"Jonathan" wrote in message
news:cPand.115563$R05.55665@attbi_s53...
I've been wondering about this for a while now, and am sure someone here
will know...

How can I find/figure out what the forecast cloud bases will be 3 or 4
days out? I've looked on ADDS, AOPA's weather, NOAA.gov, etc but not
found anything.

In other words, if I'm going flying in a few days, And I go to
weather.com and see 'mostly cloudy' in the forecast, how do I know if
that's "2000' better-file-an-ifr-flight-plan Cloudy" or "10,000'
VFR-is-no-problem" cloudy?



Peter is pretty close in his "forget it" point-of-view, but there are a few
things you can do if you want to try a guess.

The best, of course, is to be able to talk to the government's forecast
office. They have an idea of why they would have said "mostly cloudy". The
problem with this approach is.... 20 years ago, you could do just that for
free. These days, you get a free recording, but if you want anything more,
you have to go to a 3rd party fee-for-service provider. Canada used to have
a 1-900 fee-service to talk to a forecaster, but I don't see it advertised
anymore.... they just seem to say "go to the private fee-for-service
provider".

The next best thing is to know what the forecast situation is going to be.

(I digress, but frankly, I am amazed at the poor quality of Meteorological
training that ground-school seems to provide. In some ways, boaters get a
better course from the Power Squadrons. The "Meteorology" chapter of one of
the most popular Basic Training text in the US, written by probably *the*
most well known instructor in the country, has a few rather bizarre sections
in it. ... but I digress)

Back to topic:
In Canada and the US, the Computer-produced forecast runs are done twice
daily at 0000Z and 1200Z (available several hours after the nominal
run-time). The Canadian version is available on-line at
http://weatheroffice.ec.gc.ca/model_.../global_e.html ... I don't know
about the availability of the US version on line, but the Canadian one
pretty much covers all of North America.

The forecast charts from the 00Z run goes out to 144 hours, and from the
12Z run out to 72 hours.

Now, if you look at the surface-pressure forecast in the top-right panel of
each chart, you might be able to get an idea as to what the Pressure pattern
is expected for your date and place of interest.... (use the black&white
charts at the bottom of the page, the colored ones at the top are
useless.... and don't forget that the valid time: 0000Z "Tuesday", means
MONDAY EVENING in most of North America)...

All of us should have been watching the weather daily, since ground school,
and comparing it to the CURRENT surface analysis every day. By now, we
would have a pretty good idea what sort of weather we get in various
pressure regimes. All we would have to do is look at the FORECAST pressure
chart for the day in question, and we would have a pretty good idea what to
expect.

For those of us who didn't, here are a few rules of thumb to make some sort
of guess:

If the forecast chart puts you immediately to the west of a low pressure
system, or in the north or northwest flow of what appears to be cold air,
then probably 2-3 thsnd agl stratocumulus, most probably MVFR or better, but
NOT 10 thsnd.

If they are calling for isolated showers in such a flow, then probably
generally VFR cumulus except for IFR in the showers...

If closer to the centre of a High, but still in the northerly flow, probably
Cumulus so something like 3-5 thsnd VFR.

If immediately northeast of a low, especially if precipitation is being
called for, then probably IFR.

If still quite a distance northeast of a low, they are probably forecasting
the onset of the storm but it will not be there yet, so probably middle to
high cloud, like 8 thsnd or more.

The dotted lines on that chart are somewhat representative of temperature.
If they are close together, that will probably be a front. If it looks like
the wind direction is pushing such a front from the north (cold front), then
they are probably thinking thunderstorms, or at least some sort of lift
along the front, MVFR, probably IFR in any precipitation.

If southerly winds pushing the front up from the south (warm front), could
be just middle cloud 8-thousand plus, unless they calling for
precipitation... in which case likely IFR.

In the southerly, southwesterly flow south of a Low pressure system in
summer, probably Cumulus, maybe towering cumulus or isolated CB. Probably
4-5000 plus, except in the isolated showers. Also probably reduced vis in
haze, if nothing else.


In general, immediately northeast or east of the low pressure system, look
for greater possibility of IFR.
West, southwest of the low, greater possibility of VFR.
Cumulus in cold northwesterly flows might be slightly lower than cumulus in
the hot southerly flows. And cloud in the northerly flows, west of a low,
will rarely be on the deck. The hot southerly flows will produce more
severe thunderstorms out of the CU later in the day.

Also watch the catch-phrase "afternoon cloudiness". That is almost surely
an afternoon-convective-cumulus kind of day, VFR.

In poorly-organized pressure regimes, lacking a distinct low-high pressure
pattern, the forecaster is probably looking at some sort of upper
disturbance or convective instability. The upper disturbance will give you
higher cloud (8thsnd plus), the convective instability will give you patchy
towering Cumulus or patchy thunderstorms. Either case will be usually VFR
except IFR in precipitation.

All of the above are my own opinion, and working meteorologists are free to
disagree... I will not rebut. YMMV.... a lot. All kinds of complications
with mountain areas, upslope flows, etc... if you are in the foothill states
and the forecast is for an easterly or northeasterly flow, then probably
IFR.

And of course, if the forecast pressure pattern does not come to pass as
forecast, then all bets are off.

But it turns out that the computer models forecast surface pressure pretty
well. In fact, quite often the forecast is better for the third or fourth
day than it is for today. The reason is that the forecaster has no desire
to do much "interpretation" in the third or fourth day...he basically makes
a quick assessment of what the forecast model told him and he puts down
"rain" (or whatever). (In fact, it is quite probable that the text for 3rd
and 4th days is directly generated by the computer from the model).

But as that day comes closer, facts get in the way. The morning of, he is
expecting this rain, but maybe the METARs are not showing it yet. He starts
to have doubts. Maybe the lift is not quite enough to produce rain. Let's
just call it cloudy.... sure enough, three hours after his "cloudy" forecast
gets distributed to the media, the rain starts, and his "cloudy" forecast is
blown.