Weakening tropical storms and depression

Tropical storms and depressions often can produce heavy rainfall, sometimes days after landfall and at times with a distinct nocturnal maximum.  Grice and Ward (1983) conducted a 10-year study of tropical depressions and hurricanes that moved inland into Texas in hopes of separating the “wet ones” from the relatively dry ones.  They found that 5 of the heaviest 24-hour precipitation totals occurred one or more days after landfall.  They found for the “wet” and “dry” cases, the lower and mid-tropospheric were often surprisingly similar.  The biggest difference appeared to be in the upper levels and that the wet cases were usually associated with upper-level tropospheric waves and or jet streaks. 

A case in point was Tropical Storm Amelia.  Like many weak tropical storms,  the low level circulation associated with Amelia weakened once the storm came inland.  However,  at least a portion of Amelia’s 500-hPa circulation held together an contributed to heavy rainfall on two consecutive nights in Texas.  Am unofficial report of over 45 inches of rain was observed in the Texas hill country over a 2 day.  period. 

From Grice and Ward, 1983

IR image valid 1230 UTC 1 August 1978 (Caracena and Fritsch 1983)

IR image valid 0000 UTC 1 August 1978 (Caracena and Fritsch 1983)

The surface circulation weakens and only a weak cyclonic shear axis was present by 0000 UTC 1 August.  Such weakening is not that unusual for weak tropical systems once they move inland, especially into Texas.  

The convection associated with the remains of the storm also weakened.  In such cases,  Caracena and Maddox have cautioned that forecasters should. 


1) Anticipate the dissipation of thermally induced showers as boundary layer temperatures decrease.

2) Anticipate the development of deep convection at night somewhere near the core (within about 200 km radius of a weakening tropicsl storm and or an associated shortwave vorticity maximum

3)    Look for possible mesoscale features that might help      focus convection (ie.  Terrain, outflow boundaries, low level jet or area of low level cyclonic shear).

4)     Locate the most unstable are flowing into the core region.  The horizontal distribution of unstable versus stable air is a key in determining whether propagation effects might lead to a quasi-stationary convective system. 

Note that by 1230 UTC 1 August that thunderstorms had developed over the Hill Country.  By that time,  the surface circulation had died.  However, there was a 500-hPa low to the southwest that helped supply deep upslope flow into the escarpment (see below).  Low-level southeasterly flow was also present. When a storm is dying start thinking of it as a potential mesoscale convective system producer. 

Terrain (warmer colors are higher terrain)

500-hPa analysis valid 1200 UTC 1 August 1978

200-hPa-hPa winds and divergence  for 0000 UTC 2 August 1978.  The trough axis is shown by the dashed line (Caracena and Fritsch 1983).

IR image valid 0000 UTC 2 August 1978 (Caracena and Fritsch 1983)

IR image valid 1200 UTC 2 August 1978 (Caracena and Fritsch 1983)

The 500-hPa low from the remains of Amelia merged with the low over Mexico by 0000Z 2 August (not shown).  Grice and Ward noted that the proximity to an upper level trough seemed to be favorable for producing wet storms.  Note the approach of a trough at 200-hPa and that the convection developed in the region where there was strong divergence  anticyclonic curvature in the 200-hPa wind field. 


During the night the convection contracted towards the core of the system. The cloud top temperatures also warmed which can be misleading as extreme rainfall rates still can occur when the bases of the clouds lower and mean relatively humidity becomes high. 



The moisture associated with Amelia produced another MCS the next day over northern Texas. A frontal boundary that dropped south into northern Texas was the focus of this latter MCS.  Again, look for mesoscale forcing when dealing with the remains of a tropical system. 

Courtesy of Dave Roth

The remains of Elena

Sometimes the remains of a tropical system will act very similar to a mesoscale convective vortex (see http://www.hpc.ncep.noaa.gov/research/mcs_web_test_test_files/Page1207.htm for a discussion of MCVs).  These systems can be quite wet even after the surface low has weakened.   They are more liable to act as MCVs when the mid– and upper level shear is weak.  Note that the 500-hPa circulation remains removed from the westerlies and that no surface low is evident but that tropical moisture has spread northward along the western side of the ridge over the southeast. 




500-hPa height and vorticity analysis v.t. 1800 UTC 3 Sept. 1985. 

PMSL (contour interval=2 hPa) and precipitable water ( mm) analysis v.t. 1800 UTC 3 Sept. 1985.

1800 UTC 3 Sept. 1985 GOES vis imagery.

1800 UTC 3 Sept. 1985 GOES IR imagery.

A distinct swirl and apparent circulation was present on the satellite imagery but the convection was rather spotty and found around the periphery of the system rather than near its center.  This is rather typical of a mesoscale convective vortex.  Almost no convection was present by 0400 UTC 4 Sept.

0400 UTC 4 Sept. 1985 GOES IR imagery

If you treat the remains as an MCV it becomes easier to decide whether to another round of heavy rainfall might be expected.  The first step to take is to look at the low-level wind field to determine where the source region for any subsequent convection might be.  During the day,  MCVs often have convection form along the periphery but at night convection often develops directly below the MCV center (Trier et al 2000). 



At 0400 UTC there was only a few weak cells associated with the remains of the storm.