Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 240421

Area Forecast Discussion
National Weather Service Tucson AZ
920 PM MST Thu Feb 23 2017

.SYNOPSIS...Dry conditions will prevail through Saturday along with
cooler temperatures. A brief warmup on Saturday before a fast moving
system pushes across the area on Sunday. A stronger Pacific storm
will bring a chance of valley rain and mountain snow to the area on
Monday and Tuesday along with below normal temperatures.


.DISCUSSION...IR satellite imagery this evening showed
clear skies across southeast Arizona. 24/00z upper air plots show
broad troughing over the western CONUS with generally zonal flow
across the southwest states. High temperatures this afternoon were
considerably cooler compared to Wednesday, along with much lower
dewpoints. The gusty winds we saw this afternoon had dissipated for
the most part as well, with speeds now mainly less than 12 kts
across the area. We will see temperatures dip to notably lower
values overnight into Friday, with morning lows in some spots 15 to
20 degrees cooler compared to this morning. This is in part due to
the much drier airmass in place; surface dewpoints as of 24/04z were
20 to 25 degrees lower compared to 24 hours ago. The near term
forecast looks good, so no updates are needed this evening. Please
see the remaining sections for further details.


.AVIATION...Valid through 25/06Z.
Generally clear, with a few high cirrus clouds moving across the
region from west to east Friday afternoon. Surface wind generally
less than 10 kts overnight. Wind not as strong Friday afternoon,
with west winds 12 to 17 kts gusting to 25 kts. Aviation discussion
not updated for TAF amendments.


.FIRE WEATHER...A weather system passing by to the north of the area
will bring cooler temperatures to the region Friday. Gusty westerly
winds are anticipated again Friday afternoon, but not as strong as
Thursday. Otherwise, a brief warmup can be expected on Saturday
before another system passes through the region on Sunday. A
stronger Pacific storm will bring a chance of valley rain and
mountain snow to the area on Monday and Tuesday along with below
normal temperatures.


.PREV DISCUSSION...Relatively quiet weather is forecast through the
end of the week into Saturday as Arizona remains locked in zonal
flow while most of the action remains to the north. Meanwhile the
series of lows across western Canada will eventually phase into a
system that dives southward into Arizona on Sunday. Examining some
cross sections across the area reveals most of the moisture with
this system will be in 700-300mb layer. At the same time, the best
isentropic ascent with this trough will be displaced well to the
north (more than likely in the White mountains or even farther
north). Thus it would seem that this system will only bring an
increase in clouds and possibly some virga to the area on Sunday
across the deserts. Locations to the north and east of Tucson could
see a few hundredths of an inch of rain on Sunday but once again,
the best dynamics will be closer to the Four Corners and overall
impacts with this system appear to be minimal.

This system will pre-condition the atmosphere nicely for a secondary
trough that is forecast to arrive late Monday into Tuesday. Today`s
12z deterministic GFS and ECMWF share relatively good agreement
indicating a wave will arrive late Monday. This idea is also agreed
upon by the GEFS and NAEFS. Cross sections and bufr profiles depict
a much more moist atmosphere with this secondary trough although
model QPF values aren`t terribly worrisome. Looks like most valley
locales could see around 0.25 inches of rain with slightly higher
amounts north and east in the mountains. Still plenty of time to
refine QPF grids but it looks like these values would produce around
4-6 inches of snow above 6000ft. This trough will also knock
temperatures back several degrees. Raw guidance would suggest desert
highs in the 50s, which seems somewhat reasonable. Went a little
warmer on the idea that a few breaks in the clouds and a steadily
increasing sun angle is usually good for a few degrees of warming.
Nonetheless, below normal temps are a lock with this trough on
Tuesday, followed by slow/steady warming through the rest of the

Looking in the long range (Thursday into next weekend), confidence
in all forecast elements is low. Deterministic guidance is
suggesting a cutoff low somewhere in the area (each model positions
this low and its moisture in a different location across the desert
southwest) and confidence is too low to steer the forecast in one
direction or another. Individual ensemble members also depict a
variety of solutions with no clear clustering one way or the other.
I kept things dry/near climo for the time being along with near
normal temperatures until a better model consensus comes to pass.






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