Area Forecast Discussion
Issued by NWS Las Vegas, NV

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FXUS65 KVEF 221623 AAA

Area Forecast Discussion
National Weather Service Las Vegas NV
923 AM PDT SAT OCT 22 2016

.SYNOPSIS...Above normal temperatures and increasing winds are
expected today. The next storm system will bring widespread clouds
and good chances for rain to much of the region late Sunday through
Monday. Quiet weather will then return early next week before
storminess returns late in the week.

High pressure to our east and low pressure off the California coast
will keep a southwest flow over the region through today. There will
some areas of gusty afternoon winds especially over the lee side of
the mountains. Only real concern is with the fire danger as gusty
winds combined with low relative humidity will increase the
likelihood of fire spread. Afternoon temperatures still running
about 8-10 degrees above normal. No updates planned or needed.

Prev Discussion
323 AM PDT SAT OCT 22 2016

.SHORT TERM...Today through Monday Night.

Overall, the main forecast through Monday remains the same. However,
the devil is in the details, and there remain fairly significant
differences in the latest guidance. This leads to below normal
confidence, particularly in the precipitation forecast through the
short term period.

For today, expect to note an increase in southerly breezes across
the region as the next Pacific trough approaches the area. This
should lead to fairly decent mixing and above normal temperatures
this afternoon. That said, the latest guidance has definitely come
in cooler, and dropped the high temperatures by a couple degrees.
The most notable item today will be fire weather conditions in the
Spring Mountains (see FIRE WEATHER for more info).

The main impacts start Sunday and last through Monday. Winds will
increase once again, especially across Inyo, Esmeralda, and Nye
counties, where a Wind Advisory has been issued.

The big story though is the increase in moisture, which when
combined with the dynamics associated with the trough will bring
widespread chances for rain. Rain chances will begin over San
Bernardino County Sunday morning, and gradually spread northeast
through the afternoon and evening hours. This is where the forecast
gets messy. There are stark differences in the placement of the
highest moisture and rainfall between models for late Sunday into
Monday morning. Most notable is the difference between the ECMWF and
GFS. Both models have trended further west with the precipitation
bands with their 00z runs, but the ECMWF is quiet a bit further
west, placing the core of the precipitation along a Barstow-Pahrump-
Rachel line between 06-12z Monday. This is along the far western
edge of where the band was shown yesterday. Meanwhile the GFS
continues to place the core just east of Las Vegas along a Yucca
Valley-Searchlight-Colorado City line...similar to previous runs.
In the end, while I`ve increased PoPs and QPF just about everywhere,
the largest increase has been to the west of Las Vegas. We will have
to continue to monitor the trends closely and adjust rapidly as
models come into better agreement. By Monday afternoon precipitation
is likely to become more convective in nature, particularly east of
Las Vegas where some modest instability is forecast to develop.
Precipitation will taper off from west to east Monday afternoon and
evening as the shortwave trough axis pushes through the area.

.LONG TERM...Tuesday through Friday.

Models look similar to 24 hours ago on Tuesday, with the wave which
affected the region during the short term moving away to the east.
Wednesday looks subtly different now, with high pressure still
building to the south, but progressing east somewhat more quickly,
which would lead to increasing southwest flow a day earlier than
suggested yesterday, along with more high clouds. Differences
amplify somewhat on Thursday. Yesterday, the models were showing low
pressure along 130W; this morning, the low pressure is along
120-125W. Some models (including the operational ECMWF) were more
amplified with the low, suggesting a slower movement and precip
chances just reaching the Sierra crest by Thursday afternoon. Others
(including the operational GFS) depicted a weaker, faster moving
open wave, with precip chances spreading over much of the CWA
Thursday afternoon. Previous grids had an unrealistically tight
gradient Thursday night between chance PoPs and near-zero PoPs given
the uncertainty, so smeared the grid somewhat to loosen the gradient
and better express the uncertainty. The model differences get even
worse on Friday, with solutions ranging from a negatively tilted
trough quickly swinging through to a slow moving trough meandering
around to southwest flow and ridging. With such a wide range of
possibilities, stuck with the broad brush approach, with slight
chance to low end chance PoPs areawide. The uncertainty in the PoP
forecast leads to uncertainty in the sky and temperature forecasts
as well. Overall, confidence from Wednesday onward is below average.

.FIRE WEATHER...No changes were made to the Red Flag Warning for the
Spring Mountains today nor the Fire Weather Watch for Nye County on
Sunday. Winds will be marginal for the Spring Mountains today, but
with an ongoing fire and poor overnight recovery and strong
overnight winds have kept the warning going. Gusty winds look likely
to develop across Nye County on Sunday and with RH`s in the 10-15
percent range, the watch will likely be upgraded to a Red Flag
Warning later today. Increase clouds and moisture, along with fairly
widespread rain chances are expected for late Sunday through
Monday, limiting any additional fire threats. &&

.AVIATION...For McCarran...No significant weather is expected
through today. Winds will be light and follow typical diurnal
trends. The next storm system will move through Sunday and Monday
bringing enhanced southwest winds and chances of rain/lower CIGS to
the area.

For the rest of southern Nevada...northwest Arizona...and southeast
California...South winds will increase slightly today. The next
storm system will bring further increases in wind speeds as well as
chances for rain and lower CIGS to portions of the area Sunday and

.SPOTTER INFORMATION STATEMENT...Spotter activation will not be
needed through Saturday. Chances for rain and perhaps isolated
storms will increase on Sunday and Monday. Spotters should report
significant weather according to standard operating procedures.


Short Term/Fire/Aviation...Wolcott
Long Term..................Morgan

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