Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Las Vegas Nevada
935 PM PST Sat Nov 16 2019
Synopsis...dry, mild, and lightly breezy weather conditions will
continue through Monday with above normal temperatures and periods
of high clouds expected. Beginning Tuesday, a significant weather
system is expected to bring widespread rain with snow above 6000
feet. The greatest precipitation chances will be on Wednesday and
Thursday, with dry weather returning for the weekend.
Update...a mostly clear sky and light to moderate north breezes
tonight do not prompt a need for any immediate forecast changes. The
00z operational GFS solution supports the trends detailed in the
previous discussion regarding the socal closed low which will likely
produce a widespread significant precip event across our region
Wednesday and Thursday.
Aviation...for McCarran...a northwest to north wind component can
be expected the rest of the night and through Sunday morning with
speeds generally 6-10 knots. Otherwise, quiet weather will continue
through the period with dry conditions and occasional high clouds.
For the rest of southern Nevada, northwest Arizona and southeast
California...winds will remain below 10 kts across much of the area
through tonight. The exception will be down the Colorado River where
north winds 15-20 kts are likely. Stronger winds are expected at
kifp and keed on Sunday with sustained winds of 15-25 kts are
expected. Otherwise, expect it to remain dry with occasional high
clouds through the period.
224 PM PST Sat Nov 16 2019
Short term...through Monday night.
Low pressure off the West Coast of central baja will remain off
shore through Monday night allowing it plenty of time to draw
moisture out of the subtropics before it approaches the northern
Baja Peninsula Monday night. The first of the two upstream systems
moves to the pacnw coast on Monday nudging the cutoff low towards
the northern Baja Peninsula. This system stays well to the north
while a second system comes in behind it and begins to dive south.
This second system is the one we are watching that is forecast to
bring unsettled weather discussed in the next section. For the
remainder of the weekend and into the beginning of the work week,
above normal temperatures will continue with occasional high clouds.
Northerly winds between 10-20 mph will continue along the Colorado
River valley through tonight increasing to 15-25 mph with higher
gusts on Sunday. By Monday, the winds settle down with speeds under
Long term...Tuesday through Saturday.
Bottom line...as has been advertised for some time now, a big change
is coming in the middle of the upcoming week, with significant
amounts of valley rain and mountain snow on the way along with much
Details...three main players in this forecast - the slug of moisture
currently lurking just south of Cabo San Lucas, the low pressure
spinning west of the baja spur, and a northern stream low which has
not yet developed. As the southern low retrogrades slowly west
through Monday, it will entrain the subtropical moisture mentioned
above. By Tuesday morning, northern stream low pressure diving into
the Pacific northwest is forecast to split, with one piece digging
down the California coast Tuesday afternoon and evening. This will
pull the southern low and its entrained moisture northward, with
precip chances possibly beginning southeast of I-15 as early as
Tuesday afternoon. As the two lows interact (fujiwhara effect)
Wednesday, moisture will spread over our entire cwa, bringing
widespread precip chances, which will last through Thursday before
ending from north to south on Friday. As noted by previous shifts,
thunderstorms cannot be completely ruled out, but the chances are
still too small to include in the forecast. The biggest area of
uncertainty is how much rain will fall. As has been the case for a
while now, ensemble plumes show a sharp gradient from lesser amounts
to the northwest to greater amounts to the southeast. This gradient
is centered over our cwa, meaning that a very small shift to the
northwest would mean greater amounts than forecast, and a very small
shift to the southeast would mean lesser amounts than forecast.
Therefore, storm total precip forecasts (less than half an inch in
the far northwest cwa, about one inch in Las Vegas, and two inches
in southeast Mohave county) should be taken with a kilogram of salt,
and will likely change multiple times between now and the end of the
event. Similarly, there is also low confidence in forecast snow
amounts of up to a foot at the highest elevations of the Sierra, the
White Mountains of Inyo County, and the Clark County mountains. Once
this storm exits (most likely Friday, although some model
perturbations suggest Thursday or saturday), there is good consensus
on a dry weekend, although a small minority of models suggest
Fire weather...brief elevated fire weather concerns will be
possible through the weekend, especially during the afternoon
hours along the Colorado River valley. However, fire weather
concerns will be overall low given generally light breezes and min
relative humidity values above critical values.
The more significant news is the Prospect for widespread, and much
needed, wetting rains in the forecast beginning possibly as early
as late Tuesday night and persisting through at least Thursday
night, and possibly Friday.
Spotter information statement...spotters are encouraged to report
any significant weather or impacts according to Standard operating