Funnel Cloud in Arizona, December 27, 2019

Area Forecast Discussion
National Weather Service Phoenix AZ
458 AM MST Fri Dec 27 2019

.UPDATE...aviation discussion.

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.SYNOPSIS...
A significant weather system will move across the region through
Saturday morning producing a widespread area of rain and snow. Heavy
snow across mountains of southern California and moderate rainfall
in the lower deserts will gradually end Friday morning. However,
Light to moderate rain will spread into southern and central Arizona
with showers lingering until Friday night. Light accumulating snows
area also are expected across the higher terrain north and east of
Phoenix through early Saturday morning. Drier conditions will
briefly return over the weekend, although temperatures will remain
below normal with some lower desert locations seeing overnight lows
near freezing Sunday morning. Another fast moving system will swing
by the area next week.

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.DISCUSSION...
The center of the large upper trough responsible for bringing
widespread rainfall to the area over the past 24 hours is moving
into the Desert Southwest. GOES-16 water vapor imagery shows
multiple smaller-scale vort maxes embedded within the broader
trough, generally approaching the longitude of the lower Colorado
River Valley. A narrow warm conveyor belt associated with weak
isentropic uplift remains positioned over central and northern
portions of Arizona this morning, and is continuing to support
showers generally from Phoenix eastward. Colder air aloft is
spreading eastward, and is supporting a changeover from rain to snow
across the higher elevations of southern Gila County this morning. A
Winter Weather Advisory remains in effect here through Saturday
morning for elevations above 5000 feet, where 2 to 8 inches of snow
is expected. Farther west across Joshua Tree NP, a Winter Storm
Warning remains in effect through mid-morning Friday. Although most
of the accumulating snow has fallen for the event, some additional
light accumulations remain possible as snow showers spread southward
from San Bernardino County.

The primary upper low will pass through south-central Arizona this
afternoon, with cold mid-level temperatures passing directly
overhead the Phoenix area. With sufficient heating, should see low-
topped convective showers develop, and perhaps even a few
thunderstorms. The steep lapse rates and cold tropospheric
temperature profile could result in some small hail this afternoon
with stronger convective cells. HREF guidance suggests this activity
will remain scattered, and generally focused from Maricopa County
eastward. Precipitation chances should diminish after sunset across
the lower deserts, but will continue into late Friday evening across
north-central Pinal and southern Gila Counties. The upper trough
will quickly move eastward into the the central Plains early Sunday
morning, but northerly flow aloft will be maintained across most of
the Southwest. The potential for near-freezing temperatures will
exist on Sunday morning across outlying lower desert locations.
However, at this time it does not appear widespread enough nor
significantly cold enough for a Freeze Watch.

Temperatures will remain well below normal, by about 10 degrees or
so, through early next week, ahead of the next storm system
approaching from the Pacific. This system will be lacking in
moisture compared to the previous systems, partially owing to its
trajectory and orientation. Still could see isolated showers
develop into Tuesday and Wednesday, though there still remains
significant ensemble spread on the exact track of the system and
associated precipitation chances. Regardless, this doesn't appear
to be a big rain/snow maker for the region. Cool conditions are
likely to continue through late next week, but a strong
anticyclone building off the California coast may eventually
influence weather farther inland by the end of the forecast
period.

&&

.AVIATION...Updated at 1158Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Scattered showers with MVFR ceilings are expected to persist
through much of the TAF period. Thunderstorms are possible early
this afternoon before precipitation chances decrease significantly
overnight. Current guidance continues to suggest that
thunderstorm acitivty later this afternoon will be sparse and due
to this our confidence on thunderstorms developing near or over
terminals remains on the lower end and are not mentioned in TAFs.
Instead, rain showers are mentioned during what currently looks to
be the best time frame for showers/isolated thunderstorms, which
appears to be sometime between 19 and 22Z.

Once again, there is lower confidence on the wind directions
during this TAF period due to poor guidance. However, it appears
that lighter easterly winds will gradually switch to the
south/southeast by late this morning before switching to the west
early this afternoon. There is then indication that wind
directions will swing back towards the south-southeast overnight.
Wind speeds will be lighter this morning, increasing closer to
5-10 kts this afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:
Ceilings will continue to improve at KIPL with light west-
northwest winds and minimal aviation impacts. Persistent MVFR
ceilings will continue across KBLH before improving this afternoon
and evening. Winds will favor the north-northwest at KBLH with
speeds generally below 10 kts.

&&

.FIRE WEATHER...
Sunday through Thursday:
Temperatures will remain well below normal throughout the period.
Another storm system will approach the Southwest Monday into
Wednesday, bringing a slight chance of showers to the region.
However, rainfall totals are expected to be light. Humidity values
will only bottom out in the 30s and 40s with very good overnight
recoveries. Winds should mostly be on the light side through the
bulk of the period.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Winter Weather Advisory until 11 AM MST Saturday for AZZ557-558-
     563.

CA...Winter Storm Warning until 7 AM PST this morning for CAZ560.

&&

DISCUSSION...Rogers
AVIATION...Hernandez
FIRE WEATHER...Rogers