About Explosive Cyclogenesis (Meteorological Bombs)
All bombs are not created equally - Some are much stronger than others. In general,
pressure falls averaging about 1 mb per hour over 24 hours qualifies as a strong bomb.
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Recipe for Carolina Bombs - Start with a cold-air outbreak and extend it to the western
wall of the Gulf Stream (low-level baroclinicity); then mix strong upper-level vorticity advection
over the area with a good helping of jet max winds (upper-level dynamics) to get simultaneous
explosive surface cyclone deepening rates.
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Satellite imagery, your window for tracking a bomb's triggers - Typically the
vorticity maximum and jet max winds that trigger bombs preexists the surface low by days.
So these features can be followed in the water vapor imagery. They appear as well defined
bulging hooks and dark bands respectively. The bulging shape of a vort max is due to its
cyclonic spin while the darkening band of a jet streak is associated with tropopause folding
and the intrusion of dry and warmer stratospheric air. Explosive cyclogenesis can ensue
whenever these features approach highly baroclinic waters off the Carolina coast. The
stronger the vort max and/or jet streak and the faster they are moving, the more likely
a bomb will develop.
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Comma head, witness the bomb's birth - The appearance of a comma head cloud pattern
in the satellite imagery can be a signal for rapidly deepening surface low development. The
evolution of the comma head differs according to the pattern of cyclogenesis.
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Where's the bomb's heaviest precipitation? - The heaviest precipitation will not
likely be located beneath the coldest cloud tops. Rather, the heaviest precipitation will be
found beneath the tightest IR cloud top temperature gradient just east of the coldest cloud
tops and just west of the bomb's dry slot. The heavy precipitation in a bomb persists as new
comma heads emerge from near the dry slot to replace the dissipating old comma head cloud.
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Lightning data, another means for detecting bombs - Latent heat release associated
with convection can assist in the development of bombs. As rapid cyclogenesis takes place, the
convection and associated lightning strikes will persist or increase.
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Bombing out - Strong bombs tend to undergo rapid deepening for around 24 hours. Deepening continues at a lesser rate for another 12 hours and then little deepening occurs after
this 36 hour period.
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Bombs in motion - During a bomb's most rapid deepening period, the surface low
tends to speed up. Following this rapid deepening period, the bomb's forward speed then slows.
Generally following a bomb's most rapid deepening period, the storm's track is left of its
former track. In large measure, the changes in the low's path is highly correlated to the
behavior of the upper air pattern. Surface lows whose upper level trough becomes cutoff will
show more slowing and a greater jog to the left than those surface low's whose upper level
troughs do not cutoff.
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Case of January 24th, 2000 - This bomb dumped over 20 inches of snow in Raleigh, NC.
According to surface analysis performed by staff at the NWS WFO RAH, the storm deepened 28 mb in
26 hours as it tracked from the northeast coast of FL past the NC Outer Banks to off the southeast
VA coast; there was no additional deepening beyond 26 hours. Note that as the low neared Cape
Lookout NC, its forward speed slowed while its track jogged toward the north or to the left of
its previous track. This change in the cyclone's motion occurred toward the end of the bomb's
most rapid deepening period and also not long after the time that the supporting 500 mb trough
aloft began to close and cutoff. Note also that the low's fastest forward speed occurred during
the first 13 hours of its most rapid deepening period.
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NWP Forecast Errors - While projecting major cyclogenesis, the models were much too
slow in pulling the low closer to shore and too fast in moving the low away. Its not clear why
the models performed as they did. But their predictions of the low's track argued for far less
snow than the 20 inches which fell at Raleigh, NC.
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About the Atlantic Surface Cyclone Intensification Index (ASCII)
ASCII - ASCII predicts the likelihood of a bomb for a given storm moving through
its domain (i.e., off Southeast coast). ASCII is based upon a measure of the amount of low-level
near-shore baroclinicity between the western wall of the Gulf Stream and inland coastal stations
experiencing a cold air outbreak.
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Bomb Index - ASCII's Bomb Index will be automatically calculated and displayed on
its own
website. ASCII's bomb index will indicate one of the following:
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- "Bomb not likely" - ASCII is nearly perfect when it indicates "bomb not likely".
When there is only weak baroclinicity, there is insufficient low-level energy to generate a
bomb. With weak baroclinicity, even impressive amounts of dynamics do not lead to explosive
cyclogeneis, except in rare cases.
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- "Bomb is likely" - ASCII's forecast of "bomb likely" is reliable; more lows
will bomb than not. When much low-level energy is available, even so-so dynamics is likely
to generate a bomb.
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- "Indeterminate" - This means there is much variability; some lows bomb while
others do not. Here, the baroclinicity is only so-so. ACSII offers little forecast skill
about the potential for a bomb when there is just so-so baroclinicity since ASCII does not
account for the strength of the upper-level dynamics.
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A Forecast Strategy - Factors for evaluating the potential for bombs
Climatology - The reoccurrence rate of bombs off the Southeast coast is roughly
about 2 per year. Bombs which develop far enough south and close enough to shore to have a large
impact with wintry precipitation over inland NC, occur on the average of about 1 every 10 years.
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Possible Rare Event - Follow closely the model's forecast of anomalies which depart
significantly from daily climatology by referring to the WFO State College website on model
anomalies. Through an early awareness of a model's forecast of various
parameters (e.g., large departure in the 500 mb height field), you can maintain an
increased awareness of the potential for a possible bomb event.
Refer to the NWS State College / PSU Website for
Model Anomallies . Imagery available under "Model graphics Links."
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ASCII - ASCII is better than anything else we have to tell us about how much low-level
baroclinicity is out in our coastal domain. Always look at ASCII when you have reason(s) to be
concerned about coastal cyclogenesis.
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Visible Satellite Imagery for CAO - The low-level baroclinicity can significantly
increase in our coastal waters following an intense cold air outbreak (CAO). The presence of
cloud streaks in the near coastal waters may well be a sign of intense cold air moving over
the warm waters.
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Water Vapor Satellite Imagery for Tracking Upper-Level Triggers - Dark bands
(jet wind maxs) and bulging hooks (strong vort maxs) rapidly nearing highly baroclinic
coastal waters are strong indicators for explosive cyclogenesis. These triggers always
preexists, often by days, the surface low. Sometimes the surface low only appears 12-24 hours
before the rapid cyclogenesis begins.
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IR Satellite Imagery for Pattern Recognition of Rapid Cyclogenesis and Locating the Heaviest Precipitation -
When the upper level dynamics and low-level baroclinicity are favorable for bombs, the appearance
of a comma head cloud pattern indicates rapid deepening of the surface cyclone. The heaviest
precipitation can be found in the tightest IR temperature gradient within the comma head cloud,
just west of the dry slot and just east of the coldest cloud tops. The heavy precipitation persists
as new comma head clouds develop near the dry slot and replace the old decaying comma head.
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Dprog/DT - In the WFO RAH's AWIPSUSR account under ASCII, there is a procedure that
allows you to see the NWP model trends in their solutions for upper dynamics (300 mb jet and
differential vorticity 300-500 mb). Remember that even though the models may have a wrong
solution for a specific run, they may well show a more accurate solution in the dprog/dt trends
seen over three consecutive runs.
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References and Additional Readings
Explosive Cyclogenesis in the West-Central North Atlantic Ocean, 1981-84. Part 1:
Composite Structure and Mean Behavior by Fred Sanders; Monthly Weather Review, Vol 114, October 1986, pp 1781-1794.
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Images in Winter Weather Forecasting - A practical guide for interpreting satellite and radar imagery.
M.J. Bader, G.S. Forbes, J.R. Grant, R.B.E.Lilley and A.J. Waters. British Crown 1995.
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Winter Weather Forecasting throughout the Eastern United States. Part II: An Operational
Perspective of Cyclogenesis by James Gurka, et al; Weather and Forecasting, Vol 10, No 1, March 1995, pp 21- 41.
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The Effect of Gulf Stream - induced Baroclinicity on U.S. East Coast Winter Cyclones by
Joe Cione, et al; Monthly Weather Review, Vol 121, Feb 1993, pp 421-430.
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Satellite Imagery Interpretation for Forecasters by Peter Parke; Weather Service Forecasting Handbook No 6, May 1986.
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