Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Las Vegas Nevada
851 PM PST Friday Nov 28 2014
Synopsis...dry conditions as well as off and on high clouds and
breezy conditions are expected across the area through the Holiday
weekend. A storm system is looking more likely to affect the region
by the middle part of next week bringing an increase in area-wide
Update...outside of one area of cirrus which is enhancing off the
White Mountains this evening, the rest of the area is clear. The
only update to the forecast was to raise dewpoints overnight as
current observations across the area were much higher compared to
what was forecast and it is unlikely we will dry out that much
overnight at the surface.
Aviation...for McCarran...typical south-southwest drainage winds at
5-8 kts are expected overnight. Winds should lighten toward 17z
Saturday for a few hours which may trend them toward a southeast
component. After 20z Saturday winds should increase from the south
to southwest to 8-12 kts with gusts to 20 kts. A clear sky tonight
will give way to increasing clouds at or above 20k feet on Saturday.
For the rest of southern Nevada...northwest Arizona...and southeast
California...VFR conditions and light winds expected into Saturday
morning. Breezy south-southwest winds are expected Saturday
afternoon and early evening. Few-scattered high clouds at or above 20k feet at
times through the period.
issued at 244 PM PST Friday Nov 28 2014
Short term...now through Monday night...
the synoptic pattern this afternoon illustrates upper-level
troughing along the eastern Continental U.S....ridging along the intermountain
west...and a trough approaching the Pacific northwest. The area of
high pressure influencing our weather will break down tomorrow...as
trough energy pushes into the northwest Continental U.S.. flow aloft will be
generally southwest. The surface pressure gradient will strengthen
and lead to breezy south and southwest winds during the afternoon
and early evening hours. Still anticipate some downslope winds along
the Lee of the Sierra and Spring Mountains from about 11 am PST to
roughly 10 PM PST. The strongest winds will be across the Sierra
from midslope to higher elevations. Currently it looks like wind
speeds will generally be below Wind Advisory criteria...but a couple
brief isolated gusts to advisory level are not out of the question.
If our local hires models come in any stronger over the next few
model cycles an advisory may be needed. The winds will add to mixing
and keep temperatures warm...similar to today even though the ridge
aloft will be breaking down.
The Pacific northwest trough energy will track east Sunday...but a
little area of weak energy will slide across the forecast area. This
will pull in just enough moisture from the Pacific to provide a few
showers across the mountains of Inyo and Esmeralda counties Sunday.
Relatively cooler air and increased cloud cover will work into the
region as well...dropping temperatures a few degrees across the
board. By Monday...the weak shortwave energy will be well east of
the region...with ridging forecast to develop in it's wake.
Temperatures Monday will remain on the cool side..yet still a few
degrees above normal. High level clouds will linger Monday as we
await our next weather system.
Long term...Tuesday through Friday...while still only moderate at
best...confidence is increasing that a period of wet weather will
impact much of...if not the entire region during the upcoming week.
Today's 12z guidance further reduced The Gap between the operational
runs of the GFS and European model (ecmwf)...with the European model (ecmwf) continuing to gradually
accept that a broad upper level low will in fact phase with some
subtropical moisture to bring widespread precipitation into socal
beginning Tuesday morning. As a result of this slight increase in
confidence here is a first Stab as some details with this system.
For our forecast area precipitation is likely to begin over the far
western portions of Inyo and San Bernardino and Inyo counties
Tuesday morning. The as the low moves ashore and transitions to an
open wave precipitation should spread east across the remainder of
the forecast area Tuesday afternoon through Tuesday night.
Precipitations will likely become more showery and isolated through
Wednesday perhaps lingering into Thursday. With the subtropical
nature of the moisture plume and the lack of significant cold air in
the upper level low I don't expect snow levels to drop exceptionally
low with this system...likely remaining at or above 6500-8000 feet.
If this setup holds true the brunt of the moisture associated with
this system will fall over socal...however amounts of a trace to
one-half inch will be possible over the forecast area. Higher
amounts will be possible...especially in the southern Sierra where
1-2 inches of liquid precipitation is possible...resulting in significant
snowfall above the snow level. As mentioned at the beginning...
confidence is still moderate at best and the details will become
clearer over the next 24-48 hours as the upper level low develops
over the Pacific. The confidence beyond about 12z Thursday is very
low as zonal flow will likely give way to some sort of disturbance
by Friday or the weekend.
Spotter information statement...spotter activation not expected.
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