February 5th, 2024: Another Pacific Storm to Impact Arizona Tomorrow and Wednesday

 Synopsis

Another Pacific Storm is expected to affect Arizona starting tomorrow and persisting into Wednesday evening. The main rain band/Atmospheric River will impact Western AZ late tomorrow morning and afternoon  then Eastern AZ late tomorrow afternoon into Wednesday morning. Scattered Post-frontal convective showers and isolated low-topped thunderstorms can be expected Wednesday morning and early afternoon as well. Current QPF is between 0.10 and 0.50 across the lower deserts and 1 to 3 inches over the higher terrain with highest amounts along the southwestern facing slopes of the Mogollon Rim, White Mountains, and Sky Islands of SE AZ. Snow levels will remain quite high with the initial rain band/AR with freezing levels above 9000 feet and dropping to around 6000 feet by Wednesday. Snow totals of 6 to 12 inches are expected for the highest mountain peaks above 9000 feet with less than 6 inches for mountains between 5000 and 9000 feet. 

 Current Conditions

At 9:30 AM MST, IR satellite imagery displayed a strong atmospheric river stretching from the subtropics of the Eastern Pacific into Southern California.

GOES-18 Long Wave IR (Band 14) courtesy of NOAA.

Quite a brutal day yesterday across the California coast as a very strong mid latitude cyclone brought hurricane force winds, heavy rain, and blizzard conditions over the mountains. Wind gusts of 70 to 100 mph were observed along the immediate coastline of Central and Northern California yesterday! In Los Angeles, constant moderate to heavy rainfall from an Atmospheric River over the past 24 hours has lead to widespread flooding and land slides with current precipitation totals of 3 to 5 inches and still rising across the LA basin. 

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

This atmospheric river is associated with left exit region dynamics of an upper level jet streak and PVA ahead of an anomalously deep trough off the west coast.

SPC 300mb analysis valid at 16:00z.

12z GFS 500mb analysis at 12z this morning.

Within this region of ascent is an impressive conveyor belt of tropical moisture with IVT values on the order of 400 to 800 kg m/s. 

12z GFS IVT valid at 12z this morning courtesy of CW3E.

Meanwhile in our neck of the woods, mid and high clouds are beginning to move in from southwest to northeast as the Atmospheric River slowly approaches from the west. Expect these mid and high clouds to increase and thicken throughout the day today and into tomorrow morning. Southwesterly flow aloft will maintain warm temperatures across the lower deserts with high temps in the mid to upper 70s and light southerly winds this afternoon.

 Tomorrow into Wednesday Morning

Throughout the day today and into tomorrow the aforementioned trough is expected to dig southward and deepen just off the California coast. At the same time, the associated jet streak will move inland into the Southwest and put Arizona in the left exit region providing the state with upper level divergence. 

12z GFS 500mb absolute vorticity for tomorrow at 11AM MST.

12z GFS 250mb divergence for tomorrow at 11AM MST.

Within this region of ascent, rapidly increasing moisture is expected with forecast IVT values of 400 to 700 kg m/s. 

12z GFS IVT for tomorrow at 18z courtesy of CW3E.

Brisk southwesterly winds beginning tomorrow morning ahead of the main rain band/AR with sustained winds of 20 to 30 knots across the lower deserts and as high as 30 to 40 knots over the higher terrain. 

12z WRF-HRRR 10-meter winds for tomorrow at 1:00PM MST.

Throughout the morning widespread mid level clouds will thicken, but only light rain is anticipated as the low levels will initially be quite dry which is reflected by the 12Z KTUS and KPHX WRF-HRRR soundings for tomorrow morning as shown below.

12z KTUS WRF-HRRR sounding for tomorrow at 10:00 AM MST.

 

12z KPHX WRF-HRRR sounding for tomorrow at 10:00 AM MST.

By late afternoon and early evening, the low and mid levels will be nearly saturated with forecast pwats of between 0.60 and 0.80 inches across Arizona which is above the 90th percentile for February based on Tucson sounding climatology.

12z KPHX WRF-HRRR sounding for tomorrow night at 7:00 PM MST.

12z KTUS WRF-HRRR sounding for tomorrow night at 8:00 PM MST.

SPC Tucson Sounding Climatology of total precipitable water.

Expect moderate to locally heavy rainfall starting late tomorrow afternoon and persisting into early morning Wednesday. Total precipitation from this storm will range from 0.10 and 0.50 inches in the lower deserts with 1 to 3 inches over the mountains with highest amounts expected along the southwestern facing slopes due to orographic enhancement. 

12z WRF-HRRR total rainfall forecast between 12z this morning and 00z Feb 8th.

Since this is a warmer system, snow levels will remain on the higher end with freezing levels initially around 9000 feet tomorrow afternoon and early evening and dropping to around 7000 feet by late tomorrow night/early Wednesday morning. Snow totals will likely reach 6 to 12 inches above 9000 feet with locally higher amounts possible over the highest peaks and up to 6 inches between 7000 and 9000 feet.

12z WRF-HRRR total snowfall forecast between 12z this morning and 00z Feb 8th.

With moderate rainfall and strong southwesterly winds late tomorrow afternoon and evening, the evening commute will be significantly affected making driving/traveling conditions hazardous in Tucson and Phoenix metros as well as along I10 and I8. 

By mid Wednesday morning, the initial rainband/AR will move into New Mexico and the upper level trough off the west coast will rapidly lift northeastward throughout the day Wednesday. 

12z GFS 500mb absolute vorticity for Wednesday at 18Z.

12z GFS 500mb absolute vorticity for Thursday at 00Z.

As the trough axis moves eastward across Arizona, 500mb heights will fall cooling mid level temps which will aid in steepening low and mid level lapse rates. 12z KPHX and KTUS WRF-HRRR soundings only forecasting around 100 J/kg of CAPE which combined with some PVA ahead of the trough axis should produce scattered convective showers and an isolated low-topped thunderstorm or two. 

12z KPHX WRF-HRRR sounding for Wednesday at 11:00 AM MST.

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

Snow levels will also drop to around 6000 feet but only light accumulations expected.

Excessive rainfall is unlikely for the major population centers (Tucson and Phoenix), but WPC has marginal risk for Wednesday morning. There is a slight risk for excessive rainfall in western AZ as well for tomorrow afternoon. 

WPC Day 2 Excessive Rainfall Outlook.

WPC Day 3 Excessive Rainfall Outlook.

The greatest threat for flooding will remain along the Southwestern facing mountain slopes as well as in the vicinity of recent burn scars from wildfires.

Wednesday Afternoon and Beyond

The atmosphere will begin to stabilize late Wednesday afternoon and evening as a transitory ridge develops over AZ ahead of a compact shortwave diving rapidly southward along the CA coast. 

12z GFS 500mb absolute vorticity for Thursday at 00Z.

12z GFS 500mb absolute vorticity for Thursday at 06Z.

Throughout Wednesday late afternoon and evening expect showers to taper off, but by Thursday the aforementioned shortwave will move into Arizona bringing another round of rain and mountain snow to our region.

GEFS and EPS 500mb height anomalies maintaining a troughing pattern over the Southwest for at least the next 7 days. This means continued cooler weather and chances for precipitation at least into early next week.