Day 4-8 experimental outlooks
Here's an example of one graphic for the day 4-8 composite outlook from
several days ago, valid for the time period that includes the past few
days' severe weather over the southern plains and southeastern CONUS.
http://www.stormeyes.org/tornado/temp/day4...4-8_example.gif
This is, as others have told already, a purely experimental product for the time being, though the expectation is for it to be permanent perhaps
beginning next year. The Day 4-8 composite severe forecast, by the very
nature of trying to forecast a collection of mesobeta- to storm-scale
events several days out, is going to be a broadbrush outlook and
shouldn't be taken too precisely. [In other words, being barely outside the line is not appreciably different than being barely inside.] It's not clear yet that such forecasts even can be done well, consistently; but there's some reason to try based on the success of the day-3s.
The reason days 4-8 were combined instead of parsed out by single day is the growing temporal uncertainty of system timing into the future. Any chaser who has made fantasy-prog guesses knows what I mean. These really are coarsely delineated "outbreak outlooks" in a sense, because isolated/mesobeta scale events usually aren't "synoptically evident" enough to stand out as signal amidst the noise so far into the future.
Still, because of an apparently growing demand for *someone* to do an
extended severe forecast, it might as well be the SPC. [Would you rather it be anyone else?] CPC has been doing severe forecasts for their extended threat-assessment graphics (sorry, don't know the URL off my head); and these day 4-8s can be used as input to that.
So: What's left to be determined is not
whether it will become a part
of the SPC forecast suite, but
in what way. The format has changed internally a few times already during the short few months that it has been tested.
A lot of questions remain to be answered -- including best format for
service, how to wrestle with the very nebulous and sketchy science of
forecasting severe that far out, what sort of display to use, and how to
objectively verify the forecasts. None of those are trivial concerns!
The ultimate outlook might look nothing like the example above by the time it becomes operational. So until a consistent, standardized product can be developed that
1. Is shown to provide a useful service to
at least the internal
NOAA/NWS community and
2. Has been shown to be more reliable (numerically) than climatology or
a coin toss,
....then there's no use putting it out there for all to use before it's proven useful. It's NOAA-accessible mainly for CPC benefit and so NWS customers can provide early feedback on how it might best suit their needs.
Here are some of the ingredients that go into that forecast as of now:
* Deterministic extended models (spectral/GFS, ECMWF, NAM/DGEX, etc.)
* Model ensemble guidance (*very* heavily used!)
* Climatological data
* Algorithmic analog-retrieval systems that compare ensemble-forecast
patterns at various levels (i.e., 500 mb heights, 850 mb dew points, PW
and other fields) with nearest matching historic events then plot "hindcast forecast" severe probabilities accordingly -- a very promising tool!
So it will be a lot more than just a casual guessing game involving a
couple of models that one might be able to snatch off the 'net, or
speculating over fantasy progs like a lot of us do when seeking the
promise of chase nirvana sometime next week. Even then, not enough of
these forecasts have been done over enough events to say one way or
another now much, if any, skill they show.
So the word is: patience.
