Flash Flood and Damaging Wind in California, June 25, 2020

Area Forecast Discussion
National Weather Service Las Vegas NV
227 PM PDT Thu Jun 25 2020

.SYNOPSIS...Continued afternoon thunderstorm activity with brief
heavy rain and gusty outflow winds possible once again tomorrow
afternoon as upper-level low pressure system moves over head.
Another more potent system will impact the region with strong
winds over the weekend followed by cooler temperatures next week.
&&

.SHORT TERM...Through Saturday.

Concerns for the short term focus on afternoon thunderstorm
chances through Friday.

An upper-level low traversing the region through Friday over top
modest pacific moisture, resulting in afternoon instability with
ample forcing for ascent producing thunderstorms, mainly near the
upper-level low center and elongated trough axis extending from
central Nevada to the Sierra Nevada. This trough will continue
sinking southeast tomorrow, with the center coming nearly over Las
Vegas tomorrow evening. With the trough axis shearing out a bit,
lift will be a little less, lapse rates are already quite steep
across southern Nevada, and forecast minor moisture increases with
even steeper lapse rates will support afternoon CAPE values of
500-1500 J/kg. Mountains should have no problem initiating
activity, with storms following the low center and moving off the
mountains in the evening. Large inverted-v soundings indicate the
primary threat will be wind, with outflow gusts of 40-50 MPH
possible.

Saturday should be fairly quite. Lingering moisture may produce a
few buildups over the higher terrain, but not much upper-level
support for any storms that do form to de-anchor.

.LONG TERM...Sunday through Thursday.

Upper-level trough number 2 for the forecast period impacts the
area for the latter half of the weekend into next week. A rather
potent trough, especially by late-June standards, will swing
across the Intermountain West Sunday through Thursday, with the
base of the trough swinging across the area. This will result in
a belt of strong south to southwesterly winds on Sunday with a
sharp drop in temperatures to start next week. Much of next week
will be on the cool side with quiet weather as flow transitions
from northwest behind the departing trough to southwest as a ridge
of high pressure builds in near the four corners with a weak
trough off the west coast.
&&

.AVIATION...For McCarran...Gusty winds this afternoon will vary
between easterly and southeasterly through around 00z. Gusts will be
prevalent during that time with sustained speeds around 10-12 knots
gusting around 20 knots. After 00z, winds will become more southerly
between 180-220 with continued gusty winds through around sundown.
The question after 06z tonight is if an outflow from storms over
central NV will make it all the way down into the Las Vegas valley
which would shift winds to the northwest and pick speeds back up to
10-15 knots until around 10z. Confidence is building in this
scenario. Tomorrow, afternoon showers and storms in and around the
valley will be possible primarily between 21z-01z with strong,
outflow winds being a primary concern.

For the rest of southern Nevada, northwest Arizona and southeast
California...Showers and storms will continue to develop across Inyo
County and the southern Great Basin with strong winds being the
primary concern. Other areas will remain dry with breezy, southerly
winds through this evening. An outflow boundary will push southward
into southern NV which will shift winds to the north/northwest
before it washes out around the I-15 corridor. Winds behind the
boundary could come up to speeds of 20 knots gusting 30 knots.
Prevailing southerly winds will be present again on Friday. However,
there will be chances for showers and thunderstorms across all of
southern NV and southeast Cali where gusty winds continue to be a
concern.
&&

.FIRE WEATHER...Elevated to critical fire weather concerns over the
next few days across the region. Isolated to scattered
thunderstorms over the southern Great Basin will expand further
south Friday afternoon. Dry lightning may be a concern, but the
bigger concern will be gusty outflow winds. Another round of more
widespread winds and low humidity will create critical concerns
for Sunday, lingering into Monday for Northwest Arizona.
&&

.SPOTTER INFORMATION STATEMENT...Spotters are encouraged to report
any significant weather or impacts according to standard operating
procedures.
&&

DISCUSSION/FIRE...Steele
AVIATION..........Guillet

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