Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 221707

Area Forecast Discussion
National Weather Service Tucson AZ
1007 AM MST Thu Feb 22 2018

.SYNOPSIS...A passing system will bring gusty winds Friday afternoon
and evening with a few showers mainly northeast of Tucson.
Otherwise, dry conditions with below average temperatures through
Sunday. A warming trend early next week will be followed by another
winter storm system around the middle of the week.


.DISCUSSION...Visible satellite imagery depicted a few cumuloform
clouds east-to-south of Tucson at this time. Otherwise, clear skies
prevail across much of southeast Arizona. Dew points at lower
elevations valid 9 am MST ranged mainly from the mid teens to 20s,
although a few lower 30s were observed in vicinity of the Tucson
metro area southward to Nogales. These temps were about 2-10 degs
lower versus 24 hours ago. Surface temps valid 9 am MST were within
about 3 degs plus/minus (depending upon location) versus this time
Wednesday. The 22/12Z KTWC sounding depicted a thinly saturated
layer around 700 mb as was the case 24 hours ago. Otherwise, the
column was quite dry with total precip water value of just 0.23
inch. Moderate to fast swly/wly flow was observed above 700 mb.

The aforementioned cumuloform clouds should mostly dissipated during
the next hours allowing for abundant sunshine across the area later
today. However, high temps this afternoon will actually be a few
degs cooler versus Wednesday due to slightly lower heights/
thicknesses. These projected high temps will be 10-15 degs below
seasonal normals.

A broad and deep upper trough remains over the western CONUS this
morning. An impulse embedded within this trough was centered over
the Pacific Northwest at this time. This system is progged via the
22/12Z NAM12/GFS to move southeast into the Great Basin Friday, then
continue to move eastward into into the central/southern Rockies by
midday Saturday. Appears that the bulk of precipitation associated
with this system will be north of this forecast area.

However, isolated to scattered valley rain/mountain snow showers
should extend further southward to include the White Mountains,
portions of Graham County, and perhaps as far southwest as the
Catalina Mountains just northeast of Tucson by Friday evening. The
more significant impact however may be the gusty southwest to
northwest winds Friday afternoon and evening. Based on receipt of
the 22/12Z MAV/MET guidance, wind speeds early Friday evening may
encroach upon wind advisory criteria across the Upper Gila River
Valley including Safford. However, a Wind Advisory issuance will
likely not occur due to the expected brief nature of this event.

At any rate, the inherited official forecast is in outstanding shape
based on an overview of available 22/12Z models and satellite
trends. So, there are no updates necessary at this time. Please
refer to the additional sections for further detail.


.AVIATION...Valid thru 23/18Z.
A few clouds at 5k-10k ft AGL mainly near higher terrain east and
south of KTUS until mid-afternoon followed by clear skies late this
afternoon into early Friday morning. Expect a few to scattered
clouds around 4k-8k ft AGL especially northwest of KTUS toward end
of period. SFC wind this afternoon swly to nwly 6-12 kts and a few
gusts to 20 kts at KSAD/KDUG/KALK/KFHU. Less wind will occur this
evening into Friday morning, then gusty southwest to northwest winds
after valid period, or Friday afternoon and evening. Aviation
discussion not updated for TAF amendments.


.FIRE WEATHER...A few storm systems will move across the region
through next week. Dry conditions will prevail today with a slight
chance of rain and snow showers north to northeast of Tucson Friday
afternoon and Friday night. Dry conditions will then return for this
weekend and early next week. A stronger system may then bring a
chance of valley rain and mountain snow showers to much of the area
Tuesday afternoon through Wednesday night. Expect gusty southwest to
northwest winds Friday afternoon and evening, as well as next
Tuesday. Otherwise, 20-foot winds will generally be terrain driven
at less than 15 mph. Continued below normal daytime temperatures
will continue over the next week.


.PREV DISCUSSION /319 AM MST/...Our busier western CONUS pattern
continues over the next 7 to 10 days, with a mean trough position
across the region sandwiched between persistent ridges north of
Hawaii upstream and near southeastern states downstream. In between,
we get to see multiple systems carve through western states, and one
or two even hitting our area. Even when they brush by just north of
the area, it still keeps temperatures below average and brings gusty
winds at times.

A deep trough currently through the region will see a low lifting
out of the base north of our area today, and then another impulse
pushing through just north of our area over the next 48 hours.
That Friday - Friday Night system will generate snow showers in the
mountains northeast of Tucson, but not much else precip-wise.
However, we`ll see locally windy conditions Friday afternoon along
with temperatures pushed down to around 15 degrees below average
again by Saturday. Meanwhile, overnight lows will continue to be on
the cool side even as we see a brief warming trend through Friday.

A stronger system (currently cooking near the Bering Sea) is
expected to take a deeper trajectory and back a little further west
as it dives southeast of a positively tilted ridge through the
eastern Pacific and redefines the western extent of the trough.
Current medium range models have fair agreement in positioning this
storm to be slower with a deeper and a wetter trajectory around the
middle of next week, with some obvious timing differences ranging
from 6 to 18 hours. At any rate, we`ll have plenty of time to zero
in on things, but it looks like a decent storm for valley rain and
mountain snow. Snow levels could end up quite low with this one.






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