Extreme heat wave in California and the first cold snap of fall in the Northeast: Here's the weather forecast for this week

An unprecedented heat anomaly for October keeps California under extreme heat alerts, while an autumnal air mass is already bringing freezing early mornings to New England and the Midwest. Amid this contrast, the Gulf Coast is seeing the threat of torrential rain move out to sea, although Florida remains at risk of intense downpours and isolated flash floods.

California, experiencing "August-like" heat in the middle of October

A powerful upper-level high-pressure system, described by the Weather Prediction Center (WPC) as "anomalous," is dominating the weather in the West and the northern plains, bringing in air that is much warmer than usual for this time of year. The result is a heat wave that is keeping temperatures between 15 and 20 °F above normal across large parts of California.

Highs are widely in the 90s along the coast and in adjacent urban areas, with some parts of the Central Valley approaching or exceeding 100 °F. Nighttime lows, on the other hand, barely drop: temperatures in the high 70s °F are expected in Los Angeles and San Diego, limiting thermal relief overnight and increasing health risks, especially for older adults and vulnerable populations.

The National Weather Service (NWS) has issued extreme heat warnings for parts of Southern California and the inland deserts, while heat advisories are in effect for the Central Valley and the Bay Area. This pattern is expected to continue at least through the middle of the week.

The hot air isn't confined to California. A portion of that heat wave is extending northeastward, reaching the Great Basin and the northern Rockies, where average highs in the 80s are 10 to 20 degrees above normal and could break daily temperature records in some locations. In addition, the passage of a weak upper-level low brings the possibility of isolated thunderstorms on Tuesday and Wednesday in the Great Basin and the Four Corners region.

First "cold snap" of fall in the Northeast and Midwest

On the other side of the country, a high-pressure system has brought a mass of dry, cool air over the Midwest and Northeast, marking one of the first cold snaps of the fall. Tuesday's highs will remain in the mid-60s and low 70s in the Ohio Valley and the Mid-Atlantic, while in New England, many areas will not exceed 50 °F in the afternoon.

Lows overnight from Monday to Tuesday and from Tuesday to Wednesday will drop to the high 30s and low 40s in wide areas, with some locations dropping to the mid-30s due to a cooling trend in very dry conditions. This combination has led the NWS to issue frost advisories and frost warnings for early this morning in parts of New England, the Central Appalachians and southern Michigan, posing risks to agriculture and sensitive crops.

In addition, some rain and embedded thunderstorms are expected in the interior of New England by dawn on Thursday, when a new frontal system moving in from Canada will reinforce the cold air.

Gulf of Mexico: Rain moves away, but Florida remains in the crosshairs

The front responsible for the recent wet weather pattern along the Gulf Coast is slowly moving offshore, reducing the primary threat of heavy rain right along the coast, but not eliminating it entirely. Scattered showers and thunderstorms will continue to affect northern and central Florida in the coming days, with a marginal threat of excessive rainfall and an isolated risk of flash flooding.

Accumulations of one inch or more are expected each day in that area, in a "soaking" pattern that will continue at least through Thursday. This means flooded streets, potential drainage issues in urban areas, and a need to pay attention to local NWS advisories, especially where the ground is already saturated.