Area Forecast Discussion
Issued by NWS Las Vegas, NV

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FXUS65 KVEF 261435
AFDVEF

Area Forecast Discussion
National Weather Service Las Vegas NV
735 AM PDT Tue Sep 26 2017

.SYNOPSIS...Decreasing northerly breezes with gradually increasing
temperatures into mid-week for the region. Expect dry weather for
most areas with any shower and storm chances limited to northwest
Arizona on Wednesday.
&&

.UPDATE...Clouds have been increasing rapidly since about 4 AM as
100 knot northerly jet streak over northern Nevada forces upper low
over NV/UT border to dig southward. Updated sky grids for the rest
of the day and evening in an effort to account for this trend. Will
continue to monitor.
&&

.PREV DISCUSSION...
315 AM PDT Tue Sep 26 2017

.SHORT TERM...Through Wednesday night

Made no significant changes to the forecast through midweek. Quiet
and cool weather is expected to continue today with little in the
way of any kind of impacts. The trough that has been sitting over
the Southwest US will not change much over the next 24 hours, so
basically expect a day very similar to Monday with maybe a bit more
cloud cover. There could be a few gusts to 25 MPH this afternoon
near the lakes, especially Lake Mead, but they will be few and far
between. Mondays obs on the lakes were not very impressive so dont
anticipate it being an issue for today.

On Wednesday, the broad upper level trough will begin to shift
eastward and lift out...with a few waves of energy swinging through
the area. Moisture will be limited so dont expect much in the way of
precipitation as the better forcing moves through, except over
northeast Mohave County. A bit of moisture will sneak its way into
this area ahead of the exiting trough and should trigger some
scattered precipitation. A few rumbles of thunder are possible
Wednesday afternoon as some diurnal heating should allow for weak
instability to develop, especially with the cold low overhead.
Forecast model LIs support this idea, so kept the chance for thunder.

The trough will continue to lift out Wednesday night and bring an
end to any precipitation that does develop during the afternoon. In
its wake, heights will begin to build so expect a warming trend
through the second half of the week.

.LONG TERM...Thursday through Tuesday.

It still appears there will be little in the way of weather impacts
the rest of the week and early next week though confidence has
fallen in the forecast details such as wind and temperatures after
Saturday. The reason for this is considerable disagreement with the
evolution of a broad low pressure system that is forecast to move
inland across the Pacific Northwest and brush the northern Great
Basin on Saturday. The GFS has been deeper with this feature than
the ECMWF solution the past couple days and now the ECMWF has come
around to depicting a low moderate amplitude trough over the western
states Saturday into Sunday. The GFS digs the trough over
Nevada/Utah/Arizona Sunday through Tuesday while the ECMWF quickly
lifts it away across the northern Rockies and plains. Given that the
ECMWF has trended a little cooler initially like the GFS,
temperatures were lowered and winds were adjusted using ensemble
blended guidance over the weekend and early next week. If the GFS
solution ends up being closer to reality, temperatures would be
about 10 degrees below normal by Monday along with gusty north winds
Sunday and Monday. Not ready to buy totally into this idea yet, so a
trend toward the more tempered changes indicated by the ensembles
seems like the best bet for now.

.FIRE WEATHER...Expect decreasing northerly winds and gradually
increasing temperatures over the next several days. Afternoon
humidities will fall near 5-15 percent for the deserts, 20-30
percent for the mountains; and recover to 25-30 percent for the
deserts and 50-60 percent for the mountains. No critical fire
weather is expected through the period.

.AVIATION...A prevailing north-northeast wind component near or
above 10 knots can be expected from mid morning through the
afternoon hours today and Wednesday. Winds should drop off and
become light and variable in the late evening and overnight hours.
Few to scattered clouds generally above 15 Kft will be observed.

For the rest of southern Nevada, northwest Arizona and southeast
California...VFR conditions can be expected across the region as a
weak low pressure system drops over southern Nevada today and
Wednesday. North-northeast winds between 10 and 20 knots will be
prevalent across much of the region with higher gusts along the
Colorado River Valley between Bullhead City and Lake Havasu. A few
showers and thunderstorms with bases around 10 Kft will be possible
over northwest Arizona Wednesday afternoon. The rest of the region
will see generally scattered clouds with bases generally above 15
Kft.

.SPOTTER INFORMATION STATEMENT...Decreasing northerly breezes and
a warming trend are expected through the week. Spotter activation
will not be needed through the end of the week.
&&

$$

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

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