Unseasonably strong April storm, then active weather pattern to continue

It appears that the wet season in California will be bookended by two unseasonably strong storms (the earlier one being the extratropical remnants of Typhoon Melor in October, and the latter one is due in tomorrow). Northern California will have been affected more significantly by both of these than Southern California, but the entire state will see some significant precipitation this week.

A brief glance at satellite imagery this evening shows a very impressive low pressure area several hundred miles off the California coast and moving slowly east-southeast. Rapid cooling of cloud tops on the east side of the low is indicative of significant upper-level divergence and ascent through the atmospheric column. Some convective cloudiness can also be noted closer to the low center and also along the cold frontal band.

Infrared satellite imagery depicting strong April storm nearing California (NRL)


This storm system is quite strong for this late in the season, and may bring heavy rainfall, strong winds, and thunderstorms to much of the state by Monday. NorCal will see the brunt of the heavy precipitation, though the  mountains of Southern California will see significant precipitation, as well, in addition to any locations that receive thunderstorm activity. Upwards of 2 inches of rainfall is possible in the higher terrain of NorCal in a 12-hour period on Sunday, with an additional 0.5-1 inch in convection on Monday. Dynamic lifting and strong cold air advection aloft portend a significant chance of convection along the frontal band itself, though chances increase in the very unstable airmass behind the front. A jet streak is developing across Central California and will serve to enhance wind speeds at the surface as precipitation drags the strong winds aloft towards sea level. Expect gusts possibly in excess of 50 mph in favored locations, and winds of at least 25-45 mph are likely just about everywhere.

NAM depiction of strong spring storm in California (NCEP)

The low center is expected to track directly over Northern California on Monday, bringing widespread convective precipitation and a pretty good chance of thunderstorms (especially if there is any partial sunshine). Some strong storms are likely if surface heating does occur, and widespread small hail may once again occur with cold temps aloft and vigorous convection present.

An active weather pattern is expected to continue for the foreseeable future as an unseasonably active jet remains over the East Pacific. A relative weak but cold low may bring scattered showers and isolated thunderstorms to NorCal on Wednesday or Thursday, but the real uncertainty beings on Saturday. The GFS and ECMWF have begun to indicate a rather strange undercutting scenario that involves an extremely warm and moist airmass eventually moving slowly over CA sometime next week. I have extraordinarily low confidence in this scenario, and expect forecasts to change a great deal over the next week. One thing is for certain, though: this will turn out to be one of the more active Aprils in some years…


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