Excessive Heat in Arizona, August 5 to 6, 2018

Area Forecast Discussion
National Weather Service Tucson AZ
904 AM MST Sun Aug 5 2018

.SYNOPSIS...Expect near record heat with fairly minimal overnight
relief through Monday. Isolated to scattered thunderstorms will also
occur mainly east to south of Tucson. A favorable flow regime with
ample moisture will then provide increasing chances of thunderstorms
by Wednesday through next weekend and into early next week.
Temperatures will also return closer to seasonal averages.

&&

.DISCUSSION...High pressure will dominate with excessive heat and
less thunderstorm coverage to start the week. The 12Z KTWC
sounding showed a modest .1 inch precipitable water increase over
the past 24 hours, but still has unfavorable shear and
thermodynamic parameters (low SB/ML CAPE and plenty of CINH to
overcome). Another day with isolated mainly mountain thunderstorm
coverage this afternoon and evening. Meanwhile, temperatures will
climb to near record levels this afternoon and Monday (for TUS
records are 108 today and 109 Monday). Our current forecast
handles this well; no updates this morning.

Please see the previous discussion below about a strong increase
in thunderstorms and moderating temperatures this week.

&&

.AVIATION...Valid through 06/18Z.
Isolated to scattered -TSRA/-SHRA KTUS vicinity eastward and
southward this afternoon and evening. -TSRA/-SHRA generally
dissipating late tonight and early Monday morning. Otherwise,
scattered to locally broken cloud decks 10k-15k ft AGL. Surface
wind this afternoon nwly 8-15 kts with gusts near 20 kts. Surface
wind variable in direction mainly less than 12 kts at other times.
Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...Expect hotter temperatures with isolated to
scattered thunderstorms mainly from Tucson eastward and southward
through Monday. Thunderstorm coverage and chances will increase by
midweek, then scattered showers and thunderstorms Thursday into next
weekend. Gusty and erratic winds will occur near thunderstorms.
Otherwise, 20-foot winds will be terrain driven and mainly less than
15 mph.

&&

.PREV DISCUSSION...Strong high pressure aloft early this morning
was centered over west central New Mexico with a ridge axis
extending southwestward to just west of northern Baja California.
The 05/00Z KTWC sounding as well as radar depicted storm motions
depict the return of a mid-level easterly flow regime at least
across central/eastern portions of this forecast area. IR
satellite imagery and model derived wind fields suggest a more
southerly mid-level flow across western sections.

Although the 05/00Z GFS/ECMWF and their respective ensembles were
fairly similar to solutions of 24 hours ago with the general
forecast scenario, have noted these solutions were somewhat more
robust with precip potential this afternoon/evening and again Monday
afternoon/evening. Thus, there appears to be a greater potential of
showers/tstms to occur in valley locales this afternoon/evening and
again Monday versus just "near mountains" as was suggested 24 hours
ago.

The main weather impact through Monday, though, should be hotter
than normal daytime temperatures and just relatively minimal
nighttime relief. For this forecast issuance, have continued with an
Excessive Heat Warning from 10 am this morning to 8 pm MST Monday
for the Tucson metro area; Southeast Pinal County; and the Upper
Gila River Valley including Safford. An Excessive Heat Warning also
remains in effect from 10 am to 8 pm MST Monday for western Pima
County; the Tohono O'Odham Nation; and South Central Pinal County.

Relief from excessive heat will begin Tuesday as the upper high
moves northwestward into northern Arizona. However, another notable
change with the 05/00Z models is regarding the position of the next
eastern Pacific tropical system closer to southern Baja California
versus models of 24 hours ago. Assuming this position to be closer
to reality, then the potential for a Gulf of California surge
Tuesday definitely increases. Appears the models and especially the
GFS catches onto the notion of markedly enhanced precip chances
particularly during the Wednesday and Wednesday night periods.

At any rate, expect isolated to scattered showers/tstms area-wide
Tuesday afternoon into Wednesday. Scattered showers/tstms should
then occur across much of the area by Wednesday evening and late
Wednesday night. A similar precip scenario exists at this time for
Thursday and Thursday night. Modifications and refinements to timing
of showers/tstms will likely occur with subsequent forecasts.

Thereafter, the GFS/ECMWF were quite similar with moving the upper
high center northward into the Great Basin Friday into Saturday. A
deep easterly flow regime in combination with ample moisture and the
presence of an inverted upper trough (especially if the ECMWF is
reality) should translate into a potentially active period of
showers/tstms Friday into Saturday.

Although beyond this forecast period, have noted that the ECMWF
eventually develops a fairly expansive area of low pressure aloft
across southern Arizona, northern/central Baja, and Sonora Mexico
next Sunday. Meanwhile, the deterministic GFS depicts an inverted
upper trough over southeast Arizona. If reality, markedly enhanced
precip chances would likely continue late next weekend into early
next week. Given the enhanced precip-chance scenario, daytime temps
are forecast to moderate closer to seasonal normals by next
weekend.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
Excessive Heat Warning until 8 PM MST Monday for AZZ504-506-509.

Excessive Heat Warning from 10 AM to 8 PM MST Monday for AZZ501-
502-505.

&&

Meyer/Francis

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