Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 190542

Area Forecast Discussion
National Weather Service Phoenix AZ
1042 PM MST Mon Feb 18 2019

.UPDATE...Updated aviation discussion.


An unsettled weather pattern will persist through much of the week
with well below normal temperatures continuing through the
upcoming weekend. Lingering snow showers over eastern Arizona will
continue this evening into Tuesday. Drier conditions will briefly
settle in late Tuesday and Wednesday, but overnight lows are
forecast to dip down to around the freezing mark for colder desert
locations. A stronger weather system will then affect the area
Thursday and Friday bringing decent rains to much of the lower
deserts and significant snows to high terrain areas of Arizona.


A mid-level vorticity maximum associated with a sharp trough over
the Southwest is well-defined this afternoon across southern Utah
in satellite imagery. A broad area of forcing for ascent
surrounding the trough remains positioned from west-central
Arizona into east-central and northeast Arizona. This is
contributing to scattered showers across Maricopa/Pinal Counties
and into southern Gila County this afternoon. Given the cold
nature of this airmass, snow levels have fallen to 3500-4000
feet, enough to support advisory/warning-level (5-12 inches above
5000 ft, up to 2 inches 4000-5000 ft) snowfall accumulations
across parts of southern Gila County. Additionally, steep low-
level and mid-level lapse rates contributing to marginal
convective instability, reports of small hail have accompanied
some of the stronger showers this morning and afternoon. Shower
activity is expected to generally decrease this evening from west
to east, though some isolated activity may linger overnight into
early Friday across southern Gila County and far northeast
Maricopa County. Also cannot rule out patchy fog Tuesday morning,
mainly across outlying desert areas of Pinal and Maricopa counties
as clouds clear and temperatures fall overnight.

The upper trough will be slow to clear the region Tuesday before
pivoting to the northeast, which will maintain cold temperatures
around 15 degrees below normal. Additionally, sub-freezing
temperatures will be a concern into Wednesday morning across the
deserts of southwest and south-central Arizona, with temperatures
bottoming out in the upper 20s and lower 30s. The freeze watch
remains in effect for these areas, including the Phoenix area.
Hard freeze conditions are also possible over parts of southern
Gila County, including Globe and San Carlos. A very brief reprieve
in the active weather looks likely on Wednesday, but near-neutral change
in mid-level heights will occur as the significant upper trough
quickly digs southward from the Pacific Northwest.

Confidence remains in high in a potentially significant winter
storm affecting the region Thursday and Friday. A large source of
very cold air, combined with a long duration of isentropic ascent
within the warm conveyor, and DPVA associated with the primary
midlevel trough, will combine to produce a long period of
increased precipitation chances from early Thursday through
Friday. The magnitude of moisture associated with this system
will not be as substantial as previous storm systems, but given
the favorable orientation of the trough and aforementioned
attributes, that may not be much of a negative factor overall.
The biggest concern is the potential for significant snow across
the higher terrain north and east of Phoenix, with storm-total
snowfall potentially exceeding two feet over the highest peaks.
Snow levels may initially begin around 5000 ft on Thursday,
falling below 4000 ft on Friday as cold-air advection aloft
becomes more prominent. It is a near certainty that winter weather
products will be needed, and these will be addressed once the
current suite of winter weather products for today`s system
expires. Rainfall wise, south-central Arizona looks likely to
benefit the most, with southwest Arizona and southeast California
expected to be on the fringes of the heaviest rainfall. Storm-
total precipitation totals are expected to range from 0.50-0.75
inch across western Maricopa County, to 0.75-1.25 inches across
central Maricopa County and northwest/north-central Pinal County.
Rainfall should primarily be stratiform in nature Thursday into
early Friday as convective instability remains minimal, with more
cellular shower activity possible on Friday as colder air aloft
spreads eastward.

By the weekend, a quieter pattern should eventually take hold with
a brief period of quasi-zonal flow becoming established across the
Southwest. The signal for yet another trough to develop exists
into the following week, but uncertainty is quite high for this
pattern evolution.


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

Winds are expected to remain light through Tuesday morning.
Confidence in wind direction trails off by early Tuesday morning, as
winds may become variable. Skies are expected to continue to clear
up through the night with only a few clouds around 6-7 kft AGL.
Efficient radiational cooling with the mostly clear skies may
support some patchy light fog around sunrise Tuesday morning,
primarily around the outskirts of the metro. Fog confidence is too
low to include in the TAFs. Modest westerly winds, around 8-13
knots, will develop by the afternoon Tuesday. During the mid to late
afternoon, mixing should support some surface gusts up to 20 kts,
mostly across the west Valley and at KPHX. A few cumulus may develop
in the afternoon with bases around 6 kft. Winds should subside and
eventually shift easterly Tuesday night with clear skies.

Southeast California/Southwest Arizona including KIPL and KBLH:

Diurnal wind directions are expected through the 06Z TAF period.
Speeds should remain relatively light at KIPL, around 6-12 knots.
Light winds are expected through tonight for KBLH, but are then
expected to increase to around 10-15 knots during the day Tuesday
with afternoon gusts up to 25 knots. Skies should remain clear
through most of the period.

Aviation Discussion not updated for amended TAFs.


Wednesday through Sunday:
Well below normal temperatures will continue through the weekend
with a significant winter storm affecting the region Thursday and
Friday. Widespread wetting rains over the Arizona deserts with
mountain snows are likely with this system. A gradual drying and
warming trend is anticipated beginning Sunday. RH values will
remain elevated for much of the period. Winds will quite breezy to
windy on Thursday with most other days in the period seeing
lighter winds.


Spotters should follow standard reporting procedures.


AZ...Freeze Watch from Tuesday evening through Wednesday morning for

     Hard Freeze Watch from Tuesday evening through Wednesday morning
     for AZZ556-560-562.

CA...Freeze Watch from Tuesday evening through Wednesday morning for



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