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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

FXUS65 KPSR 170021

Area Forecast Discussion
National Weather Service Phoenix AZ
521 PM MST Mon Sep 16 2019

Updated 00Z aviation discussion.


A low pressure system passing mainly to the north of the region
today will continue to allow for scattered showers and 
thunderstorms to continue across portions of south-central 
Arizona into this evening. Drying conditions during the middle 
part of the week will bring sunny skies and near normal 
temperatures likely through the upcoming weekend.


Current radar shows scattered coverage of showers and 
thunderstorms across the state this afternoon. Strong to severe 
thunderstorms were currently ongoing to the south near the Casa 
Grande area at this time. This activity will continue over the 
next few hours with the best chances remaining just outside of the
Phoenix area. Dewpoints at this hour are in the upper 50s to low 
60s range across much of south-central and southern Arizona. SPC 
Mesoanalysis shows MLCAPE values upwards of 2000 J/Kg across parts
of south-central Arizona with higher values residing in the 
southern part of the state. The main concerns with these storms 
will be gusty winds along with isolated areas of dense blowing 
dust. Isolated incidences of large hail and localized flooding 
will also be possible with any of these stronger storms.

A weak jet streak ahead of an approaching Pacific trough located 
over the northwestern U.S is aiding in providing upper air ascent 
across much of the state today, which is helping to initiate the 
convection this afternoon. Upper-air analysis showed the trough axis 
located across the western fringes of the continental U.S. This jet 
streak will continue to aid in providing upper air ascent to help 
kick off isolated to scattered showers and thunderstorms across 
parts of the state this afternoon into the evening. Most of this 
convection will occur south of Phoenix with isolated showers and 
thunderstorms for the metro. Moderate instability this afternoon
will help to overcome a dry, warm mid layer aloft. CAMs indicate
additional activity developing in Yavapai County and skirting
across northern portions of the metro going into this evening.
Showers and thunderstorms taper off later this evening with only a
few light lingering showers possible.

The main pv anomaly within Pacific trough is forecast to move 
into Wyoming by late Tuesday morning with the trailing trough axis
washing out across northern Arizona. The Desert Southwest will 
continue to be under broad cyclonic flow on Tuesday, but the 
warm/dry layer aloft looks to strengthen and daytime mixing 
Tuesday afternoon should mix out much of the decent boundary layer
moisture. There may be some isolated showers and a few 
thunderstorms across the higher terrain in eastern Arizona Tuesday
afternoon, but the lower deserts will be dry and sunny.

The flow pattern aloft will change very little into Wednesday and
Thursday, but another Pacific shortwave trough is seen entering 
the Great Basin on Thursday. The airmass will still be fairly dry 
during this time with only slight chances of showers and storms 
seen over the eastern Arizona high terrain on Thursday. The 
tighter height packing will result in increased winds both days 
with afternoon gusts up to 25 mph. Once this shortwave exits to 
the northeast on Friday, significantly drier air is seen moving 
through southern Arizona as PWATs drop to below 0.5" in most 
areas. This should drop daytime surface dew points into the 20s 
and 30s while providing excellent nocturnal cooling conditions. 
This should allow for overnight lows down into the 60s across the 
rural deserts and even near 70 in Phoenix by Friday morning. High 
temperatures this week will remain fairly stable with lower 
desert readings 100-102 degrees through Wednesday followed by the
middle to upper 90s for Thursday and Friday.


.AVIATION...Updated at 0015Z. 

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: 

A southerly outflow boundary that has caused gusts up to 20 knots
at KPHX and KIWA will continue to move north into KSDL and KDVT
over the next hour. Although there is a relatively strong midlevel
stable layer aloft that should inhibit convection, recent clearing
may allow for surface-based instability to increase enough to
overcome this and allow for a few convective cells to develop as 
depicted by the 22Z HRRR. Based on this, we have included VCSH and
7-8 kft ceilings for all sites except KIWA from 0030-0130Z as the
outflow boundary attempts to develop cells over the Phoenix 
Mountains. However, confidence on this is low due to uncertainties
about instability that the 00Z balloon launch may help resolve.

The forecast beyond the next few hours is dependent on whether 
convection develops or not by 01-02Z. If no convection develops, 
winds should gradually back to the S-SE and eventually E-SE 
overnight. If convection does develop, more northwesterly to 
westerly headings will be more likely. Cells that are developing 
east of Gila Bend may push a S-SW outflow into the Phoenix sites 
that may trigger convection after 02Z. In addition, cells that 
are about 50 miles west of Phoenix may push N-NW outflow into 
Phoenix that could trigger convection after 04Z. Considering that 
the our instability and evolution of convection over next couple 
hours will dictate what occurs later this evening, we will 
probably issue an updated forecast discussion in a couple of 
hours. Regardless, no aviation impacts are expected tomorrow with 
a typical diurnal westerly shift around 19-20Z. 

Southeast California/Southwest Arizona including KIPL and KBLH: 

Main aviation weather concern will be breezy conditions as the 
pressure gradient tightens ahead of strong area of low pressure 
across the Great Basin. Westerly gusts up to 25 knots will be 
possible at KIPL and KBLH through late evening, before subsiding 
during the overnight hours. Winds should generally remain westerly
through the TAF period with gusts up to 20 knots possible on
Tuesday afternoon.

Aviation Discussion not updated for amended TAFs.


Wednesday through Sunday: 
A series of low pressure systems will move through the Great Basin
this week, resulting in a drying trend. Consequently, chances for
wetting rains will remain below 10 percent each day, even across 
the higher terrain east of Phoenix. Near normal temperatures are 
also anticipated while afternoon humidity levels will range from 
10 to 25%. Occasional breeziness will also be possible, 
particularly Thursday afternoon.


Spotters should follow standard reporting procedures.