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).
- While the "Think it'll verify?" is fairly self-explanatory, it is also jargon; simply put, a forecast "verifies" when it becomes reality.
The title text mentions store clerks who are well known for small talk about the weather as part of their sales talk. But here the clerk instead makes small talk about Derinkuyu Underground Cities, one of the most well-known archaeological sites in Turkey, in a country known for its many well-preserved ancient sites from a broad range of time periods. It would be very tempting for a Turkish archaeology geek to launch into a detailed conversation on the subject or related news -- however the subject is extremely unlikely as a topic for small-talk.
- [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.