000
FXUS65 KPSR 230522
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1022 PM MST Fri Feb 22 2019

.UPDATE... Aviation discussion updated.

&&

.SYNOPSIS...
A major winter storm will quickly exit through eastern Arizona this
evening with rain and snow tapering off across the area. Clearing
skies will result in a few chilly mornings, however dry weather and
much warmer temperatures will finally return to the region next
week. Temperatures will steadily return to near average levels
with welcome highs reaching back into the middle and upper 70s.

&&

.DISCUSSION...

Showers quickly (finally) winding down across the eastern CWA late
this evening as the large winter storm pulls eastward. Biggest
question for the overnight is fog versus frost. Current surface
dew points are in the 20s for our central and western CWA, but in
the Phoenix area into NW Pinal County they are closer to 40. Using
the UPS method, crossover temperatures would be in the upper 30s
and current forecast is for temperatures to easily drop below
that. (Widespread near/below freezing temperatures still look
reasonable with the very cold airmass in place.) With light
surface winds through the night, majority of BUFR soundings
suggest MRi (mixing) values more conducive for fog. Hydrolapses
though are a much tougher call, with some short range data
suggesting increasing moisture with height (good for fog). Recent
ACARS soundings still point to high RH values through the column,
however expect that to decrease as drier air does move in to the
mid/upper levels from the west. Thus left with the lingering
moisture near the surface. Typically that would point to fog, but
with the near/below freezing temperatures and light winds,
anticipating deposition to be the dominant process resulting in
much more frost for Saturday morning. Fog is still certainly a
possibility, but with the thinking that frost will win out any
visby reduction will be more in the 3-5 mile range (mist). Also
going to include some frost in the Imperial Valley with added
moisture from the nearby Salton Sea mixing in with near-freezing
temperatures.

&&

.PREVIOUS DISCUSSION...
A deep PV anomaly has propagated into south-central Arizona this
afternoon while the stream of more pronounced isentropic ascent and
midtropospheric frontogentical forcing has shifted into New Mexico.
However ahead of the trough axis through western Arizona and under
an impressive and climatologically rare cold core (12Z KNKX H5
temperature sampled at -34C is one of the coldest 500mb temperatures
ever recorded at this upper air site), very steep lapse rates were
supporting SBCape up to 250 J/kg and low topped convective showers
(tropopause currently below 20K ft will limit any further vertical
development to these showers). Given the very low freezing levels,
any more robust showers will support a few lightning strikes and
small hail/graupel. Coverage should be fairly numerous though the
smaller size of showers and rapid propagation should significantly
limit any flooding impacts.

Conceivably, heavier convective activity could bring snow levels
down much lower locally, however the residence time of any of these
quick moving showers will have a much more difficult time in
dynamically cooling the atmospheric column versus yesterday.
Nevertheless, overall snow levels will remain around 3000 ft and
with showers persisting into southern Gila County beyond sunset, the
Warning in the southeast part of the CWA was extended a few hours.
The combination of loss of daytime heating and passage of the trough
axis will quickly bring rain/snow to an end by late evening as
subsidence and dry air envelop the entire region.

With clearing skies and drier air moving into the area, freezing
temperatures will be a distinct possibility tonight - even at lower
elevations of south-central Arizona. The typical sheltered valleys
should fall into the upper 20s/low 30s. Any location that does have
snow cover (i.e. foothills and higher elevations) will definitely
fall even colder. However given the unusually cold period of weather
this month and common, widespread freezing temperatures very
recently, no Freeze headlines seem necessary at this time. The
greatest hazard heading into Saturday morning would be re-freezing
of ice on less traveled roads outside of major population centers.

The trough axis finally exits well to the east of the area Saturday
morning with dry westerly zonal flow taking over through Monday.
Temperatures will be slow to recover this weekend with highs still
only in the low to mid 50s Saturday over the Arizona deserts to
around 60 in the western deserts. Locations with lingering snow
cover will be notably colder for the next several days. Overnight
lows Saturday night will again be chilly, likely only a couple
degree improvement over tonight near or below freezing temperatures.
Models are now leaning to a bit quicker of a warm-up next week with
a weak ridge building across the Western U.S. Tuesday into
Wednesday. This should allow highs to climb to around 70 degrees by
Tuesday with high clouds potentially keeping temperatures in the low
70s on Wednesday before warming into the mid 70s by Thursday and
Friday.

&&

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

Skies have rapidly cleared across the Phoenix area this evening.
Winds have become very light across the entire area and will
continue as such through the overnight hours into Saturday morning
favoring an easterly flow. After further analysis, removed any
mention of reduced visibility for Saturday morning, expecting
frost instead. Saturday afternoon will be clear with light
easterly winds continuing.

Southeast California/Southwest Arizona including KIPL and KBLH:

Light winds and clear skies expected through Saturday.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Sunday through Thursday:
Dry conditions are expected through the five day period, along with
a warming trend as high pressure starts to build into the area.
Initially high temperatures will be well below normal; desert highs
Sunday will be in the low to mid 60s and nearly 10 degrees below
normal. After that steady warming will occur with highs back into
the low 70s and near normal by Tuesday. Highs will be above normal
by Thursday with mid 70s highs. Humidity levels will stay elevated
each day, with the deserts typically running 20 to 30 percent each
day. Winds will also be on the light side during the five day period.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

DISCUSSION...Iniguez/MO/Hopper/Percha/Kuhlman
AVIATION...Iniguez
FIRE WEATHER...CB

NWS PSR Office Area Forecast Discussion