Xastir has built in support for NWS Weather Alert display. You have to take a few steps to make them show up.
Get NWS Shapefiles
Run the "get-NWSdata" script provided with Xastir. Assuming you installed Xastir in its default /usr/local location, this means:
This will download all of the shapefiles needed to display US weather alerts and install them.
NOTE: NWS changes the format and name of their weather alert shapefiles often, and when they do the get-NWSdata script (and any dbfawk files associated with the shapefiles) need to be updated. If you rely on Xastir releases instead of using git, you may find that the script is out of date. This alone may be reason to avoid depending on releases.
Index your new shapefiles
Either restart Xastir or choose the menu entry "Map->Configure->Index: Add new maps" to cause Xastir to index the new shapefiles you've installed.
Connect your Xastir to a source of weather alert data
Any APRS-IS server will provide at least some weather alerts. The main servers you find at rotate.aprs.net or rotate.aprs2.net only have the most urgent weather alerts, those of "Warning" severity. To get the lower severity advisories and watches you need to connect to one of the firenet servers. Using "firenet.aprs2.net" will connect you to one of these servers just as the rotate addresses connect you to one of the many main backbone servers.
Make sure "Enable Weather Alerts" is checked under the Map menu.
Make sure "Disable All Maps" is not selected
Well, yeah. If you disable all maps, you can't display weather alert maps.
When weather alerts come in that overlap your current map they should highlight.