-75°N latitude band.  During November we can see yellow/orange colors (ridging) to the west of Alaska and a preponderance of blue (troughing) at the longitude of Alaska, and this was the pattern that produced the cold.  However, beginning about 10 days ago, a very powerful ridge emerged to the east of the Date Line, and it has been holding steady with remarkable persistence (and may generally continue to do so, if this forecast is correct).

Now here's what I think forced the transition to a more easterly ridge: an eastward shift in tropical convection (thunderstorm activity) in early December.  The graphic below shows areas of enhanced convection along the equator in blue and suppressed convection in yellow, and in the first half of November we see that convection was focused from the eastern Indian Ocean to Indonesia.  However, at the beginning of December the convection migrated into the western Pacific Ocean, and this represents a breakdown in the typical La Niña pattern over the equatorial ocean.  As I discussed in this post, the typical cold of a La Niña winter in Alaska is linked to the position of the convective activity, and so it makes sense that the North Pacific pattern would shift in the weeks following the tropical change.  This also means that if the La Niña forcing doesn't re-emerge, then sustained cold may be less likely in the rest of winter.

For the technically inclined, the eastward shift in tropical convection was brought about by a strong eastward-propagating pulse of the Madden-Julian Oscillation (MJO); see here for an introduction to the MJO.  The phase diagram below demonstrates the change of phase from November (green) to December (blue), with the key transition occurring around December 1, as noted above.  If the MJO activity eventually makes it back to the lower part of the diagram, then it may again reinforce La Niña-like patterns over the North Pacific, but this appears unlikely to happen before late January.

Deepening Cold

Temperatures below -30°F have been observed in a number of interior locations in the past week, and it seems it won't be long before -40° cold arrives in some areas.  [Update Sunday: the Norutak Lake RAWS recorded -40° this morning.]  The latest GFS MOS forecast shows -45°F for Arctic Village on Tuesday and -26°F in Fairbanks.  Here's a chart of the lowest temperatures observed each day across Alaska in the past couple of months.  The Chalkyitsik RAWS site was the location of the coldest temperature report on 17 of the 57 days shown here, although some of these were ties.  The next most frequent cold spot (7 days) has been Chicken.

In a change of scene for Fairbanks, below-normal daily mean temperatures occurred for 7 consecutive days ending Thursday, the first such period since mid-July.  It's gratifying to see that my earlier speculation of a turn to colder conditions around November 19-21 worked out well - as we all know, even a blind squirrel finds a nut occasionally.

Looking again at the MJO phase, it's interesting to note that colder conditions tend to occur about 10 days after phase 7 even when the PDO phase is positive (as it is now) - see the graphic below.  (But note that the sample size is quite small for this analysis.)  Unusual warmth is much more likely to develop soon after an active MJO wave progresses through phases 1-3, which is where the MJO has been in the past week (second image below).  We might therefore expect warmth to return to Fairbanks rather soon, although the MJO anomaly is now fading and predictability may be lessened as a result.  In any case, the tropical MJO wave is only one of many influences on sub-seasonal climate variability in Alaska, so we wouldn't want to rely too heavily on it.

When Will It Get Cold?

Temperatures in Fairbanks have returned to the "significantly above normal" category in the past couple of weeks, with a mean temperature so far this month of more than 5°F above normal.  Only 3 days this season have seen sub-zero temperatures at the airport, which is right on track with the past 3 years but well behind the usual pace.  Snowfall has been extremely meager, only 2.9" in total so far, and the second lowest on record for this point in the season; but at least now the ground is snow-covered and the scene looks somewhat wintry.

The latest long-range forecasts show no sign that significantly colder weather will develop any time soon for interior Alaska.  For example, here are the 8-14 day and 3-4 week forecasts from the Climate Prediction Center.

The raw computer model forecasts are similar, with nothing but warmth expected right through the next couple of months.  However, a look at the current behavior of the Madden-Julian Oscillation (MJO) provides a hint that cooler weather might be on the horizon for later this month.

For those who are unfamiliar, the MJO is a cycle of enhanced and suppressed rainfall that propagates eastward in the tropics, typically completing a circuit around the globe in about 30-60 days.  The latest MJO phase diagram, shown below, indicates that the MJO has moved into a period of strong activity (blue line moving farther away from the origin; the small "10" denotes the latest observation from yesterday, November 10), with the enhanced rainfall now moving across the Pacific.

The MJO is known to have significant impacts on weather patterns in extra-tropical regions, and so a period of strong and predictable MJO activity can provide useful long-range forecast information.  I looked at the impacts on Fairbanks conditions at this time of year by finding all days with significant MJO activity during November in each phase (1-8) and then looking up the subsequent temperature in Fairbanks out to 60 days in the future.  The figure below shows the results.

Notice the pattern when the MJO is in phase 7, as it currently is.  At about 8-15 days after the occurrence of phase 7, colder than normal temperatures are favored in Fairbanks, but warmer than normal conditions tend to return about 25-35 days later.  If the historical pattern holds true this year, then Fairbanks has a fair chance of turning significantly colder about 8-10 days from now.  Let's see if it works out.