August 17th: Friday through Monday Outlook; All eyes on Hurricane Hillary

 Synopsis

CAMs have been hinting at the possibility for a few showers and isolated thunderstorms overnight tonight into early tomorrow morning. Tomorrow afternoon is looking to be quite an active day for Southern Arizona. The SPC has southern and south-central Arizona (includes Tucson and Phoenix) in a Marginal Risk for severe storms tomorrow afternoon and evening. A major impact event is in store for this weekend with Hurricane Hillary forecast to move up the west coast of Baja and into Southern California. Current guidance has most of the activity remaining west of Tucson with far southwest Arizona and southeastern California in the most favorable region for thunderstorms, strong winds, and excessive rainfall.

 Current Conditions

As of 5:45 PM MST, visible imagery and radar showed numerous showers and thunderstorms across southern Arizona. As I mentioned in the morning discussion, it appeared CAMs were under-forecasting the instability today (forecasting CAPE around 600 J/kg) and this was confirmed from an 18z TUS sounding which had 1506 J/kg of CAPE. 

18z TUS sounding courtesy of the SPC.

Thermodynamics were plenty sufficient for storms in Tucson today, but a large anvil cloud from a strong thunderstorm about 20 miles southeast of the city moved over the area reducing surface heating and therefore sabotaging any storm development over the metro. 

Overnight/Early Tomorrow Morning

CAMs have been advertising the potential for a few showers and thunderstorms late tonight into early tomorrow morning across southeastern AZ. The 12z, 15z, and 18z WRF-HRRR as well as recent runs of the raw HRRR have been hinting at this. Current guidance has most of any overnight activity remaining west of Tucson, but the 18z WRF-HRRR does show a few weak echos over the Tucson metro early tomorrow morning so something to look out for. 

18z WRF-HRRR simulated maximum reflectivity for tomorrow morning at 5:30AM MST.

Tomorrow's Forecast

Synoptics

By tomorrow, the closed low off the coast of CA will deepen and open up into more of a long wave trough. In response, the mid level anticyclone builds northeastward with the center becoming displaced farther east over the Central Plains. Upper air conditions don't really change for Arizona as we will remain under southerly flow aloft. This synoptic pattern however will set the stage for the high impact event Saturday through Monday.

18z GFS 500mb absolute vorticity for tomorrow at 18z.

Thermodynamics

Sufficient amounts of moisture and instability are forecast for tomorrow with the 12z KTUS WRF-HRRR sounding forecasting pwat at 1.12 in and CAPE of 680 J/kg. The most critical quantity is the impressive DCAPE of 1945 J/kg which seems reasonable considering the drier low levels and inverted V signature. 

12z KTUS WRF-HRRR sounding for tomorrow at 1:00 PM MST.

Today the WRF under-forecast the CAPE, but the forecast CAPE for tomorrow afternoon seems reasonable considering the drier air moving in from the east. This is indicated in the 12z WRF-HRRR precipitable water map as shown below. 

12z WRF-HRRR total precipitable water for tomorrow at 12:30 PM MST.

What to Expect/Potential Impacts

With sufficient moisture and instability as well as large amounts of DCAPE, tomorrow looks to be the best day so far this week for widespread, strong thunderstorms mainly for Tucson westward. Convection is forecast to initiate over the high terrain surrounding Tucson early in the afternoon.

12z WRF-HRRR simulated maximum reflectivity at 1:00 PM MST.

Once convection initiates, CAMs forecast vigorous outflow boundaries to move west/northwest initiating new cells. The 12z, 15z, and 18z runs of the WRF-HRRR forecast convection to develop in the Tucson metro sometime in the middle of the afternoon. 

12z WRF-HRRR simulated maximum reflectivity at 3:00 PM MST.

By early evening outflow boundaries are forecast to rapidly move into south-central and southwestern Arizona initiating new cells in those areas. 

12z WRF-HRRR simulated maximum reflectivity at 5:00 PM MST.

12z WRF-HRRR simulated maximum reflectivity at 7:00 PM MST.

SPC has southern and south-central AZ in a marginal risk for severe storms tomorrow. This is due to the potential for strong, gusty outflow winds associated with the near 2000 J/kg of DCAPE expected for tomorrow. Some of the strongest updrafts could even produce nickel to quarter sized hail but as of now this looks to be isolated at best. In addition, with fast moving storm clusters and a dry sub cloud layer, excessive rainfall is unlikely but as is always the case isolated flash flooding can never be discounted. 

SPC Day 2 (tomorrow) Severe Weather Outlook.

So, are there any potential inhibitors? The only potential issue tomorrow could be leftover debris clouds from overnight/early morning convection in southeastern Arizona. If any debris clouds hang around into the late morning, it could lead to delayed convective initiation. As of now, CAMs don't appear to be advertising this but it is definitely something to watch out for. 

Otherwise, expect a busy day across southern Arizona tomorrow afternoon and evening.

Here Comes Hillary...

All eyes are on Hurricane Hillary which is currently several hundred kilometers off the west coast of southern Mexico. Hillary is currently a category 3 hurricane and is expected to move northward up the west coast of the Baja California Peninsula over the next couple of days. There is agreement in the GFS and ECMWF that Hillary will weaken as it moves northward and make landfall (potentially at tropical storm strength) in far Southern California. 

Current NHC track of Hurricane Hillary.

Synoptics: Why/How is this Occurring

The upper level pattern is pretty much perfect to pull an Eastern Pacific tropical cyclone into the southwest. Tomorrow into Saturday an anomalous deep trough (especially for August) amplifies and digs southward, and at the same time an anomalous, high amplitude ridge builds over the central CONUS. This allows brisk southerly/south-southeasterly mid level winds to develop off the coast of central Baja all the way into the Desert Southwest. Normally, Eastern Pacific Tropical Cyclones that begin to move northward up the coast of Baja get steered to the west and die off due to cooler SSTs. However with the synoptic setup this weekend, Hillary will have a much different fate as it gets "sucked up" into the upper level trough which transports the deep moisture and energy associated with it into the Southwest. 

18Z GFS 500mb absolute vorticity for Saturday at 18Z.

18Z GFS 500mb absolute vorticity for Sunday at 18Z.

18Z GFS 500mb absolute vorticity for Monday at 12Z.

Moisture and Rainfall

The amount of moisture that will be transported into Southern California and Western Arizona is staggering. Moisture is forecast to rapidly increase in far southeastern CA and southwestern AZ starting Saturday afternoon and evening with pwats climbing to near 2 inches in that region. By late Sunday pwats could sky rocket to near 2.50 inches in Yuma and El Centro! 

12z WRF-GFS total precipitable water on Saturday at 8:00 PM MST.

12z WRF-GFS total precipitable water on Sunday at 5:00 PM MST.

For perspective, the all time record pwat in Yuma based on the SPC's sounding climatology is 2.33 in! With this in mind, we could be looking at a record breaking event in the deserts of far southwestern AZ and southeastern CA. 

Current WPC QPF for this event has accumulated rainfall between 2 and 4 inches in far southwestern AZ with 4 to 6 inches expected in southeastern CA. Locally higher amounts will be possible and will depend on embedded thunderstorms in the rain bands. 

 

WPC 1-5 day QPF Forecast.

Winds

Current guidance has the strongest winds remaining farther west more towards San Diego where the center of the cyclone is expected to pass by along the immediate coast line. However, small variations in the track of the center of the cyclone would make a significant difference in the location of the strongest winds. Currently, sustained winds are expected to peak between 20 and 35 kts on Sunday across far southwestern AZ and southeastern CA with stronger gusts possible especially near or in thunderstorms. Travel conditions will be beyond hazardous for anyone planning on driving in this area this weekend.