FXUS65 KTWC 150246
Area Forecast Discussion
National Weather Service Tucson AZ
745 PM MST Sun Oct 14 2018
.SYNOPSIS...Cool overnight lows around 3 to 7 degrees below average.
Scattered showers and a few thunderstorms will return Monday into
Tuesday. Mainly dry conditions later this week before a few showers
return next weekend. Daytime temperatures well below seasonal
normals into midweek then modestly warmer next weekend.
.DISCUSSION...Satellite imagery shows mainly scattered low/mid
clouds (locally broken) hanging in across much of the eastern two
thirds of forecast area early this evening. The exception was over
central/western Pima county where most of the cloud cover has
dissipated. The KTWC 181015/00Z sounding indicates drying at all
levels versus the 12Z KTWC sounding, but especially so above 700mb.
PW values on the 00Z sounding still at 0.75", which matches up well
with the Blended Total Precip Water imagery - where values range
from 0.50 inches over the White Mountains, to nearly 1 inch over the
far western deserts. Low temperatures tonight/Monday morning across
southeast Arizona will bottom out 3-7 degrees below normal and
around 5 degrees below normal in the Tucson metro area.
Overall, expecting a dry night across most of the forecast area
tonight with Arizona between systems. There are some single digit
POPs over far eastern/northeast portions of the forecast area, with
just a stray shower/sprinkles possible. Precipitation chances are
expected to increase through the day on Monday as the next piece of
energy cuts off and retrogrades southwest through base of the trough
across the southwestern United States. Overall, the forecast package
looks to be in good shape. No updates this evening. For more
information on the latter periods of the forecast, please refer to
the PREV DISCUSSION section below.
.AVIATION...Valid through 16/00Z.
Isolated -SHRA N and W of KTUS Monday after 15/16z. Otherwise, cloud
decks generally 4k-8k ft AGL. Surface wind variable in direction
less than 10 kts thru 15/15z then turning ely/sely 8-17 kts.
Aviation discussion not updated for TAF amendments.
.FIRE WEATHER...Dry conditions tonight into Monday morning.
Scattered showers and isolated thunderstorms return Monday into
Tuesday. Dry conditions Wednesday through Friday except for a slight
chance of showers across the White Mountains. There is a slight
chance of showers especially from Tucson eastward next weekend. 20-
foot winds today will be terrain driven mainly under 15 mph. Some
gusty east winds developing Monday east of Tucson, then gusty east
to southeast winds should occur at times Tuesday into next weekend.
.PREV DISCUSSION...Our Pacific low pressure system has opened up as
it lifts into the base of the regional trough with showers drying up
this afternoon. The 12Z KTWC sounding was still holding on to 1 inch
precipitable water, but starting to show some drying between 500-
700mb. Latest CIRA layered precipitable water trends and low to mid
GOES16 water vapor channels hint at a continuation of this trend, so
we`re still on track for partial clearing tonight. Tonight will be
on the chilly side. Upper 30s possible in coldest eastern valleys
(including portions of Sulphur Springs Valley in Cochise county),
and would be even colder if we weren`t still holding on to a little
lower level and boundary layer moisture.
Reinforcing energy is expected to dip down from the north over the
next 36 hours and is then expected to lose higher latitude support
as the main trough lifts into the northern plains and high pressure
builds into the Pacific Northwest. The resulting cut-off will linger
around Arizona into mid to late week. That will keep a few showers
around (especially Monday afternoon into Tuesday), while also
keeping temperatures well below average. We might actually sneak in
our first freeze for eastern valley locations Wednesday morning, so
some potential agricultural concerns with that.
An unsettled pattern continues next weekend with some signs of a Rex
Block with a significant weakness in the flow affecting Arizona for
another chance of showers.
Beyond that, well, we might be taking another look at the tropics.
22E just formed southwest of Mexico and there`s another area of
interest southeast of that. Both the GFS and ECMWF seem to think we
aren`t done with tropical influences in our forecast.
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