Synopsis
An upper level closed low, currently centered over the lower Colorado River Valley, will continue to dig southward into Northern Mexico throughout the day today. This feature is expected to provide Arizona with cold temperatures, strong winds, and scattered, light precipitation late tonight through Wednesday. Easterly winds on the order of 20 to 40 knots are expected across the region with strongest wind speeds over the higher terrain and in Southeastern Arizona late tonight through Wednesday afternoon. Even though this upper level low is dynamically robust, it is quite moisture starved leading to only light precipitation accumulations. Snow levels will be relatively low with some light snowfall possible in some of the valley locations mainly east of Tucson and above 3000 feet. Snow accumulations over the high terrain will be light with only 1 to 3 inches possible. Besides precipitation chances, Wednesday will be a chilly day across Arizona.
The aforementioned upper level low will rapidly lift northeastward on Thursday with high pressure aloft rebuilding back into the region through the end of the week. Overnight temperatures will still be very cold, with some lower desert locations in Southeast Arizona (including Tucson) having low temps near to below freezing Wednesday night through Friday morning.
Current Conditions
At 10:00AM MST, visible satellite imagery overlaid with 500mb RAP analysis displays an upper level low over the lower Colorado River Valley, scattered convective showers across the Mojave Desert, and high clouds across Southern Arizona.
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| Visible satellite imagery overlaid with 500mb RAP analysis as of 10AM MST this morning courtesy of College of Dupage. |
Meanwhile, a sad sounding was observed in Tucson this morning with only 0.26 inches of precipitable water and a surface dew point of 16 degF.
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| 12z TUS sounding this morning courtesy of the SPC. |
Tonight Through Wednesday
Dynamics, Moisture, and Instability
The upper level low over the lower Colorado River Valley will continue to dig southward and eventually become centered over Northern Mexico later tonight with Arizona in the favorable northeastern quadrant.
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| 12z WRF-HRRR 500mb temps and winds valid for 12z tomorrow. |
1000-500mb thickness is forecast to decrease to between 546 and 540 decameters late tonight through Wednesday allowing snow levels to reach some valley locations in Southeastern Arizona above 3000 feet. However, the main issue is moisture, and models forecast the maximum in Eastern Pacific moisture transport to remain southeast of Arizona.
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| 12z WRF-HRRR 700-300mb integrated water vapor transport (IVT) valid for 12z tomorrow morning. |
In fact, model soundings for KTUS and KPHX forecast precipitable water to remain well-below 0.50 inches through the period as shown below.
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| 12z WRF-HRRR KTUS (left) and KPHX (right) model soundings valid for 12z tomorrow morning. |
Associated with the very limited moisture is poor buoyancy with CAMs only forecasting a mere 25 to 50 J/kg of MUCAPE.
Strong Winds
A major concern with this system is strong easterly winds especially over the higher terrain and in Southeastern Arizona. Models forecast the surface cyclone (nearly vertically aligned with the 500mb vort max) to become centered in Northwestern Sonora tonight through tomorrow morning. This will allow the pressure gradient to tighten and induce strong easterly winds especially in Southeastern Arizona. The UA WRF ensemble members as well as HREF mean 10-meter winds forecast wind speeds on the order of 20 to 40 knots across Southeastern Arizona late tonight through Wednesday afternoon with highest wind speeds over the highest terrain. My main concern is along the I10 corridor in Pinal, Pima, and Cochise Counties where poor visibility due to blowing dust and snow (mainly in Cochise County) will make traveling conditions extremely hazardous.
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| 12z WRF-HRRR 10-meter winds valid for 12z tomorrow morning. |
Precipitation
Unfortunately, the robust synoptic scale dynamics will not have much moisture to work with, but there will be just enough moisture for scattered, mainly light precipitation across the state late tonight through Wednesday afternoon. UA WRF ensemble members (will use 12z WRF-HRRR for display purposes) indicate that precipitation will begin in Central Arizona (including the Phoenix vicinity) after sunset and gradually shift southeastward into the early morning hours.
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| 12z WRF-HRRR simulated reflectivity valid for tonight at 8PM MST. |
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| 12z WRF-HRRR simulated reflectivity valid for tonight at midnight MST. |
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| 12z WRF-HRRR simulated reflectivity valid for tomorrow morning at 5AM MST. |
Precipitation chances are expected to be focused in Southeastern Arizona by mid morning tomorrow through the late afternoon hours as shown below.
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| 12z WRF-HRRR simulated reflectivity valid for tomorrow morning at 10AM MST. |
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| 12z WRF-HRRR simulated reflectivity valid for tomorrow afternoon at 3PM MST. |
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| 12z WRF-HRRR simulated reflectivity valid for tomorrow afternoon at 5PM MST. |
Precipitation will be somewhat spotty, so some areas could receive up to 0.25 inches of liquid and others receiving no precipitation. At this time, the best chances for greatest precipitation coverage will be in Cochise County as well as over the Mogollon Rim and Sky Islands of Southeast Arizona.
Based on UA WRF and NCEP model guidance, snow levels will likely remain above 3000 feet and to be honest I think that's a bit bullish. My take is frozen precipitation above 3500 feet, 50% chance of above 3000 feet, and 10% chance above 2500 feet. I cannot completely rule out some wet snowflakes in the Tucson valley, but it would require either heavy precipitation and/or strong evaporative cooling in the subcloud layer which seems very unlikely at this time. The best chance for snowflakes in the lower elevations would be early morning or after sunset tomorrow. Valley locations in Southeast Arizona that could receive light snow accumulations include Oracle, Nogales, Benson, Sierra Vista, and Wilcox with a dusting to 1 inch possible. Snow accumulations along the Mogollon Rim and Sky Islands will range between a dusting to locally as high as 3 inches over the tallest peaks. However, snow accumulations will be highly dependent on trajectory, frequency, and intensity of showers.
With almost non-existent CAPE values, thunderstorms are not expected, but I can't rule out a rogue lightning flash in some of the isolated, stronger cells.
Precipitation is forecast to taper off rapidly after sunset tomorrow.
Cold Temperatures
It has been somewhat consistently sunny and 75 (a legitimate country song) in Phoenix and Tucson the last several weeks. However, it will be somewhat chilly tomorrow with high temperatures forecast to remain in the mid to upper 50s in Tucson and low to mid 60s in Phoenix.
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| 12z WRF-HRRR 2-meter temperatures valid for tomorrow at 2PM MST. |
Temperatures in the higher valley locations of Cochise County will likely remain in the mid 40s as well.
Low temperatures tomorrow night/early Friday morning will be cold due to the frigid legacy airmass left behind by the upper level low. Models forecast low temps in valley locations in Southeastern Arizona to reach near to below freezing temperatures on Thursday and Friday mornings.
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| 12z WRF-HRRR 2-meter temperatures on Thursday morning (left) and Friday morning (right) at 12z. |
Thursday Through The Weekend
On Thursday, the aforementioned upper level low will lift northeastward into the Great Plains with high pressure aloft rebuilding back into the region from the west. This pattern will end any threat of precipitation with high temperatures returning to near normal through the end of the week.
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| 12z GFS 500mb cyclonic vorticity valid for Friday at 12z courtesy of Tropical Tidbits. |
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