Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Las Vegas Nevada
1010 am PDT Monday Jul 28 2014
Synopsis...scattered thunderstorms with heavy rain possible today.
Heavy rain will likely lead to areas of flash flooding. Monsoon
moisture will remain in place mostly across northwest Arizona and
southern Nevada through the week leading to a chance of showers and
Update...no significant changes planned to the forecast as the
latest observations and guidance support the trends detailed in the
previous discussion. High res models indicate the most active area
for organized convection will be Clark County northward as a large
circulation currently lifting out of northern Baja California moves over
southeast California and southern Nevada by this afternoon. The
Flash Flood Watch for the entire area is warranted but we may see
less convective activity over southern San Bernardino and Mohave
405 am PDT Monday Jul 28 2014
Short term...through Wednesday night...
at this early hour, scattered thunderstorms are occurring over parts
of Las Vegas extending east into the Lake Mead National Recreation Area
and north into Lincoln County. One cell near Nellis AFB has dumped
around a quarter of an inch since 3 am.
Decided to go with a Flash Flood Watch for the entire forecast area
from 9 am - 9 PM today as all the ingredients seem to be coming
together. The triggers will be residual mesoscale convective vortex over the western Mojave
Desert which is prognosticated to slowly lift north through central
California. This feature will help organize development in Inyo,
including the east slopes of the southern Sierra. Secondary feature
is one that the models have been consistently showing for two days
now. That is the shortwave/pv anomaly over northern Baja California that is
prognosticated to move north up the Colorado River valley helping organize
storms over northwest Arizona, southern Nevada and eastern San
Bernardino County. NAM bufr soundings indicate similar precipitable
water to yesterday, or about 150-200 percent of normal with lifted
indices -2 to -4. After today, models still show a shift north in
activity Tuesday with a shift back south with a least a slight
chance of storms for the higher terrain of the Mojave Desert
Long term...Thursday through Monday.
Thursday basically looks like an extension of what may be around on
Wednesday. A ridge aloft remains centered over eastern
Arizona/western New Mexico keeping the area in a southwest flow.
Lifted indice's are shown to be fairly stable per the GFS and whatever
instability and middle-level moisture that would be sufficient to
trigger convection remains confined to the far northern County warning forecast area and far
eastern Mohave County. We have kept in a slight chance of afternoon
and evening thunderstorms there. Otherwise expect dry conditions
with temperatures at or a little above normal. At this point, Friday does
not appear to be too different from Thursday with the greatest risk
for afternoon and evening thunderstorms in the same areas. Otherwise
it should once again be dry with temperatures at or above normal.
The real period of interest is from Friday night through Sunday
night as what appears to be either an inverted trough or just some
sort of vorticity maximum or lobe floats around the ridge and heads
across the lower Colorado River valley on Friday night or Saturday
morning before eventually heading north. The timing on this feature
varies between the ECMWF, GFS and Gem but they all show it. Given
the reasonable model agreement of this trigger feature as well as
the increase in moisture shown to push in with precipitable waters off the GFS
prognosticated to exceed a juicy 1.50 inches and sufficient instability and
cape, especially by Sunday, I have gone ahead and increased probability of precipitation
across the area. The main concern will be how much cloud cover
ultimately comes up, as this could limit heating and just result in
more showers and low-grade thunderstorms. On the other hand, the
moisture will be plentiful, and if more potent storms get going, the
risk for flash flooding will be greater. The presence of this
inverted trough or vorticity maximum or lobe will could also favor
convection developing at night. I did go ahead and also raise
dewpoints and lowered maximum temperatures a couple of degrees especially on
The GFS is slower to bring in this feature and thus eject it
compared to the European model (ecmwf). As a result, we maintained the prospects for
thunderstorms over more of the area on Monday in case it is slower
to move out. However, we should see a downward trend starting on
Monday in the amount of thunderstorms around and with more heating
see temperatures get back closer to normal.
Aviation...for McCarran...thunderstorms generally north/northeast
of the Las Vegas Valley terminals this morning. Mormon Mesa/Beatty
corridors most impacted. Additional development expected through the
reminder of the afternoon/evening will impact all corridors.
Thunderstorms will bring erratic gusty winds...heavy rain and lower
ceilings at times.
For the rest of southern Nevada...northwest Arizona...and southeast
California...isolated to scattered thunderstorms expected areawide
today. Expect erratic gusty winds...very heavy rain and lower
ceilings in and near thunderstorms.
Spotter information statement...spotter activation may be needed
later today. Spotters are encouraged to report any significant
weather conditions according to Standard operating procedures.
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