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CYCLOSTORM
Updated: Sun 27 Sep 2020 16:12 GMT
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Situation Map
KUJIRA (West Pacific)
Tropical Storm KUJIRA Situation Map
Tropical Storm KUJIRA (15W)Up
Tropical Storm KUJIRA Forecast Graphic
Tropical Storm KUJIRA Forecast Track (Joint Typhoon Warning Center)
Tropical Storm KUJIRA Storm-Centered Infrared
Tropical Storm KUJIRA Storm-Centered Enhanced Infrared
Tropical Storm KUJIRA Storm-Centered Visible
Tropical Storm KUJIRA Warning Text (JTWC)
WTPN31 PGTW 271500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/TROPICAL STORM 15W (KUJIRA) WARNING NR 004//
RMKS/
1. TROPICAL STORM 15W (KUJIRA) WARNING NR 004    
   UPGRADED FROM TROPICAL DEPRESSION 15W
   01 ACTIVE TROPICAL CYCLONE IN NORTHWESTPAC
   MAX SUSTAINED WINDS BASED ON ONE-MINUTE AVERAGE
   WIND RADII VALID OVER OPEN WATER ONLY
    ---
   WARNING POSITION:
   271200Z --- NEAR 23.3N 156.6E
     MOVEMENT PAST SIX HOURS - 330 DEGREES AT 19 KTS
     POSITION ACCURATE TO WITHIN 050 NM
     POSITION BASED ON CENTER LOCATED BY SATELLITE
   PRESENT WIND DISTRIBUTION:
   MAX SUSTAINED WINDS - 035 KT, GUSTS 045 KT
   WIND RADII VALID OVER OPEN WATER ONLY
   RADIUS OF 034 KT WINDS - 130 NM NORTHEAST QUADRANT
                            230 NM SOUTHEAST QUADRANT
                            020 NM SOUTHWEST QUADRANT
                            040 NM NORTHWEST QUADRANT
   REPEAT POSIT: 23.3N 156.6E
    ---
   FORECASTS:
   12 HRS, VALID AT:
   280000Z --- 26.1N 154.7E
   MAX SUSTAINED WINDS - 040 KT, GUSTS 050 KT
   WIND RADII VALID OVER OPEN WATER ONLY
   RADIUS OF 034 KT WINDS - 140 NM NORTHEAST QUADRANT
                            130 NM SOUTHEAST QUADRANT
                            030 NM SOUTHWEST QUADRANT
                            040 NM NORTHWEST QUADRANT
   VECTOR TO 24 HR POSIT: 340 DEG/ 16 KTS
    ---
   24 HRS, VALID AT:
   281200Z --- 29.1N 153.6E
   MAX SUSTAINED WINDS - 055 KT, GUSTS 070 KT
   WIND RADII VALID OVER OPEN WATER ONLY
   RADIUS OF 050 KT WINDS - 030 NM NORTHEAST QUADRANT
                            040 NM SOUTHEAST QUADRANT
                            010 NM SOUTHWEST QUADRANT
                            000 NM NORTHWEST QUADRANT
   RADIUS OF 034 KT WINDS - 120 NM NORTHEAST QUADRANT
                            120 NM SOUTHEAST QUADRANT
                            050 NM SOUTHWEST QUADRANT
                            040 NM NORTHWEST QUADRANT
   VECTOR TO 36 HR POSIT: 010 DEG/ 16 KTS
    ---
   36 HRS, VALID AT:
   290000Z --- 32.3N 154.3E
   MAX SUSTAINED WINDS - 070 KT, GUSTS 085 KT
   WIND RADII VALID OVER OPEN WATER ONLY
   RADIUS OF 064 KT WINDS - 020 NM NORTHEAST QUADRANT
                            020 NM SOUTHEAST QUADRANT
                            000 NM SOUTHWEST QUADRANT
                            000 NM NORTHWEST QUADRANT
   RADIUS OF 050 KT WINDS - 040 NM NORTHEAST QUADRANT
                            050 NM SOUTHEAST QUADRANT
                            020 NM SOUTHWEST QUADRANT
                            010 NM NORTHWEST QUADRANT
   RADIUS OF 034 KT WINDS - 120 NM NORTHEAST QUADRANT
                            140 NM SOUTHEAST QUADRANT
                            060 NM SOUTHWEST QUADRANT
                            080 NM NORTHWEST QUADRANT
   VECTOR TO 48 HR POSIT: 025 DEG/ 21 KTS
    ---
   EXTENDED OUTLOOK:
   48 HRS, VALID AT:
   291200Z --- 36.0N 156.6E
   MAX SUSTAINED WINDS - 060 KT, GUSTS 075 KT
   WIND RADII VALID OVER OPEN WATER ONLY
   BECOMING EXTRATROPICAL
   RADIUS OF 050 KT WINDS - 040 NM NORTHEAST QUADRANT
                            060 NM SOUTHEAST QUADRANT
                            020 NM SOUTHWEST QUADRANT
                            020 NM NORTHWEST QUADRANT
   RADIUS OF 034 KT WINDS - 120 NM NORTHEAST QUADRANT
                            140 NM SOUTHEAST QUADRANT
                            090 NM SOUTHWEST QUADRANT
                            110 NM NORTHWEST QUADRANT
   VECTOR TO 72 HR POSIT: 055 DEG/ 23 KTS
    ---
   72 HRS, VALID AT:
   301200Z --- 41.0N 166.2E
   MAX SUSTAINED WINDS - 050 KT, GUSTS 065 KT
   WIND RADII VALID OVER OPEN WATER ONLY
   EXTRATROPICAL
   RADIUS OF 050 KT WINDS - 040 NM NORTHEAST QUADRANT
                            040 NM SOUTHEAST QUADRANT
                            000 NM SOUTHWEST QUADRANT
                            000 NM NORTHWEST QUADRANT
   RADIUS OF 034 KT WINDS - 080 NM NORTHEAST QUADRANT
                            120 NM SOUTHEAST QUADRANT
                            110 NM SOUTHWEST QUADRANT
                            070 NM NORTHWEST QUADRANT
    ---
REMARKS:
271500Z POSITION NEAR 24.0N 156.1E.
27SEP20. TROPICAL STORM 15W (KUJIRA), LOCATED APPROXIMATELY 153
NM EAST-SOUTHEAST OF MINAMI TORI SHIMA, HAS TRACKED NORTH-
NORTHWESTWARD AT 19 KNOTS OVER THE PAST SIX HOURS. MAXIMUM
SIGNIFICANT WAVE HEIGHT AT 271200Z IS 14 FEET. NEXT
WARNINGS AT 272100Z, 280300Z, 280900Z AND 281500Z.//
NNNN
Tropical Storm KUJIRA Prognostic Reasoning (JTWC)
WDPN31 PGTW 271500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 15W (KUJIRI) 
WARNING NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 15W (KUJIRA), LOCATED APPROXIMATELY 153 NM 
EAST-SOUTHEAST OF MINAMI TORI SHIMA, HAS TRACKED NORTH-NORTHWESTWARD 
AT 19 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED 
SATELLITE IMAGERY DEPICTS A PARTIALLY-EXPOSED LOW-LEVEL CIRCULATION 
CENTER (LLCC) WITH EXTENSIVE DEEP CONVECTIVE BANDING OVER THE 
SOUTHERN AND EASTERN SEMICIRCLES. A 271055Z GMI 89GHZ MICROWAVE 
IMAGE INDICATES IMPROVED CONVECTIVE BANDING WRAPPING INTO THE 
NORTHERN QUADRANT OF THE SYSTEM. OVERALL, THERE IS FAIR CONFIDENCE 
IN THE INITIAL POSITION. THE INITIAL INTENSITY IS ASSESSED AT 35 
KNOTS, HEDGED ABOVE DVORAK ESTIMATES OF T2.0 (30 KNOTS), BASED ON 
RECENT ASCAT DATA AND A 271200Z ADT ESTIMATE OF T2.5 (35 KNOTS). 
UPPER-LEVEL ANALYSIS CONTINUES TO REVEAL A MARGINALLY-FAVORABLE 
ENVIRONMENT WITH A BROAD, CONVERGENT UPPER LOW POSITIONED OVER THE 
CENTER, WHICH CONTINUES TO HAMPER DEVELOPMENT, OFFSET BY ROBUST 
EQUATORWARD OUTFLOW AND DIFFLUENT POLEWARD OUTFLOW. SST VALUES (29-
30C) REMAIN CONDUCIVE FOR FURTHER DEVELOPMENT. TS 15W IS TRACKING 
NORTHWESTWARD UNDER THE STEERING INFLUENCE OF A SUBTROPICAL RIDGE 
(STR) POSITIONED TO THE NORTHEAST. 
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS 
PROGNOSTIC REASONING MESSAGE.
   B. TS 15W IS FORECAST TO TRACK NORTH-NORTHWESTWARD THROUGH TAU 24 
WITH STEADY INTENSIFICATION. AFTER TAU 24, TS 15W WILL BEGIN TO 
ROUND THE STR WITH FURTHER INTENSIFICATION DUE TO IMPROVING POLEWARD 
OUTFLOW ASSOCIATED WITH A MAJOR SHORTWAVE TROUGH DIGGING SOUTH OF 
JAPAN. THE SYSTEM IS EXPECTED TO PEAK AT 70 KNOTS BY TAU 36. TS 15W 
IS FORECAST TO BEGIN EXTRA-TROPICAL TRANSITION (ETT) AS IT RECURVES 
INTO THE MIDLATITUDE WESTERLIES NEAR TAU 48. AFTER TAU 48, TS 15W 
WILL WEAKEN AS IT ENCOUNTERS INCREASING VERTICAL WIND SHEAR (25-30 
KNOTS) AND TRACKS OVER COOL SST VALUES (23-20C). TS 15W WILL 
COMPLETE ETT NEAR TAU 72 AS IT GAINS FRONTAL CHARACTERISTICS AND 
BECOMES EMBEDDED WITHIN THE MIDLATITUDE WESTERLIES. NUMERICAL MODEL 
GUIDANCE REMAINS IN TIGHT AGREEMENT THROUGH THE FORECAST PERIOD, 
LENDING HIGH CONFIDENCE IN THE JTWC FORECAST TRACK.//
NNNN
Tropical Storm KUJIRA JMV 3.0 Data (JTWC)
WTPN51 PGTW 271500    
WARNING    ATCG MIL 15W NWP 200927131609
2020092712 15W KUJIRA     004  01 330 19 SATL 050
T000 233N 1566E 035 R034 130 NE QD 230 SE QD 020 SW QD 040 NW QD 
T012 261N 1547E 040 R034 140 NE QD 130 SE QD 030 SW QD 040 NW QD 
T024 291N 1536E 055 R050 030 NE QD 040 SE QD 010 SW QD 000 NW QD R034 120 NE QD 120 SE QD 050 SW QD 040 NW QD 
T036 323N 1543E 070 R064 020 NE QD 020 SE QD 000 SW QD 000 NW QD R050 040 NE QD 050 SE QD 020 SW QD 010 NW QD R034 120 NE QD 140 SE QD 060 SW QD 080 NW QD 
T048 360N 1566E 060 R050 040 NE QD 060 SE QD 020 SW QD 020 NW QD R034 120 NE QD 140 SE QD 090 SW QD 110 NW QD 
T072 410N 1662E 050 R050 040 NE QD 040 SE QD 000 SW QD 000 NW QD R034 080 NE QD 120 SE QD 110 SW QD 070 NW QD 
AMP
    048HR BECOMING EXTRATROPICAL
    072HR EXTRATROPICAL
SUBJ:  TROPICAL STORM 15W (KUJIRA) WARNING NR 004    
1. TROPICAL STORM 15W (KUJIRA) WARNING NR 004    
   UPGRADED FROM TROPICAL DEPRESSION 15W
   01 ACTIVE TROPICAL CYCLONE IN NORTHWESTPAC
   MAX SUSTAINED WINDS BASED ON ONE-MINUTE AVERAGE
   WIND RADII VALID OVER OPEN WATER ONLY
    ---
   WARNING POSITION:
   271200Z --- NEAR 23.3N 156.6E
     MOVEMENT PAST SIX HOURS - 330 DEGREES AT 19 KTS
     POSITION ACCURATE TO WITHIN 050 NM
     POSITION BASED ON CENTER LOCATED BY SATELLITE
   PRESENT WIND DISTRIBUTION:
   MAX SUSTAINED WINDS - 035 KT, GUSTS 045 KT
   WIND RADII VALID OVER OPEN WATER ONLY
   RADIUS OF 034 KT WINDS - 130 NM NORTHEAST QUADRANT
                            230 NM SOUTHEAST QUADRANT
                            020 NM SOUTHWEST QUADRANT
                            040 NM NORTHWEST QUADRANT
   REPEAT POSIT: 23.3N 156.6E
    ---
   FORECASTS:
   12 HRS, VALID AT:
   280000Z --- 26.1N 154.7E
   MAX SUSTAINED WINDS - 040 KT, GUSTS 050 KT
   WIND RADII VALID OVER OPEN WATER ONLY
   RADIUS OF 034 KT WINDS - 140 NM NORTHEAST QUADRANT
                            130 NM SOUTHEAST QUADRANT
                            030 NM SOUTHWEST QUADRANT
                            040 NM NORTHWEST QUADRANT
   VECTOR TO 24 HR POSIT: 340 DEG/ 16 KTS
    ---
   24 HRS, VALID AT:
   281200Z --- 29.1N 153.6E
   MAX SUSTAINED WINDS - 055 KT, GUSTS 070 KT
   WIND RADII VALID OVER OPEN WATER ONLY
   RADIUS OF 050 KT WINDS - 030 NM NORTHEAST QUADRANT
                            040 NM SOUTHEAST QUADRANT
                            010 NM SOUTHWEST QUADRANT
                            000 NM NORTHWEST QUADRANT
   RADIUS OF 034 KT WINDS - 120 NM NORTHEAST QUADRANT
                            120 NM SOUTHEAST QUADRANT
                            050 NM SOUTHWEST QUADRANT
                            040 NM NORTHWEST QUADRANT
   VECTOR TO 36 HR POSIT: 010 DEG/ 16 KTS
    ---
   36 HRS, VALID AT:
   290000Z --- 32.3N 154.3E
   MAX SUSTAINED WINDS - 070 KT, GUSTS 085 KT
   WIND RADII VALID OVER OPEN WATER ONLY
   RADIUS OF 064 KT WINDS - 020 NM NORTHEAST QUADRANT
                            020 NM SOUTHEAST QUADRANT
                            000 NM SOUTHWEST QUADRANT
                            000 NM NORTHWEST QUADRANT
   RADIUS OF 050 KT WINDS - 040 NM NORTHEAST QUADRANT
                            050 NM SOUTHEAST QUADRANT
                            020 NM SOUTHWEST QUADRANT
                            010 NM NORTHWEST QUADRANT
   RADIUS OF 034 KT WINDS - 120 NM NORTHEAST QUADRANT
                            140 NM SOUTHEAST QUADRANT
                            060 NM SOUTHWEST QUADRANT
                            080 NM NORTHWEST QUADRANT
   VECTOR TO 48 HR POSIT: 025 DEG/ 21 KTS
    ---
   EXTENDED OUTLOOK:
   48 HRS, VALID AT:
   291200Z --- 36.0N 156.6E
   MAX SUSTAINED WINDS - 060 KT, GUSTS 075 KT
   WIND RADII VALID OVER OPEN WATER ONLY
   BECOMING EXTRATROPICAL
   RADIUS OF 050 KT WINDS - 040 NM NORTHEAST QUADRANT
                            060 NM SOUTHEAST QUADRANT
                            020 NM SOUTHWEST QUADRANT
                            020 NM NORTHWEST QUADRANT
   RADIUS OF 034 KT WINDS - 120 NM NORTHEAST QUADRANT
                            140 NM SOUTHEAST QUADRANT
                            090 NM SOUTHWEST QUADRANT
                            110 NM NORTHWEST QUADRANT
   VECTOR TO 72 HR POSIT: 055 DEG/ 23 KTS
    ---
   72 HRS, VALID AT:
   301200Z --- 41.0N 166.2E
   MAX SUSTAINED WINDS - 050 KT, GUSTS 065 KT
   WIND RADII VALID OVER OPEN WATER ONLY
   EXTRATROPICAL
   RADIUS OF 050 KT WINDS - 040 NM NORTHEAST QUADRANT
                            040 NM SOUTHEAST QUADRANT
                            000 NM SOUTHWEST QUADRANT
                            000 NM NORTHWEST QUADRANT
   RADIUS OF 034 KT WINDS - 080 NM NORTHEAST QUADRANT
                            120 NM SOUTHEAST QUADRANT
                            110 NM SOUTHWEST QUADRANT
                            070 NM NORTHWEST QUADRANT
    ---
REMARKS:
271500Z POSITION NEAR 24.0N 156.1E.
27SEP20. TROPICAL STORM 15W (KUJIRA), LOCATED APPROXIMATELY 153
NM EAST-SOUTHEAST OF MINAMI TORI SHIMA, HAS TRACKED NORTH-
NORTHWESTWARD AT 19 KNOTS OVER THE PAST SIX HOURS. MAXIMUM
SIGNIFICANT WAVE HEIGHT AT 271200Z IS 14 FEET. NEXT
WARNINGS AT 272100Z, 280300Z, 280900Z AND 281500Z.//
1520092518 178N1590E  15
1520092600 182N1597E  15
1520092606 186N1601E  20
1520092612 191N1601E  20
1520092618 198N1595E  25
1520092700 206N1588E  30
1520092706 217N1577E  30
1520092712 233N1566E  35
NNNN

Satellite Imagery

Satellite Animations

What Is a Hurricane?

A hurricane (or typhoon, or severe tropical cyclone), the strongest storm on Earth, is a cyclonic (rotary) storm that derives its energy from cloud formation and rainfall, unlike frontal cyclones that derive their power from a temperature gradient.

A hurricane begins as a tropical depression with a sustained wind speed of less than 39 mph (35 knots; 63 km/hr). As the system strengthens, it becomes a tropical storm with winds from 39 to 73 mph (35-63 knots; 63-118 km/hr). Tropical storms are named in the Atlantic, East, Central and Northwest Pacific, in the South Indian Ocean, and in the Arabian Sea. When the winds are sustained (based on a one-minute average) at 74 mph (64 knots; 119 km/hr), the storm becomes: In the Atlantic Ocean, East Pacific, Central Pacific (east of the International Dateline) and Southeast Pacific (east of 160°E) a Hurricane; in the Northwest Pacific (west of the International Dateline) a Typhoon; in the Southwest Pacific (west of 160°E) and Southeast Indian Ocean (east of 90°E) a Severe Tropical Cyclone; in the North Indian Ocean a Severe Cyclonic Storm; and in the Southwest Indian Ocean (west of 90°E) a Tropical Cyclone.

The Saffir-Simpson Hurricane Scale

Category 1 – 64-82 knots (74-95 mph; 119-153 km/h). Damage is limited to foliage, signage, unanchored boats and mobile homes. There is no significant damage to buildings. The main threat to life and property may be flooding from heavy rains.

Category 2 – 83-95 knots (96-110 mph; 154-177 km/h). Roof damage to buildings. Doors and windows damaged. Mobile homes severely damaged. Piers damaged by storm surge. Some trees blown down, more extensive limb damage.

Category 3 – 96-112 knots (111-129 mph; 178-208 km/h). This is the first step of Major Hurricane. Landfalling major hurricanes have their names retired from the list of available hurricane names. For example, after Hurricane Charley made landfall in Florida as a Category 4 hurricane, its name was retired. In the future, when someone says “Hurricane Charley”, there will be no doubt which storm is meant. Category 3 storms cause structural damage to some buildings. Mobile homes are completely destroyed. Roof damage is common. Storm surge begins to cause significant damage in beaches and harbors, with small buildings destroyed.

Category 4 – 113-136 knots (130-156 mph; 209-251 km/h). Structural failure of some buildings. Complete roof failures on many buildings. Extreme storm surge damage and flooding. Severe coastal erosion, with permanent changes to the coastal landscape not unheard of. Hurricane force winds extend well inland.

Category 5 – 137+ knots (157+ mph; 262+ km/h). Complete roof failure on most buildings. Many buildings destroyed, or structurally damaged beyond repair. Catastrophic storm surge damage. All Category 5 hurricanes’ names are retired, regardless whether they ever make landfall. In the Northwest Pacific, a typhoon that reaches 150 mph (241 km/hr) is called a Super Typhoon. The damage caused by a super typhoon is equivalent to a strong Category 4 or Category 5 hurricane, depending on how strong the typhoon is. Because conditions in the Northwest Pacific favor storm formation throughout most of the year, super typhoons are much more common than Category 5 hurricanes. Every year the Northwest Pacific sees several super typhoons, while the Atlantic might see one Category 5 every few years.

SAFFIR-SIMPSON SCALE
CategoryKnotsMPHKM/HDamage
164-8274-95119-153Minimal
283-9596-110154-177Moderate
396-112111-129178-208Extensive
4113-136130-156209-251Extreme
Super Typhoon130+150+241+Catastrophic
5137+157+262+Catastrophic

Storm Surge

Historically, storm surge is the primary killer in hurricanes. The exact storm surge in any given area will be determined by how quickly the water depth increases offshore. In deep-water enviroments, such as the Hawaiian islands, storm surge will be enhanced by the rapidly decreasing ocean depth as the wind-driven surge approaches the coast. The peak storm surge is on the right-front quadrant (left-front in the Southern Hemisphere) of the eyewall at landfall, where on-shore winds are the strongest, and at the leading edge of the eyewall. Contrary to a popular myth, the storm surge is entirely wind-driven water—it is not caused by the low pressure of the eye. Another factor in the severity of the storm surge is tide. Obviously, an 18-foot storm surge at high tide is that much worse than an 18-foot surge at low tide.

Tropical Cyclone Formation

Tropical Cyclone Genesis is the technical term for the process of storm formation that leads ultimately to what are called hurricanes, typhoons, or tropical cyclones in various parts of the world.

This occurs when, in the Northern Hemisphere, the Intertropical Convergence Zone, or ITCZ, shifts northward out of the doldrums and atmospheric conditions become favorable for tropical cyclone formation after about the middle of May.

A series of low-pressure ripples develops within the ITCZ. These are known as tropical waves and progress from east to west. In the late season, they typically shift their movement toward the west-nothwest, or even northwest, after crossing 45° or 50° W longitude.

These tropical waves, ideally embedded in the deep-layer easterly flow, contain a northeast wind shift. This is typically referred to as a “convergence”, where lines of equal atmospheric pressure are pressed together between the high-pressure ridge to the north and the developing low-pressure system. The divergence that results ahead of the convergence zone gives us a notheasterly wind as the axis of the tropical wave approaches. Gusts up to 25 mph may occur. Sometimes there can be gusts to tropical storm force in stronger waves. There can be next to no weather associated with these waves, and they may pass virtually unnoticed. More typically, there are bands of disturbed weather riding the axis of the wave.

Easterly Wave
(Graphic by Robert Simmon, NASA GSFC)

When the wave passes over warmer waters (SSTs), convection and resulting rainfall are enhanced. This greater rainfall is concomitant with falling surface pressures. By the time these pressures fall to 1008mb, it is likely that the northeast wind has closed off to a southwest wind on the backside of the wave. The forward motion of the wave completes the closure on the northern side of a broad low-level center, and a tropical depression has formed.

We often hear that a tropical depression has formed, but conditions are unfavorable for further development. There are two conditions that must be present for the tropical depression to continue its development: warm SSTs (above 79° Fahrenheit/26° Celsius) and low vertical shear. A tropical cyclone derives its power from the warm waters below. In addition, a strong anticyclone directly above the low-level inflow is favorable. As a tropical cyclone is pulling in warm, moist air at the surface, it must also evacuate this inbound flow aloft. This occurs in the upper levels of the atmosphere, where high pressure facilitates the development of the cyclone by evacuating the flow from the lower levels of the cyclone. Every powerful hurricane has an equally powerful high pressure system over it. At the surface, the air spirals inward in a counter-clockwise direction, rises in the developing center, and spirals out at the top in a clockwise direction. In the Southern Hemisphere, it is reversed: clockwise inbound, counterclockwise outbound.

If the upper-level high pressure system does not develop over our cyclone, it means there is shear instead. This is a strong jet of air that is blowing directly over the cyclone, ripping the tops off the deep convection. This has the effect of breaking down the whole mechanism. This is known as vertical shear. Vertical shear usually comes from a westerly direction, and can occur if the cyclone is located in an unfavorable position near a cold front or upper-level low pressure system. Another factor that can interfere with the development of a tropical cyclone is subsidence. Subsidence is the sinking of air. This happens on the edge of an upper-level high-pressure system. Subsiding air has the effect of suppressing thunderstorm formation. This also is why a tropical storm that tries to form near an established hurricane has a very difficult time—the cyclone is on the edge of the hurricane’s upper-level outflow, and may have to contend with both subsidence and shear. The effect of shear on a cyclone can range from a failure to thrive to catastrophic collapse of the tropical cyclone’s support structure.

Tropical cyclones have the low-level circulation and the upper-level circulation (outflow), whose formation was discussed above. There is also a mid-level circulation. The mid-level circulation is similar in structure to the low-level circulation, and is critical to the survival of a tropical cyclone that is passing over land. The lowel-level circulation can be severely disrupted, or even dissipated, by interaction with land, especially mountainous terrain. If the mid-level circulation remains intact, the cyclone can regenerate rapidly when it reemerges over water, providing other factors are favorable.

Hurricane Structure
(Image courtesy NOAA)

Assuming all the ingredients are in place—warm SSTs, upper-level high pressure, and falling surface pressures—the cyclone will develop and reach a point of rapid intensification. It is one of nature’s perfect machines. As warm waters feed the convection swirling around the center, heavy rainfall lowers surface pressures, high pressure aloft evacuates the inflow, which intensifies the inflow of warm, moist air, which in turn increases the rainfall and brings about a more rapid fall in central pressure.

Eye formation begins when a tropical storm reaches approximately 65mph, provided conditions are favorable for strengthening to continue. The eyewall begins to make its appearance, usually on the eastern (Northern Hemisphere) edge of the center. As the system becomes better organized and stronger, the center contracts from about 200 miles across to roughly 90 miles at this stage. An increase in rotational velocity accompanies the smaller, more defined center. The inflow is spiralling in ever faster as it is evacuated up through the developing eyewall and out by the high pressure outflow structure. The eye begins to appear as a clear spot in the center, as the air here is sinking. The eyewall creates subsidence that not only helps clear the eye, but can also produce a feature known as the “moat”, which is an area of relatively weak convection outside the core of the tropical cyclone. The combination of eye and moat can make the storm’s core look like a doughnut. The eye can have a diameter anywhere from 10 to 40 miles across.

Tropical cyclones can exhibit a great deal of durabilty provided that the upper level support remains and the southerly (Northern Hemisphere) inflow is present. The worst thing that can happen is for this southerly (or equatorward) inflow to get cut off. Here are some examples: In 1998, Hurricane Mitch developed into a supermassive Category 5 hurricane. Nevertheless weakening began when the center moved to a position directly north of Honduras, cutting off the southerly inflow, even though the eye was still over a hundred miles off shore. Later, Mitch maintained its mid-level core against all odds over the mountains of Central America because he was able to advect moisture from the East Pacific. Reintensification to tropical storm strength was almost immediate after reaching the Bay of Campeche. Mitch never dissipated. In 1988 Gilbert hit the Yucatan near Cancún as a Category 5 hurricane. The Yucatan peninsula, though flat, extended far enough south through the critical southerly inflow zone that Gilbert never recovered, even after moving over the open waters of the western Gulf of Mexico. Contrast this with the northern Gulf coast and numerous examples from Camille to Elena and beyond, where the proximity of the eye to land is not necessarily a weakening factor, although the relatively extensive shelf water can sometimes be an inhibiting factor, if a slow-moving storm generates enough upwelling. A recent Southern Hemisphere example of this is Severe Tropical Cyclone Monica, which developed into a “super cyclone” just north of Australia. The northerly (equatorward) inflow over the Arafura Sea fueled this system.

What are the factors that contribute to the decay of a tropical cyclone? They are Upwelling, Entraining dry air, Moving over cool waters, Exposure to upper-level westerlies, and finally Landfall.

Upwelling. When a hurricane stalls, its movement is has fallen below 5mph, or its movement is erratic over a small area, the wave action caused by the strong surface winds churns the ocean surface and produces upwelling. This has the effect of cooling the temperature of the sea surface over an area 200 to 300 miles across. The result is weakening. It is possible for a hurricane to stall in one area long enough that it dissipates. In 2004, Hurricane Frances stalled off the southeast Florida coast long enough that the core of the hurricane collapsed. Frances eventually made landfall as a Category 1 hurricane, instead of the earlier expected Category 3.

Entraining dry air. Sometimes, during the peak season, when tropical cyclones approach contintental land masses, they may entrain dry air as part of their interaction with frontal troughs that carry cool, dry air behind them. It is one of the ironies of the Atlantic Hurricane Season that, just when things get going, it’s already September and the strength and frequency of cold fronts is increasing. These fronts interfere by deflecting the hurricane or injecting dry air into the circulation, or both. The dry air kills the convective masses that drive the hurricane’s engine. If the dry air entrains deeply enough, it can cause significant weakening.

Moving over cool water. Similar to upwelling, when a tropical cyclone moves over cool water (below 77° Fahrenheit/25° Celsius), it begins to weaken. Eventually this causes dissipation, particularly in the East Pacific. In the Atlantic, if the storm is caught in the mid-latitude westerlies and begins to recurve to the northeast, it generally becomes an extratropical storm by the time it has reached about 45° W. This is the so-called “graveyard” of Atlantic hurricanes. The storm becomes extratropical when it has transitioned to a cold-core, baroclinically driven system, and eventually becomes a gale in the north Atlantic, or is absorbed by a large gale. There have been times when a hurricane passes north of the Azores and hits the British Isles as a Force 8 or stronger gale, having maintained a recognizable inner core.

Exposure to upper-level westerlies. It happens that the ridge of high pressure that keeps the hurricane heading toward the west frequently breaks down. In the Atlantic Ocean, this may result in strong upper-level westerlies diving down and impinging on the northern edge of the hurricane’s upper-level support structure. Poleward outflow is restricted. A hurricane can still thrive when outflow is restricted in one quadrant. If the forces responsible for the constricted outflow bear down too strongly, the hurricane undergoes acute shearing. As outlined above, this can be devastating.

After landfall. Tropical cyclones cannot survive over land, once their access to warm SST is removed. A powerful hurricane, such as Hugo, which hit Charleston, S.C. in 1989, can project life-threatening hurricane force winds over two hundred miles inland. As the storm progresses inland, it can dump a huge amount of rain—sometimes measured in feet. The storm may evolve into a frontal cyclone that continues to cause widespread damage. The best example of this is Hurricane Camille in 1969—the strongest hurricane ever to make landfall on the continental United States with winds sustained at 190 mph and gusts well exceeding 200 mph—which roared up the Mississippi Valley and eventually exited off the East Coast. Camille maintained tropical storm strength as far as Memphis, Tennessee. Most hurricanes will diminish in strength rapidly after landfall, reaching tropical depression strength by 48 to 72 hours. The main threat from the dying storm is from tornadoes and inland flooding. The right-hand quadrant of a hurricane or typhoon (in the Northern Hemisphere, left-hand in the Southern Hemisphere) is most frequently the strong side of the storm. This is because the forward motion is added to the counter-clockwise punch of the storm. Thus, the right-hand side of the hurricane contains the strong on-shore flow. This is where the maximum storm surge flooding, and the greatest potential loss of life, will be experienced. After landfall, the friction of the circulation moving over land causes a great deal of turbulence, which results in tornadoes. These are especially likely in the forward right-hand side of the storm’s path. The dying cyclone will dump many inches of rain. The lack of access to a warm sea surface results in the death of the tropical cyclone. It usually merges into a frontal trough, or dissipates.

Global Tropics Hazards and Benefits Outlook
Graphic provided by Climate Prediction Center