Hard Freeze Warning 1, NWS Tucson, AZ

Area Forecast Discussion
National Weather Service Tucson AZ
900 PM MST Sat Nov 10 2018

.SYNOPSIS...Dry conditions persist during the upcoming week. A
cooling trend will begin Sunday followed by well below normal
temperatures Monday into Tuesday. Widespread freezing temperatures
appear likely east of Tucson Sunday night, and a hard freeze may
occur for some locations Monday night. Expect breezy to windy
conditions Monday afternoon into Tuesday. Warmer temperatures with
less wind will prevail later in the week.

&&

.DISCUSSION...Clear skies across southeast Arizona at this time with
surface winds at lower elevations variable in direction under 10
mph. Expect clear skies or perhaps just a few lower-level clouds
mainly near the New Mexico state line the rest of tonight. Low temps
at lower elevations around daybreak Sunday should generally range
from the mid 30s in the colder eastern valleys to the upper 40s
vicinity Tucson metro area and westward into the Tohono O'odham
Nation.

A quick glance at the 11/00Z NAM12 and associated MOS guidance
suggests the potential of Wind Advisory criteria being achieved
southeast of Tucson and particularly near Douglas Monday evening.
This particular guidance also favors widespread freezing
temperatures east of Tucson Sunday night, and the coldest
temperatures thus far this season east of Tucson Monday night, as
well as the potential for a Hard Freeze for some locales. Other
model guidance will likely differ somewhat in the detail, but
certainly noteworthy.

Please refer to the additional sections for further detail.

&&

.AVIATION...Valid through 12/06Z.
A few clouds around 4k-8k ft AGL mainly near New Mexico state line
and especially across White Mountains northeast of KSAD into Sunday
evening. Otherwise clear skies. Surface wind into Sunday morning
variable in direction mainly under 10 kts. Surface wind late Sunday
morning into Sunday evening wly/nwly 10-20 kts with gusts
approaching 30 kts. The strongest speeds should be vicinity KSAD.
Wind speeds tapering near end of valid period. Aviation discussion
not updated for TAF amendments.

&&

.FIRE WEATHER...Dry conditions persist into the upcoming week.
Breezy northwest 20-ft winds are expected Sunday afternoon, with the
return of gusty east winds Monday into Tuesday. East winds persist
Wednesday but at lesser speeds. A cooling trend will occur over the
next few days with well below normal temperatures expected Monday
into Tuesday. Temperatures should moderate closer to seasonal
normals later in the week. A significant decline in dewpoints will
result in daytime minimum relative humidity values of 10 to 15
percent Monday through Friday, especially across the lower deserts
west of Tucson.

&&

.PREV DISCUSSION /214 PM MST/...Water vapor imagery indicates a
shortwave trough pushing through Arizona today, bringing dry mid to
upper-level air to the region. This feature begins to usher in
cooler temperatures, but a more significant reinforcement of cold
air arrives by Monday. This is due to a second shortwave that
deepens along the Intermountain West, skimming our area toward the
northeast later on Sunday. With a well amplified ridge off the
Pacific Coast, a long fetch of northerly flow drives cold air into
Arizona.

The moisture content associated with this air mass is also
anomalously very dry. NAEFS mean precipitable water depicts the
entire region below the 10th percentile starting Monday, continuing
through Wednesday. This further assists radiative cooling at night,
with possible freezing temperatures east of Tucson starting Monday
morning. The coldest morning lows should occur Tuesday and
Wednesday, which could produce a hard freeze for the more
susceptible areas such as the Sulphur Springs Valley. However, winds
may not fully decrease each night, which could mitigate some of the
cooling.

Sticking with the theme of winds, some gusty winds will occur Sunday
afternoon, but the strongest winds for much of the area are likely
to prevail late Monday afternoon into Tuesday morning as surface
high pressure builds south along the front range of the southern
Rockies. The surface pattern results in an easterly wind regime that
generally persists through the rest of the forecast period.
Thickness and heights gradually increase later in the week as the
upper-level pattern progresses, with a building ridge over the west
by Friday. This helps temperatures recover closer to seasonal
normals.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...None.

&&

Francis
PREVIOUS DISCUSSION...Howlett

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