Area Forecast Discussion
Issued by NWS Tucson, AZ
FXUS65 KTWC 290420
Area Forecast Discussion
National Weather Service Tucson AZ
900 PM MST SAT MAY 28 2016
.SYNOPSIS...A weak and slow moving upper level low will maintain
temperatures slightly below average to start the new week. As it
passes overhead late Tuesday it will bring about a slight chance of
afternoon thunderstorms over the White mountains. Then strong high
pressure will build over the region the latter part of the week,
resulting in hotter temperatures with highs of 100+ from Tucson
across the lower deserts Thursday into the weekend.
.DISCUSSION...A warm southwesterly flow ahead of the weak low near
Southern California helped pump afternoon highs back to around
average across Southeast Arizona today. We`ll knock a degree or two
back off these values when the low pushes across the area early in
the new week.
Still on track for a strong ridge later in the new week, however we
might be a little low on our current forecast highs. For example,
both 12Z ECMWF and 18Z GFS 850-700mb thickness forecasts support
afternoon highs in Tucson around 101 on Thursday, 104 or 105 on
Friday, and 105+ Saturday (with the GFS MOS coming in lower due to a
reasonable looking moisture increase from the south). A quick look
at 00z GFS trends does nothing to dispel these notions. Tucson
International Airport will have it`s first 100, and 105 is not out
of the question Friday or Saturday depending on moisture trends.
.AVIATION...VALID THRU 30/06Z.
Expect mainly clear skies through Sunday. Expect normal diurnal
winds less than 10 kts overnight into Sunday morning before SW winds
increase again by 29/19z. Aviation discussion not updated for TAF
.FIRE WEATHER...Expect dry conditions with moderately breezy
southwest winds during the afternoon each day through Monday.
Slightly more mid level moisture is forecast during the middle of
work week with isolated thunderstorms possible over mountains well
NE of Tucson. There could even be a few decent afternoon cloud
buildups farther south. While sustained winds will generally be
lighter midweek, erratic gusts will be possible near buildups and
thunderstorms. A significant warmup is still expected toward the end
of the work week.
.PREV DISCUSSION...An upper trough along the California coast
will close off through this evening then gradually migrate eastward.
It is weak and due to being nearly detached from the westerlies,
will be a very slow mover ending up over central Arizona Monday
night and moving to the east Wednesday. This feature will have
subtle affects on our weather, namely holding temperatures a few
degrees cooler than average and maintaining southwesterly breezes
each of the next few afternoons. As it passes over the area Tuesday,
the atmosphere is unstable enough and just moist enough for a slight
chance of thunderstorms over the White mountains during the
afternoon and evening hours Tuesday and to an even smaller extent
Wednesday afternoon. If any storms develop they will be mostly dry.
Once the low moves off to the east our area will begin to come under
the influence of a fairly strong upper level ridge that built over
the west coast and will shift east Wednesday through Friday. Until
this morning, models have been pretty consistent with this idea
which would allow temperatures to heat up quite a bit. The 12Z GFS
is throwing a wrench in the works by undercutting the ridge with an
upper low with convection developing across the region next weekend.
The ECMWF is much slower and further north with this low which is
what the earlier runs of the GFS was also showing. Looking at the
12Z GFS ensemble members, almost half of them have that low to our
SW so it is not out of the question. At this time will not jump on
this idea but something to keep in mind.
As alluded to earlier, as the ridge builds so will the temperatures.
Thursday into the weekend (assuming the ridge holds) high
temperatures are expected to rise to the warmest levels yet this
year with 100+ likely across the lower deserts. Even at that I kept
the forecast highs on the conservative side, especially when you
look at low level thicknesses which suggest even warmer Friday and
Saturday. For TIA, I kept the 99 for Thursday and went 101 Friday
and Saturday. Locally run ensembles suggest an almost 50/50 chance
of 100 on Thursday at TIA, so a pretty good chance.
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