Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 181733 AAB

Area Forecast Discussion
National Weather Service Phoenix AZ
1034 AM MST Tue May 18 2021

.UPDATE...Updated 18Z Aviation Discussion.


A departing weather disturbance will bring a slight chance of
showers over central and eastern Arizona today. These showers will
result in minimal rainfall, with gusty winds being a more likely
threat. Temperatures will rebound to near normal today and then
above the seasonal average tomorrow. However, another weather
system will arrive Thursday into Friday, resulting in breezy
conditions and cooler weather.


Water vapor imagery this morning shows a closed low centered over
northern New Mexico. Several vorticity maximums are rotating
around this low, including one departing the state into New
Mexico, and a second that will soon enter the state, which is
centered just northwest of Las Vegas. RAP analysis soundings show
a dry and relatively well mixed layer from the surface up through
700 hPa. Near the top of this layer, the airmass is nearly
saturated. A weak stable layer near 600 hPa is present and is
providing some meager convective inhibition. Even so, 50-100 J/kg
of MUCAPE is evident, implying there is at least some convective

Over the next several hours there will be limited opportunity for
convective initiation given the nearest vorticity maximum is only
just now entering the state. Between 12-18Z today the
aforementioned vorticity maximum will likely pass just to the
west of the Phoenix area. CAM runs over the last day have hinted
at the potential for scattered showers as this feature passes
through but are now largely pessimistic, presumably because the
feature is somewhat diffuse and may not be generating much ascent.
To that end, precipitation is not anticipated this morning, but
an isolated shower cannot totally be ruled out.

The better chance of showers or thunderstorms still appears to
come this afternoon from about 20Z through 00Z. During this time
period a vorticity lobe along a trough axis will swing around the
larger closed low. In addition to the forced ascent that would be
expected ahead of the upper level trough axis, moisture
convergence will increase the relative saturation in the ascent
zone. These factors in combination with steep midlevel lapse
rates will create a favorable set up for showers and thunderstorms
to form over the higher terrain north of the Phoenix area. One
other difference compared to yesterday will be the stronger steering
flow. Upper level winds will shift to the north to northeast. At
500 hPa, northeasterly flow at 20-25 kts will develop by this
afternoon. This should be sufficient to advect any storms that
develop southwards towards the lower deserts. With lower levels
staying dry, little in the way of rainfall will occur at the
surface, but isolated lightning strikes and gusty outflow winds
may develop near these showers. The HREF max values (worst case
scenario) suggest wind gusts as high as 40 mph near these
showers could be possible.

Overall, ensemble output suggests around a 10% chance of getting
a storm/lightning holding together into the Valley. The chance
for accumulating rainfall at lower elevations will remain small,
but non-zero. Ensemble output also suggests wind gusts up to 25
mph will be common, but there is a 10-30% chance of getting
outflow winds up to 40 mph into parts of the Phoenix metro (northern
suburbs most susceptible).

As the low pressure system exits into the southern plains
Wednesday, shortwave ridging will temporarily build back into the
SW Conus allowing temperatures to warm back above the seasonal
normal. By Wednesday, NBM temperature probabilities show about a
75% chance of reaching 100 F or higher. Beyond, Wednesday, another
weather system will impact the region. Ensemble guidance is now in
good agreement that a closed low will develop over the Great
Basin. WPC clusters show that there is relatively minimal
variation in the big picture through day 5, with the main
uncertainty relating to how far to the southwest the negative
height anomalies will extend. These differences will affect the
magnitude of the breezy conditions that develop as well as the
degree of cooling. In either case, the Wednesday through Friday
period will likely be the windiest. Temperatures will be likely be
the coolest Saturday and Sunday. Rain remains very unlikely. Ridging
and rapidly warming temperatures are likely to return thereafter.


.AVIATION...Updated at 1734Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Current light westerly winds to increase up to 12-15 kts
this afternoon with gusts up to 25 kts. Gusts should diminish
after sunset, but speeds may remain around 8-12 kts until
midnight. Diurnal easterlies should return by 10-11Z.

Latest high-res model output continue to show the development of
scattered showers/TS over the Higher terrain well north and east of
Phoenix. Northerly steering flow aloft will allow at least some
propagation of these showers/TS towards the PHX area, but drier air
over the lower elevations will likely lead to the dissipation of
most of the showers/TS, with only a slight chance (around 10%) that
any showers/TS will directly impact the terminals. A more likely
outcome is that outflows from this activity will affect the
terminals (especially KVDT/KSDL...but even chances for this are
rather low (20-30%). Thus...not including any TS/wind shifts in the
TAFs for the time being.

Southeast California/Southwest Arizona including KIPL and KBLH:
Diurnal wind trends are favored at both terminals. KIPL will see
variable winds or a light southeast during the day before shifting
west to southwest this evening with speeds in the 10-13 kt range,
with these westerly winds continuing all night and into Wed. KBLH
will favor the south through the period with speeds up to 10-15 kts
and gusts up to 20 kts this afternoon and evening. FEW clouds around
14 kft are expected this morning with FEW cumulus this afternoon
around 10 kft.


Thursday through Monday:
A strong weather disturbance will approach the region during the
latter half of the week pulling temperatures back below normal and
leading to multiple days of strong, gusty winds. The breeziest
conditions will likely occur Thursday and Friday afternoon with
gusts 25-40 mph over ridge tops and exposed areas. With very dry
fuels, an elevated fire danger will materialize and there is some
threat of critical thresholds being met locally. Minimum afternoon
humidity levels will mostly fall to 10-15% on Thursday, gradually
diminishing to 5-10% over the weekend. Overnight recovery will be
poor to fair in a 25-50% range initially, decreasing to 15-35%
over the weekend.


Spotters should follow standard reporting procedures.




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