Area Forecast Discussion
Issued by NWS Phoenix, AZ

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332
FXUS65 KPSR 251153
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
453 AM MST Sun Feb 25 2018

.UPDATE...Updated Aviation and Fire Weather Discussions...

&&

.SYNOPSIS...
Cool conditions will continue today, and a slight warming trend is
expected Monday before another weather system moves into the area
early in the week bringing an  opportunity for showers along with
cooler temperatures. A return to drier and warmer conditions is
expected starting on Thursday.

&&

.DISCUSSION...
Early this morning, dry west/northwest flow aloft was situated
overhead; latest soundings indicated PWAT values over Arizona below
0.15 inches. IR imagery at 2 am showed clear skies area-wide.
Surface dewpoints over the central deserts were very low, mostly
ranging between 10 and 20. For today, models continue to call for
another shortwave to drop southeast and through the four corners; it
will have no impact on our weather other than to keep high
temperatures well below normal and in the low to mid 60s over much
of the lower deserts. Expect sunny skies as dry northwest flow aloft
persists. Flow aloft becomes more southwest on Monday ahead of a
developing upper low that will be moving into the area during the
early part of the week and producing widespread rains over south
central AZ. High temps will climb a bit Monday, possibly reaching
the 70s degree mark in Phoenix.

Model guidance, including operational GFS/ECMWF and GEFS ensembles,
continues to lock in on the weather system for early this week and
the best window for significant, widespread rains still looks to be
Tuesday night across south-central Arizona. Early this week a strong
shortwave will dive down the California coast and close off just off
the southern CA coast by Tuesday morning. The low will then be able
to tap into moisture from the Pacific (although no atmospheric
rivers will be present) and generate widespread rainfall over the
lower deserts as it opens up and moves inland Tuesday into Tuesday
night. Well organized frontal band will sweep east across the
western deserts Tuesday and on Tuesday night, strong dynamics will
act on this moisture to give widespread showers to south-central AZ
and the greater Phoenix area. POPs for Phoenix have been raised to
nearly 80 percent for Tuesday night, and the latest MRA MOS is up to
72 percent. Expect considerable cooling with this system given the
clouds, showers and the cool airmass that will be moving in. Highs
Tuesday will fall into the low to mid 60s from Phoenix westward, and
then fall even further Wednesday as the remainder of the cool air
overspreads the area. Look for a high near 60 in Phoenix on
Wednesday. At this time we do not expect thunder with this system as
lifted indices are generally positive over the deserts and forecast
CAPE minimal at best. We could not totally rule out an isolated
lightning strike or two with the frontal band, but confidence too
low at this time to put thunder into the forecast. Heavy rain also
not expected; we should see widespread desert rains with this event
ranging from 0.1 to 0.3 inches with locally higher amounts mainly
over higher terrain areas north and east of Phoenix.

For the rest of the forecast period - Thursday through Saturday -
model guidance is not as clear cut and GEFS ensemble members still
show quite a bit of spread. It does appear that there will be a very
large upper level low forming over much of the western CONUS later
this week with the trof axis near the Pacific NW coast by Thursday
evening. This low will tend to focus well to our north, with Arizona
on the southern flank of the low and under generally dry westerly
flow aloft. There will be embedded short waves moving thru the trof
or around the base of the main upper low, but timing of these waves
will be problematic and precip threats from any of the waves look to
be low. NAEFS POPs stay relatively modest through this time period,
mostly in the slight chance category for the southern lower deserts.
As such we will keep our POPs low, mostly single digits each day
with some slight chance values over southeast California, especially
over higher terrain locations such as Joshua Tree NP. POPs have been
raised slightly to blend better with San Diego. High temperatures
will climb slowly and should climb into the lower 70s over the
warmer lower deserts by Friday.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:

No aviation impacts through at least Monday morning under
continued clear skies. Winds will remain light and follow diurnal
patterns.

Southeast California/Southwest Arizona including KIPL and KBLH:

No aviation impacts through Monday under clear skies. Winds will
remain light through Monday morning and predominately out of the
west to northwest.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Tuesday through Saturday:
A cold weather system will move through the region Tuesday into
early Wednesday bringing widespread rain to the majority of the
Arizona deserts and rain and snow to the high terrain. Lingering
rain and snow showers is likely into Wednesday across eastern
Arizona. Winds will also be breezy on Tuesday before diminishing
into Wednesday. Minimum humidities of near 30 percent for Tuesday
and Wednesday will lower to near 20 percent through the end of
the week as high pressure takes over. A warming trend late in the
week will bring temperatures back to near normals under increasing
daytime breezy southwesterly winds.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

DISCUSSION...CB
AVIATION...Kuhlman
FIRE WEATHER...Kuhlman



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