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Area forecast discussion 
National Weather Service Las Vegas Nevada
318 PM PST Sat Dec 27 2014

Synopsis...a cold northwest flow will continue over the area
through early next week with temperatures remaining several degrees
below normal. A storm system will drop into the area by the middle
of next week leading to very cold temperatures along with some
showers and even a chance of low elevation snow.
&&

Short term...tonight through Monday night.

Northwest flow will persist over the area through Monday...becoming
westerly Monday night as the next trough drops south into northern
Nevada. Some high clouds will be streaming across the area tonight
but it looks like they should remain thin enough to allow decent
radiational cooling. MOS guidance continues to indicate low
temperatures cold enough for dra and igm to support the ongoing hard
freeze warning for tonight for southern Nye and western Clark
counties and the northwest deserts of Arizona. Also still looking at
the first freeze of the season for Needles and the freeze warning
remains in effect for that zone. Sunday the band of cirrus will
gradually shift south into the southern part of the forecast area
then south of the area during the afternoon with clearing behind it.
We should see some warming Sunday compared to today and low
temperatures Sunday night look to be a bit warmer as well. Not much
change for Monday with most areas under mostly sunny skies. A weak
disturbance could bring a light shower to northern Lincoln County
Monday. Moisture will start to increase Monday night in the far
northern part of the County Warning Area as the approaching cold trough moves into
northern Nevada. I kept a slight chance of showers in the forecast
across northern portions of Esmeralda...Nye and Lincoln counties as
a slight nod toward the somewhat faster European model (ecmwf). The much more
interesting weather follows in the long term.

Long term...Tuesday through Saturday night.

Well this year has the potential to end on a very interesting note
weather-wise in our area. A positively tilted and elongated
middle/upper level trough is forecast to drop southward across the
northern half of California and Nevada on Tuesday with energy within
the base of the trough forming a closed area of low pressure by
Tuesday evening somewhere over central Nevada or east-central
California. The models have been in remarkable consistency on
bringing an anomalously cold closed area of low pressure in or near
southern Nevada around new year's evening/day for over a week. Thus, it
is hard to discount a storm when the models show it for so long and
the operational and ensembles models trend into generally better
agreement with the track and timing with it. So with that said...

Confidence is high on a cold storm system moving across the area
between Tuesday afternoon and New Year's Day morning before pulling
east by New Year's Day afternoon. While some minor differences still
exist in the timing and track and some model run adjustment with
timing is still possible, we still feel confident that bulk of this
storm will pass through in the new year's evening to early New Year's
Day window.

Confidence is also very high that this will be a very cold storm
system with prognosticated 700 mb temperatures off the GFS and European model (ecmwf) dropping to
-12c to -16c as the center of the low moves over the area. Wet bulb
zero heights plunge to at least 2500 feet as well, however
evaporational cooling does not look to be as big of a factor as in
the Dec 2003 and Dec 2008 low elevation snow events. Initially
precipitation could start as rain as high as 4500 feet or so before the
colder air works on in. Thus eventually precipitation would be snow for
much of the area except the lowest of valley locations (below 1500
feet or so) from Tuesday night into New Year's Day. Forecast
soundings also show most of - it not all of - the thermal profile at
or below freezing except right at the surface for Las Vegas and
especially for Kingman.

What remains the shaky part of the script with this storm is just
how much moisture this system will have to work with as well as if
the center of the storm decides to shift east. The current storm
track over southeast California would be more favorable for more of
the area to see precipitation, while a track further east would
favor Lincoln, Mohave and northeast Clark counties. As a result,
probability of precipitation remain highest in these areas due to higher confidence on
precipitation having a shot at taking place there. We used a blend of the
operational GFS, ECMWF, Gem, WRF and GFS ensembles for the pop
forecast. Model precipitable waters are prognosticated to be between 0.25 and 0.30 inch
on most runs, which is about 25 percent of the mean value for
December for southern Nevada based on the upper air climatology for
this area. Looking at the study done back in the middle 1990s by
Cordero, skrbac and kosier on Las Vegas snow events the prognosticated
synoptic pattern fits the type 2 event of a Great Basin/Nevada or
Tonopah low developing. Such a storm drives its precipitation
amounts largely through dynamics and thus at this point pinpointing
which areas would get nailed with quantitative precipitation forecast the best is tough as some
areas could see a decent amount of precipitation while other
locations see very little - if anything. This pattern was also
supported by the cips analogs which favored the January 28, 1979 Las
Vegas snow event on several runs as one of the top analogs which was
not a large event. Thus, while this event could be interesting it
does not have the signs of being one of the big ones with respect to
snow amounts at least on a widespread scale.

At this point, we kept in at least a slight chance if not higher of
showers across the entire area at some point between Tuesday night
and New Year's Day. Snow amounts on average between 2000 and 4000
feet look to be between a trace to 2 inches as an early first guess,
with higher amounts likely in central and northern Mohave County
such as Kingman as well as in an areas that see any lengthy upslope
component with this storm. The main concern in Vegas will be new
year's evening night when the latest model runs show several hours of
wrap around moisture working on in and temperatures get even colder.

North of the center of the low a tight north-south surface pressure
gradient also sets up by Wednesday afternoon into Wednesday night.
This will generate gusty north to northeast winds of 20-30 miles per hour. The
latest models show less wind now for Las Vegas and the Colorado
River valley, however, if the low center shifts a bit, then the wind
could be higher.

New year's evening and New Year's Day look very cold with temperatures near
record low maximums for Las Vegas. I leaned closer to forecast
sounding highs off the GFS and European model (ecmwf) which were several degrees
below statical MOS guidance as well as the cons models.

The entire system pulls away by Thursday night leaving the area in a
relatively dry northwest flow that may not be completely cloud-free,
but dry enough to shut off precipitation. Temperatures will remain 6-12
degrees below normal with a slow moderation on up. The prolonged
cold nights will continue to create hard freeze conditions away from
the more urban/town centers as well as the Colorado River valley.
&&

Climate...here are a few stats to watch in the coming days with
respect to freezing temperatures and 'cold' temperatures in Las Vegas. All data is
for the official climate station at McCarran International Airport.

* Greatest number of consecutive days with lows above freezing...

1. Counting today - 380 days - started on December 13, 2013
2. 378 days - December 8, 2011 - December 19, 2012

* latest occurrence of the first freeze of the season...

Record: ongoing (has yet to hit 32 at mccarran)
previous record: December 20, 2012

* fewest number of days below 40 in December...

Current record: 5 days in 2010
2014 so far through today: 2 days

* fewest number of freezes in December...

Current record: one day in 1977 and in 2010
2014 so far through today: 0 days

* fewest number of days below 40 degrees in a calendar year...

Current record: 28 days in 2010
2014 total so far through today: 11 days

* fewest number of days at or below 32 degrees in a calendar year...

Current record: 2 days in 2005
2014 total so far through today: 0 days

For your information...measurable snow fell on the west side of Las Vegas on
January 7th 2005 despite no freezes at McCarran in January 2005.
&&

Aviation...for McCarran...high clouds will be over the area through
Sunday morning and will then decrease. Light and variable winds
through early this evening then typical night time drainage winds
south/southwest below 10 kts. Winds light and variable most of the
day Sunday. Looking further out...a storm moving through the area
will bring a chance of precipitation Tuesday night through Wednesday
night with potential for ceilings below 5 kft.

For the rest of southern Nevada...northwest Arizona...and southeast
California...high clouds will be over the area through Sunday
morning and will then decrease from north to south. Mainly light and
variable winds through early this evening except north winds 10-20
kts down the Colorado River. After early this evening winds will
favor typical diurnal trends through Sunday.
&&

Spotter activation is not expected through Monday. .spotters are
encouraged to report any significant weather conditions according to
Standard operating procedures.
&&

$$

Short term/aviation...Harrison
long term/climate...stachelski

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