February 1st, 2024: Starting February with a Potent Pacific Storm

 Synopsis

A pacific storm is expected to impact Arizona beginning this evening and persisting into tomorrow morning with post frontal convective showers expected tomorrow afternoon and evening. Total precipitation is expected to range between 0.50 and 1.00 inch in the lower deserts and between 1 and 3 inches over the higher terrain especially along southwestern/south facing slopes of the mountains. Snow levels will remain quite high with freezing levels starting around 8000 ft this evening a dropping to around 5000 ft tomorrow afternoon. The heaviest snowfall is expected mainly over the highest mountain peaks of the Mogollon Rim, White Mountains, and Sky Islands of Southeastern AZ with upwards of 1 to 2 feet of accumulated snowfall. 

Current Conditions 

At 9:00AM MST, IR satellite imagery displayed an atmospheric river impacting Southern California. 

GOES-16 IR satellite imagery courtesy of College of Dupage.

El Niño is in full swing as indicated by warm SST anomalies along the eastern and central basins of the equatorial Pacific Ocean as shown below.

CDAS SST anomalies as of 06z Feb 1, 2024 courtesy of Tropical Tidbits.

Another obvious El Niño signature is the strong low latitude jet stretching across the entire North Pacific as shown by the 12z GFS 250mb winds this morning.

12z GFS 250mb winds as of 12z this morning courtesy of Tropical Tidbits.

The storm currently impacting Southern California and eventually Arizona later tonight is associated with left exit region dynamics as indicated by this mornings 300mb analysis shown below. 

SPC 300mb analysis as of 15Z this morning.

This region of synoptic scale ascent has plenty of moisture to wring out with the 06Z GFS showing a decent fetch of subtropical moisture with IVT values on the order of 400 to 800 kg m/s.

06Z GFS IVT valid at 06Z this morning courtesy of CW3E.

This Afternoon through Tomorrow Morning

Throughout this afternoon expect increasing mid to upper level clouds and brisk southwesterly winds as the storm approaches. Warm advection ahead of the cold front will provide temps in the low to mid 70s this afternoon across the lower deserts. 

By this evening the aforementioned left exit region of the upper level jet and associated vort max of the upper level trough will be over AZ providing PVA and upper level divergence inducing synoptic scale ascent across the state.

06Z GFS 250mb Divergence valid for tonight at 11:00PM MST.

06Z GFS 500mb Absolute vorticity valid for tonight at 11:00PM MST.

Meanwhile, modest moisture transport from the southwest will rapidly moisten the atmosphere with forecast IVT values on the order of 300 to 600 kg m/s within the region of ascent. 

12z GFS IVT valid for tonight at 11:00 PM MST courtesy of CW3E.

This is a recipe for moderate to heavy rain rates across Arizona tonight. 12z KPHX and KTUS WRF-HRRR model soundings forecasting pwats between 0.75 and 0.90 inches by late tonight which may not seem impressive for monsoon standards but for February is quite high. 

12z KTUS WRF-HRRR sounding for tonight at 11:00PM MST.

12z KPHX WRF-HRRR sounding for tonight at 10:00PM MST.

In fact, based on TUS sounding climo, pwats in this range are well above the 90th percentile for February.

TUS Sounding Climo of total precipitable water courtesy of the SPC.

Significant synoptic scale ascent combined with ample amounts of moisture will promote moderate to heavy rain rates across the state tonight into early tomorrow morning. Due to the fast moving system and short time frame for heavy precipitation, excessive rainfall is only marginal as forecasted by the WPC. 

WPC Day 1 Excessive Rainfall Outlook.

The greatest threat of flash flooding and excessive rainfall will be along the southwestern facing slopes of the Mogollon Rim, White Mountains, and Sky Islands of Southeastern AZ due to orographic enhancement. However moderate to heavy rain rates, albeit expected only for a short period of time could lead to localized flooding across low elevations. 

The main rain band/AR will be primarily stratiform which is typically the case for AR events, but with plenty of ascent a rogue lightning flash or two cannot be ruled out. 

Models are in pretty good agreement regarding timing and rainfall amounts with 48-hour accumulations between 12z this morning and 12z Saturday morning around 0.5 and 1.0 in across lower elevations and between 1 and 3 inches over the higher terrain. Below is the evolution of precip coverage and the 48-hour QPF from the 12z WRF-HRRR.

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

12z WRF-HRRR simulated reflectivity for tonight at 9:00 PM MST.

12z WRF-HRRR simulated reflectivity for tonight at 11:00 PM MST.

12z WRF-HRRR simulated reflectivity for tomorrow morning at 1:00 AM MST.

12z WRF-HRRR simulated reflectivity for tomorrow morning at 3:00 AM MST.

12z WRF-HRRR simulated reflectivity for tomorrow morning at 5:00 AM MST.

12z WRF-HRRR 48-hour QPF from 12z this morning to 12z Saturday morning.

This is a warmer system so snow levels will remain on the higher side with freezing levels initially above 8000 ft early this evening and dropping to around 5000 to 6000 ft by tomorrow morning. Very heavy snow is expected for the highest peaks of the Mogollon Rim, White Mountain, and Sky Islands with snow totals of 1 to 2 feet in these locations over the next 48 hours. 

Tomorrow Afternoon and Evening

By mid morning tomorrow, the main rain band/AR will move east into New Mexico. However, Arizona will remain under the left exit region of the upper level jet continuing to provide upper level divergence and therefore synoptic scale ascent. 

06Z GFS 300mb winds for tomorrow at 5:00 PM MST.

At the same time, colder mid level temps will enter our region as the trough axis passes through. This will aid in slight destabilization of the atmosphere and increase the chance for convective showers and even a low-topped thunderstorm or two. The 12z KTUS and KPHX WRF-HRRR soundings indicating weak low to mid level buoyancy with CAPE around 200 J/kg by tomorrow afternoon. 

12z KTUS WRF-HRRR sounding for tomorrow at 3:00 PM MST.

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

The 12z WRF-HRRR has the most widespread shower activity across mainly southeastern AZ (including Tucson) and the higher terrain which is shown below. 

12z WRF-HRRR simulated reflectivity for tomorrow at 11:00 AM MST.

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

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

 

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

Travel conditions will be hazardous especially along I8 and I10 tonight and persisting into the morning commute tomorrow. 

Overall, this storm looks to be a good rain maker providing Arizona with much needed precipitation especially considering the poor monsoon season last summer. 

This Weekend into Early Next Week

Some lingering isolated light showers can be expected early Saturday morning across the eastern half of AZ, but should clear out by midday. Transitory ridging will develop over Arizona providing tranquil but cool weather this weekend as another, even more potent Pacific Storm begins to makes its way toward the west coast early next week. There have still been some run to run differences in the deterministic GFS and ECMWF as well as their ensemble members regarding timing and intensity for Arizona. However, expect another storm system sometime next week with more rain and snow on the way! Stay tuned!