Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 151000

Area Forecast Discussion
National Weather Service Tucson AZ
300 AM MST Sat Jul 15 2017

.SYNOPSIS...Scattered mainly afternoon and evening thunderstorms
will prevail into the upcoming week. The stronger thunderstorms will
produce locally heavy rainfall and gusty winds at times. Daytime
temperatures next week will generally be at least a few degrees
below normal.


.DISCUSSION...Isolated showers producing sprinkles or just a few
hundredths of an inch of rainfall were continuing at this time
northwest of Tucson. Otherwise, IR satellite imagery depicted
warming cloud tops associated with a decaying MCS over western
Sonora Mexico. Mostly cloudy skies prevailed across much of
southeast Arizona early this morning, although mostly clear skies
were noted east of Tucson and especially near the New Mexico border.
The isolated showers should dissipate by around 8 a.m. or so.

A broad ridge of high pressure remains anchored over the western and
central CONUS, with a 597 dm high analyzed via the 15/00Z upper
plots to be near Las Vegas Nevada. The upper air plots and the
15/00Z KTWC sounding depicted a generally northeasterly flow regime
in the 700-300 mb layer. Expect very little change in this flow
pattern thru Sunday.

Thus, showers/tstms should develop initially by late morning or
early this afternoon across the White Mountains. If the 15/00Z Univ
of AZ WRF-NAM and WRF-GFS are reality, other showers/tstms will then
develop closer to the Tucson metro area later this afternoon. As has
been the case the past couple of afternoons/evenings, this flow
regime will promote southwestward storm movements into the western
deserts this evening. Additional showers/tstms are depicted to move
from southwest New Mexico into southern Graham/Cochise Counties this
evening. The bulk of showers/tstms will end later tonight, and a
similar shower/tstm scenario exists Sunday into Sunday night.

Thereafter, the deterministic GFS/ECMWF as well as the high-res
models suggest that the mid-level steering flow will veer to more-
easterly Monday, then become southeasterly Tuesday and continue into
at least Wednesday. The 15/00Z GFS/ECMWF remained similar versus
their respective previous solutions at hinting at the potential
development of a rather large MCS to be over this forecast area
Monday night into Tuesday morning. The 15/00Z NAM12 suggests this
MCS scenario may actually occur Sunday night into Monday morning.
Have noted that the Univ of AZ WRF-NAM and WRF-GFS are not as
encouraging about this possibility. At any rate, the official
forecast continues with scattered mainly afternoon and evening
thunderstorms Monday into Wednesday.

During the latter stages of this forecast period, or next Thursday
into Friday, the mid-level flow regimes differ between the GFS/
ECMWF deterministic solutions. The GFS maintained a southeasterly
flow regime, but the ECMWF depicted a nly/nwly 700-300 mb flow. The
ECMWF depicted a weak upper low to become centered over south-
central New Mexico by next Friday. At any rate, both solutions favor
the best chances for rainfall to occur from Tucson eastward to the
New Mexico border with lesser precip chances west of Tucson. Thus,
expect a continuation of scattered mainly afternoon/evening showers/
tstms Thursday into next Friday from Tucson eastward, and isolated
showers/tstms west of Tucson.

High temperatures this afternoon will be quite close to seasonal
normals, then below normal daytime temperatures will prevail
especially Monday into Thursday.


.AVIATION...Valid thru 16/12Z.
Isolated -SHRA northwest-to-west of KTUS dissipating by 15Z this
morning. Scattered -TSRA/-SHRA developing around 19Z today
especially northeast of KSAD, then scattered -TSRA/-SHRA spreading
southwestward across the area into this evening. Brief gusts near 45
kts will occur with the strongest TSRA. The bulk of -TSRA/-SHRA will
end by daybreak Sunday. Otherwise, VFR conditions with surface wind
variable in direction mainly less than 12 kts. Aviation discussion
not updated for TAF amendments.


.FIRE WEATHER...Scattered mainly afternoon and evening thunderstorms
will prevail into the upcoming week. The stronger thunderstorms will
produce locally heavy rainfall and gusty winds. Otherwise, 20-foot
winds will generally be terrain driven at less than 15 mph. Daytime
temperatures will be at least a few degrees below seasonal normals
during the upcoming week.





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