Weather Service Report

FXUS65 KVEF 192219

Area Forecast Discussion
National Weather Service Las Vegas NV
241 PM PDT Mon Mar 19 2018

.SYNOPSIS...Moisture streaming inland off the Pacific will bring 
increasing clouds and precipitation chances in the southern Sierra 
Tuesday afternoon. Rain chances will spread eastward Wednesday and 
Thursday, with the potential for locally heavy rainfall across parts 
of the Mojave Desert and Southern Great Basin.  The warm nature of 
this weather system will limit snow accumulations to the very 
highest elevations of the southern Sierra and Spring Mountains.

.SHORT TERM...through Tuesday night

Broad, flat ridge will encompass the region during the period, while 
a trough over the eastern Pacific directs anomalously moisture laden 
air into the West Coast. Satellite shows high clouds already 
overhead. Time heights indicate moisture will progressive deepen 
with ceilings lowering to 6-10 kft AGL Tuesday afternoon/night. As 
for precipitation. 12z models continue to show most of the 
precipitation getting hung up on the southern Sierra Nevada and 
coastal ranges of California. Current forecast has around an 1" of 
QPF just east of the crest in Inyo County. Lighter amounts of 
generally a 0.10" or less spill east of the crest into the Owens 
Valley and Death Valley NP, potentially as far east as the Spring 
Mountains. As for San Bernardino County best chances would lie to 
the west of a 29 Palms-Baker line. Snow levels in the southern 
Sierra will rapidly rise to 8000 feet Tuesday afternoon and remain 
above 8000 feet Tuesday night with greatest accumulations above 
Aspendell. Even under a considerable blanket of cloudiness 
temperatures will be closer to our Spring normals while overnight 
lows will be warmer.   

.LONG TERM...Wednesday through Monday.

Main concern is still the Wednesday-Friday storm system. Satellite 
showed the low pressure center near 35N 140W this afternoon, with a 
plume of subtropical moisture (precipitable water between one and a 
half and two inches) wrapping around its eastern semicircle. By 
Wednesday, this plume will slam into the Sierra and southern 
California coastal mountains, bringing heavy rain on the windward 
sides of the ranges. There are significant timing differences among 
the models regarding how quickly and how much moisture spills over 
the crests to bring precip to the deserts. Slowed down the timing of 
highest precip chances a bit for most areas, but not as slow as the 
ECMWF would suggest. Three factors lead me to believe this storm 
will not bring major impacts to Las Vegas: one, the window for heavy 
precip in Las Vegas is only about 6-12 hours (most likely Thursday); 
two, the trajectory of the moisture plume is from the southwest, 
leading to a lot of orographic interception by the southern 
California mountains; and three, the lack of significant dynamic 
forcing to wring the moisture out of the atmosphere. The best 
forcing traverses the deserts about 12 hours after the moisture 
plume has exited to the east. With that said, the mountain areas, 
particularly southwest facing slopes, could be in for heavy rain due 
to orographic forcing. Snow levels will be 9000-10000 feet initially 
due to the warm origins of the subtropical moisture plume, then fall 
below the resort level in the eastern Sierra Thursday morning and 
down to Mount Charleston Friday morning (although most of the precip 
will likely be done by then). Two to four feet of snow will be 
possible near the Sierra crest; however, how quickly snow levels 
drop will determine how much falls in Aspendell. The Spring 
Mountains are likely to get mostly rain, with a few inches of snow 
possible at the highest elevations toward the end of the event. The 
last lingering showers should end in Lincoln County Friday night, 
with dry weather expected for the vast majority of the region 
Saturday through Monday.

.AVIATION...For McCarran...Cloudiness progressive increasing with 
ceilings lowering during the period. Suspect ceiling will be around 
10 kft MSL late Tuesday afternoon and night. Winds following typical 
diurnal trends expected through the TAF period. 

For the rest of southern Nevada, northwest Arizona and southeast 
California...Winds favoring typical diurnal trends during the 
period. BKN-OVC skies will return tonight mainly above 20k, with 
CIGs decreasing to 10-15k Tuesday with and isolated shower activity 
likely near the southern Sierra crest.

.SPOTTER INFORMATION STATEMENT...Spotters are encouraged to report
any significant weather or impacts according to standard operating 
procedures.  Rainfall and flooding reports are encouraged during 
periods of weather later this week.


LONG TERM...Morgan

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