Saturday, July 15, 2006

Postmortem of yesterday's "forecast"

One of the most important aspects of weather forecasting is (or used to be) doing routine postmortems of what went wrong yesterday. The storms in southeast Arizona yesterday (Friday, July 14 2006) did not evolve as I had expected. I had anticipated a late afternoon, large MCS event with some significant rains and winds across the Tucson metro area.

What went wrong: First, I made a novice kind of mistake by finalizing my outlook before all the observations were available. (Note that I forecast mostly using the basic observations and then may use model progs to cross check details of what I'm anticipating.) Yesterday's morning sounding that eventually arrived from Phoenix NWS/SRP showed a considerably drier lower-half of the atmosphere than did Tucson's and that advective winds were pointed toward Tucson. So, during the day that deep layer of moisture that was sampled by the TUS morning sounding gradually vanished, particularly between 700 to 500 mb. Thus, much of the day in southeast Arizona was markedly suppressed. It's not an irrelevant question to ask: "Where did that low-level moisture sampled by the Tucson sounding go?"

However, storms erupted near dark, producing a great lightning event, and I see there were a couple of severe reports around 10 pm. Two small MCSs raced west-southwestward during the night, so the outlook was not a total bust.

Looks like the drying we suffered yesterday was advecting from the higher terrain to north/northeast and mixing down - evening soundings at PHX and TUS were much different than in the morning. The TUS evening sounding remained very unstable but needed substantial mesoscale kick to lift moist part of BL air to LFC. (The 00Z TUS sounding shows an excellent example of how very strange boundary layer structures can sometimes evolve out here.) Davis Monthan AFB and TUS international airport both got measurable (TUS 0.30") and DM had gusts to 52 mph - lightning show was spectacular from the house with lots of thunder - note that once again a storm went right over the TUS airport and there was no tstm observed there (one TUS ob had the added remark "distant lightning SE-SW"). Not even a trace here, as is so often the case.

Next couple of days will be dominated by the huge, upper-tropospheric cyclone that's coming westward across northern MX (note there were a couple of impressive MCSs moving westward from TX Panhandle into NM last evening!). Our big day better be today - there's substantial cooling below 500 mb and lots of upper-level moisture coming with this not-at-all-subtle system. So I expect some substantial changes while we're under the influence of this large-scale system.

I note the following in the TUS morning forecast - POPs go to 40% for tonight and tomorrow night but are 30% for all other periods for next 7 days - high temps are forecast in the range 99 - 108F for entire 7 days with today and tomorrow being hottest (today 103 to 108 and tomorrow 100 to 105). I suspect that we may well deviate much more from climatology than this forecast does.

Should be another couple of interesting weather days!

1 comment:

  1. Steve Burk10:19 AM

    A very interesting postmortem discussion. In this case, I feel that I likely learned more from your discussion than had your original forecast verified. Thanks.