http://www.met.nps.edu/~mtmontgo/P21L.html
P21L
12N, 16W
700 hPa
ECMWF: Relatively large pouch that "dives" southwestward into the ITCZ and then "pull" northwestward. During the 48-72-hour period, ECMWF depicts an E-W elongated pouch, with a potential secondary center farther to the east but within the large pouch. I track the weak western portion during that period, which seems more consistent with the single center that is eventually depicted at 84 hours and beyond.
GFS: Unlike ECMWF, the GFS P21L pouch is easy to track. After P21L "dives" into the ITCZ, GFS depicts another circulation forming to the east of P21L. This sounds similar to ECMWF, but there's a difference. ECMWF depicted one large pouch with a couple of potential centers. GFS is depicting separate, relatively strong pouches. P21L then continues moving westward as an intense pouch, and the other pouch to the east remains strong, too.
UKMET: Like ECMWF & GFS, UKMET "dives" into the ITCZ as it emerges from Africa. Like GFS, UKMET depicts another ITCZ/monsoonal circulation to the east. However, unlike GFS, UKMET soon dissipates that other circulation, and P21L then moves along to the west as a single, easily tracked, strong pouch. Phase speed determined by "cutting the corner", which gives an average speed between the slow early period and the faster later period.
NOGAPS: Like the other models, NOGAPS "dives" into the ITCZ; even more so than the other models. Then P21L moves off to the west as an intensifying, single pouch. Phase speed determined by "cutting the corner", which gives an average speed between the slow early period and the faster later period.
HWRF-GEN: Not available.
ECMWF -6.5 v700 120h
GFS -6.5 v700 120h
UKMET -7.3 v700 & RH 120h
NOGAPS -6.0 v700 120h
HWGEN ---- ---- ---h

