Archive for May, 2013

Frost can occur above 32 degrees
May 26, 2013

By Meteorologist Mike Skurko

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Berkshire and Franklin Counties are under a Frost Advisory for Sunday night, and the National Weather Service has also acknowledged that other low-lying areas in the Pioneer Valley could also see some frost as well.  However, overnight low temperatures are not even expected to drop below 32 degrees for most, if not all locations in western Massachusetts.  How can this be?  How can the temperature stay above freezing, but areas of frost still occur?

All official temperatures are observed about 6 feet off the ground.  Accordingly, most forecast models around the world make predictions for temperatures 2 meters off the ground, not directly on the surface. That means the forecast may call for a low of 35 degrees, the official measured temperature will actually be 35 degrees, but the grass on your lawn is at 30 degrees … cold enough for frost to form, even though the “official” temperature was above freezing.

The reason for this is because cold air is more dense than warmer air.  On clear, calm nights, the ground quickly loses the heat it absorbed during the day … a process known as radiational cooling.  Since cold air is denser than warm air, the coldest air will settle right along the surface overnight.  Therefore, your lawn can cool off more than the air just a few feet above, as that coldest pocket of air remains at the lowest point to the ground.

In fact, the official definition of a Frost Advisory states that it is issued “under clear, light wind conditions with forecast minimum shelter temperature 33-36ºF during growing season.”  Just because the overnight low only drops to the mid and upper-30s (such as the forecast tonight), frost can still occur if the conditions are right.

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National Safe Boating Week
May 25, 2013

By Meteorologist Mike Skurko

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With Memorial Day upon us, many New Englanders will be getting ready for another boating season.  In preparation for this, the National Weather Service dedicated this past week as National Safe Boating Week.  Below are the daily links the NWS issued, offering reminders and statistics to review before you head out onto the water.

Life Jackets

Vessel Safety Inspection

EPIRBs (Emergency Position Indicating Radio Beacons)

Marine Forecast

Hypothermia

Thunderstorm Safety

Boating Under the Influence

NOAA predicts active 2013 hurricane season
May 24, 2013

By Meteorologist Mike Skurko

NOAA expects an active 2013 Atlantic hurricane season, with the following number of named storms, hurricanes, and major hurricanes to develop.  The Atlantic hurricane season runs from June 1 to November 30, and typically peaks from mid-August to mid-September.

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NOAA cited three main factors in their above-average forecast for this season: conductive wind patterns from Africa, warmer Atlantic waters, and lack of El Nino.  Here is a brief breakdown of why each of those factors would contribute to the development of more hurricanes.

Hurricanes begin as weak storm systems/disturbances that move off the African continent and enter the warm tropical Atlantic waters.  NOAA expects a strong west African monsoon to be in place, which will throw an abundance of these disturbances into the Atlantic … thus creating more opportunities for these disturbances to grow into tropical systems.  Favorable easterly winds must also be in place to properly carry these disturbances off of the African continent and into the Atlantic.  The photo below shows an infrared satellite image of various little storm systems moving off the African coast.

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Second, something that most people are familiar with, hurricanes need warm waters to grow and sustain themselves.  NOAA forecasters expect sea-surface temperatures to be warmer than normal, which would also be favorable for an active hurricane season.  Waters above 80 degrees Fahrenheit are best for hurricane development, which you can see on the graphic below means that hurricanes do not frequently make it northward into New England (despite Irene and Sandy over the last two years).
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Finally, NOAA also does not expect El Nino to develop this summer, which typically suppresses hurricane development due to the stronger wind shear it has been linked to in the Atlantic. Wind shear is the differences in wind speed and direction with height. Strong wind shear can effectively tear apart the top of developing storms before they can mature into tropical systems. A lack of El Nino, and a lack of wind shear, is much more favorable for hurricane development.

Probability of Precipitation
May 19, 2013

By Meteorologist Mike Skurko

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This week will be plagued by scattered showers popping up over the next several afternoons.  A quick check of some of the other guys forecasts for our region … the National Weather Service, local TV stations, a few mobile apps … and some of them have a “probability of precipitation” listed of about 40% throughout the next few days.

You are most likely familiar with the concept of a percentage being attached to a forecast with rain.  In fact, on the golf course today I overheard someone say “there’s only a 30% chance of rain today.”  But do you know exactly what this means?  What does the 30% refer to?  Does it mean it will rain 30% of the day? Does it mean 30% of western Massachusetts gets rained on?  Does it mean we’re only 30% sure it will rain?

The definition of Probability of Precipitation (PoP) is the percentage of time that precipitation will actually occur at a given point for days of similar atmospheric conditions.  For example, let’s say you wake up and the forecast calls for a 40% chance of rain that afternoon.  What that means is for 100 days of similar atmospheric conditions (similar temperatures, moisture content, size/strength of frontal systems, etc), it will actually rain on you on 40 of those days, and it will stay dry 60 of those days.  It can rain for more than 40% of the day.  It can rain on more than 40% of the county or viewership area.

Another way to think about a 40% PoP is like this … instead of 40 out of 100 days, simplify it to just 2 out of 5 days.  If each day of the 5-day workweek calls for a 40% chance of rain, that means you should expect to see rain at your location on just two of those days.

Therefore, if there is a 40% chance of rain in the forecast and all of western Massachusetts stays completely dry, it is not regarded as an “incorrect forecast”.  Likewise, if the entire state saw a downpour on one of these “40%-chance” days, it is not necessarily incorrect either.  You have to look at it as a long-term average.  If you circled every day on the calender that there was a 40% chance of rain listed for your town, then after circling 100 of those days it should have rained on only 40 of them.

Record-low tornado numbers
May 18, 2013

By Meteorologist Mike Skurko

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The last 12 months had the fewest number of tornadoes on record in the United States for a 12-month span.  Research compiled at the NOAA National Severe Storms Laboratory in Norman, Oklahoma counted a total of 217 tornadoes from May 2012 to April 2013.  Here are the top-5 “least active” 12-month spans for tornadoes of EF1 strength or greater since reliable records began in 1954:

May 2012 – Apr 2013: 217 (preliminary)
June 1991 – May 1992: 247
Nov 1986 – Oct 1987: 270
Dec 2001 – Nov 2002: 289
June 2000 – May 2001: 298

It is remarkable that this low record has come just after we had an all-time high record, with 1050 tornadoes from June 2010 to May 2011!

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Dry weather ends mid-week
May 5, 2013

By Meteorologist Mike Skurko

We have enjoyed plenty of sunshine over the last several days, but you probably know that we will have to balance that out with several rainy days somewhere down the line.  It appears that will happen over the second-half of this week.  A slow-moving, cut-off low pressure system has drenched the Southeast over the last few days (easily noticed if you watched any CBS coverage of this weekend’s golf tournament in North Carolina).

The area of high pressure that has protected New England will finally start drifting out to sea on Tuesday.  That will open the door for this cut-off low to crawl its way into the Northeast … and finally deliver us some opportunities for rain.  Good news for firefighters, bad news for sun-lovers.

Here is the progression of computer models over the next four days at 5 pm:

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Hopefully this will help close the gap between the normal amount and actual amount of rainfall we should have at this point in the season.  Over the last 60 days we’ve only had 4.27 inches of precipitation, more than 3 inches below our average for this time frame.  It does not appear that we will completely overcome that deficit on Wednesday and Thursday alone.  There is another trough that will deliver periods of rain for next weekend as well, so if we can cut this deficit in half by the end of the week, that would be a plus.

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Severe Weather Safety Awareness Week
May 4, 2013

By Meteorologist Mike Skurko

This past week, April 29 to May 3, was our region’s Severe Weather Safety Awareness Week from the National Weather Service in Taunton, MA.  Each day, a different topic was discussed by the National Weather Service, educating about different types of severe weather and informing about ways to stay safe during a severe storm.  The following topics were discussed:

Monday – Definition of a Severe Thunderstorm

Tuesday – Thunderstorms and Lightning

Wednesday – Tornado Safety in Schools

Thursday – Downburst Winds From Severe Thunderstorms Can Be Powerful

Friday – Low-topped Thunderstorms

Here is the link to the various information statements: Severe Weather Awareness Week In Southern New England