Areal Flood Warning 2, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
945 AM MST Tue Feb 28 2017

.SYNOPSIS...
Wet but cool conditions will linger as a Pacific storm system
moves through the area. Precipitation activity will rather rapidly
end from west to east through the day with skies clearing into the
evening hours. Dry conditions and much warmer temperatures will
return for the latter half of the week and the coming weekend.

&&

.DISCUSSION...
Some hydro issues this morning due to runoff from the slow accumulation
of precip yesterday afternoon and overnight. A couple of Flood
Advisories are in effect for portions of Maricopa (affects northernmost
portions metro Phoenix) and northeast La Paz County (mainly
McMullen Valley/Centennial Wash). May need to upgrade to a Flood
Warning given the rise on Cave Creek and some other reports.
Speaking of Flood Warnings, there is already one in effect for our
part of Gila County that is drained by Tonto Creek (given a big
rise on Tonto Creek as it drains into Roosevelt Lake) plus far
northeast Maricopa County affecting areas drained by Sycamore
Creek. Otherwise, shower activity is on the decrease as the upper
trough axis moves into Arizona and weak associated front begins
exiting our CWA. There will be just some isolated showers the rest
of the day over the Phoenix area with better coverage over the
higher terrain east of the metro - but pretty much wrapping there
as well by 5 pm. More later.

&&

.PREVIOUS DISCUSSION...Issued 311 am MST/211 am PST...
Precipitation activity has been on- going since last evening and
across our forecast area has been focused over northern portions
of Maricopa and La Paz counties and across much of our southeast
CA zones. Current regional radar composites show precipitation
areas have taken form of two bands, one running through SW
Maricopa county up through Yavapai and well into Coconino counties
with a second concentrated band spanning NE-SW from Lake Havasu
down thru El Centro/Imperial CA. The first band is still tied to
broad jet forced ascent from the sub- tropical jet and the
southern end of the precip band has weakened over the last few
hours as overall sinking motion of the main trough has shifted the
jet southward. The second frontogenetical band, and clear back of
the storm system, will maintain some organization with the aid of
the weakening, but still dynamic, polar jet and the concentration
of cooler temperatures aloft. Enough cooling aloft occurs to
weakly destabilize thermo profiles and some model forecasts
produce minimal amounts of CAPE (generally less than 200 J/kg).
Previous storm events this winter have had an isolated few
lightning strikes with similar CAPE forecasts, so while we can't
rule out some rumbles out there, coverage should not be enough to
warrant inclusion for the AM weather grid.

Hi-res model output (HRRR and NCAR ensembles) time the trough
axis passage at the CO River approx. 15z-16z and through the
south-central AZ deserts 17z-19z. Scattered rain showers, some
with embedded pockets of moderate to heavy rainfall, will continue
to track eastward across the forecast area through the morning
where 0.10-0.40" could be added to rainfall totals from last
evening. QPF gradient favoring upslope/terrain locales clearly
evident on precip observations via Mesowest and FCDMC across
northern Maricopa county and sites upstream. Handful of area
streamflow gauges are indicating runoff response, varying in the
discharge amounts by location but certainly indicative of flow
moving through these channels. Localized flooding and runoff
concerns remain for the foothill and mountainous locales across
many of the northern portions of the CWA but the threat for
widespread areal flooding still remains low. With cooling still
on-going aloft and showery/cloudy conditions clearing through the
day, temperatures will remain below-normal today with widespread
readings in the 60s.

The upper trough will quickly accelerate to the east and by late
afternoon mostly clear skies are possible for the western CWA.
Precipitation chances should rapidly decrease from west to east
Tuesday as a low-level frontal wave moves across the region.
Showers may linger tomorrow near the Phoenix area through early
afternoon, and across southern Gila County into the evening,
before ending altogether late tomorrow night. Still some very
remote chances for some sheltered locales to see some mist/fog
development Wednesday AM but the combo of dry advection and
elevated overnight northerly winds should inhibit significant
development.

UL heights and flow aloft will moderate as high pressure builds
over the West, but a still invigorated sub-tropical jet to our
south will lag trough heights across portions of the Southwest for
the end of the week. The large-scale pattern will remain very
active across the Pacific Northwest and Great Basin, which will
largely keep ML heights suppressed through early next week. Drier
flow off the Pacific will keep our skies mostly clear and once
ridging takes hold of the area and increases thicknesses,
temperatures will undergo a healthy warming trend for the late
week and weekend. Temperatures are likely to reach the low-80s
across many locales for the first weekend of March in the
Southwest. Substantial moisture will be hard-pressed to return to
our area, keeping rain chances near or at zero through the
extended forecast period and possibly beyond.

&&

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

Last of the weather disturbances forecast to move through the area
between 12z-14z. Cigs continuing in the 3-5 thsd foot agl range with
light rain.  Between 16z Tue and 03z Wed, drier air slowly moving in
behind the front. Cigs lifting to 6-8 thsd agl. Increasing west wind
10 to 12 knots. After 03z Wed, becoming sct clds near 6 thsd agl.
Light west wind under 6 knots.

Southeast California/Southwest Arizona including KIPL and KBLH:

Last of the weather disturbances moving out of the area after 12z
Tue. Through 13z Tue, cigs 6-8 thsd agl, sct light rain showers.
West to northwest wind 8 to 12 knots. From 16z-20z Tue, clds
becoming sct briefly bkn 6-8 thsd agl. Rain ending. Northwest wind 8
to 12 knots. After 20z Tue, sct clds 8-10 thsd agl. Northwest wind 8
to 10 knots.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Thursday through Monday...Dry west to southwesterly flow aloft with
a strong warming trend. Lower desert afternoon temperatures are
foreast to be in the lower 80s this weekend and Monday. Minimum
relative humidity in the 15 to 20 percent range. North to northeast
surface wind 8 to 12 mph Thursday and Friday, becoming light and
variable under 10 mph this weekend. Good overnight recovery.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter reports will not be needed through the weekend.

&&

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

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DISCUSSION...AJ
PREVIOUS DISCUSSION...Nolte
AVIATION...Vasquez
FIRE WEATHER...Vasquez