Flash Flood Warning 34, NWS Tucson, AZ

Area Forecast Discussion
National Weather Service Tucson AZ
1010 AM MST Wed Sep 19 2018

.SYNOPSIS...Expect widespread showers and thunderstorms producing
heavy rainfall at times through tonight. Scattered showers and
thunderstorms Thursday mainly from Tucson eastward and southward.
Rainfall chances continue to decrease Friday followed by dry
conditions across much of the area this weekend and early next week.
Much cooler temperatures will occur into Thursday with a warming
trend prevailing this weekend.

&&

.DISCUSSION...Numerous showers with embedded thunderstorms were
occurring at this time mainly across eastern Pima County extending
westward into the Tohono O'odham Nation. Other more isolated showers
and thunderstorms were across western Cochise County, and western
Pima County. Most rain gauge amounts since 7 am MST have been fairly
light, generally less than one tenth of an inch.

A very moist environment was sampled via the 19/12Z KTWC sounding
with a precip water value of 1.74 inches. The daily maximum for 12Z
September 19 was 1.68 inches as per the Storm Prediction Center
Sounding Climatology page. The flow generally above 500 mb turned
more southerly via the 19/00Z KTWC sounding, and the 19/12Z KTWC
sounding depicted even markedly stronger speeds above 500 mb.

This flow regime is in response to strong high pressure aloft
centered over the lower Mississippi River valley, and an upper
trough extending from the Pacific Northwest southward into southern
California. Blended Total Precip Water imagery valid 1330Z depicted
nearly 0.80 inch across the White Mountains, but increased sharply
to around 2.0 inches near the International border adjacent Santa
Cruz/southwest Cochise Counties.

In addition to the abundant moisture and synoptic scale flow regime
across southeast Arizona, the National Hurricane Center just updated
a disturbance over the central Gulf of California to a tropical
depression (Nineteen-E). Per Advisory Number 1, the depression is
moving toward the north near 13 mph and this motion is expected to
become more north-northeast tonight with a similar forward motion
until the system dissipates on Thursday. On the forecast track the
depression is forecast to move inland over northwestern Mexico this
evening.

The upshot is that the official forecast continues with numerous
showers and scattered thunderstorms this afternoon into tonight. A
Flash Flood Watch is in effect for all of southeast Arizona until 11
pm MST this evening. Heavy rainfall may occur late tonight
especially east of Tucson. There is the potential for the Flash
Flood Watch to be expanded until daybreak Thursday especially for
locales east of Tucson. Will scrutinize this possibility further as
additional 12Z guidance arrives. High temps today will be markedly
cooler versus the past several days.

Please refer to the additional sections for further detail.

&&

.AVIATION...Valid through 20/18Z.
Numerous TSRA/SHRA through tonight then scattered -TSRA/-SHRA mainly
KTUS eastward and southward Thursday morning. Expect MVFR
conditions and brief IFR conditions at times through the period.
Otherwise, cloud decks generally 4k-10k ft AGL. Surface wind
variable in direction mainly less than 12 kts except gusts near 40
kts with stronger TSRA. Aviation discussion not updated for TAF
amendments.

&&

.FIRE WEATHER...Numerous showers and thunderstorms into tonight will
produce heavy rainfall at times. Scattered thunderstorms Thursday
especially east of Tucson followed by isolated thunderstorms Friday
into Saturday mainly east to south of Tucson. Dry conditions to
occur Sunday into next Tuesday. 20-foot winds will be terrain driven
mainly less than 15 mph into early next week. However, gusty and
erratic winds with thunderstorms may occur at times.

&&

.PREV DISCUSSION /222 AM MST/...Current IR satellite shows
convection ongoing over Sonora with anvil clouds overspreading
southern Arizona. The 19/00Z KTWC sounding recorded 1.27 inches of
precipitable water, which is at the 90th percentile, with southerly
winds at 500 mb and above. Due to this southerly flow PWAT will be
increasing throughout the day to between 1.5 and 2.0 inches across
the CWA and will be near record values for this time of year.

This southerly flow pattern is the result of high pressure
centered over Arkansas and a trough moving into the western CONUS.
There is also a weak tropical disturbance over southern Baja that
will help moisture overspread southern Arizona. The anomalously
high amount of moisture as well as synoptic scale forcing ahead of
the trough will cause widespread showers with embedded
thunderstorms and significant heavy rain. As a result a flash
flood watch is in effect area wide today. Hi-res models and the
HREF prog storms to initiate later this morning and last through
the overnight hours.

As the trough progresses east the upper level flow will become
more westerly which will start to scour out moisture, especially
in the upper levels. Because of this storm coverage will be less
Thursday with the best chances for scattered showers and
thunderstorms from Tucson eastward. By Friday there will only be
shallow surface moisture with just isolated activity expected to
the south and east of Tucson. Saturday have opted for only a
slight chance of showers and thunderstorms over the White
Mountains. Thereafter southern Arizona will remain in a westerly
flow regime and dry conditions are expected into early next week.

Temperatures today and Thursday will be below average thanks to
the moisture and cloud cover. But this will be short-lived with a
warming trend starting Friday into the weekend. Temperatures will
return to their climatological normal or just above normal.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
  Flash Flood Watch through this evening for AZZ501>515.

&&

Francis
PREVIOUS DISCUSSION...Hardin

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