Scientific Forecaster Discussion
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Area forecast discussion...correction
National Weather Service Phoenix Arizona
1015 PM MST Wednesday Apr 16 2014
dry westerly flow aloft will persist through much of the work week
with high temperatures warming into the 90s over the warmer deserts.
A weak upper level low approaching from the east Pacific will spread
increasing moisture and clouds into the area Friday into the
early weekend...leading to a slight chance of showers across south
central Arizona and the possibility for slightly cooler daytime
temperatures. The cool down will be short lived...as high pressure
quickly returns through the late weekend.
clear skies can be expected the rest of the evening behind a
departing weak weather disturbance. Anticipate some increasing
cirrus Thursday ahead of an upper low that is within the southern
branch of the westerlies. Latest models show the low moving into
our forecast area early Friday and exiting Saturday. They also show
that the saturated/near saturated layers will be above 700 mb which
will make it harder for precipitation to reach the ground in measurable
amounts. However...there looks to be a batch of some upper level
diffluence Friday morning quickly followed by a vorticity lobe...though
the divergence q values look to be modest. Model quantitative precipitation forecast is generally not very
optimistic so any rainfall we might manage to get will not add up to
much. A concern though is that precipitation falling into a warm and
dry subcloud layer could generate gusty winds. Latest GFS indicates
that slight chance probability of precipitation might need to be expanded westward.
Otherwise...low 90s still look good for Thursday.
Previous discussion issued 220 PM...
early afternoon satellite imagery revealing a band of clouds moving
southeast through central and northern Arizona...from a shortwave feature
that is sweeping through The Four Corners area this afternoon.
Temperatures are running a handful of degrees above Tuesday...so
this shortwave should have nearly no impact to our forecast aside
from some breezes this evening. Very similar day on Thursday
expected...above normal temperatures with mostly clear skies
Thursday through Saturday...an elongated Pacific trough is
forecast to move onto the West Coast with the low circulation
centered over Southern California. Precipitable water values are
much more impressive with this system as compared to a similar setup
a few weeks ago...precipitable water values upwards of 0.5 to 0.75 inch. Comparing
previous operational model runs has shown a slower progression and a
more negatively tilted trough center forecast for Friday into
Saturday. Stronger upward vertical motion through a deep column of
moisture spreading from west to east will increase shower chances
Friday afternoon through Saturday. A broad area of strong diffluence
in 200-300 mb layer associated with the low will be the upward
movement needed for shower and possible thunderstorm initiation.
Cooler temperatures aloft with the cold core of the system...500 mb
temperatures cooling to -16c to -18c...will set up an inverted v-like upper
air sounding patter...which translates to stronger downburst wind
probabilities and increased instability. Minor adjustments made to
the pop grids and little to no change in temperature forecast was
made. While chances for showers look much better with this system
significant rainfall is not expected. Dry sub-cloud layers from high
based storms may create gusty winds and strong outflows. Chances for
blowing dust may be possible and something to consider with
following forecast packages.
medium range operational and ensemble model forecasts are in decent
agreement for the early part of next week's forecast compared to
their performance just 24 hours ago. High pressure from over the
weekend will transition eastward beginning Monday...as a large and
potent Pacific storm moves into the West Coast. In general...lower
storm heights and the potential for the upper jet as far south as
Arizona could bring significant cooling by the early to middle part
of next week (potentially into the low to middle 80s attm). The bulk of
the best wave energy looks remains to our north...so only blended a
few percent of climatology probability of precipitation into the extended grids for now.
south-central Arizona including kphx...kiwa...and ksdl...
weak weather disturbance passing into New Mexico tonight. Surface
winds will be light overnight. However...models depict west winds in
the low levels above the surface increasing to 20-25 kts around 2000
feet mean sea level between 06-07z and dissipating by 09z. The increase in west
winds probably not quite enough to insert low level wind shear in the Phoenix area
tafs but the potential should be noted. Otherwise...surface winds
become southwest and west in the afternoon though gusts should
remain well below 20 kts. Some increasing cirrus Thursday afternoon
ahead of an approaching upper low.
Southeast California and southwest Arizona including kipl and kblh...
southwest and west surface winds will weaken overnight though a
southerly component will prevail in the winds above the surface.
South and southeast surface winds will develop by midday Thursday
with gusts to 20 knots. Cirrus will be on the increase toward
morning ahead of an approaching upper low.
Aviation discussion not updated for amended tafs.
Saturday through Wednesday...
dry westerly flow aloft with warmer afternoon temperatures are
expected. However by Wednesday a strong Pacific storm is forecast to
move into the western states...resulting in very strong southwest
winds and a chance of showers. Otherwise...minimum humidities Sat
through Tuesday should remain in the 10 to 15 percent range...increasing
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