09 March 1999:
Heavy Snow Over the Mid-Atlantic Region:
A CSI Perspective

David Schultz

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Scott Bachmeier writes:

You may have heard about the "DC snow bust" earlier this week, (09 March) when the Washington area was "surprised" by up to 11 inches of snow. Naturally, my morbid curiosity led me to collect a few GOES images and place them at http://cimss.ssec.wisc.edu/goes/misc/990309.html.

I'm always looking for subtle banding features in the satellite imagery, as potential indicators of precipitation enhancement in regions of slantwise convection. Low and behold, there was some banding in the vicinity of DC http://cimss.ssec.wisc.edu/goes/misc/990309_hires_vis14.GIF.

I found it interesting to read the NWS forecast discussions for this event. From the Blacksburg VA office (at 08/2121 UTC):

"...by mid morning...expect moderate PVA and strong WAA to bring precip burst across the area...Ertel Potential Vorticity fields support CSI enhanced upward vertical velocity...and possibly some elevated convection..."

From the Washington office (at 09/1935 UTC):

...enhanced precipitation banding seen on 88D from eastern panhandle WV along and either side of the Potomac including the DC metro area looks suspiciously like CSI banding but the potential for that not indicated in model derived vertical cross sections...

I don't have access to GEMPAK...and I'd have to order the AWIPS dataset for this event...any chance you have the 09/00Z and/or 09/12Z models handy to do a quick'n'dirty diagnosis?

Scott Bachmeier University of Wisconsin - Madison | SSEC | CIMSS

Dave responds:

Thanks for the showing me this case, Scott. It's a great case and I think the diagnostics are pretty straightforward. Give credit to the Blacksburg forecast office for their foresight in recognizing the potential for enhanced precipitation rates!

Here are four-panel CSI diagnostics from the RUC-2.
00-h forecast from 09/00 run
06-h forecast from 09/00 run
00-h forecast from 09/12 run
06-h forecast from 09/12 run
00-h forecast from 10/00 run

Surface data from IAD (Washington/Dulles Airport)

From these plots, you can see that conditional symmetric instability (not conditional instability) was found in eastern Virginia after 9/12. By 9/12, the air above the frontal surface (above 800 mb over Virginia) was nearly saturated (>90% RH) and ascent was forced by the frontogenesis at this level. Thus, the three ingredients (instability, moisture, and lift) needed for slantwise convection were present. By 9/18, the upper-level forcing arrived and S+ was recorded at IAD. At 9/18, there were four inches of snow on the ground. At 10/00, there were 9 inches, followed by another inch in the next 6 hours. As the snowfall rate decreased, it appears as if the instability was no longer present (presumably, released during the period of heavy snowfall?) and the deep forcing (lower-level frontogenesis supported by upper-level forcing) was no longer present.

Here is an email I received from the Steve Zurbick at the Sterling, Virginia (office responsible for DC) Weather Service Office. I agree with many of his comments. This was a difficult forecast produced under a difficult situation!

If you have any further questions about the research discussed here, or desire a manuscript, please feel free to write to me: david.schultz@noaa.gov.

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Last update: 7 May 1999

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