097 
FXUS65 KVEF 291701
AFDVEF

Area Forecast Discussion
National Weather Service Las Vegas NV
1000 AM PDT Sat Sep 29 2018

.SYNOPSIS...Cooler and wetter weather is in store for southern 
Nevada and surrounding states as we move through this weekend. 
Tropical moisture from Hurricane Rosa will meander into our area 
late Sunday and into the early part of the work week. This moisture 
will result in widespread rainfall, which will be accompanied by 
thunderstorms and possible flash flooding. Flash Flood Watches have 
been issued for much of the region effective for Monday and Tuesday.
&&

.UPDATE...The 12Z GFS continues to support the forecast trends 
detailed in the previous discussion with respect to the incoming 
tropical moisture from Rosa and associated heavy rainfall potential. 
It appears the majority of the moisture will move up through 
Arizona, but there will be plenty pulled up over southeast 
California and southern Nevada to be lifted by a large closed upper 
low off the West Coast. The 12Z GFS indicates precipitable water 
values of 1.5 inches surging up into southern Nevada Monday night 
and a wide south-north oriented swath of precipitation develops apart 
from the more southwest-northeast oriented area of Arizona 
associated with the forecast track of the decaying core of Rosa. PoP 
and QPF grids will be adjusted slightly by this afternoon based on 
the latest guidance, but no major changes are anticipated.
&&

.PREV DISCUSSION...227 AM PDT

.SHORT TERM...through Sunday night. 

Dry and warm weather will persist until early next week as a result 
of a high pressure system situated over our southwestern region. 
Triple-digit temperature days look to be behind us, though above-
average high temperatures in the lower-to-mid 90s should continue 
through the weekend. A cut-off low pressure system will begin to 
push into Northern California Saturday, resulting in high clouds and 
gusty conditions. Expect sustained winds upward of 15-25 mph, with 
gusts up to 30-40 mph across south-central Nevada. Higher gusts are 
possible in localized areas and along higher terrain.

.LONG TERM...Monday through Friday. Complicated forecast coming up 
as our area deals first with the remnants of Rosa and then with a 
non-tropical low pressure system in the westerlies. Rain chances 
associated with Rosa will already have spread as far northwest as I-
15 by Monday morning, and will increase and overspread the entire 
area Tuesday. Heavy rain will be a risk, and kept the Flash Flood 
Watch in place. Timing becomes a problem beginning on Tuesday as the 
models disagree on how quickly to move the low in the westerlies. 
For example, by Wednesday morning, the operational GFS shows the low 
centered near Point Conception with an atmospheric river type 
feature aimed along I-15 through our area, while the operational 
ECMWF has the low centered about 300 miles farther west and depicts 
an AR-type feature aimed straight north into the San Francisco Bay 
area. Model spread only increases thereafter, so made few or no 
changes due to lack of confidence. With the increasing clouds and 
rain, temperatures will come down significantly from what has been 
experienced recently. Heavy rain and flash flooding will be the main 
threats.
&&

.AVIATION...For McCarran...Breezy southwest winds are expected to 
continue through the overnight hours with a sustained wind of 10 
kts. Mid-morning Saturday, winds will adopt a more southerly 
component and pick up to around 18 kts with gusts to 28 kts 
possible. Peak gusts expected to occur in the late afternoon hours. 
After sunset, gusts drop off a bit with peak gusts around 20 kts 
possible and wind direction becomes more diurnal. Scattered clouds 
around 25 kft expected through Saturday evening.

For the rest of southern Nevada, northwest Arizona and southeast 
California...Breezy south-southwest winds are expected to continue 
through the overnight hours. Mid-to-late morning Saturday, winds 
will pick up to around 20 kts with gusts to 30 kts possible. Peak 
gusts expected to occur in the late afternoon hours. After sunset, 
gusts drop off a bit, and continue to do so overnight. Scattered 
clouds around 25 kft expected through Saturday evening.
&&

.FIRE WEATHER...Gusty winds are expected across south-central 
Nevada this afternoon as an approaching weather system moves into 
our area. Expect sustained winds of 15-25 mph with gusts 25-35 mph. 
Localized gusts of 40-50 mph are possible. Relative humidities will 
be low across much of the region, but should reach fire weather 
criteria for a few hours in zones 460, 461 and 462. We have a Red 
Flag Warning out for zones 460, 461 and 462 from 1pm until 7pm 
today. Moisture, as well as calmer winds, will be moving in by 
Sunday so there are few fire weather concerns beyond today.
&&

.SPOTTER INFORMATION STATEMENT...Heavy rainfall is expected Monday 
and Tuesday, and perhaps beyond. We encourage spotters to report any 
significant weather or impacts in your area.
&&

$$

UPDATE...Adair
SHORT TERM/AVIATION/FIRE WEATHER...Varian
LONG TERM...Morgan

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