925-hPa winds, frontogenesis (orange contours) and 700-hPa omega (upward motion is red and white area) valid 1200 UTC 16 Sept

A pool of cooler drier air is usually associated with mid- and upper level troughs, especially the stronger ones.† Note the almost cold air damming look to the isotherms over Virginia and the Carolinas.† Clearly a front was present and showed up on the 925-hPa 0000 UTC Sept. 16 analysis.†† As the Floyd interacted with the trough and moved north,† the thermal gradient tightened to its north and east.† Strong frontogenesis and isentropic lift was taking place along the thermal boundary

The 4-panel GOES-8 loop shows water vapor imagery (top right) and three goes sounding channels.† The two bottom panels of the loop are sensitive to mid- (left) and low-level (right) moisture.† Note how the clouds increase to the west and northwest of the storm and that the heavy rainfall across North Carolina begins well in advance of Floyd.†

The track and speed of Hurricane Bertha were very similar to that of Floyd making it an interesting storm to study since it rainfall amounts and rainfall distribution were different than occurred during Bertha.† The axis of heaviest rainfall during Bertha was to the right of the track across the southern Middle Atlantic States but shifted to the west side from New Jersey northward.†

Bertha and Floyd were both Category 2 hurricanes at landfall.† However,† the circulation associated with Bertha was smaller.† An even bigger difference between the systems was the thermal structure around the storm.†††

Loop courtesy of Rod Scofield

Image from Kelsch lecture

Composite radar reflectivity valid 1200 UTC 16 Sept. From Atallah and Bosart 2003.

925-hPa height and winds (black), temperature (blue lines, oK),† valid 0000 UTC 16 Sept 1999

925-hPa height and winds (black), temperature (blue lines, oK),† valid 1200 UTC 16 Sept 1999

The radar echoes and region of rainfall elongated along the frontal band.† Such an elongation of the precipitation pattern is typical when a front is present.† The axis of precipitation of left of track (LOT) storms often is aligned parallel to the movement of the storm which increases the period of heavy rainfall.†† Bosart (personal commuciation) believes that the LOT cases on average produce a larger total volume of rainfall than right of track (ROT) cases.† The idea is supported by two six-case composites (shown below) that one of his students, Josh Darr, assembled for his M.S. thesis at SUNY Albany.

Strong extra-transition 6 case composite cases, precipitation contoured in mm.†

Composite of non-transition cases, precipitation contoured in mm.†

1000-500 hPa thickness (dashed black lines, contour interval=60 m), 1000-hPa geopotential height (solid black lines, contour interval=30 m) and 850-200 hPa wind shear (shaded, contoured every 5 m s-1, starting at 20 m s-1) ,† for right of track cases. Thick black line is the trough axis. (from Atallah et al, submitted to Mon. Wea. Rev. 2006)

Precipitation primarily right of the track (ROT).storms.

The composites of storms with the bulk of their precipitation to the right of the track from the Atallah et al. study are shown below.† There are several notable differences in the ROT composite from the pattern typically associated with LOT storms.† The axis of the 850-200-hPa shear is typically oriented more westerly than the more north-south oriented axis associated with the approaching trough during LOT cases. Also,† the location of the upper level jet streak is much farther removed for ROT cases than for LOT cases.† The baroclinic zone as shown by the leading edge of the 1000-500 hPa thickness is also much farther removed from the storm.† Also,† implied 1000-hPa circulation is much weaker, especially on the northern and west side of the system where there is a much weaker gradient implying that the easterly low level winds are quite a big weaker for ROT storms than LOT. storms.†

Donít be fooled by the compositeís location of either LOT or ROT cases. 11 of the 16 cases for the ROT composite made landfall over the Gulf of Mexico and 5 made landfall along the east coast, usually near the Georgia-South Carolina border.† 8 of the storms transitioned from a ROT storm to a LOT storm.†

850-700-hPa PV (shaded in cool colors every 0.15 PVU starting at .45 PVU where 1 PVU=10-6Kkg-1 m2 s-1) and 300-200-hPa PV (shaded in warm colors every 1 PVU starting at 2 PVU) and winds (barbs, knots convention).†† Thick dashed contours indicate areas of positive PV advection in the 300-200 hPa layer.† The thick black line represents the axis of cross sections that except for at T+24 are not shown.† (from Atallah et al, submitted to Mon. Wea. Rev. 2006)

The more zonal look to the pattern helps keep any northern stream PV anomaly well removed from the low level PV center associated with the tropical system. The right hand composites lack the negative tilt.†

250-hPa winds and heights valid 0000 UTC 13 July 1996, the red dot indicates the positions of Bertha

925-hPa height and winds (black), temperature (blue lines, oK),† valid 0000 UTC 13 July 1999

During Bertha the 292oK isotherm dipped southward around Bertha which helped make the isotherms almost parallel to the winds to the north and northeast of Bertha (see figure above). At 00 UTC 13 July, no frontal boundary was present near the storm.† The lack of warm advection suggested that the heavy rainfall would be confined to near the center or to feeder bands on its east side.† Also, the jet was well removed from the storm with generally westerly flow across the storm,† the latter suggests that the storm may have been affected by shear.† Also, the orientation of the upper level jet was more west to east than north to south when compared to Floyd.† The implied shear would tend to keep the convection on the east side of the system and at that time, the heaviest rainfall was located to the east of storm.† The structure of the storm is an important component in predicting the rainfall with a storm.†