Archive for January, 2012

What causes freezing rain?
January 22, 2012

By Meteorologist Mike Skurko

Freezing rain is widely regarded as the most dangerous winter weather hazard.  While a light round of freezing rain does not get as much fanfare as a major snowstorm, it creates a glaze of ice over the roads and sidewalks that often go overlooked…and can cause just as many, if not more, accidents than an unplowed couple inches of snow.

Freezing rain is caused by an above-freezing layer within the lowest layers of the atmosphere, along with surface temperatures that are below freezing.  As we know in western Massachusetts, a very chilly rain can occur when the temperatures are in the mid-30s.  However, if the temperatures at the surface (i.e. roadways and sidewalks) are still freezing, these raindrops will freeze after hitting the ground.

This is different from sleet, which is when the raindrops re-freeze before hitting the ground…creating those little round ice pellets that “ping” off of your car.  This is also different from snow, which stays frozen or partially-frozen as it hits the ground as well.  In the case of freezing rain, you have a situation where pure liquid water reaches and settles on the ground.  This liquid then freezes into ice as it sits atop roadways, power lines, and other surfaces.

It is the same idea as a Zamboni resurfacing an ice rink.  The Zamboni puts down a layer of liquid water that is above freezing.  However, as it sits on top of the frozen surface, that liquid freezes into a new layer of ice.

To forecast freezing rain, the simplest things meteorologists look at are the temperatures for the surface (the typical numbers reported for highs/lows or for an hourly forecast) as well as temperatures in the lower-levels of the atmosphere (numbers you don’t see on TV).

Let’s take a look at the forecast for Monday morning, using a computer model run for Pittsfield as an example.  This model is showing a surface temperature of 30 degrees at 10 AM.  However, the temperature at about 5000 feet is projected to be 2.6 degrees Celsius (about 37 degrees Farenheit).  This would be one clue to help suggest that temperatures will be above freezing in the atmosphere, which would support raindrops, but will be falling onto roads in Pittsfield that are still below freezing.

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Snowfall Totals: Jan. 21
January 21, 2012

By Meteorologist Mike Skurko

Observations have been pretty scarce across western Massachusetts from our snow this morning…but here are the final totals we do have so far:

Agawam   2.8 inches

East Longmeadow   2.8 inches

Southwick   2.8 inches

Pittsfield   2.5 inches

Chicopee   2.3 inches

A few other reports came in during the middle of this winter weather.  Listed below are those snowfall accumulations and the time the report was made.  The total in parenthesis represents an estimated final snowfall total based on extending the data a few hours until the snow exited on radar.  It’s not an exact process, but filling in the gaps this way seems to yield results that correlate well with the final reports we do have.

Using Westfield as an example, the report around 11:40 am would have been at about 80% of the way through the length of the storm…based on its arrival and departure times gathered through observations from Barnes Municipal Airport and on radar.  In addition, automated reports from Barnes produced .05 inches of liquid water precipitation through noon, which coincidentally is also 80% of the total .06 inches of water reported by the end of the storm.  If 2.1 inches of snow represents 80% of the final total, then the final total would have been 2.6 inches.  Using the totals above, this fits in nicely with the 2.8 inches reported in Southwick and Agawam.

Westfield   2.1 inches, 11:40 am (estimated 2.6 inches total)

West Springfield   2.0 inches, 11:54 am (estimated 2.6 inches total)

Blandford   2.0 inches, 12:13 pm (estimated 2.5 inches total)

South Hadley 2.0 inches, 10:13 am (estimated  2.8 inches)

Ware 1.0 inches, 9:14 am (estimated  1.8 inches)

Radiational cooling
January 15, 2012

By Meteorologist Mike Skurko

What causes temperatures to plummet tonight unlike an average January night?  A meteorological process known as radiational cooling is typically responsible for these sub-zero nights across western Massachusetts (or any other part of the country).

During the day, shortwave radiation from the sun warms the earth.  During the night, however, this energy is returned back.  This “heat” is emitted from the earth and back into outer space in the form of longwave radiation.  It is a part of earth’s natural energy balance.  Just like you absorb and burn calories on a daily basis, the earth will absorb and lose energy on a daily basis.

Clear and calm nights provide the optimum conditions for this longwave radiation to be lost very rapidly, and thus cool the earth very quickly.  Clear nights provide an uninterrupted path for this longwave energy to return to outer space, whereas clouds would bounce it back towards the earth and keep the warmth trapped in (this is when we talk about the clouds acting like a blanket over us and keeping us warm).  Calm winds provide less mixing of the air, also allowing this radiation to freely return on a direct path to space.

The more this longwave energy can escape the earth, the faster our temperatures will fall.  This process does not happen only in the winter.  It can also provide us with those refreshingly cool nights in the summer.

Know your watches & warnings
January 13, 2012

By Meteorologist Mike Skurko

With snow continuing across the Berkshires earlier this evening, the National Weather Service made the upgrade from Winter Weather Advisory to Winter Storm Warning.  All of these watches and warnings that western Massachusetts may see during the winter have specific criteria.

For example, if an average of 6+ inches of snow is expected across a large area (such as half a county) in a 12-hour period…that falls under the category of Winter Storm Warning.  With more and more spots across the northern Berkshires now on their way to that mark, the upgrade was made because initial forecasts didn’t expect that much “warning criteria” snow.

To serve as a guide throughout the winter…here is a list of the common winter-related watch/warning criteria from the National Weather Service in Boston:

WINTER STORM WARNING: When the following is expected within the next 36 hours…snow exceeding 6 inches averaged over a forecast zone in a 12-hour period, or snow exceeding 8 inches averaged over a forecast zone in 24-hour period.

WINTER STORM WATCH: When conditions meeting a winter storm warning are expected within the next 36 to 48 hours

WINTER WEATHER ADVISORY: When any one of the following four are expected within the next 24 hours…(1) 3 inches of snow averaged over a forecast zone in a 12-hour period, (2) wind gusts of 25 to 34 mph accompanied by falling snow occasionally reducing visibility to less than a quarter-mile for more than three hours, (3) widespread blowing snow reducing visibility to less than a quarter-mile, (4) sufficient ice is expected to develop on road surfaces…issued at forecaster’s discretion

BLIZZARD WARNING: Wind gusts above 35 mph and considerable falling snow reducing the visibility to less than a quarter-mile for the majority of  a 3 hour period

ICE STORM WARNING: 1/2 inch of ice accretion due to freezing rain

WIND CHILL ADVISORY: Wind chill index expected to be between -15 and -24 degrees for at least 3 hours

WIND CHILL WARNING: Wind chill index expected to be less than -24 degrees for at least 3 hours

2012 Winter Prediction
January 1, 2012

By Meteorologist Mike Skurko

Happy New Year!  As we roll the calendar over to 2012, I thought it would be worthwhile to take a look at what may be in store for western Massachusetts as we get ready for the heart of winter.  If you recall, last January was incredible…a record 54.3 inches of snow fell, including that unforgettable January 12th when we got about 2 feet of snow in one day.  It would be very hard to have back-to-back record-breaking Januarys, so that should give us at least some sigh of relief in western Massachusetts.

Long-range weather prediction is not easy.  I will be the first to admit that the forecast can change just 5 days out, so a spot-on forecast for a few weeks or months out would be impossible.  However, climate research has equated certain global weather phenomenon to long-term forecasts for various regions, which we will explore here.  The two biggest predictors for a New England winter deal with the highly publicized El Nino/La Nina and the lesser-known Arctic Oscillation.

La Nina

We are currently under the influence of La Nina…a cooling of ocean water in the equatorial Pacific.  La Nina has been linked to above-average precipitation across northern tier states (a very strong La Nina was in place during our hectic winter last year).  Temperatures are generally warmer than normal across a larger portion of the country, particularly the south, during La Nina as well.

The strength of this current La Nina is not as great as last year’s, and in fact it is expected to reach its peak intensity around January before further weakening by early spring.  Still, history tells us that we should see slightly above average precipitation here in New England with a La Nina.

Arctic Oscillation

It’s hard to believe that the early part of 2011 was so unusually cold, even though I just stated above that La Nina tends to bring warmer conditions for a large part of the U.S.  For the Northeast, our temperature trends for an entire winter are also governed by the Arctic Oscillation.  This refers to those very cold outbreaks that we see come down from Canada…like when television reports use the term “Arctic blast”.  The magnitude of the Arctic Oscillation determines how often those bitter-cold outbreaks occur, and how strong they are.

The last two winters, the Arctic Oscillation was in a strongly negative phase…which meant some brutally cold outbreaks for us.  To put an example in numerical terms, the -4.3 value for the month of February 2010 was the most negative phase value since the 1950s…the same month of the “Snowmaggedon” storm in the nation’s capital.  The point is that strongly negative phases can provide plenty of cold air for widespread snowstorms…rather than mixed rain, snow, and ice storms

Right now the Arctic Oscillation has been mostly positive for the last few months.  A lack of colder air would suggest more mixed rain/snow/ice storms than simply snow.  So while La Nina would suggest more precipitation across our region, many forecasts all calling for more rain and ice in the mix than last year due to a less-negative mean Arctic Oscillation through the course of winter.  This would limit any extreme cumulative snowfall totals for the months ahead.

Forecast:

To sum it all up, Bradley Int’l sees an average snowfall of 49.0 inches each season.  I would expect somewhere in the neighborhood of 55 inches for the entire season…which means I predict 42.7 inches of snow from January through the end of winter (6 inches above average for that time frame) with one or two fewer major snowstorms and a few more rounds of rain and ice in the mix with our winter weather in the beginning of 2012.