Air Mass Thunderstorms
July 24, 2007
Today's forecast includes the slight chance for an "air mass thunderstorm". What is that and how does it compare to stronger storms ?
Meteorologist and educator Jeff Haby defines the Air Mass Thunderstorrm:
"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 to occur."
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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.)
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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.
$$
WTB
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.
The NOAA Fire Weather website provides Fire Weather Forecasts.
