Friday, October 21, 2016

Snow Cover - Will It Last

A fair portion of the interior now has at least a little snow on the ground after yesterday's modest snowfall.  Half an inch of accumulation was reported at Fairbanks airport, but the snow depth was rounded up to 1".  It's little more than a dusting of snow, so what are the odds that it will last and prove to be the start of the winter snow pack?

An analysis from a couple of years ago (see here) indicates that we're now late enough in October that there is less than a 20% chance that a one-inch snow cover will melt out to a trace or less.  This suggests that this year is likely to be one of the 10% or so in which the first measurable snowfall doesn't melt out.  On the other hand, the forecast for next week looks warm, with major storminess in the Bering Sea importing warm air over interior Alaska.  About half of all years reach 40°F or warmer after October 25 in Fairbanks, so if it doesn't snow any more - which looks likely in the short-term - then I'd say there's a fair chance of seeing bare ground again.

The chart below shows the number of days with zero or trace snow cover occurring after the first measurable snowfall, versus the date of the first snowfall on the horizontal axis.  Obviously the earlier the first snow occurs, the more subsequent snow-free days there tend to be before winter sets in.  But it's interesting to see that very late first snows do tend to melt out - which makes sense if the weather pattern is unusually warm in those years.

The outlier on the lower left is from the infamous early winter onset of 1992 - the only time the winter snow pack was established in September, and mid-September at that.

Thursday, October 20, 2016

MOS Forecast Bias

As Fairbanks awaits the much-delayed first accumulating snow of the season today (according to the NWS), I wanted to look back at the forecast discrepancy I noted a couple of weeks ago to see how things turned out.  The chart below shows the automated computer forecasts of daily high temperatures from the GFS MOS product (black line) and the actual outcome in red.  Note that I've taken the forecasts for "day 3", for example a forecast for Thursday's high temperature produced on Monday evening.

The results are amazing: the MOS forecast was about 18-20°F too cold for more than a week.  Given the usually strong performance of MOS, the error is very surprising.

I've done some digging on potential causes for the problem, and there seems to be a partial answer in the way the MOS equations are developed.  First, recall that the MOS (Model Output Statistics) technique uses multiple regression to estimate relationships between observed weather variables and predictors in the "raw" model forecasts (e.g. 850mb temperature).  To better capture differences in the relationships between summer and winter, the equations are developed separately for April through September and for October through March (reference here); so the regression switches on October 1 and April 1.

What does this mean for Fairbanks?  Well, the vast majority of days between October 1 and March 31 have snow on the ground, and so the "winter" model will reflect the physics of a snow-covered landscape; for example, when high pressure develops with low humidity and clear skies, the model will predict a substantial drop in temperatures.  However, in the minority of cases when snow is absent, then the model forecast will be too cold.  In other words, the model is effectively assuming snow cover to be present, although it does not use snow cover as a predictor.

This information partially explains the huge cold bias in early October this year, because the weather pattern was just the kind of set-up that is normally cold in winter - high pressure, clear skies, and low humidity.  However, if the snow-cover explanation were completely adequate, then we would expect that similar errors would have occurred in other years with no snow cover in October.  The charts below show the same analysis for selected years in the past.

First, in 2015 the MOS forecasts were too warm (as expected) during the period of unusual snow cover in late September and early October, but there was no obvious bias later in October after the early snow melted off.

October of 2013 was very unusual with its lack of snow, and MOS showed a modest cold bias then, but nothing like what we've seen recently.

Finally, I looked at two earlier years in which strong high pressure occurred in combination with zero snow on the ground during October.  In 2003, high pressure aloft peaked at the beginning of October, and there was a slight cold bias in the forecasts - but nothing too alarming.

In 2009, a ridge of high pressure developed on about the 7th of October, and once again the MOS forecasts were too cold, but not drastically so.  Bear in mind that the GFS model and the MOS equations have changed over time, so the comparison to 2016 isn't quite fair, but if anything the newer forecasts should be better.

In conclusion, it's still not clear what caused the enormous errors in the MOS forecasts recently.  Part of the problem is that the winter MOS equations aren't suitable for predicting temperatures during snow-free conditions, but in previous years this hasn't caused a major issue.  The good news is the NWS forecasters ably detected the recent bias in the MOS output and adjusted their forecasts accordingly.  As a meteorologist myself, it's comforting to know there's still room to improve on what the computers provide.

Monday, October 17, 2016

Windy and Colder

Strong winds have been blowing in Fairbanks since yesterday morning, as high pressure to the north is creating an intense pressure gradient across the state.  The strong easterly flow has imported the season's first decently cold air mass, with 850mb temperatures dropping to -14°C above Fairbanks by yesterday afternoon.  Saturday morning's balloon sounding revealed the first entirely sub-freezing temperature profile of the season in Fairbanks, and this is much later than usual - the normal date for this event is September 28, and the record latest was in 1969 (October 20).  Fairbanks also recorded its first sub-freezing calendar day yesterday, with a high of 28° F - but no doubt it felt a good deal colder than that.

The graphics below show the sea-level pressure analysis (top) and 500mb height analysis (bottom) from 3pm AKST yesterday afternoon, courtesy of Environment Canada.  The development of an upper-level high pressure system over Chukotka is consistent with the high-latitude blocking patterns and negative Arctic Oscillation that have been observed so far this month.

Here's a surface weather display from about the same time.

Webcam images from Ester Dome yesterday showed an abundance of blowing dust in the Tanana River valley as a result of the strong winds.  The simple animation below shows the period when winds began to gust over 20 knots (23 mph) at Fairbanks airport; blowing dust was reported at the airport for several hours.

Sustained winds of 20 knots are quite unusual in Fairbanks at this time of year, as the following chart demonstrates.  In the nearly 20 years since the ASOS platform began reporting at Fairbanks airport, there have been only 2 other occasions with 20+ knot winds in the month of October, and September and November have seen zero and 1 such event respectively.  Winds like this are more commonly observed in late winter and spring.

In terms of wind direction, however, the current event (northeasterly winds) is much more typical in Fairbanks; the chart below shows that nearly all wind events of this magnitude are directed either from the northeast or from west to just south of west.  However, the easterly component is rarely observed in summer: since 1998, all of the June-September strong wind events have come from the western half of the compass (and one from the north).

Thursday, October 13, 2016

Freeze-Up Progress - Or Lack of It

The situation in Barrow deserves another quick mention today, because it's so extraordinary - the warmth is unprecedented for the time of year in the modern observational record.  There's no snow on the ground - not even a trace - and this is the latest date in the year when this has been observed (1920-present); and this isn't because it's been too dry, it's just been too warm.  Not a single day so far this autumn has seen a high temperature below freezing, and we're now 9 days past the previous record in this respect.  As the chart below shows, temperatures have not decreased at all in the last month - in fact the trend is slightly up.

Another perspective on the unprecedented warmth is to look at the accumulation of freezing degree days so far this season.  Barrow has managed a grand total of 4.0 FDDs so far this autumn, which is easily the lowest on record (2012 had 24.0 through October 12).  On the flip side, thawing degree days have accumulated at a record pace this month, with 24.5 TDDs for October 1-12.  The chart below shows the numbers for each year since 1930 (click to enlarge).

Here's the current webcam photo from Barrow - no sea ice and no snow.

Away from the warming influence of the Arctic (an odd concept, perhaps), freeze-up is beginning on interior rivers.  The webcam photos below show the situation on the Yukon River at Dawson YT and at Beaver.

For good measure, and for future reference, here are some shots from North Slope locations today.  Toolik Lake has frozen over in the past 48 hours.

Tuesday, October 11, 2016

Excessive Warmth in Barrow

It's that time of the year again when Barrow's temperatures reveal the impact of the loss of Arctic sea ice when compared to what used to be normal in the latter part of the 20th century.  Simply put, Octobers now are really warm relative to what they used to be, and there's little variance from year to year as the climate has become more or less maritime at this time of the year.  However, this month is standing out even relative to recent warm years, as Rick has noted in recent messages on Twitter:

To facilitate the comparison to how things used to be, I created climatological normals of daily high and low temperatures for 1961-1990 and 2002-2015; October of 2002 was the first in the modern run of very warm Octobers.  The chart below shows that the daily normals have risen for just about every day on the calendar, but the difference in October is most profound: from October 1 through October 29, the 2002-2015 normal low temperature is higher than the 1961-1990 normal high temperature.

Looking at the variance of daily temperatures, we find that the magnitude of daily variations has become much smaller from mid-September all the way through mid-winter, and this is especially true for daily maximum temperatures.  The peak percentage difference occurs on October 10, when the standard deviation of high temperatures has dropped by over 50% between the two periods shown here.  This is easy to understand physically: with sea ice now far to the north in October, it's just about impossible to get cold air into Barrow, so the lower part of the temperature distribution has been eliminated.

Here are some webcam photos from northern Alaska this afternoon, showing the relatively balmy scene.

Toolik Lake is still mostly open:

There's no snow on the ground at Kavik:

Teshekpuk Lake remains unfrozen, with bare tundra on the shore:

And there's just a little ice on one of the small lakes adjacent to Teshekpuk Lake:

Evidently the Colville River remains largely unfrozen at Umiat as well:

Saturday, October 8, 2016

Peak Climatological Cooling

First, an update on temperatures: the first 0°F observations this season occurred today at the usual cold spot of Chicken and also at the Chalkyitsik RAWS.  Remarkably, the latter site had measured +1°F each of the 3 previous nights.  Circle Hot Springs has been to 3°F and the Salcha RAWS made it to 4°F today.  There was a bit of ice running on the Yukon River at Dawson, Yukon Territory, today, where the temperature has been struggling to get above freezing in recent days, even under clear skies.

We're approaching the date on the calendar when climatological normal temperatures drop most rapidly in interior Alaska; this occurs around October 24 in Fairbanks, at which point daily normal mean temperatures are dropping more than 6°F per week.  Based on published normals, the station with the most rapid drop is Woodsmoke (7°F per week), as noted in this post from 3 years ago:

The very rapid cooling at this time of year means it is quite likely that any given week will be cooler than the previous week in Fairbanks.  But how likely is quite likely?  I got to wondering about this, because of course any given day is only slightly more likely to be cooler than the last, but a full month is certain to be cooler than the last at this time of year.  How do these probabilities vary with calendar date and with the length of the averaging period?  The chart below answers this question, which I've been curious about for some time.

It's interesting to see that the rate of cooling in mid-October is so great that any given day is more than 60% likely to be cooler than the previous day, and any given week is 80% likely to be cooler than the last.  It's still possible, though unusual, for consecutive 2-week periods to defy the seasonal trend, but by the time we get to 20-day periods this is virtually impossible - so for example, the last 20 days of October are nearly certain to be cooler than the 20 days prior.

Looking at calendar months, it's interesting to note that August has never been warmer than July in Fairbanks (although it's bound to happen one of these years), September has never been warmer than August (although remarkably in 1969 it came close), and of course October and November have never been warmer than the preceding months respectively.  On the other side of the year, April, May, and June have always been warmer than the calendar months immediately prior.

An unrelated but surprising detail on the chart is that the series for consecutive single days (black line) doesn't drop below zero until early August: so even in late July, each day is more than 50% likely to be warmer than the previous day.  This is inconsistent with the 1981-2010 normals, which show the climatological peak of daily mean temperature on July 3 in Fairbanks.  More surprising still is that the other series don't show the same thing, so for example 10 day periods ending in late July are considerably more likely to be cooler than the one before.  I suspect the anomaly for single days has to do with some skewness in the temperature distribution, but a bit more investigation would be needed to clarify this.

Tuesday, October 4, 2016

Gradually Cooling Off

The coolest temperatures of the season so far were observed in the interior last night, with several locations dipping into the single digits - as befits the time of year.  The Chalkyitsik RAWS dropped to +4°F, which is the lowest temperature observed so far in Alaska this season as far as I'm aware.  Other lows included:

7°F   Beaver Creek RAWS
8°F   Circle Hot Springs COOP
9°F   Eagle airport and RAWS
9°F   Chicken COOP

At Fairbanks airport the first indisputably hard freeze occurred the night before, with a low of 22°F.  Yesterday was also the first day with a daily mean temperature of 32°F.  If these conditions were to continue, it wouldn't be long before some ice would be running in the rivers, but the forecast shows a warming trend.

[Update October 5: this morning was colder, with a low of +1°F at the Chalkyitsik RAWS and +4°F at Chicken. Closer to Fairbanks, the Goldstream Creek COOP had 11°F, and it was 9°F at the Salcha RAWS.]

Speaking of the forecast, I couldn't help noticing a major discrepancy today between the GFS MOS (automated computer) forecast and the rest of the guidance for Fairbanks.  Here's this morning's MOS bulletin, with the "N/X" line showing low temperatures well down in the teens from Thursday through Monday, and with high temperatures staying near or below freezing for several days despite clear skies.

A quick look at this evening's NWS forecast for the airport shows a dramatically different picture, with high temperatures above 40°F and lows not far below freezing.

The GFS model forecast itself (from which the MOS output is derived) shows increasing warmth aloft in the next week, with 850mb temperatures rising well above freezing by Friday, and with very low relative humidity suggesting clear skies.

The raw model 2m temperature forecast shows conditions similar to the NWS forecast and drastically different from MOS.

So what's going on with MOS?  Frankly I have no idea, but it's clearly wrong - and this is interesting, because MOS usually serves as a baseline or "first guess" for forecast systems; it is generally much better than raw model output, because it's a statistical regression based on historical observations.  I briefly wondered if perhaps MOS is assuming there is snow on the ground already, but this is most unlikely as the historical probability of having snowcover in Fairbanks at this date is less than 20%.

To show how unlikely it is that Fairbanks will see a daily high temperature below freezing any time soon, we can look at how cold the upper-air temperatures need to be to produce a sub-freezing day.  The chart below shows the afternoon 850mb temperatures on all such days from September 20 through November 19, beginning in 1957; we can see that 850mb temperatures are always well below freezing when daytime conditions remain below freezing in September or early October.  As the autumn advances and the sun sinks, nights lengthen, and snow cover becomes more likely, it becomes possible to have sub-freezing days when 850mb temperatures are well above freezing, but early October is too early for that to happen.