Thunderstorm, Flood Threats Continue in the Southwest

This past week has featured some wild weather across parts of California and the Southwest. This is all thanks to a slow-moving upper-level storm system that has been wandering around the Southwest for nearly two weeks.
Perhaps the most significant flooding so far occurred on Thursday afternoon across portions of interior Los Angeles County and southwestern San Bernardino County. Excessive rain caused major flooding, mudslides and closures of a stretch of I-5.

Latest Southwest Radar

Looking back, October started with several bouts of thunderstorms that caused flash flooding across parts of the Desert Southwest. In the second week of the month, portions of Texas and New Mexico received over a month's worth of rain in one day.
(MORE: Texas Rain Floods Roads, Causes Rock Slide)
Last week targeted California and more rain is yet to come into the beginning of a new week farther east.

Thunderstorm and Locally Heavy Rain Threat

Monday's Forecast

Forecast Rainfall

Flash flood watches continue into early Monday for portions of southeast California adjacent to Nevada, southern Nevada, southern Utah and northwestern Arizona, as slow-moving thunderstorms moving over mountainous or desert terrain may quickly trigger local flash flooding of normally dry washes and arroyos, as well as additional debris flows.
Although the rain should help alleviate some fire conditions over interior parts of the region, parts of western and south-coastal California that need rain the most will not see much in the coming days.
(MORE: Interactive Radar and Flood Alerts)
Furthermore, frequent lightning strikes may start new wildfires.
(Forecast: Albuquerque | Phoenix | Las Vegas)
Through early Monday morning, the thunder threat expands from the Great Basin into the northern and central Rockies, persisting in parts of the Desert Southwest. A few severe storms with gusty winds or hail cannot be ruled out in southeast California, southern Nevada, northwest Arizona and southwest Utah.
Late Monday-Wednesday, a new low pressure system aloft will form over the Southwest. Scattered showers and thunderstorms with locally heavy rain are possible Monday and Tuesday from parts of the northern Rockies into New Mexico and Arizona.
As the the low pressure system moves east later this week, drenching rains are possible in parts of New Mexico, west Texas and Colorado.
(MAPS: 7-Day Planner)
Strong thunderstorm winds may also produce areas of blowing dust, quickly reducing visibility, mainly over Arizona.
Winds gusted up to 62 mph in southern Arizona on Friday, associated with a thunderstorm complex that evolved into a haboob over the Phoenix area.

Meteorology 101: Meandering Upper Level Low

The odd progression of an upper level low across the region is largely to blame for the inclement weather pattern.
Origins of the low can be traced back to the first few days of October. The cutoff low swung into southern California on Oct. 4 and brought with it near-record low temperatures aloft for early October into the area.

The low, detached from the jet stream, drifted east, and later, began to pull moisture northwestward from the Gulf of Mexico. This contributed to the development of heavy rainfall across the southern High Plains.

An upper level low moving through the Southwest, once again, teams with an approaching trough to help draw moisture northeastward from the Pacific.

What makes this setup even more odd is that the low proceeded to drift south and then move west underneath a parent ridge of high pressure, back over the eastern Pacific.
The ridge generally kept above average temperatures locked in across the West last week, especially across the locations that have had little to no rainfall or clouds to keep temperatures down.
(MORE: October Heat is Breaking Records in the West)
Recently getting pulled back north by an approaching trough, the low is gradually becoming absorbed by the jet stream.
Through this week, what is left of the disturbance is forecast to move east through the Plains, finally departing the Southwest after impacting the area for at least two full weeks.
At the same time, another area of low pressure is moving into northern California. This will bring the risk of more rainfall and flash flooding, albeit somewhat further north, through early this week.
MORE: Western Wildfires (PHOTOS)