A slow-moving upper low and associated frontal system currently offshore of California will be steadily advancing southeastward through Thursday and will then advance inland across the Southwest on Friday.
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Strong forcing associated with this coupled with a notable increase in Pacific moisture out ahead of it will lead to bands of heavy showers and thunderstorms through this evening and into the overnight hours across portions of central and especially southern California.
The heaviest rains will be situated across the coastal ranges and especially the Transverse Range tonight where the better moisture transport and heavier rainfall rates are expected to set up.
The heavy rainfall threat will persist through Thursday across southern California, and over the next 24 to 36 hours, the rainfall totals across the Transverse Range may approach or even locally exceed 4 to 8 inches.
These rains are likely to result in significant concerns for flash flooding with a notable threat to the area burn scar locations where debris flows and mudslides will be possible.
Urban flash flooding will also be possible in the lower elevations closer into the Los Angeles basin.
Some rainfall rates with this event may approach or locally exceed 1 inch per hour with the heavier showers and thunderstorms, and the persistence of these rates will lead to much of the broader threat of potentially significant flash flooding.
The Weather Prediction Center has highlighted a Moderate Risk of excessive rainfall to address these concerns going through Thursday.
By Friday, this storm system will be accelerating off to the east and will begin to overspread the Southwest where areas of locally heavy rain and at least some isolated concerns for flash flooding will continue.
As the heavy rainfall threat begins to exit southern California, areas of central and southern Arizona will begin to see these impacts.
However, the rainfall totals should be much more beneficial and modest by comparison.
In general, the threat of winter weather over the next couple of days will be rather limited due to the lack of cold air across much of the country, but the storm system coming into California and the broader Southwest will foster some locally heavy accumulating snowfall tonight and Thursday for the high terrain of the southern Sierra Nevada.
Gradually the threat of accumulating snowfall will increase also into the San Juan mountains of southwest Colorado, with a much heavier threat of snow here expected by this weekend.
Given the presence of a lot of Pacific air across the country, and general lack of southward advancing Arctic air from Canada, the temperatures across large areas of the country will be well above normal for this time of the year.
This will especially be the case across areas of the central and northern Plains, and the Midwest, where high temperatures will be as much as 15 to 25 degree above normal for the remainder of the week.
Relatively cooler and more unsettled weather should begin to arrive across the Pacific Northwest by Friday as a new cold front arrives and advances inland.
Locally heavy showers with the front can be expected given the influx of Pacific moisture, and this will translate into areas of heavy snowfall for the higher elevations including the Cascades and eventually the northern Rockies. ■