Flash Flood in Arizona, February 6, 2019

Area Forecast Discussion
National Weather Service Phoenix AZ
954 PM MST Tue Feb 5 2019

.SYNOPSIS...
A weather disturbance sweeping through the region will produce
widespread light rain across central and eastern Arizona through
Wednesday morning. Much drier and cooler air will rapidly rush into
the area by Wednesday evening bringing an end to the rainfall
chances for the remainder of the week. Although sunny skies will
prevail during the latter half of the week, temperatures will
generally remain below normal.

&&

.DISCUSSION...
The front is currently pushing through the Phoenix metro area but
the rainfall has primarily remained along a corridor stretching
from New River towards the southwest to Tonopah. Rainfall amounts
have been the heaviest along the higher terrain north of Phoenix
with approximately half an inch in those desert communities, and
much less elsewhere. Much of the Valley has remained dry.

The high resolution models have finally caught onto the drier
trends for the majority of the Phoenix metro. As such, late
evening and early night PoPs have been cut but may still be a bit
optimistic. Models now indicate, as has been trending the last few
hours, of most of the showers missing the central Phoenix area and
redeveloping further east where higher terrain may enhance any
rainfall potential.

In Southeast California, persistent strong wind gusts persisted
longer into the evening so the wind advisory was extended until
06z.

&&

.PREVIOUS DISCUSSION...
A large western Conus trough was propagating inland early this
afternoon with a notably deep PV anomaly rotating into western NV.
This system will continue to swing eastward over the next 48
hours pulling the entire trough axis through the forecast area,
however with the main cold core and strongest forcing remaining
over northern AZ and UT. Nevertheless, a moderately steep frontal
boundary will sweep through the area tonight acting upon a modest
low level moisture field and incorporating a favorable orographic
component producing fairly widespread showers late this afternoon
through Wednesday morning over central and eastern Arizona.

Moisture transport will be maximized late this afternoon and evening
ahead of the aforementioned frontal boundary, however regional
soundings sampled moisture profiles almost exclusively trapped below
the H7 layer with PWATs barely above 1/2 inch. Therefore, much (if
not all) of SE California and far western Arizona will likely be
rain shadowed by this quick moving system. Gusty winds will be the
greatest threat in this area flirting with advisory criteria, and
the typical SW corner of Imperial County will experience the
strongest gusts requiring a short wind advisory through late
afternoon.

Further east into central AZ along the moisture ridge, ascent and
weak instability will be sufficient to produce low topped convective
elements and numerous showers. While overall lapse rates are not
impressive, shallow SBCAPE approaching 250 J/kg should be
released along the front. The environment will slowly lose capping
above H7 late this afternoon and convective updrafts could
eventually reach a -10C layer leading to a few embedded lightning
strikes into the evening.

While rainfall chances from the Phoenix metro eastward look very
good (especially foothills and higher terrain), SREF QPF plumes
have steadily been falling over the past day and match a similar
trend in high resolution model output. Given the more showery nature
to the rainfall, some lower elevation may miss rain; however most
within the metropolitan area should see totals in a 0.10-0.25 inch
range. Any deeper convection could result in locally higher amounts.
Of greater concern is the prolonged moist upslope flow over higher
terrain east of the Phoenix metro. This area has been quite wet over
the past several days with nearly saturated soils which will
correspond to any additional rainfall going almost totally into
runoff. Larger river channels will be able to handle this runoff,
however smaller washes and creeks could experience detrimental
impacts and a Watch has been issues for the Tonto Basin area where
forecast suggest notable water rises by Wednesday morning.

The frontal boundary will quickly move into eastern Arizona Wednesday
morning with more pronounced subsidence and dry air advection
punching through the forecast area. While a few lingering showers
may be possible over the higher terrain of Gila County, a vast
majority of the CWA will stay dry after sunrise. Despite the
clearing skies and mostly sunny conditions spreading into the area,
temperatures will remain below average under the lower heights aloft
and cooler air filtering south.

Over the Thursday-Friday time period, drier westerly flow and flat
ridging will define the weather pattern yielding mostly sunny skies
but continued below normal temperatures. With clear skies, low
dewpoints, and light winds, Thursday will likely be quite cold
during the morning hours with potential for freezing conditions over
the most favored sheltered valley communities. Even high
temperatures will stay quite cool on Thursday with readings
struggling to reach the upper 50s and lower 60s. Friday morning
looks to be almost as cold as the previous day, but for the most
part lows will stay just above freezing. Highs will climb a bit
however under the influence of the weak ridge; look for readings
climbing back into the mid 60s over the central Arizona deserts and
in the lower 60s further to the west.

Over the latter portion of the extended forecast - Saturday into
Monday - unsettled conditions will continue across the desert
southwest as fast and rather zonal flow will allow a series of
mostly dry low pressure systems to race east and across the area.
POPs will generally be in the single digits and close to climo but
an isolated shower could occur over higher terrain areas east of
Phoenix as well as over Joshua Tree national park. The pattern will
keep the deserts on the cool side as highs each day stay in the 60s
and below seasonal normals.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:
A band of showers associated with a cold front will move through
the Phoenix terminals through midnight. A few showers, especially
out east, will linger through the early part of the morning before
drier air begins to filter into the region. Mainly BKN to OVC
cloud decks aoa 4 kft will remain over terminals through midnight
before skies gradually clear from west to east, with clear skies
expected by Wednesday afternoon. Winds will favor the west and
remain elevated through most of the TAF period. Another round of
gusty conditions will develop Wednesday afternoon with gusts to 25
kts.

Southeast California/Southwest Arizona including KIPL and KBLH:
Brisk westerly winds, with gusts as high as 30 kts, will likely
continue through the early morning hours. Winds will subside at both
terminals by late tomorrow morning, but will increase again by the
afternoon with gusts up to 20 kts. SCT to BKN clouds aoa 5-7 kft AGL
will continue to gradually give way to clear skies by midnight.

&&

.FIRE WEATHER...
Thursday through Monday:
Drier air will generally dominate through the period, but a weather
system on Saturday may bring a slight chance of showers to higher
terrain areas. Minimal RHs will lower into the 20s each day through
the period with good overnight recoveries. Below normal temperatures
are expected to persist through the week. Winds will generally be on
the lighter side through Friday with breezy conditions at times
starting Saturday.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Freeze Watch from late Wednesday night through Thursday morning
     for AZZ533-552.

     Flash Flood Watch through Wednesday evening for AZZ556.

     Hard Freeze Watch from late Wednesday night through Thursday
     morning for AZZ560-562.

CA...Wind Advisory until 10 PM PST this evening for CAZ562-566-567.

&&

DISCUSSION...Deemer
PREVIOUS DISCUSSION...MO/CB
AVIATION...Hernandez
FIRE WEATHER...Kuhlman