Heavy Snow in Arizona, February 17 to 18, 2019

Area Forecast Discussion
National Weather Service Las Vegas NV
315 PM PST Sun Feb 17 2019

.SYNOPSIS...An abnormally cold weather pattern will persist over our
region through the coming week. This will allow cold low pressure
systems to drop out of Western Canada and bring chances for snow
down to the desert floors. The first system will arrive today into
Monday, and a potentially colder and more vigorous storm could
impact the region Thursday and Friday.
&&

.SHORT TERM...through Monday night.

The ingredients are still coming together for fairly extensive snow
showers to develop across the lower deserts of Southern Nevada and
nearby surrounding states this evening and tonight. Recent satellite
loops showed a vorticity lobe rotating along the Inyo/San Bernardino
County border and will move across southern Clark County through
this evening. The vorticity advection will provide increased upward
forcing over the area from Clark County southward to I-40 in
conjunction with increasing instability as colder air in the H7-H5
layer moves overhead. This will support the expected scattered to
numerous showers with snow levels lowering to between 2000 and 2500
feet MSL this evening. The HRRR, CAMs and HREF models have been
depicting a focus of showers directly over Clark County this evening
until after midnight the approaching cold front and an embedded
closed circulation aloft coincide. This would be favorable for at
least a few hours of sustained showers with the majority of the Las
Vegas Valley seeing some snow. Confidence continues to increase that
we will see around an inch or so of snow over the elevated western
and southern parts of the Las Vegas Valley. We will continue to
monitor the developing situation through this evening.

Showers will taper off late tonight and early Monday morning on the
back side of the low pressure system, but isolated to scattered snow
showers in the cold northerly flow will continue...especially over
Mohave County. Dry air will finally overspread the entire region by
Monday night with brisk north winds and low temperatures at or below
freezing across all zones Death Valley and the Colorado River Valley
from Laughlin to Lake Havasu.
&&

.LONG TERM...Tuesday through Sunday.

Cold conditions will continue through much of the week followed by
some slight moderation over the weekend. High temperatures are
forecast to be 15-20 degrees below normal Tue-Fri and 8-12 degrees
below normal over the weekend. Low temps will be exceptionally cold
for the time of year with freezing temperatures in many lower
elevation locations. Most of the Las Vegas Valley will see temps at
or near freezing each morning into the weekend.

An expansive low pressure trough over the Rockies on Tuesday moves
east with our next weather maker taking shape over western Canada.
Strong high pressure off the West Coast guides this energy southward
into the Pac NW and Great Basin Wednesday to the southwest deserts
Wednesday night and Thursday. This cold Canadian airmass settles
over our area dropping snow levels at the same time moisture is
forecast to increase. This combination along with broad scale
uplifting could result in accumulating snow in lower elevations.
Confidence is high regarding the weather pattern With models
consistent in forecasting a large, cold trough impacting our area
Wednesday night and Thursday.

Models continue to adjust solutions in where the components of the
trough set up and the associated details including location/timing
of precipitation, snow levels and rain/snow amounts. The latest
round of model solutions focus the most significant precipitation
amounts over Mohave County. There remains high variability in when
and where models decide to close off a low and this information will
come into play when deciding the higher resolution details and what
those details will mean with regard to impacts. The latest GFS has a
broad, closed low forming over eastern California/Nevada Thursday
morning continuing to deepen through the day, with a compact
circulation centered over southern Nevada Thursday afternoon. The
trough axis slips east of our area by midday Friday. The ECMWF is of
similar strength but slower in developing the closed circulation
over southern Nevada and features an impressive jet that drives the
trough deeper into Baja and northern Mexico. The Canadian, although
similar, is the outlier of the three solutions closing the low off
over southwest Utah and then moving it to the east-southeast. With
all these model adjustments, it is too early to talk about the
smaller scale details such as snow amounts and locations, impacts
and types of products that may be needed and when.

The European MOS keeps the cold air entrenched into the weekend even
though the weather pattern moderates. The GFS MOS is warmer and may
be better handling the warm up the pattern suggests given the time
of year and where our temperatures should be overall.
&&

.AVIATION...For McCarran...Winds are expected to be primarily out
of the south/southwest although they may be erratic and gusty at
times with all the shower activity around the area. Still looking
favorable for prevailing showers this evening however timing of
onset looks a little later between 05-07Z, lasting through about 11-
12Z. Cloud decks down to 5k feet still looks about right with the
precipitation. Low confidence but a distinct possibility is lower
clouds and visibility due to snow showers. A small chance of a
shower or two lingering after daybreak Monday but dry conditions
should return by mid day with BKN CIGS between 7-9k feet. Winds mid
morning through the afternoon hours on Monday will be from the
northwest 10-12 knots with gusts up to 20 knots.

For the rest of southern Nevada, northwest Arizona and southeast
California...Scattered showers across much of the region through
tonight with CIGS down to 4-6k feet in shower activity. Winds will
generally follow diurnal patterns except where influenced by shower
activity. Westerly winds gusting to 25-30 knots will continue in DAG
through at least the evening hours. Gusty northwest winds are
expected across much of the region on Monday with lingering showers
over mainly Mohave County.
&&

.SPOTTER INFORMATION STATEMENT...Scattered to numerous snow showers
will occur today and tonight across the Mojave Desert. Spotters are
encouraged to report any significant weather or impacts according to
standard operating procedures.
&&

SHORT TERM...Adair
LONG TERM/AVIATION...Salmen

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