Areal Flood Warning 31, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
1217 PM MST Fri Dec 24 2021

.UPDATE...
Updated Aviation discussion.

&&

.SYNOPSIS...
Showers will continue through this morning with additional rain
chances this weekend. Today's rain will be beneficial for most
areas. But, wet roads will make road travel more hazardous and
flooding potential exists over any burn recent scars as well as in
some normally dry washes...especially Tonto Creek. An unsettled
weather pattern is likely to continue into next week with
additional chances of precipitation while temperatures remain at
or below seasonal normals.

&&

.DISCUSSION...
Showers are spreading throughout the forecast area marking the shift
towards a cooler and wetter regime. So far, most rainfall totals
have been under 0.10" with a few pockets of higher amounts. Shower
activity will continue to increase from west to east this morning as
latest guidance and observational trends show the bulk of activity
falling now through the early afternoon. Activity will wane from
west to east this afternoon while chances beyond trend downwards
each successive day this weekend.

Rainfall rates of 0.10-0.25 in/hr have been observed so far and
should persist throughout the event. Temporary localized higher
rates cannot completely be ruled out, especially for areas subject
to upslope flow. Rainfall totals will generally rise from west to
east across the CWA with 0.10-0.50” most likely for southeastern CA
and southwestern AZ, and 0.50-1.50” for south-central Arizona. The
latest forecast mirrors ensemble means with widespread totals of
0.75-1.25” across the Phoenix metro while higher totals, close to 2
inches, focus over the higher terrain north and east of Phoenix.

The main driver for the weather shift is an upper level low that
shows well on water vapor imagery just of the SoCal coast. The on-
going steady stream of moisture advection will intensify later this
morning as the main circulation ejects into the Southwest today.
Integrated Water Vapor transport guidance still shows 50-80% chance
for 500-750 kg/m/s of water vapor streaming into the region which
means this atmospheric river will be seasonally significant and near
climatological maximums in the NAEFS record. When all is said and
done, this amount of moisture means most places will see at least
some rain and some places may see their wettest December day(s) in
decades.

The rain will be beneficial for most areas but there is a limited
flood concern within the WPC’s latest marginal Excessive Rainfall
Outlook. The lower rainfall rates should give plenty of time for
soils to absorb the rain but burn scar areas and some normally dry
washes could see some flow. Tonto Creek is forecast to exceed action
stage by early this afternoon, potentially inundating low water
crossings.

The forecast beyond remains ambiguous but ensemble guidance is
hinting at another shortwave moving out of the Gulf of Alaska and
southward along the West Coast Tuesday. More shortwaves are also
possible later in the week but guidance uncertainty remains fairly
high. Regardless of uncertainties, successive waves will reinforce
the upper level trough that will lead to cooler temperatures at or
even slightly below seasonal normals through the week. Fog will also
loom for some early mornings following very wet periods, especially
within any river valleys or low spots where cooler air may pool.

&&

.AVIATION...Updated at 1915Z.

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

A strong short wave within southwesterly flow aloft is tapping
into rich moisture from lower latitudes to produce widespread MVFR
ceilings and IFR visibilities in showers this morning. The wave
is in the process of weakening as it moves across Arizona. Hi-res
models are in good agreement that the shower activity will
decrease in coverage significantly after about 21Z. But, before
then, the low ceilings and visibilities will continue (locally IFR
ceilings) with a considerable amount of fluctuation from one TAF
site to another at any given hour depending upon the localized
shower intensities and proximity of hills/mountains to the
airfield. Even after 21Z, scattered showers and lingering low
level moisture will continue to cause problematic ceilings though
not as bad as the previous hours. If any form, they could easily
bring brief LIFR conditions. There is about a 10% chance of
thunderstorms this afternoon (too low to reflect in the TAFs).
Expect much better conditions this evening but with lingering sct
cumulus and bkn altocumulus/altostratus. Overnight, lower ceilings
may develop AOA FL050 and some spotty very light showers may
develop during the day Saturday - but nothing like today.

Southeast California/Southwest Arizona including KIPL and KBLH:

The main band of showers and lower ceilings is east of the TAF
sites. But lingering low level moisture and some very modest
instability will keep SCT040-060 and BKN080-100 going through the
afternoon could even have some localized BKN040-060. The potential
for thunderstorms at the TAF sites is less than 10% (higher to the
east and southeast). Strong southwesterly winds with gusts of
25-30kts (possibly as high as 35 kts) can be expected through the
afternoon before weakening this evening. Couldn't rule out brief
reductions in visibility at KIPL but that is more likely in
desert areas west of the airfield.

&&

.FIRE WEATHER...
Sunday through Thursday:
Temperatures will remain cool at or slightly below normal. Low level
moisture will remain present keeping Min RH values at or above 40%
while overnight recovery surge above 60-70%. Winds may be concerning
at times with occasional gusts of 20-30 mph some days, but which
days will be the breeziest remains ambiguous. Showers will decrease
Sunday-Monday but another wave of showers seems more likely Tuesday.

&&

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

DISCUSSION...12
AVIATION...AJ
FIRE WEATHER...12