How do they know...

[Dan GTX]

Ok, real world example here, and this is probably the main thing that bugs me and what I want to learn more about.

As of this post, this is the outlook for zip 15108 (near where I live):
Late Afternoon: A chance of showers and thunderstorms. Partly sunny, with a high near 81. Windy, with a southwest wind around 30 mph, with gusts as high as 48 mph. Chance of precipitation is 40%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

Yesterday, it gave me a 60% chance for "Severe Thunderstorms", and now it's been downgraded to basically a "40% Chance of Rain".

The low pressure system is out in Michigan and there's no merging of fronts anywhere near here.

It doesn't help I have no idea what RUC or NAM models mean... lol

What do I need to read or better understand to help with figuring out how they get their forecasts? (aka: Which maps/models do I need to have a better understanding of?)

[MClarkson]

The RUC (Rapid Update Cycle) and NAM (North American Mesoscale) are both models run by the NWS. They essentially break down the US into a set of grids and then move different atmospheric variables from grid to grid according to the fundamental laws of physics. You can access their output here:

http://mag.ncep.noaa.gov/NCOMAGWEB/a...MODEL+GUIDANCE

A good starting point is comparing some of the model forecasts with what actually happens in your region.

[Chad Ringley]

Agree with the above, the raw NAM and RUC models may be difficult to decipher for a newcomer, but the radar reflectivity and four-panel charts with both upper air and surface parameters are a great place to start. I'd use the RUC for same day events, NAM for 1-3 day events, and the GFS out a few days past (beyond 5 or 6 remains highly questionable).