“The January 21-22, 2007 Winter Storm and its Impact on Southeast Arizona” Glenn Lader, NOAA/National Weather Service Tucson, AZ Robert Kolbert

Introduction A significant 500mb low moved south from Southern Utah early on January 21 st south into the Southern Arizona deserts late January 21 st and January 22 nd This system took an entirely overland track which is typically cold but has very little moisture A unique aspect of this storm is that it followed a rather significant storm January 19 th that moistened up the lower levels

Standardized Anomalies Courtesy: Rich Grumm, NWS State College, PA

Water Vapor Loop & RUC 500mb heights 9z January 20-18z January 22

RUC 500mb Heights/Vorticity Loop 18z January 21 to 21Z January 22 This system had extremely impressive dynamics to work with including strong frontogenetic forcing, vorticity advection and upper level divergence as it approached Southern Arizona.

Four Panel mb Q Vectors, Div Q GFS, NAM80, NGM, NAM40 valid 00Z January 22

2330Z January 21 IR Image IR imagery showed cloud tops of -40C coinciding with tropopause near 400mb on the 0Z KTUS sounding indicating strong lift and convective precipitation. It was during this period that snow mixed in across the city of Tucson even with temperatures still in the mid 30’s before sunset

Hourly Radar Loop 18Z January 21-16Z January 22 A solid area of precipitation moved into the Tucson area early-mid afternoon with a frontal band which significantly cooled the atmosphere, even across the lower elevations into the mid- upper 30’s. Snow levels were initially above 3000 feet As precipitation continued into the early evening, snow levels dropped to valley floors including city of Tucson (2500 feet).

Hourly Radar Loop 18Z January 21-16Z January 22 After a short lag of precipitation, a second round of precipitation organized across Southwest Tucson and moved across the Southern and Eastern Portions of the city between 130Z and 03Z This second round was right along the interface between the dry slot wrapping in near the center of the upper level low and moist inflow This portion was mostly snow and gave the city measurable snowfall anywhere from a dusting to isolated 3” with temperatures hovering near or just above freezing

12Z January 21 and 00Z January 22 KTUS Sounding

Evolution of event across Southeast Arizona After 03Z the precipitation came to an end in Tucson as the dry slot moved in and skies cleared As temperatures and dewpoints dropped into the upper 20’s, freezing fog developed and all bridges in Tucson saw icing conditions with numerous accidents Farther east across Cochise, Graham and Greenlee County, snow continued all night into January 22 nd. Areas along the pivot point along the moist axis from Bisbee (elev 5500ft) to Safford (elev 2800ft) received very impressive snowfall with amounts ranging from 8 to 10 inches.

Snow Totals Across Southeast Arizona Pima County/Southeast Pinal County Central Tucson, Elev 2500ft, 1-2 inches Vail, Elev 3200ft, 3 inches Mt. Lemmon, Elev 7810ft, 12 inches Oracle, Elev 4510ft, 6 inches Santa Cruz County Sonoita, Elev 4900ft, 3 inches Patagonia, Elev 4000ft, 3-4 inches Cochise County Bisbee, Elev 5500ft, 8.3 inches Sierra Vista, Elev 4600ft, 2-4 inches Paradise, Elev 5670ft, 8.3 inches Graham/Greenlee County Safford, Elev 2800ft, 6-9 inches Clifton, Elev 3520ft, 3 inches Hannagan Meadow, Elev 9560ft, 24 inches

How did the numerical models do? 12Z January 18 th ECMWF (left) and GFS (right) The models were quite late getting a handle on this system. All guidance 3-5 days ahead were showing trough staying east with little moisture across Southeast Arizona.

How did the numerical models do? 12Z January 19 ECMWF, GFS and UKMET The guidance trended west with the 12Z January 19 th. This was especially the case with the ECMWF and UKMET. Even with this run the models showed limited moisture availability. The 0Z January 20 th GFS and UKMET made the first real aggressive change to a moister/wetter solution. The NAM was the last to adjust the upper level low track west across Southern Arizona. It took until the 12Z January 20 th run for the models to have a consistent handle on moisture availability and track.

Concluding Thoughts A storm moved through two days prior to this event which moistened up the atmosphere and provided adequate moisture for significant precipitation. This also likely contributed to the numerical models being underdone with qpf. 500mb heights being 2-3 standard deviations is unusual, but not unheard of. We see similar systems several times per season. The main difference with this event was the moisture availability and strong lift. Tucson averages measurable snowfall once every five years. If the storm had been a bit farther west, the 6-9 inches that occurred at 2800 feet in Safford (a record amount for them) could’ve been in the city of Tucson. Being in the desert by no means precludes us from having significant winter weather!