FXUS65 KTWC 240410

Area Forecast Discussion
National Weather Service Tucson AZ
910 PM 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


.DISCUSSION...The weak ridge over the region is becoming a bit more
dirty with time allowing high level moisture to pass overhead. This
will result in some high cloudiness at times through Tuesday morning
then high clouds will increase during the afternoon hours.  With the
passing clouds and warmer air mass, temperatures are generally 2-7
degrees warmer than 24 hours ago.  High temperatures Tuesday will be
very similar to what we had today.  Beyond that things may get a bit
more interesting and that is described well in the previous
discussion found below.


.AVIATION...Valid through 25/06Z.
SCT-BKN clouds above 20k ft AGL tonight into Tuesday morning
becoming BKN-OVC Tuesday afternoon. SFC winds will remain less than
10 kts and terrain driven through the forecast period. Aviation
discussion not updated for TAF amendments.


.FIRE WEATHER...Well above-average temperatures will continue
through the week. Dry conditions will prevail through mid week
before a slight chance of showers and thunderstorms develops for the
White Mountains Thursday afternoon. Thunderstorm chances then expand
to the mountains surrounding Tucson and areas east Friday afternoon
into Saturday. Any lightning would enhance fire initiation concerns
and gusty outflows may also occur. Otherwise, winds will generally
remain light, with normal afternoon gusts due to daytime heating.
Wind speeds will then increase for the weekend.


.PREV DISCUSSION...The main story is shaping up to be possible
convection in eastern areas later this week into the weekend.

High pressure building in from the west will dominate our weather in
the short-term, accompanied by warmer temperatures and dry
conditions today and tomorrow. Surface dew points have fallen into
the teens as expected this afternoon. GOES total precipitable water
in the .3 to .4 inches range. A weak and shallow 5kt flow from the
northern gulf (CA) is helping to push 30 degree dew points into
western Pima county as we speak, but that won`t get very far this

Another low has split from the northeast Pacific and into a position
well off the coast of Baja. As it dirties up the base of the ridge
and comes through on Wednesday, we should manage just enough mid
level moisture for another round of virga similar to (maybe a little
less than) Sunday. Not much in the way of measurable rainfall though
with any significant moisture southeast and east of our area.

After Wednesday, we`ll continue to see enough mid level moisture
with weak disturbances embedded in the flow to bring a few
thunderstorms to the eastern mountains later in the week. Friday
afternoon and evening still looks particularly interesting with
increasing GFS/ECMWF H7 theta-e trends from central through
northwest Mexico.

Subtle model differences in flow consolidation northwest of our area
is still making Saturday a hard call. A more progressive GFS is
shunting storm chances east of our area by Saturday afternoon, while
the ECMWF is less aggressive in reconsolidating the flow south and
southeast of the Gulf of Alaska and is thus slower. That could mean
a very busy Saturday. It`s worth noting that over the past 24 hours
the ECMWF has doubled down while GEFS plumes are slowly increasing
QPF, PW and surface based CAPE values for Saturday. Considering
opposite model timing biases, splitting the difference often works
well for timing on days 4 and 5 for Pacific features.






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NWS TWC Office Area Forecast Discussion