Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
331 AM EST Tue Feb 25 2020

Valid 00Z Wed Feb 26 2020 - 00Z Wed Mar 04 2020

...Heavy rain quite possible this coming weekend into next week...

Upper ridging around 170W will surge northward later this week
ahead of a system off Asia lifting toward the Aleutians. This will
allow troughing and eventually a closed low to fill in the void
over the region from the northeast Saturday and then move westward
across the islands Sun-Mon. The ECMWF has been leading the models
in this scenario and the latest 00Z models were in good agreement
with this evolution (rather than stalling the system over or just
to the east of the Big Island). At the same time, an upper high
will crest atop the upper low Sunday in a Rex block configuration,
but it will be short-lived as each will have its own momentum to
pinwheel around the other next week. At the surface, models show a
1040+mb high settling near 37N/152W Sat/Sun then drifting slowly
to the east Mon/Tue. The pressure gradient of ~20mb over ~1000mi
will yield breezy trades especially Friday onward.

Upper low will bring cold/unstable airmass through the region with
500mb temps potentially down to around -17C per the 00Z GFS/ECMWF
and freezing levels near 9500ft/700mb. This would support some
convection in addition to potentially heavy snow on Mauna Kea and
Mauna Loa. Several inches of rain are possible as the upper low
passes through a given point as deep tropical moisture may get
pulled north and northwest on the east side of the upper low.
Highest probabilities lie over the Big Island, then decreasing to
the northwest amid increased uncertainty.


Fracasso