FXUS65 KTWC 171024

Area Forecast Discussion
National Weather Service Tucson AZ
324 AM MST Sun Dec 17 2017

.SYNOPSIS...A winter storm system will bring periods of valley
rain and mountain snow showers today and tonight, along with
colder temperatures. Dry weather and warmer temperatures will
return mid next week before another storm system moves into the
area late in the new week into next weekend.


.DISCUSSION...Satellite imagery is showing our low pushing from
the northern Gulf of California into northwest Sonora this morning
on a slow track through Southeast Arizona over the next 24 hours.
It`s wound up a bit and is a little friskier than I though we
would see at this point. It also has dipped just far enough south
to wrap better moisture in from the south and southeast. Surface
dewpoints are up 25 to 30 degrees over the past 12 hours into the
lower 40s in areas from Tucson southward. Satellite total
precipitable water estimates have climbed into the .5 to .75 range
which isn`t bad for winter.

Decent lift from the core of the low into the eastern and northern
flanks, with multiple convective bands noted on satellite and
radar imagery. More than a few lightning strikes with some storms.
Our initial band is pushing into Maricopa county this morning,
with another band moving up from the south over the next few
hours. This will continue today with periods of showers and
thunderstorms and breaks in between. Current overall snow levels
are between 6500 southwest to 7500 feet east, with isolated lower
levels due to convective processes. Snow levels will lower to as
low as 5000 to 5500 feet as the low shifts directly overhead by
early this evening with convection again temporarily lowering
levels in some areas. The low will fill a bit as this happens, so
we may see snow levels lift a bit this afternoon before dropping
again tonight as the cooler air filters in. Storm total precip
amounts through Monday morning around .2 to .4 in valley
locations, and .5 to a little over an inch in mountains.

We`ve expanded the winter weather advisory to include the
southern slopes of the White Mountains (northern Graham and
Greenlee counties) and lowered it from 6500 to 6000 feet. We may
see one or two isolated highest locations a little higher than
advisory levels, but not expecting it to be a widespread thing.

A shortwave ridge will push temperatures back up around mid week,
then another system will impact the area the second half of the
week into next weekend. Even if this system is a glancing blow,
it should be strong enough and deep enough to bring strong winds
and much cooler temperatures by Thursday or Friday. We also may
see our coldest overnight lows of the season so far somewhere in
that period.


.AVIATION...Valid thru 18/12Z.
Cloud decks generally around 4-6k ft AGL with layers above. SCT
valley SHRA, ISOLD TSRA and mountain SHSN will continue this
morning, with better coverage this afternoon and evening. Brief MVFR
cigs/vsbys may accompany the strongest convective showers/tstorms,
especially near terrain. Winds mainly southeasterly at around 8 to
12 kts, though occasional gusts to 25 kts may occur in/near stronger
SHRA or TSRA. Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Low pressure will bring increased relative humidity
and scattered precipitation to the area through Monday. Easterly
winds today will give way to relatively light/diurnally driven winds
next week along with dry weather for most of the week. There is the
potential for another system late the work week that could bring
some breeziness and possibly some precipitation to the area. Overall
no fire weather concerns through late next week.


Winter Weather Advisory until 11 PM MST this evening for




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NWS TWC Office Area Forecast Discussion