From 11:40 a.m. 3
May 2010:
Hi Everyone,
Prompted by the group QPF forecasts from last
Th.......
A critical part of national forecasting is to
develop a good situational awareness of current and impending weather patterns.
For example, the record-breaking rains in parts of the TN Valley the last few
days occurred with three consecutive soundings from BNA that showed PW values
> 50 mm (0000 UTC 2-3 May 2010). How unusual is it to have PW values > 50
mm at this time of the year at BNA? To find out, go to the below PW climo link
Precipitable water climatology for NWS raob stations:
http://www.crh.noaa.gov/unr/?n=pw
Then click on BNA:
http://www.crh.noaa.gov/unr/include/pw.php?sid=bna
The all-time PW records for April and May at BNA are
~44 mm and 53 mm, respectively, while +2 standard deviation PW values are ~35
mm and 42 mm for April and May, respectively. A blend of the April and May PW
climatologies is appropriate for an event that occurs at the very beginning of
May. By any measure, the PW values in BNA (and surrounding areas) were
"off the charts" for this time of the year.
Given an appreciation of the situational awareness
of the PW climo and the fact that the GFS was predicting 50+ mm PW values in
the region on Fri-Sun from earlier last week, a red flag should have been
raised in everyone's mind that an extreme weather event was possible. Given
further that a large-amplitude, slow moving trough with quasi-unidirectional
winds (translation: training echo situation) with embedded short-wave disturbances
(translation" lifting mechanisms") was forecast to the cross the
region with a boundary in place, there was likely going to be a high risk of an
extreme heavy rain event.
There is no more important *general* forecast skill
than being able to anticipate the possibility of extreme weather events a few
days in advance and the subsequent follow-through required to gear up for the
event through the watch and warning process should the extreme weather event
look to be increasingly likely closer to the time of the event.
CIMSS GOES-13 IR loop of historic TN Valley flooding:
http://cimss.ssec.wisc.edu/goes/blog/archives/5432
Lance