Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 261541

Area Forecast Discussion
National Weather Service Tucson AZ
841 AM MST Wed Jul 26 2017

.SYNOPSIS...Daily rounds of thunderstorms will continue in parts
of southeast Arizona through Thursday but storm coverage should
become more widespread Friday into this weekend. As storm coverage
increases, high temperatures will cool slightly this weekend.


.DISCUSSION...Morning sounding shows 1.53 of precipitable water,
which is down from 24 hours ago but is more evenly distributed
throughout the column compared to the drier air aloft that was in
place Tuesday morning. 500 mb temperature at -5 deg C, a shade
cooler than Tuesday. Sounding also shows the 30-35 kt jet segment
from 200-250 mb that helped sustain convection in Sonora and into
far southern AZ overnight into early this morning.

No distinct remnant mesoscale circulation detected on satellite
imagery in this area although a couple of weak disturbances near
the Sonora/Chihuahua line are present. New 12z NAM shows the
northernmost and weaker of these disturbances translating over
southeast AZ today, but with no jet support above it. The stronger
disturbance looks like it will remain south of AZ today and the
12z NAM has correspondingly reduced its precip coverage for today.

06z and 12z U of A WRF NAM also showed decreased tstorm coverage
from what the 00z run showed for today with thunderstorm coverage
limited to mountains and locales along the international border
from Nogales eastward.

With models showing the 500 mb temperature warming to between -3
and -4 degs C this afternoon, believe a reduction in rain chances
for this afternoon is in order. With lower rain chances, we should
be more likely to attain the high temperature forecast today so
will leave those as is.


.AVIATION...Valid thru 27/18Z.
Isolated to scattered TSRA/SHRA this afternoon and evening with
MVFR cigs and vsbys, mainly from KTUS/KSAD SE and along the Rim.
Otherwise, VFR conditions with cloud decks generally 5k-10k ft
AGL. SFC wind variable in direction mainly less than 12 kts,
except in the vicinity of TSRA where gusts may exceed 35 kts.
Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Isolated to scattered showers and thunderstorms
are expected through Thursday, mainly east of Tucson. An upswing in
thunderstorm activity will occur Friday through this weekend. Near
normal daytime temperatures are expected through Friday with below
normal readings this weekend. 20-foot winds will remain below 15
mph, with the exception of strong outflow winds in the vicinity of


Early morning water vapor imagery reveals a decaying complex of
storms over Sonora, with quite a bit of cloud cover and moisture
making its way northward into Arizona. Some of this cloud cover is
showing up on radar but nothing has developed in terms of showers or
storms as of 09Z. Blended TPW imagery suggests values close to 1.7-
1.8 inches resides just south of the border. Looks like our period
of drying out is on the cusp of ending.

Exactly how today`s forecast plays out depends if we get any remnant
MCVs or weak inverted troughs from the decaying Sonoran convection.
Past 5 or 6 runs of the HRRR along with UA WRFs suggest a fairly
typical monsoon afternoon with storms developing east of Tuscon
around noon and converging on the city between 3-5 pm. NCAR ensemble
isn`t nearly as enthused about storms making it into Tucson but
agrees that storms are plausible along the AZ/NM border. While I
think it`ll be less active than what we saw last week, today should
see an increase in storm coverage compared to yesterday. PoPs have
been increased slightly on the premise that we`ll see a weak trough
move through the region this afternoon.

Today should see the beginning of the return of deeper moisture
across the area with nearly all deterministic guidance along with
larger scale ensembles pointing to a period of anomalously high PW`s
across the area Thursday through early next week. Hi-res models are
depicting a fairly active day across the area on Thursday, Friday,
and again late Friday night into Saturday morning. Latest blended
00Z guidance has arrived with somewhat higher PoP values than
previous runs and I have trended PoPs upward a bit into the weekend.
By early next week there is are hints that conditions will try and
dry out in the west with storms confined to areas east of Tucson,
although I`m hesitant to buy into much of a drying trend unless we
get a large scale pattern shift w/ southwesterly winds. Since that
doesn`t appear to be in the cards at this point, I`ll hang on to
higher than normal PoPs at all sites into next week.

As for temps, expect highs at or above 100 today in the deserts.
This should be short lived with increased moisture on the way and
highs should only top out in the mid 90s this weekend into next






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