Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 231208
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
508 AM MST Fri Feb 23 2018

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

&&

.SYNOPSIS...
Cool conditions will continue to affect the region through the
weekend with several passing weak weather systems. A few showers may
develop on Friday, mainly across northern Arizona, as a weak storm
system affects the Southwest. The system will also result in breezy
conditions this afternoon. A slight warming trend is expected early
next week before another weather system moves into to the area early
next week bringing another chance for showers along with cooler
temperatures.

&&

.DISCUSSION...
Early this morning, little had changed with regard to the large
scale weather pattern - a large upper trof remained in place across
most of the western CONUS. Latest plot/raob data showed dry westerly
flow aloft over southern Arizona and IR imagery showed skies genly
clear over most of southern Arizona and far SE California. For the
most part, the cool and somewhat unsettled weather pattern that we
have been under for some time will not change over the next week and
we will continue to be dominated by cyclonic flow and cooler than
normal high temperatures. Periodically, disturbances will move
through the long wave trof and across our area giving us a threat
for precipitation.

The first of these threats will be today, although model guidance
has been trending drier and drier with this system for the past few
days. A short wave over Nevada is poised to move southeast and
across Arizona later today, with the strongest dynamics and most
significant moisture to affect the northern half of the state. We
are starting with a very dry airmass over southern Arizona;
yesterday desert dewpoints were in largely in the teens and though
they have been climbing, are still mostly in the 20s across the
south central deserts as of 2 am. The trajectory of the approaching
system is largely over land and no consequential moisture advection
from the Pacific is expected. Latest SREF plumes forecasts suggest
little if any QPF over Phoenix with this system; most members show
0.00 accumulation with one or two maybe spitting out 0.01 inches.
MOS POPs continue to drop; the latest MAV MOS for Phoenix shows just
a 9 percent chance of rain this afternoon and the MET MOS shows just
1 percent. As such, we will keep POPs low, mainly in the slight
chance category for the northern half of our area. Most likely we
will see isolated showers at best over the lower deserts with
slightly better chances over higher terrain areas north and
northeast of Phoenix (such as portions of southern Gila County).
This system will usher in another shot of cooler air, lowering high
temps today into the mid to upper 50s over much of the lower deserts
and Phoenix should see a high around 58 today. After the short wave
quickly exits east this evening, we should clear out and we will
have a rather cold morning coming on Saturday morning. Many lower
deserts will fall into the 30s and although some colder outlying
deserts may drop to around freezing, we don`t expect enough areal
coverage of freezing temps to warrant the issuance of a freeze
warning at this time.

For the weekend into early next week, we will see continued dry
westerly flow aloft, with any passing disturbances remaining well to
our north. High temps this weekend will be slow to climb, remaining
well below normal and mostly in the low to middle 60s. A bit more
warming is on tap for next Monday as Phoenix approaches the 70
degree mark.

The latest operational guidance from the GFS and ECMWF have come
into better agreement and both call for another upper low to move in
from just off the California coast and across Arizona bringing a
chance of showers to the area Tuesday into Wednesday. Timing issues
between the models have been mostly resolved; the best window for
rain will probably be Tuesday evening into early Wednesday with best
coverage to be over south central Arizona (as is usually the case).
POPs have only been tweaked with this system and will stay mostly in
the 30-40 percent ballpark for areas from Phoenix east. The system
will usher in more cool air and another cooing trend, dropping high
temperatures back into the 60s Tuesday and Wednesday.

After that models become very inconsistent and any agreement really
goes off the rails; ensemble member spread from the GEFS is extreme.
GFS operational run suggests a flat ridge developing over the area
while the ECMWF spins up a HUGE upper closed low over most of the
western CONUS with AZ on southern flank under somewhat moist
westerly flow. Due to very low confidence, we will not buy into any
of the individual solutions but go with a broad brushed approach
keeping modest POPs in the forecast for Thursday - mostly slight
chances or single digit numbers. Temps will remain below seasonal
normals, of course.

&&

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

Winds will shift to the west earlier than normal as a cold front
sweeps through the area. This will result in a breezy afternoon
and early evening with sustained wind speeds near 10 to 15 kts
and gusts to 20 kts. An increase in low-level moisture will allow
BKN cloud decks around 6 kft to develop late this morning and last
through early evening before skies clear.

Southeast California/Southwest Arizona including KIPL and KBLH:

Strong west winds will again be an issue today with gusts to 25
kts at both KIPL and KBLH through early evening with some
slackening of gusts by early evening. Skies will generally remain
clear, but a period of FEW to SCT strato-cu is likely late this
morning through the afternoon. Skies will be clear tonight with
winds slowing diminishing.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Sunday through Thursday:

Drier westerly flow aloft will dominate Sunday and Monday,
allowing temperatures to gradually climb and approach seasonal
normals by Monday. Minimum RH values will drop into the 12 to 18
percent range through Monday with improvement into the 20s
starting next Tuesday. A weather system will likely bring good
chances of wetting rains on Tuesday with chances lingering into
Wednesday. Light winds through Monday will increase for Tuesday
and Wednesday as the weather system moves through the area.

&&

.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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