Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 180331

Area Forecast Discussion
National Weather Service Tucson AZ
831 PM MST Sat Mar 17 2018

.SYNOPSIS...A weather system will bring a chance of valley rain
and mountain snow late tonight into Sunday morning. Expect cooler
temperatures Sunday and Sunday night before high pressure brings
a warming trend the first half of the new week. Another system
will brush by mainly north of the area later in the week.


.DISCUSSION...Shortwave trough axis is now located across central
California with the associated cold frontal band now just pushing
into Arizona across the lower Colorado River Valley. Latest IR
satellite shows mostly clear skies from Tucson westward with some
residual cirroform cloudiness east of Tucson. There is not a lot of
cloud coverage with the cold front upstream of our area near Yuma
but as the front pushes to the east it will pickup a bit of Gulf of
California moisture. This moisture combined with weak forcing and
ascent with the front are expected to result in a few showers on
Sunday morning. Latest HRRR runs and 00Z NAM are certainly not too
excited about shower coverage Sunday morning with the HRRR
consistently not depicting much of anything. Will not make any
changes to the forecast at this time, but if anything our PoPs might
be a touch too high. Bottom line is this is really a minor system
with precipitation amounts generally under 0.05" in the valleys with
slightly higher amounts in the mountains. Just made some minor
changes to tweak for latest trends in sky cover overnight, otherwise
the previous forecast remains unchanged.


.AVIATION...Valid through 19/06Z.
Approaching trough will result in increasing cloud cover late
tonight along with VCSH around KTUS Sunday morning. Looks like best
chance of rain will be centered on 14Z plus/minus a few hours, with
showers exiting the region before 18Z. Any rain that occurs at KTUS
will be light with no restrictions to vsby and only a brief drop in
cigs to around 8kft. KOLS/KDUG likely too far south to see any rain
but will likely see an increase in clouds. Expect southwest winds
around 10-15 kts to become westerly Sunday with afternoon gusts of
20-25 kts possible mainly east of KTUS.

Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Isolated showers expected late tonight into Sunday
morning but any rain/snow will be light. Mostly clear skies and dry
weather are expected from Sunday night through mid/late week.
Another trough will approach the region on Thursday bringing the
next opportunity for rain to the area. As for winds, expect the
strongest winds of the week to be the rest of today into early
Sunday morning. Afternoon breeziness is expected each day Monday
through Wednesday with typical diurnal patterns expected. Gusty
southwest winds should return next Thursday and Friday.


.PREV DISCUSSION...Winds picking up this afternoon ahead of a storm
system digging into the region from the northwest (below advisory
levels but gusty). Most of the action will be north of our area with
this system as it pushes across the region over the next 36 hours.
The associated cold front currently near far western Arizona looks
just a tad slower, and should push across SE AZ between 3AM and
early afternoon. Some upslope and moisture increase ahead of the
front, with a few orographically assisted light showers developing
overnight. Our best rain chances will be immediately ahead of and
with the front, which times out to between 5AM and 9AM MST for
Tucson Metro. With little dynamics and a relatively poor moisture
pool this far from the main system, storm totals will be on the
light side. Say around .05 in valleys and 1/3 of an inch in
mountains. Snow levels falling to around 5500-6000 feet behind the
front with 1 to 3 inches of snow expected in the mountains.

Solid cooling behind the system with daytime highs down 10 or 12
degrees tomorrow, and overnight lows dropping to around 5 to 8
degrees below average for mid March early on Monday morning. That
means mid to upper 30s around Tucson, and mid 20s to lower 30s in
colder valleys east and south of Tucson. Those colder valleys
typically aren`t done freezing yet, and this year is no exception
with a freeze just a few days ago (average last freezes in April for
Santa Cruz, Cochise and Graham counties).

Strong high pressure the first half of the new week. Temperatures
will jump quickly back to 8-10 degrees above average by Wednesday.
Basically some flavor of 80s just about everywhere.

It looks like the storm track is going to have difficulty digging
back into our neck of the woods, so a system pushing across western
states the second half of the new week will probably remain north of
our area. A glancing blow with gusty winds and some cooling, but
best precip chances should be north of us. A common theme even in
early portions of our transition season. We`ll see if we can get any
moisture help off the Pacific, but trajectories aren`t as favorable
this far south.





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