Area Forecast Discussion
Issued by NWS Tucson, AZ
FXUS65 KTWC 301604
Area Forecast Discussion
National Weather Service Tucson AZ
904 AM MST Wed Nov 30 2016
.SYNOPSIS...A cold start to the morning, with another widespread
freeze or hard freeze for Southeast Arizona tonight. Temperatures
will remain below average through the week, and another storm
system may bring a chance of valley rain and mountain snow Friday
into the weekend.
.DISCUSSION...Water vapor imagery and 30/12Z upper-air plots show a
large area of low pressure centered over the Upper Midwest/Great
Lakes region with the associated trough axis extending southwest
into the central and southern plains. Meanwhile, closer to home a
dry northwest flow aloft continues across the Desert Southwest, with
a weak disturbance currently moving southeast across northern Baja.
The 30/12Z KTWC sounding revealed a very dry airmass in place with a
precipitable water value of 0.18 inches, which is 0.28 inches drier
than 24 hours ago.
IR satellite imagery shows clear skies across much of the
southwestern states. With clear skies expected to continue
overnight, and with light to near calm winds, expect another night
with freezing and subfreezing temperatures for early Thursday
morning. That said, freeze and hard freeze warnings continue late
tonight and into early Thursday morning for parts of my forecast
area. Will take a closer look at low temps for tonight and may make
some slight adjustments.
As of 15Z (8 AM MST), temperatures across the region ranged from the
lower 20s to the mid 30s, with the Tucson Intl Airport reporting a
temp of 36 degs, after achieving a low of 29 degs. These current
readings seem to be on track with the inherited afternoon high
temps, so no changes are planned at this time.
See previous discussion below for details beyond today.
.AVIATION...Valid through 01/18Z.
Clear, with normal diurnal wind trends generally less than 10
kts. Aviation discussion not updated for TAF amendments.
.FIRE WEATHER...Dry conditions will occur through Thursday. A low
pressure system will then move into the region Friday and Saturday,
resulting in a slight chance of valley rain and mountain snow. This
system may also produce some gusty east winds. Dry conditions are
expected behind the system late Sunday into Tuesday of next week.
.PREV DISCUSSION...Another round of clear and cold conditions
tonight into Thursday morning.
Another impulse from the north Friday and Saturday as the large
scale trough through most of the country lifts into the Great Lakes
region and the ridge positions upstream and downstream of our area
amplify again. Moisture forecast fields are shaky, but we should see
another chance of valley rain and mountain snow late in the week. It
doesn`t look quite as vigorous as our most recent series of systems,
and it`s a play in one act with no table setter to maximize the cold
air (unlike our recent 3 act play). So overall less of an impact,
but it will keep temperatures below average at least through the
weekend. Another 1 to 3 inches of snow possible above 5000 feet.
Shortwave ridge Sunday with temperatures jumping back to about
average for early December on Monday, but then we see if the pattern
can hold and bring another system before the middle of next week.
Hard Freeze Warning from midnight tonight to 9 AM MST Thursday
Freeze Warning from midnight tonight to 9 AM MST Thursday for
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