Hawaii Extended Forecast Discussion NWS Weather Prediction Center College Park MD 720 AM EST Fri Dec 22 2017 Valid 00Z Sat Dec 23 2017 - 00Z Sat Dec 30 2017 There is still some spread among the guidance regarding a front likely to affect the state around the middle of next week but differences are a little less extreme than yesterday, while preceding evolution looks more agreeable than seen previously. Initially expect a band of drier air to drift south across the state behind a departing wavy front and upper shortwave/compact low. Then on Saturday a weakening front with only modest accompanying rainfall will reach about as far south as the northwestern islands, followed in another day or so by a weak front that approaches but may not quite reach the northwestern islands. Progression of surface highs to the north, with trailing ones weaker than the first, will lead to brisk northerly winds becoming gradually less strong and directed from the northeast and east-northeast over the course of the weekend. The trend toward easterly trades may bring in enough moisture to support a modest increase in windward-focused shower activity Sunday onward. During early-mid week most guidance agrees in principle that a sharpening upper trough should approach/reach the area, but with differences in shape that affect the progression of the associated front. As noted earlier the guidance spread for frontal timing has narrowed, by way of the GFS slowing down by about a day over the past 24 hours and the ECMWF speeding up somewhat. Now the 00z CMC is on the flat/fast side (not supported by its mean) while the 00z ECMWF/ECMWF mean are slower side by a small margin. The 00z GEFS mean shows a slower progression than GFS runs and in fact toward the end of the period holds onto somewhat higher precipitable water values over the state than the 00z ECMWF due to the latter being more amplified with the southern part of upper trough. Recent trends/consensus still favor progression slower than the GFS, near an ECMWF/GEFS mean/ECMWF mean compromise. This front will provide some enhancement to rainfall but with specifics determined by timing that still has a fair degree of uncertainty. Rausch