"Thunderstorms require an unstable troposphere, adequate moisture and enough lift to
realize the unstable air. An air mass thunderstorm is in reference to a relatively
weak thunderstorm that usually does not last much longer than an hour, moves slowly and does
not produce severe weather.
Another term that means the same as an air mass thunderstorm
is a garden variety thunderstorm. Air mass thunderstorms tend to occur within a maritime
tropical air mass. The wind shear is weak and this is why the air mass storms
are not severe, do not last long and do not move quickly. The main threats from
an air mass thunderstorm are lightning and brief heavy rain.
The instability tends
to be weaker with air mass thunderstorms. For example, the CAPE value for
an air mass thunderstorm will often be more than half as less as CAPE values
associated with severe thunderstorms. Since wind shear is less in the air
mass thunderstorm environment, the Helicity values are often less than those
associated with severe thunderstorms. The lifting mechanisms that generate
air mass thunderstorms tend to be weaker. While a severe thunderstorm may have lifting
from a strong approaching cold front, strong upper level jet streak or a strong
low level convergence boundary, the lifting for an air mass thunderstorm tends to
be more subtle such as lifting generated by daytime heating and weak low level
convergence. Because of this air mass thunderstorms tend to occur in the
afternoon hours. Air mass thunderstorms are common in the summer along and
near the Gulf coast states and in other areas where the conditions are right for them
What are CAPE values ?
From the NWS glossary:
Convective Available Potential Energy. A measure of the amount of
energy available for convection. CAPE is directly related to the
maximum potential vertical speed within an updraft; thus, higher values
indicate greater potential for severe weather. Observed values in
thunderstorm environments often may exceed 1000 joules per kilogram
(J/kg), and in extreme cases may exceed 5000 J/kg.
However, as with other indices or indicators, there are no threshold
values above which severe weather becomes imminent. CAPE is represented
on an upper air sounding by the area enclosed between the environmental
temperature profile and the path of a rising air parcel, over the layer
within which the latter is warmer than the former. (This area often is
called positive area.)
An example of CAPE in our local weather:
The CAPE value for Orange, MA for 6PM today is 35.16. For Worcester, 16.24, For Windsor Locks, 5.75. These are very low values.
Worcester MA for 5PM on July 10 had a CAPE value of 4197.84
and Orange, MA that same day for 6PM had a CAPE value of 4842.09
The NWS Taunton office watches the CAPE values to determine the likelihood
and severity of storms. For that day, July 10, an afternoon update exemplifies this, Note the "best chance" locations for the most severe conditions and possible tornadic activity:
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 423 AM EDT TUE JUL 10 2007
.SYNOPSIS... A MEANDERING FRONTAL BOUNDARY ACROSS SOUTHERN NEW ENGLAND WILL MOVE NORTH TUESDAY. SUMMER-LIKE HEAT AND HUMIDITY WILL BE WITH US THROUGH MIDWEEK. A SERIES OF FRONTAL PASSAGES AFTER THURSDAY SHOULD BRING UNSETTLED WEATHER.
.NEAR TERM /UNTIL NOON TODAY/... A QUIET CAPE LOADING MORNING ENVISIONED WITH WHEREVER THE QUASI-STNRY BOUNDARY IS AT NOON BEING ONE OF THE MECHANISMS TO POTENTIALLY UNLEASH BIG CAPE DRIVEN TSTMS. PATCHY DENSE FOG ALONG THE COAST AND A FEW INTERIOR VALLEYS.
A Special Weather Statement was in effect that evening for severe storms:
SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE TAUNTON MA 1102 AM EDT TUE JUL 10 2007
CTZ002>004-MAZ002>006-008>012-014-026-NHZ011-012-015-RIZ001- 101715- HARTFORD CT-TOLLAND CT-WINDHAM CT-WESTERN FRANKLIN MA- EASTERN FRANKLIN MA-NORTHERN WORCESTER MA-CENTRAL MIDDLESEX MA- WESTERN ESSEX MA-WESTERN HAMPSHIRE MA-WESTERN HAMPDEN MA- EASTERN HAMPSHIRE MA-EASTERN HAMPDEN MA-SOUTHERN WORCESTER MA- SOUTHEAST MIDDLESEX MA-NORTHERN MIDDLESEX MA-CHESHIRE NH- EASTERN HILLSBOROUGH NH-WESTERN AND CENTRAL HILLSBOROUGH NH- NORTHWEST PROVIDENCE RI- INCLUDING THE CITIES OF...HARTFORD...WINDSOR LOCKS...UNION... VERNON...PUTNAM...WILLIMANTIC...CHARLEMONT...GREENFIELD... ORANGE...BARRE...FITCHBURG...FRAMINGHAM...LOWELL...LAWRENCE... CHESTERFIELD...BLANDFORD...AMHERST...NORTHAMPTON...SPRINGFIELD... MILFORD...WORCESTER...CAMBRIDGE...AYER...JAFFREY...KEENE... MANCHESTER...NASHUA...PETERBOROUGH...WEARE...FOSTER...SMITHFIELD 1102 AM EDT TUE JUL 10 2007
...SEVERE THUNDERSTORMS ARE POSSIBLE IN PARTS OF SOUTHERN NEW ENGLAND THIS AFTERNOON AND EVENING
THERE IS A CHANCE OF SEVERE THUNDERSTORMS IN INTERIOR SOUTHERN NEW ENGLAND THIS AFTERNOON AND EVENING. THE MOST LIKELY FORM OF DAMAGING WEATHER WILL BE STRONG STRAIGHT LINE WIND GUSTS...WITH SMALLER CHANCES OF LARGE HAIL AND POSSIBLY A TORNADO. THE BEST CHANCE FOR THUNDERSTORMS WILL BE IN NORTHWEST MASSACHUSETTS AND SOUTHWEST NEW HAMPSHIRE...WITH A SMALLER CHANCE IN NORTHERN CONNECTICUT...NORTHWEST RHODE ISLAND...CENTRAL MASSACHUSETTS...AND SOUTH CENTRAL NEW HAMPSHIRE. THUNDERSTORMS WILL BE MOST LIKELY BETWEEN 2 PM AND 8 PM.
THE THUNDERSTORMS WILL BE CAUSED BY STRONG HEATING OF HUMID UNSTABLE AIR...AS WELL AS THE COLLISION OF THAT HUMID UNSTABLE AIR WITH COOLER AIR BLOWING IN FROM THE EAST.
Another type of thunderstorm frequently seen in the western US is called a "dry thunderstorm".
A Dry Thunderstorm is a "high-based thunderstorm when lightning is observed, but little if any precipitation reaches the ground. Most of the rain produced by the thunderstorm evaporates into relatively dry air beneath the storm cell. May also be referred to as "dry lightning"."
These storms often spark wildfires, as has been the case recently in Nevada, Utah and other western states.
With "seabreeze" season on the way, it has reminded me of something that is never talked about, not even whispered about when it comes to comparing the climate of different cities: How LONG it stays near the high temperature of the day. Sound boring? Well, maybe a little perhaps, but let me spice things up. Here's a paradox for you: How can the spring and summer weather in Eastern Massachusetts be colder by day than our neighbors in northern New England (Burlington, Vt. as an example) even though Burlington has a colder average high temperature on record? The answer is that because of late day seabreezes, even though we will average warmer readings during the peak warming of the day, the early evening cooldown means we spend LESS TIME hovering near that warm high temperature. That's right! Temperatures are computed by the daily high and daily low when cities are compared to each other, but it would be a far different "picture" if the temperature was computed hour by hour. If you think that is impressive, considering how much more that rule "counts" right on the immediate coast where sometimes it can be in near 90 in May midday, followed by 50 just hours later due to the seabreeze! A little natural air conditioning can go a long way..
The seabreeze circulation dominates the immediate coastlines around the world, and can control the diffusion of pollutants, as well as the temperature and precipitation. We are investigating how different weather models simulate the seabreeze circulations along the east and south coasts of New England."
"New England weather and climate is arguably one of the most varied in the world. It includes extremes of both hot and cold temperatures, droughts, heavy rainfall, hurricanes, tornadoes, blizzards, and more. These great variations in New England weather are influenced by many factors which relate to its physical geographical setting, including its latitude and coastal orientation." Coastal Boundary Layer Influence on Pollutant Transport in New England
Wayne M. Angevine CIRES / NOAA Aeronomy Lab
Christoph Senff and Allen B. White CIRES / NOAA Environmental Technology Lab
Michael Tjernström and Mark Zagar Stockholms Universitet
Robert Talbot AIRMAP, University of New Hampshire
A Geographical Study of New England Temperatures P. E. Church Geographical Review,
Vol. 26, No. 2 (Apr., 1936), pp. 283-292
Winter forecasts in New England sometimes refer to the potential for 'WINDEX events'. WINDEX itself stands for 'winter instability index'. It is used to assign a value to the probability of (non-Lake Effect) snow squalls.
You calculate the difference between the T1 and the T5 on the NGM, if it is over 10, and especially over 15, you then look at the lifted index.. if it rises significantly after the proposed event, and the RH at R1 is over 50%, you will have instability squalls.
T1: Temperature in the 35 mb closest to the surface
T5: Temperature at 800 mb
R1: Relative Humidity in the 35 mb closest to the surface
Lifted Index: Instability index calculated by lifting parcels from the
surface to 500 mb and subtracting the temperature of its environment
from the parcel temperature.
WINDEX V2.0 – An
improved forecast technique to assess the threat of snow squall
potential over a given area (Weir Lundstedt)
the early 90’s a wintertime instability index (WINDEX) was
developed to assess the potential for snow squalls along frontal
boundaries across New England. It was that study which attempted to
quantify moisture, lift and instability in the low levels of the
atmosphere needed to produce snow squalls in an environment unstable
in respect to shallow upright convection. WINDEX V2.0 is the second
version, or next generation of WINDEX which makes use of improved
model resolution, better understanding of dendritic growth and the
utilization of the latest software of BUFKIT to produce what is
believed to be an even more robust technique to forecast snow
squalls. A review of the new parameters will be presented along with
a recent case study from the winter season of 2003.
Lundstedt currently works for the National Weather Service (NWS) as
an aviation weather forecaster in Nashua, New Hampshire. He has been
working in the Center Weather Service Unit (CWSU) within the Boston
Center Air Traffic Control Facility since the fall of 1992. His
duties include providing Air Traffic Controllers timely weather
forecasts for such things as icing, turbulence, thunderstorms and IFR
conditions for the safe and efficient movement of aircraft over the
northeast corridor of the United States. Before working at the CWSU,
Weir entered the NWS in the fall of 1986 and worked as meteorologist
intern for over 5 years at the Weather Service Office (WSO) in
Concord New Hampshire. While there he published WINDEX – A
wintertime instability index to aid in forecasting snow squalls.
Weir’s interest in snow squalls stem back to his days at Lyndon
State College where he watched forecast “flurries”
occasionally turn into several inches of snow. He graduated with a BS
degree in meteorology in 1986.
Snow Squalls in Current
Mesoscale Forecasting Models (Dr. Frank Colby)
this part of the talk, we will concentrate on the ability of
currently available forecasting models to forecast the occurrence of
snow squalls in advance. We will compare the operational National
Centers for Environmental Prediction Eta model output with output
from an NCAR/PSU Mesoscale Model (MM5) run made in the UMass Lowell
Weather Lab. The Eta model ran on a 12 km grid, but the output is
typically viewed on a 40 km grid, due to Internet bandwidth
limitations. The MM5 model ran with 3 nested grids: 36 km , 12 km,
and 4 km. The 4 km grid output will be highlighted here, illustrating
the importance of the fine-scale grid in simulating snow squalls.
Frank Colby is a
Professor of Meteorology at the University of Massachusetts Lowell,
in the Department of Environmental, Earth, and Atmospheric Sciences.
He has been a Professor since 1995, and has been at the University
received his B.S. from the University of Michigan in 1976, his M.S.
from the Massachusetts Institute of Technology (M.I.T.) in 1979, and
his Ph.D. in Meteorology from M.I.T. in 1983, under thesis advisor
Professor Fred Sanders. Dr. Colby’s research interests include
a blend of analysis, forecasting, and modeling, especially relating
to the atmospheric boundary layer.
Application of the Wintertime Instability Index (WINDEX) During Two Significant Snow Squall Events in Central New York and Northeast Pennsylvania
NOAA/ National Weather Service
winter, intense snow showers, often termed "snow squalls" are
associated with the passage of surface frontal boundaries or troughs of
low pressure, and are common to the Northeastern United States. Snow
squalls pose a significant, short-term forecast problem for
meteorologists concerned with public and aviation safety. Although snow
squalls are not known for heavy snow accumulation, they do typically
produce heavy snowfall rates, very poor visibility, and enough of a
snow accumulation to wreak havoc on travelers as they pass through an
The Wintertime Instability Index (WINDEX) method for
assessing the synoptic environments favorable for snow squalls was
developed in 1993. This legacy technique was developed during an era
when only a limited subset of parameters from the Nested Grid Model
(NGM) was routinely available to operational forecasters. These
parameters were related to moisture, low level instability, surface and
upper level forcing. The method was modified in 2005 to take advantage
of the much larger suite of forecast data currently available on a
routine basis to National Weather Service (NWS) forecasters. The
updated technique now incorporates higher resolution, point specific,
numerical model data via the NWS Buffalo's Forecasting Toolkit (BUFKIT)
thermal profile analysis software.
Two events, February 17,
2005 and November 24, 2005 (Thanksgiving Day), were examined using the
WINDEX method. These events were chosen due to their significant,
negative impact on late afternoon and evening travel in the Binghamton,
NY and Scranton, PA areas on the dates noted. Upper level and surface
synoptic charts, radar and satellite imagery, and plan views of the
North American Mesoscale Model (NAM) forecast output were examined for
each case, and will be used to lay the synoptic groundwork for each
event. The specific WINDEX criteria were analyzed from hourly forecast
soundings via the BUFKIT software, and tabular model output from the
NAM. Forecast lapse rate data between 950 and 800 hPa for the two cases
was examined, as these levels correlate best to the T1 and T5 sigma
levels from the NAM model output parameter used in the current WINDEX
This presentation will outline the WINDEX method, and
then demonstrate it's effectiveness on short term forecast and warning
operations at NWS Binghamton, NY on February 17, 2005, and Thanksgiving
Day 2005. The hourly forecast lapse rate data prior to the development
of the snow squalls will be examined, and recommendations for using the
lapse rate forecast data in lieu of the legacy T1 and T5 differential
will be presented.
Feb 13, 2007 - Yesterday saw the effects of yet another Arctic Air Mass in New England.
What is the difference between "polar air" and "arctic air" ?
Polar air masses are cool or cold and dry, but not as cold as an arctic air mass which is extremely cold, contains very little moisture and forms at or near the Arctic Circle.
Arctic air masses form primarily the winter over the Arctic Basin, Greenland, and the northern interior of North
Despite the difference by definition in temperature, "arctic air is cold aloft and extends to great heights, but the surface temperatures are often higher than those of polar air.." (AMS Glossary - arctic air)
"Air masses are basically large weather systems with homogeneous temperature and moisture characteristics that are created and classified by the geography where they formed. If an airmass originates over water, it is called a maritime airmass. If it forms over land, it is called a continental airmass. These two airmass types are moisture-dependent, so if an airmass is a maritime one, it tends to be humid, while a continental one tends to be dry.
Airmass type also depends on how much sunlight the ground under it receives in a given location over a period of time. Air masses closer to the tropics get more sunshine, and are thus warmer than air to the north. Such air masses are referred to as tropical, while air masses which form closer to the poles are colder, and are called polar. An airmass which forms over the arctic ice cap near the pole itself is termed an arctic airmass. Using these broad groups of airmass types, meteorologists are able to classify any airmass, in general terms, across the entire planet."
Air Masses are described in nice detail with graphics at the Meteorological Service of Canada - Air Masses:
FORMATION AND CLASSIFICATION OF AIR MASSES
Maritime arctic (primarily Canadian)
FACTORS THAT DETERMINE WEATHER WITHIN AN AIR MASS CHANGES IN AIR MASSES WEATHER ASSSOCIATED WITH AIR MASSES
And from the Archives, the United States Department of Agriculture, Weather Bureau, Monthly Weather Review.. This 1940 document includes a section on the formation of continental arctic air.
"FORMATION OF CONTINENTAL ARCTIC AIR" "It has been shown by Wexler3 that the formation of continental Arctic air masses * comes about mainly as the result of cooling of the lower atmosphere by radiation processes over the northern snow-covered continents and ice fields during the winter night. If these areas represent a true cold source, then air masses reaching there from other localities must always be at higher temperatures at first, then become Arctic in character through cooling. Since a migration of air masses from one part of the earth to another is occurring constantly, the building up of continental Arctic air from previously warmer air masses, their outbreak into lower latitudes after having achieved the low temperatures, and their replacment by new air masses which are relatively warm in the beginning but undergo the same cooling to form again more Arctic air, are continuous processes."
Feb 13, 2007 - The Valentine's Day Storm this year will bring damaging winds (hurricane force) to the coastal areas and the Cape and Islands due to a "bombogenesis" or rapid intensification of a storm.
"Bombs" often result in Nor'easters, as is the case tomorrow. A heavy
accumulation of snow as well as sleet, freezing rain, and at times
possibly rain make this a complex dangerous storm.
This storm is similar to what I recall Thanksgiving 72's being, where the heaviest snows migrated to Albany and they got 24". I'd expect they will get that again. Springfield is more or less on the border of the big amounts, but tends to 'downslope' on a brisk east wind aloft. Albany will probably be more prone to a northerly wind, so they may keep the totals high. Watch for a 'surprise' in the middle of this one tomorrow.. more on that in future updates!
"Bombogenesis" is a term which some folks scoff at, but is an actual meteorological slang word adopted by most eastern seaboard meteorologists for a strengthening coastal storm, similar to a quickly deepening hurricane. Some have even adopted the new term "Wintercane", but this is just a re-hash of an older term I believe, "Neutercane" which was popular after the Blizzard of 1978.
Bombogenesis is the extreme intensification of a cyclone or low-pressure storm system characterised by a drop in surface pressure of at least 24 millibars in 24 hours.
Intense winds arise as the height contours build
around the center of rotation with the number of height contours increasing rapidly in the developing stages of the "bomb".
This rapid intensification typically occurs between
a cold continental air mass (see associated post on Continental Air Masses coming later today) and warm ocean waters or between a cold polar air mass and a much warmer air mass.
Read more about bombogenesis at Jeff Haby's HabysHints. Learn more about Low Pressure systems At the University of Illinois WW2010 page "Low Pressure Centers".
"Categorizing storms according to their sea level pressure patterns is another way of classifying storms. By plotting central sea level pressure against time a pressure profile can be created. All storms do not conform to the same sea level pressure pattern throughout its life. Some storms can be classified as classical storms. These are storms where their pressure profiles are "V" shaped or have a dip pattern in pressure. Oscillating storms are those with insignificant pressure changes, while ascending storms increase in pressure over time. Descending storms are those that decrease over time in pressure only."
For information on how to read weather charts, computer models, etc, on the internet, I recommend Peter Chaston's book Weather Maps, 3rd Edition, which you can order by clicking here or by emailing me .
With the anniversary of the Blizzard of '78 approaching, more folks are searching for it on the web. We'll talk a little about blizzards in general here at ToddGross.com and work-up a little info on that event as well as a few other major winter storms.. coming up in the February. 'Til then, the Massachusetts Office of Coastal Management has a nice PHOTO GALLERY of the blizzard of 1978..
It was a storm that changed the way Americans prepare for significant winter weather events. The NWS marked the 20th anniversary in 1998 with this Press Release.
The National Weather Service defines a blizzard (and a blizzard warning) as:
(abbrev. BLZD)- A blizzard means that the following conditions are expected to prevail for a period of 3 hours or longer:
Sustained wind or frequent gusts to 35 miles an hour or greater; and
Considerable falling and/or blowing snow (i.e., reducing visibility frequently to less than ¼ mile)
Issued for winter
storms with sustained or frequent winds of 35 mph or higher with
considerable falling and/or blowing snow that frequently reduces
visibility to 1/4 of a mile or less. These conditions are expected to
prevail for a minimum of 3 hours.
Does it have to be actually SNOWING for there to be blizzard conditions ? NO! Blowing snow combined with high winds can create blizzard conditions.
blowing snow—Snow lifted from the surface of the earth by the wind to a height of 2 m (6 ft) or more above the surface (higher than drifting snow), and blown about in such quantities that horizontal visibility is reduced to less than 11 km (about 7 statute miles). As an obstruction to vision, it is encoded BS in a surface aviation weather observation and as BLSN as an obstruction to vision in a METAR or SPECI observation. Blowing snow can be falling snow or snow that already accumulated but is picked up and blown about by strong winds. It is one of the classic requirements for a blizzard. (AMS Glossary)
Where did the term 'blizzard' come from ? "In the 1870's, an Iowa newspaper used the word "blizzard" to describe a snowstorm.
Previously, the term blizzard referred to a canon shot or a volley of musket fire. By the
1880's, the use of the word blizzard was used by many across the United States and in
England." Source : Blizzards - NWS Flagstaff, AZ
This week's question involves May 21's
crazy weather. It actually snowed during the downpours in some areas..
at 45 degrees!! How can that happen?
ANSWER: A very cold pool of air moved in above, and thunderstorms erupted at the same time. As you can see by a typical thunderstorm structure to the left, warm air is drawn into the thunderstorm ahead and from the sides of the approaching storm. However, JUST ahead of, and accompanying rain, there is a DOWNDRAFT of cold air from above. This often results in what is known as a "microburst" which is a cold rush of air down that spreads out at speeds up to 100mph when hitting the ground causing tornado like damage. This area of the storm is sometimes accompanied by hail, large chunks of ice that are blowing around up and down in the top of the thunderstorm, getting bigger and bigger until they fall as ice out of the cloud. But snow??? Rare, but it happens in the "near winter" season. If the downdraft is strong enough, it will simply accompany huge snowflakes that form which rush down from the cloud and don't have time to melt, even if the temperature near the ground is 45-52 degrees. That is why there is snow on record in places like Rochester, NY in August as an example, and why it snowed in Harvard, Ma. briefly last weekend (5/21) in the Boston area! CLICK ON THE IMAGE, ASIDE FROM THE COLOR, IT IS HAND DRAWN BY WEATHER EXPERT/METEOROLOGIST/PROFESSOR JEFF HABY.
This incredible shot from Newport, RI from our Todd Gross Support team member Barry shows the twin rainbows that graced most of our skies prior to sunset Sunday evening. The storm came in with a bang, but left to the east, and the sunlight shining back on the raindrops as it was setting in the west set off an incredible rainbow display. You see, the raindrops, reflect and refract the light BACK at you, splitting white light into the array of colors that you see...
Clikc on the image for the full impact.
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