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Area forecast discussion 
National Weather Service Phoenix Arizona
1250 PM MST Thursday Apr 17 2014

Update...updated fire weather discussion.

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Synopsis...
another warm afternoon expected today before a Pacific storm moves
into the area Friday. The Pacific low will spread increasing moisture
and clouds into the area...supporting slight chances for showers from
the Colorado River east across south central Arizona and into the
higher terrain. Daytime temperatures for Friday and Saturday will
fall to more seasonal values. After the storm and clouds move to the
east on Sunday...temperatures quickly warm back into the 90s for
next week with mostly sunny skies.

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Discussion...
upper level height analysis currently shows a ridge axis moving into
our region from the west and forecast to pass to our east by
midnight tonight. Given the clear skies across Arizona...and
increasing thin clouds across western California ahead of a Pacific
low...we should expect similar highs today compared to yesterday.
A couple degree pullback across southeast California is
likely due to the increasing high clouds this afternoon. These high
clouds are a precursor to the small and compact shortwave trough
that will affect our region Friday and Saturday. Overall...this
Pacific system will be moisture starved below 10k feet...but ample
middle level moisture and sufficient upper dynamics should result in
virga showers and isolated sprinkles across the lower deserts and
light showers across higher terrain areas starting late Friday
morning or early afternoon.

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Previous discussion...
Pacific low pressure will move into so-cal early Friday...spreading
middle and high level moisture from the subtropical Pacific ahead of it.
First area of upper low difluence and 400-200mb Omega move into
western Arizona by Friday afternoon...so slight chance probability of precipitation were
expanded to the lower Colorado River valley. Low end slight chances
(mostly 10 percent) then expand into the south-central deserts for
late Friday and Saturday as the low slowly travels through the area.
Additional vorticity energy on the back of the low prognosticated to sweep
through the area Saturday...providing some additional lift to
persist slight chances before the system exits to the east overnight
Saturday/early Sunday. Mostly cloudy to overcast skies are expected
Friday with a very slight cool down in temperatures...mostly into the
80s for the Lower Desert elevations.

Models have been generally been consistent with the overall moisture
profile for this system...generating nearly saturated/saturated middle-
level sounding profiles with very dry sub-cloud layers and inverted
v traces. While chances for light shower and isolated thunder
activity continue look decent with this system...wetting or even
measurable rains are not expected. The warm and dry sub-cloud layers
lend themselves more towards high-based development with the
potential for gusty winds and outflows. That being said...consistency
in the model quantitative precipitation forecast placement and amounts has varied even in recent
forecast runs. Also considering past under-performances this Spring
with other east Pacific systems moving into the southwest...broad
brush slight chances with near zero quantitative precipitation forecast values from Friday midday
through Saturday midday were adjusted into the pop grids.

Dry westerly flow transitions into the area Sunday...quickly warming
temperatures back into the 90s for early next week. Broad area of
Pacific troughing still prognosticated to sweep into the west late Monday
into Tuesday...although latest model runs vary in their depth and
strength as it moves in from the northeast Pacific. The set-up still
looks promising for a return to cooler (and more seasonal)
temperatures by middle part of next week...but any chances for rain
look pretty remote. Maintained some influence of climatology in the
pop grids given the presence of troughing...but unless the models
expand the southern extent of the system...even those single digit
values may be overdone.

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Aviation...
south-central Arizona including kphx...kiwa...and ksdl...
a gradual increase in middle and high clouds is expected over the next
24 hours ahead of a low pressure system. With the low in our
vicinity...winds will also retain more of a southerly component
through Friday afternoon. However...through 18z Friday...winds will
remain diurnal and generally less than 10 knots. Given the dry
conditions near the surface...there also is the potential for virga
producing localized downburst winds...perhaps as early as Friday
morning but more likely Friday afternoon.

Southeast California and southwest Arizona including kipl and kblh...
southerly winds will be the primary aviation weather impact...with
occasional gusts to around 20 knots this afternoon. Winds will abate
overnight before increasing again Friday afternoon ahead of a low
pressure system.

Aviation discussion not updated for amended tafs.

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Fire weather...
Sunday through Thursday...
westerly flow will bring a warming and drying trend to the region
through Tuesday. Thereafter...a cooling trend is expected as an area
of low pressure moves through the western states. The biggest impact
with this system will be an increase in wind...particularly on
Wednesday. Low rhs during the midweek period will also present an
increased fire danger threat.

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Psr watches/warnings/advisories...

Arizona...none.

California...none.

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Discussion...Kuhlman
previous discussion...Nolte
aviation...Hirsch
fire weather...Hirsch