Area Forecast Discussion
Issued by NWS Tucson, AZ

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661
FXUS65 KTWC 171616
AFDTWC

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

.SYNOPSIS...Low pressure will bring periods of valley rain and
mountain snow to the area through early Monday morning, 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...
Slow moving area of low pressure remains positioned just south of
Pima County this morning per regional satellite and radar
observations. We`ve had several rounds of precip thus far with most
locales observing anywhere from a few hundredths of an inch to
upwards of a quarter of an inch. Mountain snow reports have started
to come in from Mt. Lemmon with anywhere from 2-4 inches observed as
of 8am MST. With another band just starting to move into the Tucson
area from the south, I`d easily expect those totals to double by
this afternoon.

Forecast continues to look on track with various rounds of precip
expected through the day today. Last few runs of the HRRR depict
precip tapering off by late afternoon but until the upper low and
the associated area of strong isentropic lift to its east exit the
area and move into New Mexico, I`m hesitant to pull back on PoPs.
Going to stick with the idea of on/off valley rain/mountain snow
showers through early evening with things tapering off closer to
midnight. No changes to the Winter Weather Advisory, going to leave
the elevation at 6000ft. Even though the background snow level is
probably closer to 6500 ft, embedded convective elements within the
precip bands has been pulling the snow level down to around
5500-6000ft within the heavier showers.

As for totals, latest blended QPF available from NBM and the
inherited forecast seem to be matching observed amounts quite well,
thus no major changes needed.

&&

.AVIATION...Valid thru 18/18Z.
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.

&&

.PREV 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.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 11 PM MST this evening for
AZZ510>514.

&&

$$

Leins/Meyer/Carpenter

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