Area Forecast Discussion
Issued by NWS Tucson, AZ
FXUS65 KTWC 181001
Area Forecast Discussion
National Weather Service Tucson AZ
301 AM MST SUN SEP 18 2016
.SYNOPSIS...Dry conditions will occur this weekend then increasing
moisture from the south should provide a return of some showers and
thunderstorms Monday through Wednesday. This will be followed by
another strong drying trend late next week. Expect above average
daytime temperatures Sunday and Monday, then temperatures
returning closer to normal by the middle of next week.
.DISCUSSION...Still dry with precipitable water values well below
average (currently around .5 to .7 inches across SE AZ), and
surface dew points in the 40s. Another day with no storms, but we
could develop a few cumulus fields near higher terrain border
areas later this afternoon. That`s because moisture is on the way
back for the first half of the week.
The dry westerly flow we`ve been seeing under a regional trough is
disappearing quickly as the jet temporarily shifts back northward
and the flow weakens. Our three main weather players over the next
several days will be the cut-off low left behind just off the
southern California and northern Baja coast, a strong ridge
building through northern Mexico and west Texas, and a developing
tropical system south of Baja.
First the ridge will dominate with thicknesses supporting
temperatures around 6 to 10 degrees above average the next 2 days.
The tropical system has intensified to tropical storm strength
(named Paine), and is currently around 390 miles south of Cabo San
Lucas. Considering the dominance of the flow around the high pressure
ridge, the current NHC track looks solid. It keeps Paine on the
west side of the Baja Peninsula through late Tuesday before the
higher shear area decouples the storm west of the central Baja
spur. Both operational models and tropical model track guidances
are in good agreement as the sheared and rapidly decaying remains
are expected to be drawn north into the cut-off low as it once
again picks up higher latitude support and is lifted through
northwest Arizona on a quick trip to the central plains.
No Newton repeat for our area, but we will have precipitable water
values back near or above 1.3 to 1.4 inches by Tuesday afternoon.
However, by then we will probably have plenty of cloud cover to go
along with it so not much help from solar insolation destabilizing
the lower atmosphere. So, this does not look like a significant
tropical even for SE AZ. More like clouded over, with a few areas
if isolated to scattered showers Monday night into Wednesday. A
few lightning strikes here and there but not major league
Another decent westerly flow under a trough through the central
Great Basin should push the moisture back out of the picture late
in the week. Not sure if it will come back after that with our
monsoon flow pretty much gone at that point.
.AVIATION...Valid through 19/12Z.
SKC or FEW clouds at 10-15k ft AGL into this evening, with FEW-SCT
mainly high clouds streaming into the area late in the period.
Terrain driven sfc wind at less than 10 kts through the morning,
becoming NWLY this afternoon at around 10 kts. Aviation discussion
not updated for TAF amendments.
.FIRE WEATHER...Dry conditions will prevail today with lower relative
humidity. Moisture will increase Monday, with isolated showers and
thunderstorms mainly south to southwest of Tucson spreading
northward into Tuesday. Then, expect isolated to scattered showers
and a few thunderstorms area-wide Tuesday and Wednesday. Precip
chances will taper off from west to east Wednesday night through the
end of the work week. 20-ft winds will be mainly terrain driven at
15 mph or less through Wednesday, with some westerly to
southwesterly breeziness Thursday and Friday afternoons.
CLIMATE...Sunday and Monday (Sep 18-19) appear to be the hottest
days through the coming week. The current forecast of 100 degrees
for Tucson International on Sunday would be the first 100 degree
day since September 5. Our average last 100 degree day occurs on
September 18, with the latest ever recorded on October 16.
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