A World of Weather
Fundamentals of Meteorology

Laboratory Exercise #4

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On the afternoon of November 9, 2009, Tropical Storm Ida bore down on the central Gulf Coast. By early morning on November 10, Ida was already on its way to becoming extratropical, a transition that wasn't surprising, given the inevitable cooling trend over land during late autumn and the corresponding seasonal increase in vertical wind shear.  The 12Z surface analysis on November 10 encapsulates the transition of Tropical Storm Ida to an extratropical low. Indeed, note the incipient front connected to the tropical-storm icon marking Ida's official position...the "mixed" nature of this surface representation indicates that the developing system had both tropical and extratropical characteristics at this time.

The 12Z run of the GFS model on November 10 predicted that the mid-latitude low-pressure system would become a memorable storm. To get a sense of the simulated evolution of the storm, run your cursor over the forecast times along the top of the progs.  In case you're wondering, the labels, f06, f12, f18 and f24 (for example) correspond to the 6-hour, 12-hour,18-hour and 24-hour forecasts (respectively).  Additionally, the individual progs associated with these specific forecast times were valid at 18Z on November 10, 00Z on November 11, 06Z on November 11, and 12Z on November 11 (respectively).  In a nutshell, running your cursor over the forecast times allows you to look at a loop of progs so that you can get a sense of how the model predicts weather patterns and systems will evolve with time.

(a)  Focus your attention on the upper-left panel of the six-hour prog (500-mb heights and 500-mb absolute vorticity).  What kind of 500-mb feature did the GFS predict to lie above the extratropical remnants of Ida at 18Z on November 10?  Briefly explain. 

(b)  Slowly run your cursor over the forecast times between 6 and 48 hours.  Describe the interaction between the feature you identified in part (a) and a similar feature in the northern branch of the 500-mb westerly flow of air.  Were these features predicted to eventually merge? Please explain.  Recalling the general discussion in Chapter 16, what's the formal name that meteorologists give to this kind of interaction between the northern and southern branches of the 500-mb flow?

(c)  Focus your attention on the upper-right panel of the 48-hour prog.  At this forecast time, what was the predicted central pressure (approximately) of the mid-latitude surface low that absorbed the extratropical remnants of Ida?  Proper units are a must!

(d)  Still referring to the 48-hour prog, qualitatively describe the magnitude of the predicted pressure gradient along the Middle Atlantic Seaboard (North Carolina to New Jersey) at this time.  Was the mid-latitude surface low predicted to be exclusively responsible for this pressure gradient?  Please explain.

(e)  The predicted pressure gradient along the Middle Atlantic Coast was consistent with the eventual wind observations on November 12. To see what we mean, check out this meteogram at Wallops Island, VA, which we assume to be representative of Middle Atlantic coastal stations around this time. Based on this meteogram, do you think that the National Weather Service likely issued a gale warning for parts of the Middle Atlantic Coast? Please explain.  Did this system qualify as a nor'easter (even though it had its roots in the Tropics)? Please explain. 

(f) Slowly run your cursor slowly over the forecast times after 48 hours.  For the Middle Atlantic States, was this expected to be a short-lived (less than 24 hours) or a more protracted event?  Please explain, specifically citing the evolution of the system on the upper-left and upper-right panels. 

(g)  In addition to the winds discussed in part (e), what other types of significant conditions should you have expected along the Middle Atlantic Coast?  To help you formulate your thoughts, check out the following photographs taken along the Middle Atlantic Coast during the height of the storm. Photograph #1 was taken at Wallops Island, Virginia, and Photograph #2 was taken in Chincoteague, Virginia

(h)  Write a short paragraph that relates the speed and persistence of the coastal winds discussed in parts (e) and (f) to the threats documented in part (g).  Please compose the paragraph in such a way that the general public will easily grasp the basic meteorology and oceanography of this event.  Please include a general reference to the times of high astronomical tides

(i)  In light of your answer in part (f), check out the satellite-derived rainfall (in millimeters) from November 6-13, 2009.  Over interior Virginia, what was the greatest rainfall in inches?  Try this online converter.  What lifting mechanism likely contributed to the heavy rain over interior Virginia?  Please explain.