Riverbeds will roar to life early this week across the dry desert of the Southwestern US as remnants of Hurricane Rosa trek northeastward. Trailing Rosa will be an upper-level system that will only make matters worse. It will produce rainfall over central and southern California, enhance rainfall over the Southwest, and keep rain chances in place after Rosa’s remnants eject into the Rocky Mountains. Conditions will therefore be ideal for a widespread flash-flooding event between Sunday and Tuesday night as several inches of rain, or half of a year’s worth, deluge the region in a span of 48 hours.

This is your Sunday Storm.

As of 5pm September 30, Rosa was a 70 mph Tropical Storm with minimum central pressure of 985 mb and located roughly 250 mi southwest of Punta Eugenia, Mexico. Rosa is expected to weaken further later tonight as she gets torn apart by shear and cool sea-surface temperatures (SSTs) just off the coast of the Baja California peninsula. Landfall is expected along the northern Baja California peninsula, between Colonet and El Rosario Monday evening as a weak Tropical Storm.

The mountainous terrain of the peninsula will eviscerate the tropical storm’s upper-level circulation. Less than 12 hours after making landfall on the Baja California Peninsula, Rosa will have already crossed the Gulf of California and will have made landfall again as a tropical depression somewhere in northwestern Senora, Mexico just southeast of Yuma, Arizona. The depression will quickly merge with the jet stream and become post-tropical after crossing the border into Arizona as an open wave.



There is unusually high confidence about the magnitude and location of this significant rainfall event as evidenced by the spaghetti plot of deterministic model tracks below from 12z Sunday, courtesy of Tropical Tidbits. Notably, there is very little spread among the commonly used deterministic models in regards to track. With Rosa expected to become a relatively small-scale post-tropical wave, the heaviest precipitation will fall within the vicinity of the remnants’ ultimate track.

Rosa’s outer bands have already reached southern Arizona. Guidance has increasingly suggested that these downpours will overspread across most of Arizona and southeastern California through Sunday night. The bands will continue to push northward as moisture is drawn by the jet stream and by a weak shortwave trough over northern Arizona. Modest warm advection alongside the shortwave will enhance the coverage of precipitation, as it will indirectly produce downpours associated with Rosa’s moisture .

The heaviest rain will fall Monday evening through early Tuesday as the remnants’ center approaches Arizona. Guidance depicts two separate concentrations of heavy rainfall; one in the southern Sonoma Desert and the other on the Colorado Plateau. Lighter accumulations of rain will fall between these areas, but that does not necessarily reduce the flash flooding risk.

Initially, the heaviest rain will be focused in the vicinity of the low over the south-central Sonoran desert east of Yuma. The left exit region of the remnant wave’s impressive low-mid level jet will drop the heaviest and most widespread downpours directly across the center of the state. Phoenix, Scottsdale, and Prescott will all be in the path of these concentrated downpours. Up to 4″ of rain will fall in these cities, which could be enough to grind life to a halt.  The rocky desert has very poor drainage, so water will quickly pool and gush down slopes that face the Sonoran basin.





North of the Senoran desert, orography will make flash flooding much more dangerous. The southerly wind direction will force winds to ride up the Colorado Plateau in central and northern Arizona. The resulting orographic lift will result in exceptionally heavy rainfall over the Colorado Plateau, where 3-6″ of rain  with locally higher amounts could fall in cities like Flagstaff. The rocky soils of this landscape have very little capacity to drain water. Consequentially, most of the rainfall will rush down mountainsides and charge through empty river-beds with the momentum of a bullet train. This will be more than enough force to drag rocks from slopes, contributing to a significant risk for rock-slides. The floodwaters will pour into the northern Sonoran basin and plow through many structures in their path until slowed by friction. Though rainfall here will be less than the lower Sonoran basin, that the floodwaters here will result from upsloping to the north will result in a significant flash flooding risk. It is for these reasons that the WPC has placed this region under a moderate risk for excessive rainfall.

Excessive rainfall is not an exaggeration. The significance of this rainfall event is perhaps best realized in the context of the deserts’ low average annual precipitation. The map below from the Climate Prediction Center (CPC) shows that much of the Southwest receives less than 10″ of rain per year. For example, Phoenix’s average rainfall per the CPC’s set of 1981-2010 climatological normal is only 8.03″. Rosa could produce half of that total in just two days.

Southern Utah, western Colorado, and northwestern New Mexico will face the remnants of Rosa Tuesday afternoon into the overnight hours before the wave completely dissipates in the Rocky Mountains. Torrential rainfall is expected here, especially along mountain slopes facing the southwest. Fortunately, the jet stream will swiftly carry the moisture eastward, so the downpours will not last long enough to pose as high a threat as in Arizona. Hence, these areas have a slight or marginal risk of excessive rainfall.

While Rosa’s remnants pound the desert southwest and Rocky mountains with up to a half-year’s worth of rain, downpours and flash flooding unrelated to Rosa will threaten the southern half of California, Nevada, and northern Utah. The approach of a strong upper-level trough will draw pacific moisture into the West. Rosa’s latent heat will help invigorate the jet stream and increase moisture transport and divergence over the West. The large-scale nature of this trough will produce vast areas of large-scale lift near the jet-exit region and produce several days of downpours. Southwestern mountain slopes will accumulate the most rain, but gravity will draw that rain into valleys and basins.

Though Rosa’s remnant circulation will not make the journey across the Rocky Mountains, her moisture certainly will. The upper-level trough responsible for heavy rain in Nevada, California, and Utah will carry the moisture northeastward into the Midwest. A flash flooding event will already be ongoing early in the week. The resurgence of moisture from Rosa may cause another significant flooding event Thursday and Friday. All interests in the Midwest should check back with WeatherOptics as we continue to follow the progress of Rosa and her remnants.



 

Author

Josh is a lifelong nature and weather enthusiast as well as the Head Meteorologist at WeatherOptics. He began regularly forecasting for New Jersey, Long Island and New York City in 2014 on social media, contributing to community pages such as SBU Weather. He holds degrees in Physics and in Atmospheric and Oceanic Sciences from Stony Brook University, from which he graduated in 2018. In the Fall of 2018 Josh will start graduate school for his M.S. in Marine and Atmospheric Sciences at Stony Brook, continuing his research on approaches to non-convective wind gust forecasting.

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