Title text: At least if you're really into, like, Turkish archaeology, store clerks aren't like 'hey, how 'bout those Derinkuyu underground cities!' when they're trying to be polite.
Social norm accepts casual small-talk as an ice breaker for interaction — usually it is always safe to talk about the weather without hitting any disagreements as there are rarely any personal view points about the weather — in contrast small-talk is never about political subjects or similar where chances are that there are strong personal view points.
In this strip Cueball is an expert on meteorology and weather forecasting. When Hairy makes a comment about the weather, Cueball launches into a detailed technical discussion, not realizing Hairy is simply trying to engage in small-talk. Only weather experts would have this problem, but this topic is a common opening for a conversation in casual small-talk. Cueball switches to small-talk once he realizes that Hairy is confused and didn't expect this level of technical information.
As to the jargon:
- Jet streams are strong air currents high in the atmosphere which have a big influence on the weather.
- 18z is 18:00 UTC (6 PM in Greenwich, England, 10 AM in California). See ISO 8601 at Wikipedia. The letter "Z" is used as 'Zulu' in the NATO phonetic alphabet, meaning just UTC.
- GFS is the Global Forecast System (also known as NCEP-GFS). It is a computer model used by the National Weather Service to predict the weather up to 16 days in advance. The model is run 4 times a day and the output is distinguished by the UTC hour it was started (18z in this case).
- Part of the prediction is the atmospheric pressure expressed in mbar (or mb). 960 mbar is very low pressure, which is usually associated with seriously bad weather (record low pressure for Minnesota was 963 mbar till 1998).
- "Think it'll verify?": A forecast "verifies" when an analysis of observations at the forecast time are found to match the forecast. Cueball is asking if Hairy thinks the prediction of a 960mbar low will be shown to have been correct.
The title text clarifies the problem weather nuts like Cueball here have: Unlike other geeky pursuits (like, say, the Derinkuyu Underground Cities, one of the most well-known archaeological sites in Turkey) weather is a fairly common small talk subject. As a result, weather geeks have to be constantly vigilant so as not to launch into technical monologues.
- [Cueball and Hairy are talking.]
- Hairy: So, how 'bout this weather?
- Cueball: I know, right? The whole jet stream layer is nuts!
- Hairy: Um, sure...
- Cueball: The 18z GFS forecasts 960mb by Tuesday. Think it'll verify?
- Hairy: What?
- Cueball: ...Right. Sorry. Uh, yeah! Weather sure has been crazy.
- Weather geeks have it tough.