Area forecast discussion
National Weather Service Phoenix Arizona
950 am MST Thursday may 23 2013
Synopsis...
a low pressure center over the Pacific northwest is expected to keep
the region under dry southwesterly flow aloft...with breezy to windy
conditions at times the rest of this week and into early next week.
Anticipate some modest cooling today with temperatures remaining
near seasonal normals through the weekend. Additional cooling is
likely next week with highs across the lower deserts in the upper
80s to middle 90s.
&&
Discussion...
the upper low centered over the Pacific northwest continues to have
vorticity lobes rotating around it. The next one is rounding the bottom
of the trough that the low resides in with the axis of it near the
California coast this morning. The bulk of it will pass to the north
of Arizona. Water vapor imagery indicates a small perturbation
moving into Arizona ahead of the next vorticity lobe which may help
enhance breeziness temporary today in portions of southern Arizona
but overall will have little noticeable impact. 12z plot data shows
winds at 850 mb were lighter than yesterday but at 700 mb they were
stronger. Net effect should be a little less wind than
yesterday...more noticeably over southeast California. Anticipate
high temperatures today to be a little cooler than yesterday...more
noticeably over southeast California. Made some adjustments to dew
point/relative humidity grids for today but otherwise no changes at this time.
&&
Previous discussion issued 258 am...
the main weather feature remains far removed from the Desert
Southwest...but will have some minor impacts over the coming days.
Breezy to windy daytime conditions remain to be the biggest impact
with some locations yesterday gusting to near 40 miles per hour. Pressure
gradients today will still be on the higher side...but not quite
like yesterday and Friday will be lesser than today. Strongest winds
today are expected to be across southern Gila County with afternoon
gusts approaching 35 miles per hour. The dry airmass in place will persist into
the weekend...but an increase in middle and upper level moisture by
Saturday night should lead to increasing high cirrus. Temperatures
over the next several days will change very little...but will be 4-7
degrees off yesterday/S highs due to the slight cooling influence of
the deep closed low over Pacific northwest. The blocking upper level
pattern with the Pacific northwest low and the high amplitude ridge
over the plains into southern Canada will start to break down by the
weekend. However...reinforcing shortwave impulses dropping down from
the northern Pacific will keep a large scale troughing pattern
across the western United States for the foreseeable future. This
will keep the Desert Southwest under a modestly warm southwesterly
flow through the weekend...with temperatures remaining around or
slightly below seasonal normals.
Uncertainty increases after this weekend with previous model runs
differing in the progression of a stronger northern Pacific system.
Latest model guidance is seems to be in better agreement...but
confidence is far from high. A strong upper jet maximum is forecast to
approach the West Coast next Monday...but weakening as it does. Even
with the depicted weakening...both the GFS and European show a
developing 500mb low diving southeastward into the Desert Southwest
Monday night into Tuesday. Both models generally agree with the
intensity of the system with slight timing differences. Based on the
overall good agreement now...feel that a cooler first half of next
week should occur. Have updated temperatures for Tuesday and
Wednesday next week...lowering highs anywhere from 3 to 10 degrees.
A decent amount of middle and high level moisture should accompany the
system...but for now only expecting a couple days of partly to
mostly cloudy skies with chances for any precipitation less than 10
percent.
&&
Aviation...
south-central Arizona...including kphx...kiwa...ksdl...southeast
California/southwest Arizona...including kipl and kblh...
Dry southwest flow aloft will keep skies clear to mostly clear
through the taf period. Gusty winds are expected to redevelop at the
terminals this afternoon...but with speeds remaining lighter than
what we saw yesterday. Winds will then diminish later this
evening...and revert back to the usual diurnal trends in direction
at the phx area terminals.
Aviation discussion not updated for amended tafs.
&&
Fire weather...
Saturday through Wednesday...
low pressure is expected to remain along the West Coast from
Saturday Onward into Monday...then move inland on Tuesday and
Wednesday...keeping Arizona and southeast California under a dry
southwest to west flow aloft through the period. Breezy to windy
conditions will develop during the afternoon and evening hours each
day...with peak wind gusts in excess of 25 miles per hour possible. Single
digit minimum humidities will continue through Monday...with
slightly higher values on Tuesday and Wednesday. Overnight recovery
will be poor over south-central Arizona and poor to fair over
southwest Arizona and southeast California. High temperatures will
remain near or a bit below seasonal normals from Saturday Onward
into Monday...with somewhat cooler temperatures possible on Tuesday
and Wednesday as the low pressure moves inland into the Desert
Southwest.
&&
Psr watches/warnings/advisories...
Arizona...none.
California...none.
&&
$$
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Discussion...aj
previous discussion...Kuhlman
aviation...percha
fire weather...percha
