Area Forecast Discussion
Issued by NWS Tucson, AZ

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441
FXUS65 KTWC 231558
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
858 AM MST Mon Apr 23 2018

.SYNOPSIS...Dry conditions with unseasonably warm daytime
temperatures through midweek. Increased moisture will then bring
isolated to scattered showers and thunderstorms mainly east of
Tucson Friday into Saturday. Dry conditions to return next Sunday.

&&

.DISCUSSION...High pressure building in from the west will
dominate our weather in the short-term, accompanied by warmer
temperatures and dry conditions today. Surface dew points are
holding steady in the 20s this morning, but we will have room for
those values to crater into the teens or lower this afternoon with
a drying trend below 700mb. Likewise the 12Z KTWC sounding with
precipitable water at .45 inches will likely drop closer to 1/3 of
an inch this afternoon. Western areas may be an exception to this
trend with the HRRR showing a weak low level flow from the Gulf of
California spreading a shallow moisture increase into western Pima
county later this afternoon.

The main story is starting to shape up to be possible convection
in eastern areas later this week into the weekend. Please see the
previous discussion for additional details.

&&

.AVIATION...Valid through 24/18Z.
SKC-FEW clouds above 20k ft AGL today, becoming scattered to
broken tonight into Tuesday morning. Normal wind trends generally
less than 10 kts, with some gustiness in the afternoon due to
standard surface heating. Aviation discussion not updated for TAF
amendments.

&&

.FIRE WEATHER...Unseasonably warm temperatures are expected this
week, with readings topping out about 10 degrees above normal for
most locales. Dry conditions will prevail through mid-week, then
isolated to scattered showers and thunderstorms develops mainly
east of Tucson during the afternoon hours by Friday into
Saturday. Any lightning would enhance fire initiation concerns and
gusty outflows may also occur. Winds will generally be light and
terrain driven this week becoming stronger this weekend.

&&

.PREV DISCUSSION...Clear skies or just a few cirriform clouds
across southeast Arizona early this morning as per IR satellite
imagery and surface observations. Upper ridge axis was essentially
adjacent the West Coast. Meanwhile, a deep upper trough was well
west of the Pacific Northwest coast, and a broad area of low
pressure aloft was well west of the Baja California spur, and
centered generally near 22N/132W or such.

Sunny skies or just a few cirriform clouds for much of the area
today. A few cumuloform clouds may also occur later today east of
Tucson. The 23/00Z deterministic GFS/ECMWF were quite similar with
depicting a gradual increase in moisture mostly above 500 mb to
occur by Tuesday afternoon and especially Tuesday night into early
Wednesday. The aforementioned upper low is progged to gradually move
northeastward and transition toward a negatively-tilted open wave
while moving northeastward across this forecast area Wednesday. At
any rate, precip-free conditions will continue through Tuesday and
likely for much of the area into Wednesday.

The caveat for Wednesday is that lower-level moisture should
increase particularly near the New Mexico state line as a cold
front moves southward into southern New Mexico. Based on the various
23/00Z models, have opted for precip-free conditions across the
White Mountains Wednesday. At this time appears that any
thunderstorms that may develop Wednesday afternoon should remain
just north and east of AZZ510 (White Mountains of Graham and
Greenlee Counties).

The upper ridge on Thursday is progged to amplify with the ridge
axis extending from southern Arizona northward into the Pacific
Northwest. Meanwhile, a gradual increase in lower and mid-level
moisture is also depicted via the 23/00Z solutions to occur in an
axis from northern Chihuahua Mexico northwestward toward the central
portions of the Mogollon Rim. A slight chance of showers and
thunderstorms exists Thursday afternoon and night across the White
Mountains, and further south into the Chiricahua Mountains of
eastern Cochise County.

Have noted that the 23/00Z GFS/ECMWF and their respective MOS
guidance have increased precip potential Friday into Saturday from
Tucson eastward versus their respective solutions/guidance from 24
hours or longer ago. The GFS generally depicts the potential for
higher rainfall amounts while the ECMWF depicts somewhat higher
PoPs.

Have some reservation toward this scenario in that particularly the
GFS seems to historically shunt lower level moisture too far west
into this forecast area this time of season. However, given model
trends as well as neighboring WFO`s gridded data fields, forecast
confidence is sufficiently high to state that isolated to scattered
showers/thunderstorms should occur Friday into Saturday mainly east
of Tucson.

The best period for any thunderstorms is during the afternoon/early
evening hours. The high-based nature of expected thunderstorms would
have the potential to produce gusty outflows. As noted in the Fire
Weather section of this product, there is an enhanced potential for
fire initiation due to lightning. Thereafter, the GFS/ECMWF depict
increased southwesterly flow aloft next Sunday as an upper trough
deepens over the Great Basin. This flow regime should shunt deeper
moisture well northeast of the area. Thus, precip-free conditions
are forecast areawide next Sunday.

Expect only very minor daily high temperature changes this afternoon
through Saturday. High temps during this period will generally range
about 6-10 degrees above normal depending upon location. A degree or
two of cooling is on tap next Sunday but high temps to remain
several degrees above normal.

&&

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

&&

$$

Meyer/Francis

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