Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 180338
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
838 PM MST Tue Jan 17 2017

.SYNOPSIS...A series of storms will spread valley rain, mountain
snow and gusty winds over Arizona Thursday night into Saturday.
Snow levels will lower through that time potentially reaching the
valley floors east of Tucson by Saturday. After a break in the
weather another storm system is likely early next week.

&&

.DISCUSSION...Still a little residual moisture as evidenced by the
isolated shower activity earlier. We may have enough boundary
layer moisture for patchy fog again overnight, but not as much as
we saw this past morning.

After a shortwave ridge tomorrow, our main weather story is still
shaping up to be the two systems lining up for the second half of
the week. A table setting system will have more impact for
northern areas Thursday night, then the follow-on system will
carve deeper into the mean trough position, bringing colder and
wetter conditions for most of the area. Our current Winter Storm
Watch covers this well, but looking at the latest analysis and 00z
model data, our snow level forecast may not be quite low enough
with thicknesses supporting lower values. Something to ponder
going forward.

Please see the previous discussion below for additional details.

&&

.AVIATION...Valid through 19/06Z.
SCT locally BKN clouds around 5k-9k ft AGL and SCT-BKN clouds
above 20k ft AGL diminishing overnight with mostly clear skies
after 18/10z. Surface wind will be variable in direction mostly
less than 10 kts then becoming westerly 6-12 kts Wednesday
afternoon. Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...Dry weather into early Thursday with generally
light winds and elevated humidity levels. Then a series of storms
will spread a good amount of precipitation across the area late
Thursday into Saturday with snow levels near 6000 feet Thursday
night and Friday dropping near or a bit below 5000 ft Saturday.
Gusty west to southwest winds are expected at times through this
period especially across the mountains. Dry weather returns Sunday
into Monday before another storm affects the area early next week.

&&

.PREV DISCUSSION...Wednesday into early Thursday will be a dry
period as a short wave ridge builds across the area. winds will be
light with plenty of sun Wednesday and temperatures only a couple
of degrees cooler than normal.

The "fun stuff" begins Thursday night and continues into Saturday as
a two part weather punch affects the region with significant
precipitation expected. The first system will spread precipitation
across southern AZ mainly Thursday night into early Friday morning.
There has been a bit of back and forth on the start time but at this
time I feel it will be the evening hours generally. Models were
running reasonably moist with this system until yesterday when they
dried it out but they have now returned to their previous thought.
Snow levels with this first system will start out near 6500 feet and
drop to around 5500 feet and after doing a QPF blend of models and
WPC forecast then trimming a bit still ended up with advisory snow
amounts over our mountains from the Catalinas north. Will also be
looking at rather breezy conditions mainly across the mountains as
this passes.

The precipitation will taper off the middle part of Friday then
begin to pick back up again later in the day with the bulk of the
precipitation occurring Friday night into early Saturday morning with
this second system. This will be the heavier precipitation producer
with snow levels Friday evening around 6000 feet lowering through
the night, potentially to 4500 Saturday morning. The bulk of the snow
is expected above 5500 feet where warning amounts are possible Friday
night into Saturday. Again it will likely be windy mainly across the
mountains especially the southern ones where 25-35 mph winds and
higher gusts are expected.

So, with a middle of the road QPF forecast, valley rainfall amounts
of 0.50 - 1.20 inches is likely with locally higher amounts. For
snow, across the southern mountains (border area) we could end up
with 2-4 inches of snow for Thursday night and 5-12 inches Friday
night. For the Catalinas northward could end up with 5-8 inches
Thursday night and 8-14 inches Friday night. At this time for
simplicity and coordinating with neighbors went with a Winter Storm
Watch above 5500 feet for the entire period which will likely be
broken up into advisories and/or warnings and adjusted in the future
as details become clearer.

This system moves east leaving behind a dry Sunday with another
system passing through the region early next week. While this
continues to modeled as a rather vigorous and cool system it dosn`t
look as wet as this weekend system.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Thursday evening through Saturday
morning for AZZ510-511-514.

Winter Storm Watch from Friday afternoon through Saturday
morning for AZZ512-513.

&&

$$

Meyer/Cerniglia/Glueck

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