Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 131547
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
846 AM MST Tue Feb 13 2018

.SYNOPSIS...Dry conditions with above normal temperatures today.
Expect showers to develop Wednesday morning with widespread showers
Wednesday afternoon into Thursday as a storm system moves across the
area. Heavy rainfall may occur especially across the mountains.
Lingering moisture will maintain a slight chance of showers Friday
and Saturday, then dry conditions to return Sunday.

&&

.DISCUSSION...The latest satellite imagery indicates mostly cloudy
skies across central/western Pima county early this morning with
mostly clear to partly cloudy skies from Tucson eastward. Surface
observations across southern Arizona at 8 am MST indicated rather
mild temperatures with readings generally in the lower to mid 40s
for areas south and east of Tucson and temperatures in the 50s from
Tucson northwest and westward. Dewpoints were generally in the upper
20s to upper 30s across the area, although higher dewpoints in the
40s were noted over central/western Pima county.

A strong positively tilted trough extends from the northern Great
Basin southwestward to off the central California coast, with a 552
dm low in the southwestern section of the trough off the central
California coast, west of Morro Bay. This upper low is expected to
slide southwest through Wednesday morning before moving eastward in
to the region. This upper low will tap into a deep subtropical
moisture feed on Wednesday, which satellite imagery already shows
the beginnings of moving into the western coast of Mexico and the
southern portion of the Baja peninsula.

As the upper low moves further southwest/offshore today, the flow
will turn more southerly in advance of the system tomorrow. Expect
some breezy southerly winds across southeast Arizona this afternoon,
although wind speeds will be down considerably from yesterdays
levels. Overall, the forecast package looks to be in great shape,
especially given the increased POPS/QPF totals for the upcoming
precipitation event starting Wednesday. No updates this morning. For
more information on the latter periods of the forecast, including
the upcoming storm system, please refer to the PREV DISCUSSION
section below.

&&

.AVIATION...Valid thru 14/12Z.
SCT-BKN clouds 4k-8k ft AGL mainly KTUS area westward will gradually
dissipate this afternoon. Elsewhere, expect SCT-BKN cirrus clouds
above 20k ft AGL, increasing in coverage overnight tonight. Surface
wind sly/swly 5-10 kts increasing to 10-15 kts with gusts to 25 kts
this afternoon. Wind quickly easing to S/SE 5-10 kts after 14/03Z.
Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...Mostly dry conditions are expected today with
lighter but still somewhat gusty south to southwest winds.  Then a
storm system is expected to bring much needed rains to the area
later Wednesday into Thursday with a few showers possibly lingering
into Friday. Significant rains are possible along with an extended
period of high humidity levels which will help the fine and 100 hr
fuels recover from current all time lows. Snow will be limited to
above 8500 ft. After the somewhat breezy conditions this afternoon,
winds will generally be light through Sunday.

&&

.PREV DISCUSSION...IR satellite imagery and surface observations
depicted partly cloudy to cloudy skies from Tucson westward into
western Pima County early this morning. Meanwhile, clear skies or
mostly clear skies prevail across eastern sections of this forecast
area. Surface winds valid 3 am MST were sly/swly at 5-10 mph.

A deep upper level trough early this morning extended from the Great
Basin southwestward to well west of the southern California/northern
Baja California coasts, or to around 30N/130W. Within this upper
level trough, a 552 dm low was centered near the central California
coast. This upper low is progged to move southwestward and become
briefly centered west of the southern California coast near 31N/123W
by Wednesday morning.

A few sprinkles or showers producing just a few hundredths of an
inch may occur this morning west-to-northwest of Tucson. Otherwise,
dry conditions are expected into tonight. The upper low will fill on
Wednesday, but another shortwave trough will move southward toward
southern California from the Pacific Northwest Wednesday into
Thursday. Southwesterly flow aloft will draw deep moisture across
southeast Arizona, especially late Wednesday into Thursday. As noted
in the Monday evening Forecast Discussion, NCEP GFS and NAM
Integrated Water Vapor Transport(IVT) fields depict values greater
than 500-600 kg/m-s will move across southeast Arizona. These values
qualify as a Category 2 (out of 5) Atmospheric River event.

At any rate, expect showers to develop especially south of Tucson
Wednesday morning followed by scattered to numerous showers
Wednesday afternoon into Thursday. Precip chances were again raised
somewhat for this period. Per coordination with the NWS Weather
Prediction Center, forecast liquid rain amounts for Wednesday
afternoon through Thursday were also raised. The 13/00Z GFS/ECMWF
were slower versus previous solutions with ejecting deeper moisture
east of this forecast area Thursday night into Friday. Thus, have
maintained chance-category PoPs generally from Tucson eastward, and
a slight chance of showers west-to-northwest of Tucson. Rain amounts
should be reduced Thursday night into Friday versus the heavier
amounts expected prior to that time.

The official forecast rainfall amounts by Friday evening have been
revised upward versus previous forecasts. Thus, rain amounts are
expected to generally range from around 0.50" to 1.00" in the
valleys, and generally from 1.00" to 2.00" across the mountains.
Localized rainfall amounts approaching 3.00" for some mountains as
noted in the Monday afternoon Forecast Discussion are certainly not
out of the question.

Forecast confidence regarding snowfall accumulations remains quite
low at this time. Snow levels are still expected to be fairly high,
with any snow accumulations of significance likely limited to above
8500 feet or 9000 feet. Given the increased rain amount totals, then
snow accumulations by Friday evening for Hannagan Meadow may be near
10 inches; snow accumulations for Mount Graham may range from 12-15
inches; and snow accumulations for the highest peaks of the Catalina
Mountains may approach 12 inches. For point of emphasis, these
amounts are preliminary, and forecast confidence is quite low.
Little if any snow accumulations should occur below 8000 feet.

A slight chance of showers continues Saturday mainly east to south
of Tucson, then dry conditions area-wide Sunday. There is a slight
chance of showers across the White Mountains next Monday with dry
conditions elsewhere. High temperatures today will be a few degrees
above normal, then a cooling trend to occur with below normal temps,
especially Thursday. A warmup is on tap Friday into Saturday, with
highs Sunday remaining at least a few degrees above normal. Temps
returning closer to seasonal normals should occur next Monday.

$$

.TWC WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

DISCUSSION...Zell
PREV DISCUSSION/AVIATION/FIRE WEATHER...Francis/Cerniglia

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