River Flood Warning 1, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
503 AM MST Fri Mar 13 2020

.UPDATE...Updated Aviation Discussion...

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.SYNOPSIS...
A slow moving and wet storm system will slowly exit to the east
today but will still result in lingering showers over mainly south
central Arizona into tonight. Isolated afternoon thunderstorms are
also possible. After a brief dry period this weekend, another colder
but not as wet storm system is expected toward the middle of next
week.

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.DISCUSSION...
Early this morning, showers (and isolated thunderstorms) continued
to dissipate and move off to the east in association with one of the
vort maxes/lobes rotating around the large Pacific low. Subsidence
behind this exiting system has led to quite a bit of clearing across
the deserts to the west of Phoenix per the latest IR imagery. During
the morning hours there may be patchy fog developing but confidence
is somewhat low as RH values are still well short of 100 percent
most places and lingering breeze or redevelopment of low cloud decks
would inhibit fog formation. Model guidance, including ensemble
output such as GEFS, remains consistent in calling for the large
upper low to finally open up and lift to the northeast and across
Arizona today into the evening hours. As the final vort trof swings
through the deserts, instability climbs (MLCAPE ramps up)and CAMs
suggests showers and isolated thunderstorms will develop again over
mainly south central Arizona. This seems reasonable, and thunder
continues to be mentioned in the forecast for the afternoon hours
across south-central Arizona. POPs remain rather high again today,
over 70 percent over the central deserts, before tapering off
gradually during the evening hours. This should be the final day
that we have to deal with this very persistent, wet, upper level low.

Most of the greater Phoenix  area saw event total rainfall amounts
between 1 and 2 inches, and much of the western deserts (including
western Maricopa County) picked up between 2 and 4 inches. Phoenix
so far has seen 4 consecutive days with a measured rainfall amount
of 0.25 inches or more, tying the record for the longest such streak
which last occurred in 1932. At this time, we still have a flash
flood watch in effect for most of the area, but the watch is only in
effect for south-central Arizona during the afternoon hours. We may
decide to drop the watch for zones to the west of Maricopa County
after further collaboration with neighboring offices. High
temperatures today will stay well below seasonal normals with
readings in the mid to upper 60s over most of the lower deserts.
Modest warming can be expected Saturday as highs climb into the low
70s, although readings will still be a bit below normal levels.

After the upper low finally exits to the east tonight, we can expect
a bit of a break in the weather for a few days as weak high pressure
returns from the west. High temperatures will climb into the mid to
upper 70s Sunday into Monday under partly to mostly sunny skies.

It seems clear that the desert southwest has seen the weather
pattern become very active and we can expect a virtual parade of
potentially wet weather systems to follow over the next 7 to 10
days. Ensemble guidance from sources such as NAEFS, CMC and GEFS are
in pretty good agreement that another potentially wet Pacific low
pressure system will set up off the central California coast by next
Tuesday afternoon, and then slowly move through the area Wednesday
into Thursday bringing scattered to numerous showers along with
isolated mainly afternoon thunderstorms. The low is similar in some
respects to the system that is currently impacting the state, with
one of the primary differences being that it will probably not have
the same moisture tap (IVT plume) that the current one experienced.
Moisture may be a bit lacking with this next storm but locally
enhanced rains would occur should thunderstorms develop. It may also
be a bit colder as snow levels are forecast to fall below 5000 feet
so there is some potential for a bit of higher elevation snow in
both Joshua Tree NP as well as southern Gila County. High
temperatures will also cool off sharply, falling into the mid 60s
next Wednesday.

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.AVIATION...Updated at 1200Z.

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

There is a break in the shower activity which should last until
around Noon. Will see FEW to SCT lower clouds through the rest of
the morning with a BKN 5-7K feet deck likely reforming this
afternoon. There is fairly high confidence instability type
isolated to scattered showers will redevelop this afternoon with
an isolated thunderstorm or two possible. The coverage is still
somewhat of low confidence, so will mention VCSH for now. Shower
chances diminish quickly into the evening with skies eventually
scattering out. Winds through the period should mostly follow
diurnal trends, but any shower activity could create some erratic
wind shifts.

Southeast California/Southwest Arizona including KIPL and KBLH:

Anticipate some cumulus to fill later this morning into the
afternoon, possibly becoming BKN. A predominate southwest to
westerly wind is expected through the period, staying fairly
elevated with some afternoon gusts of 20-25kts.

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.FIRE WEATHER...
Sunday through Thursday: Drier conditions will move in by Sunday and
Monday before wetter and more unsettled weather is expected into the
middle part of next week, beginning later on Tuesday. Humidity
levels will stay elevated through the period with minimum readings
generally 25-40%. High temperatures will stay near seasonal normals
through the weekend with well below normal temperatures likely by
the middle of next week. Winds will be breezy at times through much
of the period, especially during the daytime hours. Gusts will
mostly reach into the 20-25 mph range.

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

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.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch through this afternoon for AZZ534-537>560-562-
     563.

CA...None.
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DISCUSSION...CB
AVIATION...Kuhlman
FIRE WEATHER...Kuhlman/CB