Hawaii Extended Forecast Discussion NWS Weather Prediction Center College Park MD 715 AM EST Mon Dec 25 2017 Valid 00Z Tue Dec 26 2017 - 00Z Tue Jan 02 2018 The dominant system of interest during the period will be a strongly sharpening shortwave that will approach the state later today and Tuesday, then progress across the state Wednesday into Thursday as energy upstream briefly slows the progress of the overall trough. The combination of strong dynamics aloft, with the initial shortwave likely taking on a negative tilt, and leading cold front will generate a strong southerly flow of moisture and potential for some strong convection and/or heavy rainfall. Guidance is agreeable in principle, with differences generally within the typical error range for the time frame of interest. Drier air will move into the region from northwest to southeast during the latter half of the week. Models suggest precipitable water values may increase a bit for the Saturday-Monday period but remain below average for the season. After the upper trough passes by, models/ensembles still differ on the details but as a whole have trended eastward with the location of any potential closed low compared to guidance from 24 hours ago. GFS details aloft continue to result in its runs showing a stronger inverted surface trough east of the state versus most other guidance. Thus as high pressure progresses eastward to the north of the islands, expect the transition from northeasterly to more easterly trades to occur a little sooner than in the GFS. From late week onward anticipate relatively light shower activity focused over windward terrain. Rausch