Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
240 AM EDT Mon Apr 6 2026

Valid 00Z Tue 07 Apr 2026 - 00Z Tue 14 Apr 2026

...Heavy rainfall and flash flooding potential increases midweek...

The synoptic pattern is forecast to undergo a significant shift
as a pair of upper-level troughs move east of the Dateline,
effectively disrupting the trade wind flow. Shortwave energy in
the base of the first trough will approach from the northwest on
Tuesday, driving a cold front that is expected to remain west of
the state. However, low-level winds will veer to the south ahead
of this boundary, drawing a deep plume of tropical moisture
northward over the islands. Precipitable water (PW) anomalies will
continue to climb through midweek as a second, more amplified
shortwave digs toward the chain. Guidance indicates that a surface
low will develop and strengthen along the lingering boundary as
an upper low begins to close off northwest of the islands by
Thursday. This evolution into a "Kona Low" type setup will further
amplify the southerly flow, driving PW values well above normal.

Widespread heavy rainfall and organized convection are likely to
develop by Thursday, with the potential for flash flooding
remaining elevated through the end of the week. There is good
model agreement among the GEFS, ECENS, and CMC ensembles regarding
this heavy rain threat, particularly for the western islands where
probabilities for 24-hour totals exceeding 2 inches climb over 50
percent Thursday-Friday. While confidence decreases by the weekend
regarding the exact track and strength of the closed low,
southwest flow aloft will likely persist. With a lingering
boundary in the vicinity and continued high PW anomalies, the
threat for additional heavy rainfall and flooding may persist
into early next week.

Pereira