Heavy Rain in Arizona, December 9, 2019

Area Forecast Discussion
National Weather Service Phoenix AZ
143 PM MST Mon Dec 9 2019

.SYNOPSIS...
An upper level disturbance moving through Arizona today will
bring scattered showers and a few thunderstorms mainly over
southern Arizona through this afternoon. Drying conditions and
mostly clear skies will then affect the region through the rest
of the week with temperatures gradually warming to above normal by
late week.

&&

.DISCUSSION...
The latest water vapor imagery shows a trough centered over
northern Baja California. Cold air aloft has moved over Arizona
increasing instability that has triggered heavy showers and
isolated thunderstorms capable of producing heavy rain and small
hail. Visible imagery shows decent cumulus development over the
southeastern quadrant of Arizona with increasing growth around the
Phoenix area. Current radar scans show a line of small storms
that formed along a low level convergence boundary stretching from
Cave Creek south past South Mountain. Just as we have seen all
morning, these storms are capable of producing small hail and
heavy rain. Additional isolated storms are possible into the late
afternoon primarily for the Phoenix area eastward. Farther west
in southeast California and western Arizona, subsidence is keeping
skies mostly clear while a tight post-front pressure gradient is
creating breezy conditions. As the low progresses eastward,
subsidence aloft will spread across the forecast area ending our
rain chances. For late tonight and early Tuesday, fog may form
given the ample low level moisture and saturated grounds where
ever skies may clear.

The forecast area will trend both warmer and drier for the remainder
of the forecast period. A weak trough will pass by to our north
keeping atmospheric heights dampened through Wednesday. However,
ridging quickly builds behind in the wake of trough with H5 heights
nearing 582 dm (90th percentile of climatology for the season) by
the end of the week. This will translate to afternoon highs
reaching the low 70s by Thursday and lasting through Saturday.
Another trough late next weekend will skirt by to our north
cooling our temperatures by 5-8 degrees.

&&

.AVIATION...Updated at 1745Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

The extensive fog that was across the region has pretty much burned
off now at all of the PHX area terminals at this hour, except for
KIWA, which is expected to clear by 19Z. Shower/TS activity that
also occurred earlier is gradually shifting off to the east. This
afternoon should see continued gradual clearing/lifting of CIGS with
most showers remaining off to the east, thus have reduced shower
coverage in the TAFS to just VCSH. Further clearing tonight will
bring back the risk of fog again tomorrow morning, but drier air
moving into the region should limit fog development. At this point,
have introduced VCFG in the TAFS for now. any fog that does develop
should clear by 17Z Tues, with mainly CLR skies the remainder of the
day. Westerly winds this afternoon should turn easterly early this
evening, with easterly winds persisting until Tues afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:

Now expecting just FEW-SCT CIGS in the 5-6k foot range this mourning
to clear this afternoon and evening, with CLR skies then persisting
through Tuesday. Gusty northwesterly winds at both SE CA TAF sites
to diminish this evening and tonight, with much lighter winds on
Tuesday.

&&

.FIRE WEATHER...
Wednesday through Sunday:
Dry conditions will prevail first under a weak but dry upper level
disturbance on Wednesday followed by increasing high pressure
aloft into the weekend. Minimum humidity levels initially will be
around 30-40% early in the period, but will fall to 25-30%
beginning Thursday. Overnight recovery will still be good to
excellent. Winds through the entire period with be fairly light
with little to no breeziness.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch through Tuesday afternoon for AZZ556.

CA...None.
&&

DISCUSSION...Deems
AVIATION...Sawtelle
FIRE WEATHER...Kuhlman

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