DCA86IA017

While en route, the company dispatcher notified American flt 844 (via acars) that Harlingen (hrl) had unforecast fog & was 300' obscd with 1/2 mi vis. At 2335, as the flt bgn a dscnt fm FL 310, the same ceiling & vis were rprtd. However, as the flt was clrd for an ILS rwy 17R apch, houston ARTCC adzd the wx was (in part) 200' obscd, 3/8 mi vis with fog. Wxmins for the apch were 200' ceiling & 1/2 mi vis. Two apchs were made using the autoplt, but both were terminated with missed apchs due to wx. The crew started to divert, but recd a wx update (via acars) that the wx was improving, so a 3RDILS was atmtd. After activating rwy lgts, the capt hand flew the apch. As he transitioned to a visual apch, the dscnt rate increased & the acft touched dwn short of the rwy, dmgg the lndg gear & apch lgts. The crew made a go-arnd, then diverted with no further incident. A company wx observer at hrl noted the 0010 wx was 200' obscd & 1/2 mi vis & the 0018WX was 100' obscd & 1/4 mi vis, but the crew did not contact the observer, as reqd, bfr making the apchs.