Freeze Warning 3, NWS Tucson, AZ

Area Forecast Discussion
National Weather Service Tucson AZ
308 AM MST Mon Oct 26 2020

.SYNOPSIS...Much colder temperatures are on the way as a storm
system settles in over the area into Tuesday. This system will
bring a few showers and mountain snow showers into Tuesday. As
the storm moves out of the area, near or below freezing low
temperatures are expected across portions of southeast Arizona,
especially the normally colder valleys east and south of Tucson
Tuesday and Wednesday. Then above normal high temperatures return
to the area at the end of the week into next weekend.

&&

.DISCUSSION...A band of light showers are developing from near
Sells northeast to Globe this hour with dewpoints rising ahead of
this band. The surface cold front will catch up with this pre-
frontal band near dawn and it will then translate across Pima
county this morning and areas east this afternoon. This will be
the best chance for precipitation today and likely the only
chance for Tucson with this storm as it moves across, and even at
that it will be a brief hit. Dewpoints will quickly crash behind
the front with gusty westerly winds which interestingly would
bring about elevated fire weather conditions where significant
precipitation doesn't occur. The freezing levels also crash behind
the front with any lingering mountain showers allowing for
dropping snow levels. The only mountains that might get decent
snow amounts are Mt Graham and the Whites with a skiff of snow
elsewhere at higher elevations due to the lack of significant
precipitation.

The system moves off to the east tonight with a chance of showers
continuing basically east of Willcox through Tuesday. In the
higher valleys out east, such as the Sulphur Springs Valley we
may see a few snow showers Tuesday morning thanks to the cold
temperatures.

That is the big story with this storm system, the cold
temperatures. The cold advection will be strong behind the front
along with a significant drop in dewpoints which will allow for
significant cooling, especially with any clear skies and light
winds. Low temperatures the next two mornings will be quite cold,
especially given how warm it has been.

Given the time frame and confidence level of the sub-freezing
temps, I went ahead and converted the Freeze/Hard Freeze watches
into warnings. The areal coverage is tweaked a bit due to the
latest forecast low temps and there are a few areas in the Tucson
Metro that are in the freeze warning, the normally colder
locations like NE part of the metro. For the best view, please see
the detailed hazards viewer at www.wrh.noaa.gov/map/

For areas from roughly Tucson westward, the coldest morning is
likely to be Tuesday morning while Tucson eastward Wednesday
morning should be the coldest. Some of the typically colder areas
in Cochise county may dip into the teens Wednesday morning which
will be a shock to the system!

These cold temperatures will be short lived however as a rapid
warmup will occur Thursday with high temperatures bouncing back up
to normal then continuing a slow climb warmer into the weekend. By
Sunday we will be 7-11 degrees warmer than normal once again and
may touch 90 in Tucson......again.

&&

.AVIATION...Valid through 27/12Z.
Clouds will briefly thicken and lower to 3k-5k ft agl between 11-17Z
this morning due to a strong cold front moving across the area. This
front will be accompanied by ISO-SCT SHRA from KTUS north and east.
Based on lower CIGS and SHRA, brief MVFR could be possible. As the
front passes, breezy W/WNW sfc winds of 10-15 kts, with gusts around
20 kts will develop during the late morning and afternoon. Skies
will clear with only FEW clouds possible. SFC winds will diminish by
evening with FEW/SCT clouds between 9-13k ft agl returning after
27/06Z. Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...As the front quickly swings through SE AZ later
this morning, there will be rapid drying behind the front. Dew
points will drop from the 40s/50s to single digits. Expect RH in the
single digits from Tucson south and west. With breezy winds also
occurring during the afternoon, elevated fire weather is likely
across the western half of the area. Winds will generally be out of
the W/WNW 10-15 mph with gusts to 25 mph.

Much colder temperatures remain in the forecast through Wednesday
with scattered showers northeast and east of Tucson. Mountains could
see some light snow accumulations, with the best chance and greatest
amounts across the White Mtns. Dry conditions return after Wednesday
and persist the rest of the week. Minimum relative humidity values
are expected between 10-20 percent with slightly higher values
across the mountains. 20-foot winds will be less than 15 mph Tuesday
and Wednesday becoming easterly Thursday into the beginning of the
weekend.

&&

CLIMATE...Thru Sunday Tucson was recording the warmest October on
record. The upcoming cold snap ends any chances that it would
occur. Likely to end up around 4th/5th warmest.

The forecast high of 60 degrees on Tuesday, October 27th, for
Tucson is one degree off the daily record low maximum of 59
degrees from 1996.

As mentioned above, some of the low temperature guidance for
Willcox, be it the NBM, GFS_BC or ECMWF_BC, suggests that near
all-time record October low is possible Wednesday morning. The
record for Willcox is 15 degrees on October 27, 1970. If the low
temperature does fall below 23 degrees it would mark the coldest
October low for Willcox in 50 years.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
Hard Freeze Warning from midnight tonight to 8 AM MST Tuesday
for AZZ503-504-506>509.

Hard Freeze Warning from 9 PM Tuesday to 8 AM MST Wednesday for
AZZ503-504-507>509.

Freeze Warning from midnight tonight to 8 AM MST Tuesday for
AZZ502>509.

Freeze Warning from 9 PM Tuesday to 8 AM MST Wednesday for
AZZ503-504-506>509.

&&

Public...Cerniglia
Aviation...Michael
Fire Weather....Michael

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