Flash Flood Watch 8, NWS Tucson, AZ

Area Forecast Discussion
National Weather Service Tucson AZ
1046 AM MST Thu Nov 28 2019

.SYNOPSIS...A strong Pacific storm will bring significant valley
rain and mountain snow later today into Friday. Areas of flooding
possible. Windy conditions and much cooler temperatures are also
expected. Chilly overnight lows to end the week, with warmer
afternoon highs early next week. Another storm system may move
across the area next Wednesday.

&&

.DISCUSSION...As expected, our initial deep moisture influx
associated with the tropics brought 1 to 3 inches for much of
Cochise county with a few higher amounts. Heaviest areas hit were
southwest Cochise county and upstream over far northern Sonora.
The San Pedro River continues to be a concern with a flood warning
from the border to Fairbank through the day.

Otherwise we tweaked the forecast to allow for a little longer
break in between systems, lowering PoP values for the day and
slowing the onset of the best valley rain and mountain snow
chances Friday morning. Please see the previous discussions below
for additional details.

&&

.AVIATION...Valid through 29/18Z.
SCT-BKN clouds decks at 4k-7k ft MSL and BKN-OVC clouds at 10k-
14k ft MSL. Isolated to Scattered -SHRA into this evening, with
widespread valley RA and mountain SN across the entire forecast
area late tonight and Friday. Brief MVFR conditions today and
mountain top obscurations, with widespread MVFR to IFR conditions
tonight into Friday. SFC wind SLY/SELY at 10-15 kts thru 28/19Z,
becoming SLY and increasing to 15-22 kts with gusts to 25-30 kts
between 28/19Z and 29/04Z. SFC wind becoming SLY/SWLY aft 29/04Z
and increasing to 25-30 kts with gusts to 35-40 kts. Aviation
discussion not updated for TAF amendments.

&&

.FIRE WEATHER...Scattered valley rain and mountain snow showers will
continue across southeast Arizona into this evening. Widespread
valley rain and mountain snow is then expected tonight through
Friday. Significant snowfall will occur in the mountains, mainly
above 7000 feet. The precipitation will come to an end from west to
east late Friday night into Saturday. Strong and gusty winds will
also occur late this afternoon through midday Friday. High winds
will be possible in the mountains and some Cochise County valleys.
Dry and warmer conditions are then expected Sunday into Tuesday of
next week. Another storm system may impact the region mid to late
next week.

&&

.HYDROLOGY...Widespread 1" to 3" of rain across Cochise county has
led to significant rises in creeks and rivers with a few creeks
briefly exceeding bankfull. San Pedro River has seen significant
rises at Palominas and was above bankfull as of 10 AM, but the
rate of increase is now behind the forecast curve (which tops out
at 17 feet early this afternoon). Any additional rainfall today
should be light, before increasing again Friday morning.

&&

.CLIMATE...Daily rainfall records fell yesterday with more likely
falling over the next 2 days with some locals, mainly east and
south of Tucson, recording top 10 wettest November days. Its
possible that all-time wettest Novembers, will be approached or
exceeded, especially across Cochise county. It happened at Douglas
airport yesterday.

&&

.PREV DISCUSSION... /issued 402 AM MST Thu Nov 28 2019/
Doppler radar was showing light to moderate rain southeast of a
Clifton-Safford-Tucson-San Miguel line with bulk of the moderate
rain across Cochise county. Rainfall amounts across Cochise county
in the past 18 hours have mostly been in the 1"-3" with locally
higher amounts. This has lead to heavy runoff in creeks and
washes. Several areal flood warnings and small stream flood
advisory are currently in effect across Cochise county along with
a flash flood watch.

For today will see some easing of rainfall rates across Cochise
county showers. Today may end up a bit better than what current
forecast PoPs are before the part two impacts the area tonight
into Friday.

09z WPC surface analysis has cold front over far western Arizona
with latest guidance having this front hanging across wrn Arizona
into this afternoon before it starts to move across the rest of
the state tonight into Friday morning. This front will bring
another round of heavy rain, first starting out in western Pima
county later this afternoon into this evening and then sweeping
across the rest of the area tonight into Friday morning. Snow
levels will start to drift lower Thursday evening, to around 8000
feet initially. The strong cold front will push snow levels
rapidly lower Friday morning, bottoming out near 5000 feet Friday
night. Our best moisture and precip will correspond with snow
levels above 6000 feet just ahead of and with the FROPA. 1 to 2
feet of snow above 7000 feet with up to 3 feet near mountain
peaks. Around 6 inches possible at 6000 feet. Winds also a strong
with very strong winds pre-frontal with 40-50 mph common with
localized spots briefly around 60 mph.

Partial clearing from the west late Friday with a few snow
showers hanging on in the mountains Friday night into Saturday
morning. This system still expected to push quickly into the
central and northern plains Saturday with a dry westerly flow
behind it for most of the weekend. Chilly mornings, but a warming
trend for daytime highs by Sunday.

We could see another system by the middle of next week.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from 11 PM this evening to midnight MST
Friday night for AZZ510>514.

Flash Flood Watch from 5 PM MST this afternoon through Friday
afternoon for AZZ501>506-509>511-514-515.

Wind Advisory from 11 PM this evening to 2 PM MST Friday for
AZZ501>515.

Flash Flood Watch through Friday afternoon for AZZ507-508-512-
513.

&&

Meyer

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