Scientific Forecaster Discussion
fxus65 kvef 171226
Area forecast discussion
National Weather Service Las Vegas Nevada
426 am PST sun Nov 17 2019
Synopsis...another couple days of overall fair weather is
expected while the atmosphere primes itself for a major pattern
change. By Tuesday, a significant weather system is expected to
bring widespread rain with the possibility for the first
significant snowfall for elevations above 6000 feet. While
confidence is still very low on details such as rain/snow
amounts, confidence overall is increasing for the potential for
significant impacts associated with this system through Thursday
and possibly Friday.
Short term...through Monday.
While overall, very little in the way of impacts or even sensible
changes will occur between now and early Tuesday, the upper level
flow pattern will be taking shape for the mid to late week period.
This is characterized by a well advertised and evident low just
west of the Baja Peninsula working to pull up tropical moisture
from the south. This feature will be meandering around this
general area for the next 48 hours or so before becoming
influenced by deep diving energy from the Pacific northwest.
Until then, some local enhancement of the surface pressure
gradient today is already resulting in gusty north winds along
the Colorado River valley. Expecting this to persist through much
of the day today, however mostly localized to the
Laughlin/Bullhead City areas, with lower winds elsewhere. Boaters
on Lake Mohave from Cottonwood Cove and south could see some
enhanced wave action, possibly getting to 1-1.5 feet at times.
Lesser winds are expected Monday with no weather impacts expected.
Long term...Tuesday through Friday.
The main focus of the forecast package this morning was on the
well-advertised storm system expected to impact the region
beginning Tuesday and persisting through possibly Friday. At the
synoptic scale, more clarity is being scene between ensemble
members and deterministic outcomes alike, which is good news.
However, the more subtle characteristics are still very much
unclear, which makes things like rain/snow totals difficult to
What is becoming more clear...
cloud cover is expected to increase substantially on Tuesday as
moisture filters into the region from the south to the north. This
will signal the beginning of deep moisture advection into the area
arriving just in time for large scale ascent that will be driven
in part by the aforementioned baja low, but more significantly
from the secondary deep diving low from the Pacific northwest.
Gefs plumes are trending earlier with each run the onset of quantitative precipitation forecast
along the lower Colorado River valley, which could be as early as
Tuesday early afternoon. This is also evident in some ecs members
as well. Thus nudged up the pops for this area in the grids.
Next, the interaction of the baja low and the (lets call it the cali
low), appears to be more or less agreed upon in the model outcomes
to begin late Tuesday night. The interaction of these two system
should serve to focus the southerly, moist, low level flow with
broad divergence over Arizona. Thus greatly enhancing quantitative precipitation forecast
generation as these variables phase and resulting in heavier
rainfall rates and increased rainfall coverage. At current, this
lines up the best along and east of the Colorado River, putting
Mohave County in the bullseye for the highest totals overnight
Tuesday into Wednesday morning. Latest estimates show potentially
up to an inch of rain across Mohave County before sunrise
Wednesday already with model trends continuing to increase these
values. This would obviously be a concern for flash flooding and
watch considerations certainly appear valid.
Beyond that, here is still what is unclear...
as the baja low continues to be ingested into the deepening and
broadening cali low, it will be ejecting vorticity maxima itself
into the Southern California, lower Colorado River valley, and
western Arizona region. The dominant flow should then become the
cali low, leaving the baja low effects to be mostly moisture at
this point. What this does over the course of Wednesday is enhance
the precip shield northward across southern Nevada, with mid level
convergence enhancing rain production in a very moist atmosphere
into Clark County. This could be particularly problematic for the
higher elevations of the Spring and Sheep Range mountains as very
efficient processes for snowfall production could occur. This
however hinges on these more subtle features taking shape in the
locations currently seen in the 00z and 06z model suites. If these
features speed up, this would result in earlier onset but overall
less totals, and if it slows down, the opposite. The concerning
thing is the trends are currently of slowing. Thus, while the
actual totals to be expected are still quite low confidence, it is
fair to say the potential for significant snowfall for the
Spring/Sheep Range (and to a lesser extent the eastern sierra), is
increasing. At current, this best potential looks to be late
Wednesday into Thursday morning. A consideration for winter
weather products will likely be warranted for this possibility as
Also still unclear is the eastward propagation of this cali low.
A bit more agreement has been noted with the 00z-06z runs showing
the low center over the Nevada/Arizona border Thursday evening.
Thus, it would appear that lingering shower activity could indeed
persist through Friday morning, this system should be exiting
the region and returning to drier conditions by Saturday morning
at the latest.
As key elements of this forecast are still unclear, it is
encouraged for all interested parties to continue to stay aware of
the latest forecast changes between now and next week as these
details will undoubtedly evolve.
Fire weather...another couple days of dry conditions but
relatively weak winds are expected which should keep fire weather
conditions from reaching critical thresholds.
A major pattern change will bring much needed wetting rains to
the region as well as high elevation snows. Higher elevation snow
amounts could be significant as well which should significantly
improve the fuel situations in those places. Lower elevation areas
will be dependent on how much rain occurs this week, but expecting
enough rainfall activity to result in at least short term fuel
improvement for much of the region. Drier conditions expected to
return by the weekend.
Aviation...for McCarran...light and diurnal winds expected
generally below 6 kts with a northeasterly component during the day
and a southwesterly component overnight. Quiet weather will continue
through the taf period with dry conditions and occasional high
clouds aoa 25 kft.
For the rest of southern Nevada, northwest Arizona and southeast
California...winds will be light and diurnal with winds remaining
generally below 8 kts. The exception will be down the Colorado River
valley where kifp has gusted to 30 kts from the north-northeast
overnight and will increase through the day today. Relief from gusts
will come in the form of 15 kt northerly sustained winds after
sunset. A similar story for keed expected but to a slightly lesser
degree with maximum north gusts to 30 kts expected this afternoon.
Quiet weather will continue through the taf period with dry
conditions and occasional high clouds aoa 25 kft.
Spotter information statement...spotters are encouraged to report
any significant weather or impacts according to Standard operating