A very active weather pattern developed across Southeast Arizona in early February 2009 as the pattern transitioned from a very mild and dry pattern across the Western US during much of January to a broad trough early in February. The first system in this active pattern was a closed mid/upper level low which moved across Southeast Arizona late February 7th into the day on February 8th. This event will not be focused on during this write-up but was an efficient valley rain and mountain snowfall producer with widespread quarter to half inch valley rainfall and up to 12 inches of snow in the mountains above 6000 ft. Most importantly this system helped ramp up the lower level moisture as it moved through 48 hours ahead of the next storm that produced the low elevation snowfall on February 10th.

The February 10th storm was the result of a ~538dm neutral to slightly positively tilted 500mb trough, about three standard deviations below normal that swung through Arizona. This storm system started in the Gulf of Alaska and rapidly dived Southeast into the lower latitudes with the aid of 130+kt flow on the backside of the trough across the Eastern Pacific. As it pushed into the area there was little break between the first system mentioned above and the second one for the White Mountains given the favorable moisture and upslope flow. For the remainder of the area though, there was a solid break on February 9th between events. As the trough approached during the day on February 9th, Southeast Arizona experienced a cool day with highs generally in the upper 50s and breezy to locally windy conditions with local gusts in excess of 40 mph as the pressure gradient increased.

As the trough and resultant cold front moved Southeast through the area late in the evening on February 9th and through the early morning on February 10th, conditions were quite favorable for brief moderate to locally heavy precipitation. This was due to impressive dynamics and lift from low to mid level frontogenetic forcing, strong vorticity advection and height falls in addition to upper level divergence. Precipitable water values were just under a half inch with leftover moisture from the previous event, plus a favorable marine breach per the 12Z February 9th San Diego sounding in addition to low level South to Southwest flow just ahead of the front adding some Gulf of California moisture to the mix. A band of moderate precipitation was observed on radar pushing Southeast with coinciding IR satellite imagery depicting -25C to -30C cloud tops. Even though the cloud tops were not too high/cold, this precipitation was convective in nature as it pushed through. The heaviest precipitation was observed along and just behind the rather sharp cold front as it pushed Southeast through the area from about 06Z to 13Z on February 10th. One hour observed temperature drops as the front and precipitation moved through were quite impressive for this part of the country and were on the order of 10 to 15F.

Once the front passed through the area, dynamic cooling resulted in the moderate rainfall band turning briefly to snow as low as the city of Tucson at about 2500 feet about 2am MST. Elevations near and just above 3000ft to about 4000ft including the east side of the city of Tucson and the Vail/Corona De Tucson areas became just cold enough to see anywhere from 1 to 3 inches of snowfall. Snowfall amounts quickly increased above 4000ft with most such as Oracle in Southeast Pinal County and many sites in Cochise County seeing 2 to 6 inches. Snowfall amounts rapidly increased above 6000ft with the mountaintop locations such as Mt. Lemmon and Hannagan Meadow officially recording 16 inches of snowfall. However, at elevations above 6000ft where the event was snow for the duration, the winds were quite strong which resulted in blowing and drifting snow with drifts in excess of 4 feet. In fact, these areas were quite close to blizzard conditions but came short of meeting the wind criteria for greater than 3 hours. Given these conditions, snowfall estimates were likely on the lower end of what occurred at these high elevation mountain locations.

This was not a long duration event with lower elevation sites not seeing precipitation for in excess of 6 hours. After the heaviest precipitation moved through which resulted in the dynamic cooling, temperatures in lower elevation locations below 3000ft tended to see temperatures increase by a couple of degrees. When all was set and done, precipitation totaled from as low as about 0.10" to in excess of 0.50" especially on the mountain tops. On average a quarter of an inch of precipitation was common for the lower elevation locations. Once the precipitation ended, skies quickly cleared resulting in a cold day with highs in the 40s to low 50s.

The model guidance handled the evolution of this quite well. By about 5-6 days prior to the storm system it became evident rather quickly that there would be a secondary cold trough behind the first one that moved through on February 7th/8th. Once that became clear it was just a matter of honing in on the details such as how just how cold would the system be along with the exact track, timing and moisture/QPF.

In the short term, the NAM12 did a particularly good job with the timing and QPF of this system. This is not of great surprise given the greater resolution of the NAM which is better able to handle the orographic enhancement along the higher terrain. The GFS was too broad brushed and a bit high on QPF particularly for the lower elevations. With regard to temperature profile there were only minor differences between the GFS/NAM solutions which both handled the temperature profile quite well. Speaking of the temperature profile, it is quite interesting to note that while 1000mb-500nb thicknesses bottomed out in the mid 530 dm range across Southeast Arizona with 500mb temperatures down to -30C, 700mb temperatures down to about -13C and 850mb temperatures as low as -1C to -2C, the core of these coldest temperatures in the mid levels were progged by the models to come through after the precipitation ended. For example, when it was briefly snowing in Tucson with a surface temperature of 35F, this was just after the frontal passage in the heaviest precipitation band when the parameters to get snow based on the models in Tucson were not as favorable with 1000-500mb thicknesses only near 540dm which usually would correlate to a snow level close to 4000ft under ordinary circumstances. However, there was probably a stronger temperature/thickness discontinuity along the front than the models had progged in addition to dynamic cooling which resulted in snow levels down to 2500ft that quickly.

When all was set and done this was a well advertised and verified event for Southeast Arizona!