Friday, February 11, 2011

2011-02-06 Freezing rain thunderstorms in Albany, NY

It's not often that thunder occurs in the northeast during the month of February.  When it does, there's probably only a few lightning strikes occuring over a relatively broad area.  Not this time.  Checking out the cloud-to-ground lightning maps in the area reveal lightning frequencies on par with some of the better summer-time convection in the same area.
Figure 1  Cloud to ground lightning map for the northeast US from 00 - 03 UTC.  The lightning strikes colored in white represent those within the first 12-30 minutes of this image time.  The yellow represents the 31-50 minute time frame.  Red indicates 51-70 minutes and so on.

 These thunderstorms formed as a line of ordinary cells aligned north-south axis.  The cells were more discrete in the southern portion of that line passing through western Massachusetts.  This radar capture below in figure 2 shows the cells as they passed just east of the Albany area.

figure 2

This kind of lightning frequency requires a substantial amount of instability.  Fortunately, the Albany sounding, launched just before 00Z provides an opportunity to sample the environment just ahead of the convection.  Indeed, the profile showed a region of strong elevated instability with steep lapse rates starting above the frontal surface at nearly 2 km AGL (see figure 3).
Figure 3.  A sounding from Albany, NY on 6 Feb 2011 00 UTC.

This convection fired up in the front end of a dry slot wrapping around the big upper-level wave to the north.  Such a configuration reminds me of a study by Carr and Millard (1985) where a dry slot aloft allows surface heating to commence while cooling continues aloft with unsaturated ascent ahead of the trough.  However, it's clear from the sounding that a saturated frontal inversion would prevent any surface heating from occurring in this event. 

Figure 4.  GOES12 IR image with cloud to ground lightning taken 6 Feb 2011, 0325 UTC.  Each color in the lightning represents 15 minute intervals.

So this event does have elements of dry slot convection and it also happens to fit a synoptic pattern in which the convection forms near or just north of the surface low as thundersnow events often do within the case selection process documented by Market et al. (2002).  Note the sea level pressure field in figure 5 relative to the active convection.

Figure 5.  Sea level pressure field relative to the composite reflectivity from the SPC mesoanalysis page. 

There is one major difference between this event and those studied by Market et al. (2002).  This event featured freezing rain.  But further north, the convection extended to a location where the sounding remained below freezing and snow could reach the ground.  Look at this RUC two hour forecast sounding in figure 6 from 02 UTC near Burlington, VT and the same midlevel instability appears there.

Figure 6.  The 00 UTC RUC 2 hour forecast valid 02 UTC 06 Feb 2011 for Burlington, VT.
Was this event forecasted?  Well, it was by some of the ensemble members in the SREF  model suite as you can see by this ensemble model sounding visualization in figure 7.

Figure 7.  Ensemble sounding display in BUFKIT from the 15 UTC 05 February 2011 9 hour forecast at Albany, NY.

Most of the permutations from the Regional Spectral Model showed significant instability in the midlevels.  Even though only 1/3 of the SREF members showed instability, that still amounts to a 30% chance of thunderstorms.  With the given hazards presented by these storms when considering the subfreezing lower atmospheric profile, it stands to good reason that identifying this thunderstorm potential ahead of time is a good idea.  This is especially true since the output in the SREF model agrees well with the synoptic patterns identified that commonly produce convection.  In fact, the SREF model output shown in concert with the pattern matching may help to overcome any inhibition for including thunderstorms in a forecast when it is climatologically and geographically rare.

Carr, Frederick H., James P. Millard, 1985: A Composite Study of Comma Clouds and their Association with Severe Weather over the Great Plains. Mon. Wea. Rev., 113, 370–387.

Market, Patrick S., Chris E. Halcomb, Rebecca L. Ebert, 2002: A Climatology of Thundersnow Events over the Contiguous United States. Wea. Forecasting, 17, 1290–1295.

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