Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 201155 AAA
AFDPSR

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
455 AM MST Wed Feb 20 2019

.SYNOPSIS...
Unseasonably cold air over the region is resulting in a freezing
or near freezing start to today. Today will then bring mostly
sunny skies, but high temperatures will only top out into the
middle to upper fifties across much of the lower deserts. A major
winter storm will then begin to affect the region tonight with
snow starting to fall over higher terrain areas and widespread
rain over the bulk of the deserts by around sunrise Thursday.
Steady light to moderate rain will fall across the Arizona
deserts on Thursday, likely persisting through Thursday night.
High terrain heavy snow is forecast for areas north and east of
Phoenix on Thursday, lasting through a good portion of Friday.
Well below normal temperatures, but drying conditions are
forecast for the weekend into early next week.

&&

.DISCUSSION...
The main forecast concern over the next week is obviously the
major winter storm quickly approaching tonight into Thursday.
Before that moves in, unseasonably cold temperatures continue
across the region with low temperatures later this morning likely
dipping just below or to around freezing across much of the
Arizona lower deserts. The cold air will keep highs today in the
50s for the Arizona deserts to around 60 across southeast
California.

The next Pacific storm system is already taking shape across the
Pacific Northwest and is forecast to quickly dig south
southeastward into southern California late tonight into Thursday
morning. Little if anything has changed in the latest forecast as
the center of the trough digs southward tonight across Nevada,
settling over southern California Thursday afternoon as it
essentially stalls out for 12 hrs. Models continue to depict a
near perfect synoptic set-up for a major winter storm across
Arizona as cold air already in place adds to the strong moisture
advection off the Pacific late tonight into Thursday. Strong low
and mid level southwesterly flow will allow PWATs of around 0.50"
into southern Arizona by Thursday morning. Impressive system
dynamics will combine with the moisture to provide a long duration
rain and snow event for much of Arizona, likely lasting through
Friday afternoon across the eastern half of Arizona.

The most difficult forecast question is where snow levels will
end up during the storm. The cold and dry air initially in place
should lead to quite low snow levels tonight through late Thursday
morning. Snow levels should initially be as low as 3000 ft and
evaporative cooling within the dry boundary layer should aid in
lowering temperatures in the boundary layer. Models have pushed
up timing of the initial shower development, as early as this
evening west of Phoenix. Rain and snow should then spread across
much of central Arizona and then southern Arizona between midnight
and sunrise Thursday. The initial lower snow levels should allow
for at least a mix of rain and snow as low as 2500 ft tonight and
Thursday morning with accumulating snows at around 3000 ft. This
will likely impact the bulk of higher terrain areas north and east
of Phoenix, including the Globe area where a few inches of snow
seems likely. Winter Storm Warnings have now been issued for
elevations above 3000 ft across northeast Maricopa and southern
Gila Counties.

During the day Thursday, warm air advection into southern Arizona
will raise snow levels to between 4000-4500 ft. This will keep the
bulk of the accumulating snow in the mountains, but it will still
likely impact the higher passes in southern Gila County and
especially for anyone traveling north of Phoenix. Steady light to
moderate rainfall will fall over the south-central Arizona deserts
nearly all day Thursday and Thursday night, slowly tapering off
from west to east Friday morning. Shower activity will be much
more scattered across southeast California and southwest Arizona.
QPF amounts for Thursday and Friday range from as little as a
tenth of an inch over the southwest Arizona and southeast
California deserts to over 2" over northeast Maricopa and southern
Gila Counties. Rainfall amounts for Phoenix will likely top 1" in
many locations with the highest amounts likely falling just north
and east of Phoenix. The 2" QPF amounts northeast and east of
Phoenix will result in heavy snows with snow totals ranging from
a few inches at around 3000 ft, as much as a foot between
4000-4500 ft, and much higher amounts above 4500 ft. The highest
peaks in the Mazatzals, Superstitions, and eastern Gila County may
see more than 2 feet of snow by Friday evening.

The trough will slowly drift eastward on Friday with the upper
level cold core moving over Arizona by Friday afternoon.
1000-500mb thickness are forecast to drop to between 528-534dam
as the trough moves through allowing snow levels to drop closer to
3000 ft again. Wrap-around cold core showers will still be likely
Friday afternoon, mainly north and east of Phoenix. We can`t rule
out some snow or sleet with some of these showers over areas like
New River and Cave Creek and possibly down to elevations as low as
2000 ft due to the expected convective nature of the showers.
Friday will be quite chilly with the bulk of the day staying in
the 40s over the Arizona deserts. Highs Friday should top out
right around 50-52 degrees, which would end up breaking the
coldest daytime high for Phoenix for the date (currently set at 54
degrees).

The rain and snow should come to an end sometime Friday evening
with the trough finally exiting to the east Friday night.
Overnight lows Friday night and Saturday morning will again be
quite cold with the coldest desert locations dipping down to
around freezing. Drier conditions should then prevail this
weekend into much of next week as models generally show the active
Pacific low pattern staying to the north of our region. This
doesn`t mean we are in store for a big warm up anytime soon. Highs
will slowly climb this weekend, possibly reaching 60 on Sunday.
Further warming is seen for next week with highs climbing into
the middle to upper 60s by Tuesday and possibly 70 degrees for
Wednesday.

&&

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

For most of the day, aviation impacts should be minimal; a dry west
flow aloft will start to spread mid/high clouds in to the area by
afternoon with most bases aoa 14k feet. However, later tonight we
can expect significant thickening and lowering of cloud decks as a
potent winter storm system moves into the area from the northwest.
By mid evening we can expect BKN decks 7-8k feet developing with
isolated showers, and after midnight, CIGS will lower between 3 and
5k feet in the greater Phoenix area as showers become more
widespread. Visibility should remain above 5sm for most of the night
but could start falling below that into Thursday morning if showers
become heavy enough. Winds today should tend to follow normal
diurnal tendencies, but may remain southerly through much of
afternoon ahead of the approaching system. A period of southwest
winds late this afternoon and early evening should turn back more
towards the south or southeast by 05z with some gustiness into the
morning hours.

Again, the most significant aviation impacts should not begin until
after midnight tonight with much of the day Thursday impacted by the
low CIGs, Visibilities and widespread showers.

Southeast California/Southwest Arizona including KIPL and KBLH:

Through the morning hours there will be no aviation impacts, as some
mainly high cloud starts to spread across the western deserts ahead
of an approaching Pacific storm system. Expect increasing and
gradually lowering cloud decks through afternoon and into the
evening with CIGS dropping to 14k feet. By mid evening, around 03-
04z, we should see CIGS falling between 6k and 8k feet with isolated
showers developing. Winds may be the bigger issue today; look for
south/southeast winds up to 10kt or so through early afternoon
before picking up from the southwest and gusting to 25kt or more
into the evening hours. South/Southwest winds should persist much of
the night tonight with persistent gustiness. Do not expect gusts
strong enough to result in any significant blowing dust however.

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 remainer of the five day period.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Freeze Warning until 9 AM MST this morning for AZZ530-533-
     535>540-542>544-546-548>555-559.

     Winter Storm Warning from 11 PM this evening to 5 PM MST Friday
     for AZZ545-557-558-560>563.

     Hard Freeze Warning until 9 AM MST this morning for AZZ556-560-
     562.

CA...Freeze Warning until 8 AM PST this morning for CAZ569.

     Winter Weather Advisory from 4 PM this afternoon to 10 PM PST
     Thursday for CAZ560.

&&

$$

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



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