Forecast-Discussion

000
FXUS65 KPSR 201156
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
456 AM MST Sat Oct 20 2018

.UPDATE...Updated aviation discussion.

&&

.SYNOPSIS...
Warm temperatures with readings slightly above normal will be seen
today, but more unsettled weather is on the way later today
through early next week. Mainly slight chances for showers will
begin this evening favoring higher terrain locations, especially
across central Arizona. There will be a slight cool down starting
Sunday with additional rain chances from Phoenix eastward. Rain
chances shift mainly east of Phoenix for Monday into Tuesday.
Drier conditions will move into the region by the middle of next
week with temperatures remaining near seasonal normals.

&&

.DISCUSSION...
A weak closed upper level low is currently centered just off the
southern California coast with southerly flow increasing across
the Desert Southwest. Southeasterly flow below 700mb is allowing
for strong moisture advection into southeast and central Arizona
this morning. Latest IR imagery and metar obs show evidence of
this moisture advection with clouds blossoming across south-
central Arizona. The upper low to the west will provide little
support for any showers today and forecast soundings show a
rather warm and dry layer just above 700mb. Breezy easterly winds
early this morning will become gusty after inversion break with
gusts to around 25 mph across much of eastern and central Arizona.
Models have backed off on shower chances this afternoon across
the lower deserts, so rain chances have been removed for all areas
except for the high terrain northeast of Phoenix. The additional
warming aloft and the breezy winds today will give a boost to
daytime highs as much of the lower deserts approach 90 degrees.

Continued moisture advection along a southeast to northwest
corridor oriented through Tucson and to just west of Phoenix
should eventually allow for some isolated showers and possibly a
few thunderstorms this evening. PoPs have been lowered a bit this
evening with best chances residing in the moisture advection
corridor west of Phoenix. Much of the shower activity should be
upslope induced as dynamics aloft will continue to be weak. The
threat for any stronger thunderstorms still seems fairly low, but
we can`t rule out the a rogue storm producing small hail and sub-
severe gusty winds. As rain chances for Saturday look less then
previous runs, models are now shifting better chance rain chances
over south-central Arizona for Sunday morning. This is likely due
to a northeastward shift in the upper level low position and with
better jet dynamics coming into play.

Mid-level winds will also begin to shift more out of the south
and even out of the southwest across southwest Arizona starting
Sunday morning. This will bring drier air into south-central
Arizona starting Sunday afternoon while shifting the bulk of the
shower or thunderstorm activity mainly east of Phoenix. The upper
low will also become temporarily more stationary later Sunday
while a stronger jet enters the base of the trough. This will keep
rain chances focused mainly east of Phoenix for Monday into
Tuesday with best chances early Monday night due to a brief
increase in dynamics aloft. Temperatures Sunday and Monday will
edge downward a bit from today, or closer to seasonal normals.

Finally the upper level low is forecast to open up and shift to
the northeast of the region later on Tuesday, but broad scale
troughing should still remain over the region through Wednesday.
This will keep temperatures stable before a gradual warming trend
commences starting Thursday. Models are still showing a strong
ridge eventually shifting eastward off the Pacific into the
Southwestern U.S. as early as Friday, but there are some model
timing differences. Either way it seems likely we will see more
significant drying and temperatures a few degrees above normals
going into next weekend.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:

Low-level wind shear will remain a concern through late morning as
strong southeasterly flow near 30 kt persists just above the
surface. Mixing by late morning should result in decoupling, with
winds becoming gusty into the afternoon. Gusts to 25 kt will be
possible, with direction remaining generally out of the east
throughout the period.

Later this evening, there is still potential for isolated showers
and thunderstorms to develop across the Phoenix area, but this
remains a low confidence scenario. Activity will remain very
isolated and the probability of any storm directly affecting
terminals is low. An accompanying lowering of cigs to near 8
thousand ft is expected if thunderstorms can develop. Occasional
showers are expected to persist overnight into early Sunday
morning. Low- level wind shear will also become a concern once
again overnight as a nocturnal southeasterly jet redevelops, as 30
kt of flow becomes established above 2000 ft by 08Z.

Southeast California/Southwest Arizona including KIPL and KBLH:

No significant aviation issues next 24 hours. Expect winds to favor
the north to northeast at KBLH with speeds mostly below 15kt. Winds
at KIPL to favor the west through late morning, then the
north/northeast and then back to west by Sunday evening. Expect some
mid level clouds to move into the area by Saturday morning with most
bases 8-12kt feet. CIGS are possible but most likely clouds stay in
the FEW-SCT ranges.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Monday through Friday:
Low pressure over the region will keep wetting rain chances
elevated during the Mon-Tue timeframe across south-central
Arizona. The best chances for rain continue to be over the high
terrain north and east of Phoenix. There is also a potential for
isolated thunderstorms as well on Monday. Dry conditions and
slightly lower humidities should return during the Wed-Fri period
as winds aloft become more westerly. Winds should remain at 15-
20mph or less area-wide. Minimum humidities in the 25-50 percent
range Mon (lowest over SE CA) will fall into the 20-40 percent
range (lowest over SE CA) by Thursday.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

DISCUSSION...Kuhlman
AVIATION...Rogers/CB
FIRE WEATHER...Rogers/Percha/Hernandez

NWS PSR Office Area Forecast Discussion