( ( A ( falling barometer ) ) ( ( indicates ( ( the arrival ) ( of ( ( ( a ( low ( pressure ( air system ) ) ) ) , ) ( which ( often ( means ( stormy weather ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (VBG falling) (NN barometer)) (VP (VBZ indicates) (NP (NP (DT the) (NN arrival)) (PP (IN of) (NP (NP (DT a) (JJ low) (NN pressure) (NN air) (NN system)) (, ,) (SBAR (WHNP (WDT which)) (S (ADVP (RB often)) (VP (VBZ means) (NP (JJ stormy) (NN weather))))))))) (. .)))
A falling barometer indicates the arrival of a low pressure air system, which often means stormy weather.
(ROOT (S (NP (PRP You)) (VP (MD can) (VP (VB expect) (ADJP (JJ stormy)) (SBAR (IN if) (S (NP (NP (DT a) (NN center)) (PP (IN of) (NP (JJ low) (NN pressure)))) (VP (VBZ is) (VP (VBG moving) (NP (PRP$ your) (NN way)))))))) (. .)))
You can expect stormy if a center of low pressure is moving your way.