welcome to the forum! I was in Eugene last week for biz... hung out at Alton Baker for a few minutes eating a sandwich
The 18-55 will do landscapes pretty well though you might want to stop down a bit - maybe around f5.6 or so (put your cam in AV mode and turn the back dial). If you are in a dark area, you'll probably want a tripod so you can keep the iso as low as you can so you have less grain and higher resolution. Make sure you use the 2 second timer to reduce camera shake... If you want a higher quality and wider lens for landscapes, I think I've seen a couple used Sigma 10-20 on the forum marketplace for under $400 recently. If you have the dough, the Sigma 8-16 is very good as is the Pentax 12-24.
For macros, the Pentax DFA 100 2.8 wr is awesome, but if you want to save a bit of $, the tamron 90 2.8 and Sigma 70 2.8 or Sigma 105 2.8 are also very good. In fact, I'll be selling my 105 2.8 pretty soon - I might even be able to meet you down in Eugene in a few weeks if you'd like to see it in person first...
Storm chasing, huh? Sounds pretty exciting as long as the weather cooperates!
Good luck w/ the camera - it's a great hobby, but it does take time to learn.