Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 191712
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1012 AM MST Mon Feb 19 2018

.UPDATE...Updated aviation discussion.

&&

.SYNOPSIS...
Temperatures will cool drastically today and Tuesday as
a low pressure system drops southward through the Great Basin and
into the Desert Southwest. Winds will become very breezy today
with light rain and snow chances for the higher terrain through
early Tuesday morning. In the wake of the low pressure system,
very cool weather will persist only warming back to near average
late this weekend.

&&

.DISCUSSION...
A pronounced PV anomaly was driving down the Sierra spine with an
impressive 100kt H5 jet rounding the trough base bringing 10-16dm
height falls into southern NV/UT. Locally, 6-8dm heights falls have
been measured and objective analyses shows an increasing pressure
gradient and pre-frontal LLJ developing. Regional observations are
now show a more extensive surface reflection tapping the higher
momentum air with increased daytime mixing. Advisory level winds are
already being reported in SW Imperial County; and the eastward
spread of stronger winds aloft will translate across the higher
peaks of central AZ later today.

Otherwise, a favorable orographic component continues east of
Phoenix, though the strengthening midtropospheric wind profiles seem
to be pushing most of the shallow convective showers further away
from the forecast area and original ascent/generation zone. However,
some enhanced convergence along the pre-frontal LLJ zone has allowed
some deeper isolated convection within the Phoenix metro. This zone
may shift through the metro during the day, and have made some minor
adjustments to POP distribution. Also, given the current observations
and the frontal approach, made some changes to hourly temperatures
as daily highs were quickly being reached with deeper mixing this
morning. With the strength of this system, some locations will
actually see temperatures falling during the afternoon.

&&

.PREVIOUS DISCUSSION...
/531 AM MST Mon Feb 19 2018/
The prominent orographic lift along with the higher moisture has
allowed weakly convective showers to generate over the higher
terrain north and east of Phoenix. Tucson`s sounding indicates
roughly 100 J/kg of mixed layer CAPE while model analysis suggests
similar vales are currently being observed over Gila County.
Given that moisture and the strong southwesterly flow (i.e.
orographic lift) will remain fairly steady through this afternoon,
expecting showers to continue for the higher terrain until
moisture shuts off behind the front early Tuesday morning.

The associated surface front with the aforementioned trough is
currently stretched across southern Nevada but will continue to
race down towards our CWA this morning. Models indicate this front
will rip through Blythe around 8am this morning and into the
Phoenix metro this afternoon/evening. While the front is expected
to be dry for lower elevations, we are expecting some very
breezy/windy conditions with it. Consequently, we have a wind
advisory for portions of SE California and also for the higher
terrain of Gila County. For the Phoenix area, winds will not quite
be at advisory criteria but gusts up to 30 mph could be realized.
Winds are forecast to quickly settle down by late Monday night.

Behind the front, colder and much drier air will begin flooding
into our area. By Tuesday afternoon, 850 mb temperatures will only
be around 1-2 C over Phoenix which would translate into highs in
the mid-50s! This is despite the fact that skies will be crystal
clear under the late February sun. Believe it or not, this could
match or best the coldest day so far this winter which was 57
degrees observed on December 21st. Temperatures are forecast to
plummet Tuesday night after sunset with widespread 30s expected by
Wednesday morning in the lower deserts. In fact, the coldest
lower desert locations could briefly drop into the upper 20s
during this period!

This long wave trough is forecast to stick around through at
least Friday. However, the warm February sun will undoubtedly
modify this airmass on both Wednesday and Thursday bringing a bit
of warming each day. Yet another shortwave will rotate through
this tough come Friday which looks to be relatively cold in
nature but will not be packing much in the way of moisture. PWs
are forecast to be around 0.5 inches at best but strong PVA
forcing may accompany this trough if indeed it comes to fruition.
Really, the main impact would be some breezy conditions along with
some light snowfall above 4000 ft. However, there are still model
disagreements with the ECMWF & GEFS offering a much drier
solution than the GFS.

Quasi-zonal flow develops for the weekend and possibly into early
next week bringing tranquil weather.  This pattern would allow
temperatures to gradually rise back to near normal while winds stay
relatively light.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:
Gusty winds and low ceilings will be the two main aviation impacts
through the TAF period. An abundance of low-level moisture has
resulted in SCT to BKN cloud decks in the 4 to 6 kft range. These
clouds decks will gradually lift and dissipate throughout the day,
with mostly clear skies expected overnight. A chance for showers
will persist through most of the day but will be confined
primarily north and east of Phoenix terminals. Southwest winds
with gusts to 30 kts will be possible this morning through around
midnight. Thereafter winds will subside and remain out of the
west.

Southeast California/Southwest Arizona including KIPL and KBLH:
Main aviation weather impact will be the windy conditions and
blowing dust. Winds will be out of the northwest with gusts up to
30 to 40 kts, particularly at KIPL. These gusty winds will result
in areas of blowing dust and associated reductions in visibility
through late this evening. Thereafter winds will subside. Outside
of any dust, skies will be clear.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Wednesday through Sunday:
A series of low pressure systems will move through the Desert
Southwest through the weekend. This will help to reinforce the
below normal temperatures across the region, while there will only
be a slight chance of precipitation with the system Friday and
Friday night. Below normal RHs are also expected particularly
Wednesday, though winds will remain below critical thresholds
through the period. A warming trend is likely by Sunday, though
temperature will remain below average.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Wind Advisory until 9 PM MST this evening for AZZ557-558-563.

CA...Wind Advisory until 8 PM PST this evening for CAZ560-562-566.

     Wind Advisory until 10 AM PST this morning for CAZ563-565-567.

&&

$$

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DISCUSSION...MO/Wilson
AVIATION..Hernandez
FIRE WEATHER...Hirsch



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