Low Pressure System Off Coast Brings Hawaiian Moisture, Showers to Humboldt

A 552 DM low pressure system sitting 85 miles off the Humboldt coast is drawing a moisture plume that stretches back to the Hawaiian tropics, setting up the county for late-season showers, possible thunderstorms tonight, and temperatures well below the April norm through midweek.

The system's 552-decameter reading on the 500-millibar pressure level signals a moderately deep trough capable of sustaining organized precipitation. What makes this week's setup particularly potent is its moisture source: tropical Pacific air from near Hawaii channeled northeast to the Northern California coastline, a pattern meteorologists classify as a Pineapple Express-type atmospheric river. A meteorologist tracking the system has shared detailed model analysis of its trajectory and water vapor content.

The National Weather Service is calling for a 100 percent chance of precipitation across portions of Humboldt County tonight, with showers and possible thunderstorms arriving after 8 p.m. Overnight lows are expected to drop to around 41 degrees. Rainfall totals between a half-inch and three-quarters of an inch are possible through tonight alone.

Wednesday brings continued unsettled conditions, with showers likely through the morning, a high near 51 degrees, and an additional tenth to quarter-inch of rainfall before the system begins pushing east. By Thursday, mostly clear skies return, though overnight lows will settle near 39 degrees as cooler, drier air fills behind the departing low.

April typically marks the close of Humboldt's wet season, when Pacific storms weaken and tropical moisture connections become increasingly rare. A low this deep, still drawing an active plume from near Hawaii at the start of the month, makes this week's system an outlier worth tracking through its full run.