This is a summary of the BBCs 5 day forecast for my location
for January.
What I've done is look at the forecasts and compared them to
each other. I haven't correlated whether the weather they
forecast actually happened, just how often the forecasts changed
(since a 5 day forecast that changes the very next day can't
be relied on, so isn't any use).

The "hit rate" for this month was virtually the same as for
December, with the same number of different weather categories
used and the same (exactly the same!) number of correct
forecasts. The only difference being the actual weather that
was forecast. In some ways, I'm pleased by that as it shows
that the analysis of the forecasts is repeatable, even if the
actual data changes.

forecast forecast on: times num of days
-- date -- on day D-1 D-2 D-3 D-4 correct changes ahead
Sun 2 Jan WC M S SI S 0 4 0 !
Mon 3 Jan WC* F WC SI WC 2 4 0
Tue 4 Jan M SI M WC WC 1 3 0
Wed 5 Jan HR WC SI WC D 0 4 0 !
Thu 6 Jan LR WC SI HR LRS 0 4 0 !
Fri 7 Jan HR HR SL LS HR 2 3 1
Sat 8 Jan SI SI WC SI HR 2 3 1
Sun 9 Jan S S S SI LRS 2 2 2
Mon 10 Jan WC D SI HR LRS 0 4 0 !
Tue 11 Jan WC* S S WC HRS 1 3 0
Wed 12 Jan HRS HR LR WC WC 0 3 0 !
Thu 13 Jan LR HR HR HR HR 0 1 0 !
Fri 14 Jan HRS HR HR LR HR 0 3 0 !
Sat 15 Jan WC WC SI LR WC 2 3 1
Sun 16 Jan GC GC WC LR HR 1 3 1
Mon 17 Jan HR S SI WC HR 1 4 0
Tue 18 Jan SI SI WC SI HR 2 3 1
Wed 19 Jan SI* S SI SI SI 3 2 0
Thu 20 Jan WC WC WC WC GC 3 1 3
Fri 21 Jan S S WC S WC 2 3 1
Sat 22 Jan GC WC WC WC SL 0 2 0 !
Sun 23 Jan GC* SI LR HR HR 0 3 0 !
Mon 24 Jan GC* GC GC WC WC 2 1 2
Tue 25 Jan WC HRS LRS LR WC 1 4 0
Wed 26 Jan LR LR LR LR LRS 3 1 3
Thu 27 Jan WC WC LRS HR HR 1 2 1
Fri 28 Jan SI S GC GC GC 0 2 0 !
Sat 29 Jan GC WC WC LRS WC 0 3 0 !
Sun 30 Jan WC WC WC D GC 2 2 2
Mon 31 Jan WC* SI GC WC GC 1 4 0

For last 30 days: avg 2.80 different forecasts per day
On average forecasted 0.63 days ahead
Correct 4 days ahead: 0 times
Correct 3 days ahead: 2 times
Correct 2 days ahead: 5 times
Correct 1 day ahead: 12 times (40.0%)
Number of times the weather was same as yesterday(*): 6 times (20.0%)
Number of days every forecast was wrong(!): 11 times (36.7%)

Key:
D: Drizzle 0 occurrences, predicted 3 times over-forecast
F: Fog 0 occurrences, predicted 1 times over-forecast
GC: Grey Cloud 5 occurrences, predicted 10 times under-forecast
HR: Heavy Rain 3 occurrences, predicted 20 times over-forecast
HRS: Heavy Rain Showers 2 occurrences, predicted 2 times under-forecast
LR: Light Rain 3 occurrences, predicted 9 times about right
LRS: Light Rain Shower 0 occurrences, predicted 7 times over-forecast
LS: Light Snow 0 occurrences, predicted 1 times over-forecast
M: Mist 1 occurrences, predicted 2 times under-forecast
S: Sunny 2 occurrences, predicted 11 times over-forecast
SI: Sunny Intervals 4 occurrences, predicted 18 times about right
SL: Sleet 0 occurrences, predicted 2 times over-forecast
WC: White Cloud 10 occurrences, predicted 34 times about right