Rosa’s Rains Drenching Southwest U.S.; Caribbean System Bears Watching

Tropical Depression Rosa is no more, but its moisture will continue to spur locally heavy rains across and beyond the Southwest U.S. this week. The NOAA/NWS National Hurricane Center issued its final advisory on Rosa at 11 am EDT Tuesday. At that point, the storm’s remnants were centered along the east coast of Baja California with top winds of just 30 mph. One drowning death has been attributed to Rosa in the city of Caborca, Sonora, on the Sea of Cortez about 100 miles southwest of Tucson, Arizona.

As of noon MDT Tuesday, Phoenix had recorded 2.22" of rain since midnight, making Tuesday the city's tenth wettest day in records going back to 1895. There's a chance that ranking will go higher if a shower or thunderstorm happens to move over Phoenix later Tuesday.

Here are Phoenix's Top 10 wettest days ever (since 1895).

1 3.30" 2014-09-08
2 2.91" 1939-09-04
3 2.81" 1911-07-02
4 2.68" 1911-07-01
5 2.62" 1926-09-26
6 2.43" 1970-09-05
- 2.43" 1951-08-27
8 2.32" 1988-10-14
9 2.24" 1923-11-10
10 2.22" 2018-10-02 https://t.co/f2GrGmnmfF

— NWS Phoenix (@NWSPhoenix) October 2, 2018

Steering currents around a strong low off the California coast are driving Rosa’s remnant circulation inland, together with an impressive slug of moisture for so late in the warm season. Las Vegas, NV, set a record at 8 am EDT Tuesday morning for the most moisture in the atmosphere on any date in October—and in fact, for any date outside of June through September—in records that extend up to 2014. This is based on precipitable water, the amount of moisture in the air above a given point. Such records date back to the establishment of routine balloon-borne radiosondes, which have been launched twice daily across the nation since the late 1940s.

Las Vegas, NV:  1.39” (old record for Oct-May = 1.15”)

It’s likely that Yuma and Phoenix would have set precipitable-water records as well, but radiosondes are no longer launched from those sites. In the close-but-no-cigar category, we have:

Flagstaff, AZ:  1.03” (old record for Oct-May = 1.09”)
Tucson, AZ:  1.62” (record for Oct-May = 1.74”)
Grand Junction, CO:  1.00” (record for Oct-May = 1.05”)

Figure 1. Forecast for precipitation over the 7-day period starting at 8 am EDT Tuesday, October 2, 2018. Rainfall could top 5” across large parts of the Southern and Central Plains this weekend, after Rosa’s remnants and subsequent systems help replenish moisture across the Southwest. Image credit: NOAA/NWS/WPC.

Total rainfall from Rosa’s moisture could reach 10” in isolated parts of far northwest Mexico, up to 6” in the mountains of central Arizona, and as much as 4” in other favored parts of the Southwest U.S. Flash flood watches extend from the lower Colorado River basin into eastern Nevada, most of Utah, and southwest Colorado. The heaviest rains will be patchy, associated with localized showers and thunderstorms and where winds are blowing upslope against higher terrain. This is a common pattern for decaying tropical cyclones that move into the Southwest U.S. from the Northeast Pacific.

CLOSURE: I-17 and Peoria: Underpass is flooding. Peoria EB/WB is blocked at I-17. #phxtraffic #azwx pic.twitter.com/JRL0dXbMwC

— Arizona DOT (@ArizonaDOT) October 2, 2018

Though some areas might see only minor amounts of rain, the influx of moisture from Rosa should help to alleviate drought conditions now plaguing the Southwest, especially the Four Corners area. Some additional rain will be triggered later this week by the upper low off the California coast as it moves ashore and interacts with residual moisture from Rosa. The upper low heralds a major pattern change, as discussed by weather.com’s Chris Dolce. Cold, snowy conditions will invade the Northern Rockies this weekend, and heavy rains will set up across the Great Plains, while unusually warm air remains over the easternmost U.S.

Figure 2. Strong thunderstorms are evident east of an ill-defined area of low pressure in the southwest Caribbean Sea in this infrared satellite image from 1745Z (1:45 pm EDT) Tuesday, October 2, 2018. Image credit: NASA/MSFC Earth Science Branch.A

An area of concern in the Caribbean

A broad area of low pressure in the southwest Caribbean could evolve into a tropical cyclone over the next few days. October is the prime season for tropical development in the Caribbean, where waters warm to some of the highest temperatures in the Atlantic basin. Early in the season, wind shear often inhibits development in the Caribbean, but shear tends to relax by early autumn. The wall of wind shear that’s been in place across the northern and eastern Caribbean for most of the last few weeks is projected to lift northward over the next few days, possibly opening the door for development.

Tuesday saw a dramatic blow-up of intense thunderstorms southwest of Hispaniola, well east of the low pressure center (see Figure 2). This large storm complex was especially impressive for midday, given that tropical systems often have their strongest showers and thunderstorms (convection) during nighttime hours. We have no scatterometer passes to indicate any low-level rotation below the storm complex, but any such storm complex that might form closer to the broad surface low could be a forerunner of overall tropical development.

In its tropical weather outlook issued at 2 PM EDT, NHC gave the Caribbean system a near-zero chance of development through Thursday, but a 20% chance during the period from Thursday through Sunday. The 0Z Tuesday ensembles from the GFS and European models (updated with 12Z Tuesday data) are more bullish: about one-third of the European ensemble runs and 60% of the GFS runs show a tropical cyclone in the Caribbean during the 3- to 5-day period. The ensemble odds are even higher during the 6- to 10-day period. Operational model runs show a broad area of southeasterly flow building by next week across the Gulf of Mexico, Caribbean, and adjacent northwest Atlantic, so all of these areas will need to be watched for potential development over the longer term. If the current area of low pressure ends up hugging the west end of the Caribbean while moving slowly north, it could become a Central American Gyre, a large monsoon-type low that can produce torrential rains onshore.

Watching tropical disturbance in Caribbean this week. With big upper trough in the region, hard to see a path forward w/ low shear, though east US breaking ridge could potentially provide a chance if TUTT splits. For now, watching for something messy/wet to come north next week. pic.twitter.com/gsFEKAVI5J

— Levi Cowan (@TropicalTidbits) October 2, 2018

Leslie still plodding across Central Atlantic

Little has changed with Tropical Storm Leslie since Monday. Leslie’s top sustained winds were 65 mph as of 11 am Tuesday, and the storm was located about 500 miles east of Bermuda, heading southwest at 8 mph. The strongest thunderstorms around Leslie have now shifted toward the northwest half of the storm, a sign that Leslie is getting the upper hand on westerly wind shear that had been shunting convection eastward.

Figure 3. Infrared image of Tropical Storm Leslie at 1520Z (11:20 am EDT) Tuesday, October 1, 2018. Image credit: NASA/MSFC Earth Science Branch.

The shear over Leslie will remain moderate (10 – 15 knots) over the next few days, and Leslie has now drifted south across sea surface temperatures that are 26°C (79°F), the traditional minimum for tropical development. There’s a good chance that Leslie will become a hurricane by Wednesday (as predicted by NHC). Later this week, it will reverse course and moves back north across its own upwelled water toward cooler SSTs, most likely denting any gain in strength. A curve back toward the east appears likely by this weekend.

Swells and rip currents from Leslie will continue to affect the U.S. East Coast, and eventually Atlantic Canada, as Leslie continues to churn. Leslie could be in the picture well into next week that as a tropical or subtropical storm. The bulk of the GFS and European ensemble members show Leslie producing tropical-storm-force winds for at least the next 10 days!

For more on Kong-rey and Walaka, the two powerful cyclones in the Northwest and Northwest Pacific that reached Category 5 strength early Tuesday—the first simultaneous Cat 5s on record involving these two basins—see the writeup from Dr. Jeff Masters posted earlier today.