The figure above indicates the forecasted high in orange (with error bar denoting the range of ensemble forecast members), average high for this time of year in orange (dashed line), forecasted low in blue (with error bar), average low this time of year in blue (dashed line), and forecasted wind speed in purple. Note that the wind speed forecast is sustained wind (not gusts), and it is based on only one model (not an ensemble).
An upper-level low pressure system digging southward along the West Coast will cut off from the polar jet stream and slowly shift eastward across Arizona the next few days. In association with this low, a subtropical jet core with wind speeds over 90 mph will spread across Arizona in the upper atmosphere. Cooler air aloft will move across the state in the upper atmosphere tonight and tomorrow, making the atmosphere increasingly unstable. A moist, southerly low-level jet is forming and will bring warm, moist air northward from the Gulf of California and the subtropical Pacific across the state, further destabilizing the atmosphere. As a result, showers and thunderstorms will become likely tonight through Saturday. Most of the shower and thunderstorm activity will occur ahead of an approaching slow-moving surface cold front that will probably stall temporarily along the California, Arizona border until finally moving eastward on Sunday.
Of greatest concern is the potential for severe weather between Saturday morning (5 am) and Sunday morning (5 am) ahead of the cold front. Severe weather is defined as damaging wind gusts (at least 58 mph), large hail (at least 1” diameter), or tornadoes. The Storm Prediction Center is indicating between 15%-30% probability that severe weather will occur within 25 miles of any given location from central to northwestern Arizona (http://www.spc.noaa.gov/products/outlook/day2otlk.html). Stay tuned to local weather service products for current watches and warnings and take appropriate shelter as needed. This storm event could be similar to the Oct. 5-6, 2010, event when several severe thunderstorms storms dropped up to baseball sized hail in the Phoenix area, and a total of 11 tornadoes touched down in northern Arizona.
The strength of the vertical wind shear will increase this evening, and we will start to see lines of thunderstorms or multicellular thunderstorms propagating from the south-southwest toward the north-northeast at about 20 mph. The motion of the thunderstorms will increase into Saturday morning. On Saturday, there will be enough turning of the wind with height that isolated rotating thunderstorms could develop. If rotating storms develop (called supercell thunderstorms) they will exhibit a hook shape on the radar and the strongest ones will likely move toward the northeast at about 30 mph. Supercell storms are also capable of producing large hailstones.
Expect the chance of showers to decrease on Saturday night to Sunday as drier, cooler westerly wind moves first in the upper atmosphere then in the lower atmosphere behind the cold front. Skies will become clear by Monday, and morning lows will drop into the mid 40s by Monday morning. The precipitation totals this weekend are expected to average around once inch across central and northern Arizona, with higher precipitation totals up to 3 or 4 inches in some areas where echo training (or thunderstorm redevelopment occurs).
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Official National Weather Service forecast