Saturday, October 3, 2015

Wet September

The total liquid-equivalent precipitation in September in Fairbanks was 3.74", which is the greatest amount on record since 1925, when 5.61" fell in September.  Normally I only look at historical station data since 1930, the beginning of the Weather Bureau era (when data quality became much better), but in this case the 1925 record seems indisputable and is worthy of mention.

Below is a copy of the September 1925 observations from the University Experiment Station in Fairbanks.  A remarkable 4.16" of rain fell in one week in mid-September, including more than 1" on two separate days.  Since 1930, the highest weekly precipitation amount for dates entirely within September is 2.24", which occurred just a few days ago and also last year from September 1-7.  So the event in 1925 was quite an extreme outlier.

In attempting to confirm the veracity of the 1925 data, I looked at the 20th Century Reanalysis, which is a model estimate of historical atmospheric conditions based solely on observed sea-level pressure and sea surface temperature patterns.  The reanalysis model "predicts" historical precipitation, so if the reanalysis shows a significant precipitation signal on the same dates as the claimed Fairbanks rainfall, this provides good evidence that the event really happened.

The maps below show the daily mean precipitation rate estimated by the model from September 12-17, in units of mm/s, so a value of 0.0002 (the upper end of the color scale) is equivalent to 0.68" per day.  The reanalysis shows a rather pronounced precipitation signal over the interior on the 13th-14th and again on the 16th, which lines up nicely with the Fairbanks observational record.  I'm actually impressed that the historical reanalysis, based only on sparse pressure and sea surface temperature data, is able to reproduce the event; the 20th Century Reanalysis is a really nice scientific accomplishment.

As an aside, while looking at record wet months in the history of Fairbanks, I noticed that 5 of the 12 calendar months have set precipitation records (based on the 1930-present period) since 2002: these are listed below.  The probability of this happening by random chance is only 3.3%, which indicates that the change to wetter conditions (by this metric) is statistically significant.

April 2002 (3.06")
May 2004 (1.96")
June 2014 (3.56")
July 2003 (5.96")  Second place is July 2014  5.78"
September 2015 (3.74")

Three of the other months (January, August, and October) set their records in the 1930s, which was a volatile time for Alaska climate, as we've noted before.

Friday, October 2, 2015

Snow and Upper-Level Wind

I mentioned the other day that it was unusual to see southerly flow aloft during a heavy snow event in Fairbanks-land, because southerly flow is normally associated with downsloping in the interior, and this precludes sustained, significant precipitation; chinook winds are dry.  However, chinook winds also involve deep southerly flow from near or at the surface to levels far aloft.  In the recent storm, significant southerly flow was confined to levels above 700 mb, as shown by the wind vectors in the following Fairbanks soundings from Tuesday morning and Tuesday afternoon.

In the recent event, despite southerly flow in the middle troposphere, widespread deep ascent was generated by intense synoptic-scale forcing across the eastern interior.  The series of maps at the end of this post shows the evolution of the flow pattern at several levels and documents how the storm system evolved.

Looking at historical data from Fairbanks, it's clear that heavy snow events are much more commonly associated with upper-level winds from directions between southwest and northwest.  The chart below shows the wind vectors at 3pm AKST for each day since 1957 on which at least 6 inches of snow was observed.  Note that the markers show the direction the wind is coming from, so the wind vector points from the marker to the chart's origin.  Tuesday's event is represented by the only marker on the vertical axis (500mb wind from due south). Not only are winds more typically westerly during heavy snow events, wind speeds tend to be higher than they were on Tuesday, often 50 knots or higher at 500mb.

The next two charts show corresponding results for 700mb and 850mb winds.  There's a nice clustering of wind vectors from a direction just south of west, with quite substantial velocity.  On Tuesday afternoon the 700mb wind was almost calm (3 knots from 130°) and the 850mb wind was a light 7 knots from the northwest.

The 500mb chart above showed two historical snow events with an easterly component to the flow; the reanalysis 500mb maps for these events are shown below, along with the recent event (third map).  The October 1963 and November 1996 events look rather similar to the most recent one, with a strong trough over western Alaska.

Below are the Fairbanks wind scatterplots for less substantial snow events with 3-5" in a calendar day.  The main difference seems to be that wind speeds aloft are usually less when snow amounts are smaller.

To document the recent event in more detail, here is a series of maps showing the pressure and wind patterns at several levels.  First, the MSLP evolution (times in AKST):

3pm Monday

9pm Monday

3am Tuesday

9am Tuesday

3pm Tuesday

9pm Tuesday

3am Wednesday

Next, the 500mb analyses: what a potent trough!

3am Tuesday

3pm Tuesday

3am Wednesday

Here are the 250mb analyses:

3am Tuesday

3pm Tuesday

3am Wednesday

And the 700mb maps; the stippling shows relative humidity - note the trough and high-humidity frontal zone over eastern Alaska on Tuesday afternoon.

3am Tuesday

3pm Tuesday

3am Wednesday

Finally, the Wednesday afternoon 850mb analysis shows the extremely strong temperature gradient and frontal zone over the far eastern interior.  The dashed contours are at 5°C intervals and the 850mb temperature goes from +14°C in western Yukon to -6°C at Fairbanks.  The deep ascent that produced the snowfall was directly associated with the broad (and vertically tilted) zone of temperature contrast.

Wednesday, September 30, 2015

Storm Totals

From the NWS public information statement... storm totals:

COLLEGE HILLS.............................16.5 INCHES
SOUTH FOX.................................15.5 INCHES
UPPER MCGRATH ROAD........................14.5 INCHES
MIDTOWN FAIRBANKS.........................13.6 INCHES
UAF.......................................13.0 INCHES
KEYSTONE RIDGE............................13.0 INCHES
EAST FARMERS LOOP.........................12.9 INCHES
UNIVERSITY WEST...........................11.1 INCHES
NORTH POLE................................10.0 INCHES


TRIMS CAMP................................12.0 INCHES
NENANA.....................................8.5 INCHES
EAGLE......................................6.0 INCHES

Here's the scene at Nenana this morning... plenty of snow, but no river ice - an uncommon situation.

Historic Snow

More to come on this event, but yesterday's official snowfall of 11.2" in Fairbanks is the greatest calendar day snowfall on record for September (1930-present); the previous record was 7.8" on September 13, 1992.  The storm total of 11.9" (through midnight last night) is the second greatest, as the mid-September event of 1992 brought 17.4" over 5 days.

The past week has brought a total of 18.6" of snow in Fairbanks (through midnight).  It's remarkable to consider that only 15 winters (1930-31 through 2014-15) have seen more snow in a single week - see the chart below.  If 1.5 more inches fall before this event is over, which seems possible, then only 11 winters will have seen more snow in a week.  Only 1 winter in the last 15 brought such an onslaught of snow in one week (2010-11).

Some more factoids: a snow depth of 11" (last night's reading) isn't usually reached in Fairbanks until December 8.  In nearly a quarter of years it isn't reached until after the New Year (for example, the winters of 2000-01 through 2002-03, and 2005-06 through 2007-08), and in 1940-41 and 1952-53 the entire winter had less snow on the ground.

As my post the other day illustrated, this depth of snow has never melted off completely before spring.  However, the forecast looks very warm, and so it seems the odds still favor the reappearance of bare ground before the permanent snow cover is established in Fairbanks.

Here's a loop of infrared satellite images showing the evolution of the deep cloud cover during the period of heaviest snowfall in Fairbanks yesterday.  Blue colors show cold, high cloud tops, and orange indicates a warm ocean or land surface; note the warmth in the Yukon Territory (it was 66°F in Carmacks).

Monday, September 28, 2015

Winter Storm Warning

The highly unusual September weather continues in Fairbanks, with a Winter Storm Warning now in effect for another round of snow, and potentially even heavier than Friday's dump.  The latest forecast calls for 6-12", which is really amazing for the time of year.

It probably goes without saying that the upper-level flow pattern associated with this storm is very unusual, but it's worth pointing out one aspect that seems particularly strange: heavy snow is expected in Fairbanks with a strong southerly flow aloft.  Assuming the snowfall pans out as expected, this is indeed rare.  Looking at historical data from 1948, I found 58 distinct heavy snowfall events that brought 8" or more of snowfall in association with 500 mb wind speeds of at least 30 knots on any of the surrounding days.  Only 3 of these events saw 30+ knot winds from a direction between 90° and 200° (i.e. easterly to just west of southerly): November 19-20, 1964, December 18-19, 1968, and October 16-17, 1974.  Southerly winds aloft typically bring strong down-sloping to the central interior of Alaska, which is very unfavorable for precipitation.  It will certainly be interesting to see if the computer models are correct in suggesting that it's different this time.

The series of maps below shows the predicted 500 mb height and wind fields at 12-hour increments beginning at 3pm today and ending at 3am on Wednesday.  The potent trough is already swinging down across western Alaska and will cut off south of the Kenai Peninsula, then pull northwards.

The 850 mb forecast shows colder air working its way in aloft tonight and becoming easily cold enough for snow in Fairbanks.  A striking frontal zone sets up across the eastern interior by tomorrow afternoon.

The sequence of 12-hourly precipitation accumulation maps clearly shows the zone of substantial snowfall associated with the frontal zone stretching northeast from the Kenai Peninsula to northeast Alaska.  It will certainly be interesting to see how this one plays out.

Update September 29: here's the 3am AKST 250 mb analysis from today.  The densely stippled areas show the highest jet speeds.  The right-entrance region and the left-exit region of the jet maxima are both favorable for deep tropospheric ascent; this zone extends from about Valdez to Fairbanks in this analysis.