Sierra Vista, AZ Weather

Synopsis

Dry conditions are expected across Southeast Arizona into Thursday. Scattered showers and thunderstorms will return to the region Friday, followed by a lull in storm activity Saturday, then widespread showers with isolated thunderstorms Sunday into Monday. Although moderate intensity rainfall with pockets of heavy rain is expected both Sunday and Monday, locally heavy storm-total rainfall amounts will be possible, especially in the higher terrain. Afternoon high temperatures will be near normal today, warming to a couple of degrees above normal Tuesday through Thursday, then lowering back below normal Friday through the weekend.

Discussion

Quiet day across all of Southeast Arizona today with sunny skies and light winds observed across the area. The upper pattern early this afternoon is defined by an upper trough across the Pacific Northwest and the northern portions of the Intermountain West, a strong upper-low in the Eastern pacific near 33N/147W, and a generalized zonal flow extending from this feature into the Desert Southwest. The latest GOES precipitable water imagery indicated a dry airmass across our neck of the woods, ranging from 0.25-0.45 inches.

An upper ridge building into the Gulf of Alaska will try to phase the upper-low in the NW with the EPAC low, resulting in an amplified mean trough. Strong northerly flow developing on the backside of this trough will help weaken the closed circulation over the Eastern Pacific, eventually allowing this system to open into a progressive the second half of the week. This feature will weaken as it moves east, but southerly low to mid-level flow developing out ahead of the trough should encourage a northward surge of tropical moisture (the extreme northerly extent of Hurricane Vance) through the Gulf of California and into southwest Arizona Thursday night into Friday. The 26/12Z deterministic GFS indicated decent 700-850mb moisture flux convergence west and northwest of Tucson between 30/18Z and 31/00Z Friday. Expect scattered showers and thunderstorms as early as Thursday night but more likely during the day Friday across Southeast Arizona with this moisture push. Rainfall amounts will generally be a tenth to third inch across the majority of the CWA with this first system. However, given the fact the N/S orientation of the moisture plume aligns with the stronger southerly low-level flow, we may see areas where training of storms may increase the potential for heavier rainfall amounts across central and western Pima/SE Pinal counties Friday.

The aforementioned ridging in the Gulf of Alaska will then push another shortwave south, reinforcing the mean trough over the Eastern Pacific late Friday into Saturday. This will close off another upper-low near 27N/130W by 01/00Z Saturday afternoon before shifting this feature east toward Arizona Sunday. This low will tap into a deep plume of subtropical moisture as it makes its way into Arizona with the GEFS still indicating PWATs in this plume 2.5 to 3.0 standard deviations above normal. Examining both the 26/12Z GEFS and ECMWF ensemble data with respect to an atmospheric river event thru the Center for Western Weather and Water Extremes, the mean IVT for this second event Sunday is very close to 500 kg/ms which would lean it toward an AR2 event, but the shorter duration (around 24 hours) of the moisture plume limits the event more toward and AR1 scale classification. In any event, this is looking like it will be a heavy precipitation event for Arizona, with the strong SWLY flow likely focusing the heaviest precipitation into central AZ and especially the Mogollon Rim Sunday with strong orographic lift. The northern portions of our CWA including SE Pinal, Graham and Greenlee counties will be the main focus in Southeast Arizona.

In general, moderate-intensity rainfall will produce a half to one inch of storm-total (Sunday and Monday) rainfall in the valleys, with amounts between 1 to 2 inches in the mountains. Heavier amounts to 2.50 inches will be possible on the SW facing slopes of the higher terrain from Tucson north and northeast including the Catalina and Rincon Mountains. Since the rainfall is occuring over a longer duration of time, I anticipate rainfall runoff will result in water flowing within normally dry washes and roadway dips with significant ponding of water in low-lying and poor drainage areas possible. Heavy flows may be present in creeks and rivers across northern Graham and Greenlee counties like the San Francisco, Blue and even Gila rivers, potentially impacting localized areas into early next week depending on the magnitude of the rainfall event.

I guess the good news for the kiddos is that at this time, it still looks like Halloween may be the lull in storm activity in between these two distinct weather systems Friday and Sunday: but timing can change, and residual moisture and generalized instability may result in a low-end chance of showers.

Aviation

Valid through 28/00Z. SKC thru the remainder of the forecast period. SFC wind 10 kts or less thru the forecast period, favoring a WLY/NWLY direction during the afternoon/early evening hours and terrain driven/variable in direction at other times. Aviation discussion not updated for TAF amendments.

Fire weather

Dry conditions are expected across Southeast Arizona into Thursday. Scattered showers and thunderstorms will return to the region Friday, followed by a lull in storm activity Saturday, then widespread showers with isolated thunderstorms Sunday into Monday. Although moderate intensity rainfall with pockets of heavy rain is expected both Sunday and Monday, locally heavy storm-total rainfall amounts will be possible, especially in the higher terrain. Afternoon high temperatures will be near normal today, warming to a couple of degrees above normal Tuesday through Thursday, then lowering back below normal Friday through the weekend. 20-foot winds will generally be 15 mph or less through Thursday. Breezy southerly winds are anticipated Friday, with breezy to locally windy southwest winds expected Sunday and Monday. Minimum RH values will be in the 18-30 percent range in the valleys today (higher in the mountains) and in the 15-25 percent range Tuesday and Wednesday. Minimum RH levels will increase late in the week and over the weekend, with values at 40+ percent.

Watches, warnings and advisories

None.