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HankScorpio-vs-World t1_j3lnlxi wrote

There are so many things that can alter the answer to your question…. Some places have much more variability in their weather patterns… to the degree that the same month one year may be 5 rain days and another year it’s 20 rain days, the average of rain days will be the number of rain days each year averaged over a period of time (maybe 10 years) so in this example the average might be 12 rain days but it may be as many as 20 days during several years when a particular weather pattern is prevalent.

Things that alter the local weather pattern are things like altitude of the place and height of surrounding mountains, major high altitude air currents like the jet stream, proximity to the coast and sea temperature, proximity to seawater currents like the Gulf Stream, low level winds like the trade winds, ares of desert, proximity or not to the equator and position within a large landmass all create local pockets of weather patterns that are unique often to a very small geographic area. So where you are the answers may be very different compared to where I am.

Some places are even “high or low” air pressure generators that begin to propagate patterns of weather in different ways as the wind direction changes. So if winds are easterly it’s pattern “A” if winds are westerly it’s pattern “B” if winds are very low then pattern “C” becomes more likely.

A weatherman will probably tell you, you are far less likely to see the average number of rain days or the average temperature occur because the average is just that the number calculated mathematically. Now that may be a good average because there is never much variation or a bad average calculated from two extremes… normally weathermen know that in some places it will regularly be very high or very low, the average itself May happen very very rarely.

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kajorge t1_j3n75lh wrote

To add to this, there are large-scale structures that DO make excellent predictors of weather to come. For instance, a "bomb cyclone" just hit the western coast of California. These mid-latitude cyclones follow somewhat predictable patterns.

Over the US, winds tend to blow from the west, especially along the Pacific coast, so it was pretty much guaranteed that this weather system would move from west to east. In that link you will see a satellite picture of the storm, which has the tell-tale cyclone shape, like a comma with two tails. The clouds on the western tail form a narrow band, which is indicative of the tall storm clouds associated with a cold front. The eastern tail is wide, made of low-hanging stratus clouds that form due to an incoming warm front. In the center of the comma is a swirling occluded front where the cold and warm fronts meet.

Experiencing a few days of little precipitation and overcast skies does not necessarily tell you a whole lot about the weather to come. But if you know that the large scale formation looks like this cyclone and you experience those overcast days followed by a couple clear days, then on the clear days you can bet that you are in the cloudless space between the comma tails, so the cold front is coming, bringing with it heavy rain.

But as HankScorpio here said, using monthly averages as a prediction tool is not a great idea, simply because of the potential variability of year-to-year climates.

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