Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 210956
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
256 AM MST Thu Feb 21 2019

.SYNOPSIS...
A major winter storm will affect the region today with valley
rain and mountain snow becoming widespread by mid to late morning,
continuing off and on through tonight before tapering off from
west to east Friday. Precipitation will be best east of the Lower
Colorado River Valley. Significant accumulating snow can be
expected for elevations above 3000 feet with very heavy snow above
4000 feet. Otherwise, expect much below normal temperatures
through Saturday before a slow warming trend starts. A return to
dry conditions are forecast into next week with temperatures
gradually warming back to near normals by late next week.

&&

.DISCUSSION...
The strongest weather system of the winter season and likely of
the past few winters is beginning to affect the Desert Southwest
early this morning. A deep trough is currently being carved out
across the Southwestern U.S. with the trough center over Nevada.
Further deepening is anticipated over the next 12-18 hours as the
center of the trough shifts southward over southern California.
The positioning of this impressive late winter trough will provide
a nearly perfect scenario for heavy snow across the high terrain
of Arizona. As stated in previous discussions, upper level
dynamics and low level upslope forcing will be quite strong and
persistent over the majority of Arizona this morning through much
of tonight.

Current objective analysis shows light rain and snow becoming more
widespread over southern California and the northwestern 3/4 of
Arizona. As the atmosphere saturates from the top down and
forcing increases, rain and snow will increase in intensity later
this morning for much of Arizona. Little has changed with the
current forecast package with high QPF amounts still shown over
all but southwest Arizona and southeast California. The best
timeframe for moderate to possibly heavy rain and snow over the
eastern half of Arizona looks to be this afternoon and evening
with the heaviest precipitation shifting southeastward into
southeast Arizona by sunrise Friday. The highest impacts will
obviously be the heavy snow across elevations above 3500-4000
feet. Snow levels continue to be a bit difficult to handle with
initial levels around 3000 ft early this morning and gradually
rising to around 4000 ft this afternoon before dropping back down
again on Friday. Snow accumulations are still likely to occur down
to around 3000 ft early this morning and on Friday, changing over
to rain below 4000 ft between 18Z today and 12Z Friday.
Elevations around 4000 ft and above should stay mostly snow
during the bulk of the event with accumulations above 1 foot,
possibly as much as 30" over the highest peaks in the Mazatzal
and Superstition Mountains and across far eastern Gila County.
Total rainfall of 1-1.5" is likely for lower elevation areas from
central Phoenix and to the north and east. This will likely lead
to some localized flooding of washes, but no Flood Watches should
be needed.

The trough will finally begin shifting east through Arizona and
Utah on Friday with the cold core moving over the area. At this
time, the main forcing will push to the east into New Mexico, but
steep lapse rates under the cold core and forcing from a strong PV
anomaly over central Arizona will aid in scattered rain and snow
showers from Phoenix eastward Friday afternoon into the early
evening. The colder air with the trough will lower snow levels
closer to 3000 ft again and with the convective nature of the
showers we can`t rule out rain mixing with snow or graupel as low
as 1500-2000 ft elevation. Drier air will quickly work in from
the west later Friday with skies becoming mostly clear for the
lower deserts by late Friday evening. After chilly daytime
temperatures mostly in the 40s to around 50 for the lower deserts,
overnight lows into Saturday morning should again end up around
freezing. The coldest desert spots may dip into the upper 20s to
around 30 degrees.

The trough finally exits well to the east by Saturday morning with
dry westerly zonal flow taking over through Monday. Temperatures
will be slow to recover this weekend with highs still only in the
low to mid 50s Saturday over the Arizona deserts to around 60 in
the western deserts. Overnight lows Saturday night will again be
quite chilly, likely only a couple degree improvement over the
previous night`s near freezing temperatures. Models are now
leaning to a bit quicker of a warm-up next week with a weak ridge
building across the Western U.S. Tuesday into Wednesday. This
should allow highs to climb to around 70 degrees by Wednesday and
into the lower to middle 70s for the end of next week.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:
A major storm system will bring low ceilings, showers, and breezy
conditons to the area. Cloud decks will continue to lower, as
moisture and rain continues to spread into the region. By early
tomorrow morning and lasting through the remainder of the TAF
period, we expect sct to bkn cloud decks in the 1 to 3 kft range,
and ovc skies above 3 kft. Showers will begin to move into the
vicinity of the terminals within the next few hours, with
widespread showers by mid-morning and also continuing for the
remainder of the TAF period. Light to moderate southeasterly winds
this evening will become breezier by early tomorrow morning.
Winds will then start to favor the south by the late morning and
remain breezy with gusts to 20-25 kts. Winds will shift to the
southwest in the early evening and remain elevated near 10 to 15
kts.

Southeast California/Southwest Arizona including KIPL and KBLH:

Cloud decks will continue to lower, as moisture and isolated
showers continue to spread into the region. Can expect sct to bkn
cloud decks near 3-4 kft and ovc skies above 5 kft by late
tomorrow morning. The best chances for rain seem to be later in
the morning and afternoon. Winds will favor the west and remain
elevated near 10 to 15 kts with gusts up to 20-30 kts during the
mid-morning and afternoon.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Friday through Tuesday: A lingering powerful winter storm will
result in scattered to numerous showers of desert rain and mountain
snow across south central Arizona Friday, along with high
temperatures nearly 20 degrees below seasonal normal. The storm will
exit to the east Friday night resulting in a drying and gradual
warming trend for the weekend into early next week. Despite nearly
constant warming each day, highs will remain below normals through
next Tuesday although they will climb into mid to upper 60s over the
warmer lower deserts. Humidity levels will remain elevated through
the period with drier deserts still mostly above 20 percent each
day. Winds will be locally breezy from the northwest Friday, then
turn lighter for the remainder of the five day period.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Winter Storm Warning until 5 PM MST Friday for AZZ545-557-558-
     560>563.

CA...Winter Weather Advisory until 10 PM PST this evening for CAZ560.

&&

$$

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



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