RAMMB Satellite Case Studies

An Undular Bore off the Massachusetts Coast

Published: April 23, 2003

On April 16, 2003, the passage of a sharp cold front (temperature dropped 56 deg F at Blue Hill Observatory in less than 18 h) was associated with the development of an undular bore off the Massachusetts coast, as noted by the GOES 12 visible satellite loop below:

GOES-12 Visible Loop

Long, thin bands of low cloud form parallel to the front (probably indicated by the leading cloud arc) and travel southward across the Cape Cod region. East of the Cape, the pattern is better defined with 7 or 8 narrowly spaced bands. In this case, the wave train was excited when the cold front interacted with a strong marine layer inversion caused by the advection of unseasonably warm air out over the chilly Atlantic. Record highs of 84 and 85 were set at Boston and Blue Hill Observatory, respectively, while Chatham out on Cape Cod (just below the northward bend of the Cape), remained in the 50's. This marine inversion is shown nicely in the skew-T plotted from Chatham's rawindsonde observation:

Chatham sounding for 12Z April 16 (before frontal passage)

Chatham sounding for 00Z April 17 (after frontal passage)

For comparison, the postfrontal skew-T at 00Z on the 17th is also shown above, about 4 hours after the frontal passage.

Following are some surface observations from Chatham during the time window of interest (the most recent obs are at the top):

From the satellite images, it appears that the leading cloud arc passes through Chatham at 2015Z, which coincides nicely with a windshift observed at the surface at 2017Z and a peak wind of 31 kts at 2020Z. Pressure was rising rapidly during the cold front passage.

This images from this event show some similarities to the undular bore observed on the Texas coast a few years ago, as chronicled in a 'Picture of the Month' feature in Monthly Weather Review (Clarke, 1998).

In both cases, it appears that a sharp cold front interacted with a strong temperature inversion ahead of the front to cause the bore. In the Texas case, it was a nocturnal inversion. In the Massachusetts case, it was a strong marine boundary layer inversion. In the Texas case, the cloud system was not associated with surface temperature or dewpoint change, just a prefrontal pressure trough and wind, whereas the bore in the Massachusetts case appears to be nearly coincident with the front.

For more on undular bores, see Smith (1988).


Clarke, J. Christopher 1998: An Atmospheric Undular Bore along the Texas Coast. Mon. Wea. Rev., 126, 1098-1100.

Smith, R. K. 1988: Traveling waves and bores in the lower atmosphere: The "Morning Glory" and related phenomena. Earth Sci. Rev., 25, 267-290.


Jonathan Vigh