Below follows some useful links for the weather part of flight planning. The links are in no particular order but loosely divided into different categories.
Most of these links are from a compilation by Scott Dennstaedt. Scott runs the excellent web site Avwxworkshop where you can find detailed discussions about weather for pilots and how to use various weather forecasting products. I can highly recommend this source for weather know-how essential for pilots.
Aviation weather links in general
Surface Analysis
Current
Loop 21 hours
Loop 3 days
Storm Prediction Center (overview)
Enhanced SPC overview
Day 1
Day 2
Day 3
Outlook and Forecast Discussions
NWS Outlook
NWS Discussions
Extended Outlook Discussion
Short Range Forecast Discussion
0–7 Days Fronts
Area Forecast
NexRad Image
Stability and instability
Lifted Index Maps
Surface Based CAPE
Most Unstable CAPE
Convective Outlooks
Some imagery
Current Ceilings
Current Visibilities
Water Vapor Satellite
Models
NCEP Models
RUC Models
SREF Aviation Products
Simulated Reflectivity
Latest Simulated Reflectivity
RUC Soundings
South America
Weather Underground
GFS Model Output
Direccion Meteorologica de Chile
Model Soundings
To use the model soundings for South America, choose the GFS as initial data source, up to 72 hours for the forecast time and then put in the nearest airport identifier (SCCF for Calama, SCEL for Santiago). Hit SIMPLE java plots, or, for a more detailed sounding plot: Java-based plots.
Aviation Weather (GEOS IR images among many other things)
Meteo Centrale (Forecast weather for 48 hours – images and movies)