Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 232340 AAB
AFDPSR

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
440 PM MST Tue Apr 23 2019

.UPDATE...Updated aviation discussion.

.SYNOPSIS...
A low pressure system has increased shower and storm chances in
south-central Arizona through this evening. As high pressure builds
over the region in the wake of the low, a warming trend will begin,
starting over southeast California and southwest Arizona today and
then in south-central Arizona tomorrow. Thursday through Friday,
highs in the lower deserts will be approaching or exceeding 100
degrees. Anticipate cooler temperatures by late in the weekend and
early next week.

&&

.DISCUSSION...
The upper level pattern shows a pronounced upper level closed low
pressure system centered over NW MX and the SSE AZ border with a WSW-
ENE oriented ridge axis from N CA across the Intermountain West and
Central Rockies. IR satellite and WV imagery is showing an increase
in cloud cover across the region as a pair of vort max disturbances
push southwest across the area along the northern periphery of the
low. As a result, the chances for showers and storms have increased
this afternoon and early evening from Phoenix and Casa Grande
eastward.

High-res models and HREF members are fairly bullish on developing
scattered convective showers, storms and outflow winds for good
portions of S Gila, Pinal and E Maricopa counties beginning early to
mid afternoon with the activity lingering into the evening and
tapering off by/after 01-03Z. Surface heating with warm daytime
temperatures combined with cold air aloft, near -21C at H5, will
contribute to increasing instability this afternoon. HREF mean CAPE
by mid afternoon is forecast in the low 100s j/kg with max CAPE
values forecast at 250-750 j/kg. Moisture also continues to increase
this afternoon with SPC mesoscale values already above 0.5 in. as
they head towards values near 0.75 in.

The main impacts from today`s storms will be gusty outflow and
convective downdraft winds and dry lightning which will potentially
impact both aviation and fire weather concerns. This is supported by
relatively dry inverted V boundary layer profile combined with
forecast DCAPE values near 1400 j/kg by 00Z. Sustained easterly
component outflow boundary winds near 15-20kt will likely experience
local convective gusts to 30-35kt closer to the storms. Other
notable impacts will include abrupt windshifts and erratic winds,
especially near the storms. Due to the gusty winds, there is also a
slight chance of locally lofted dust. Rainfall amounts with today`s
storms are also forecast to be isolated to spotty and mostly modest.

Also for today a warm up will begin across SE CA and SW AZ with
temperatures climbing into the mid 90s in those areas. For Wednesday
through Thursday the warming trend spreads east as the low pressure
system exits to the east and high pressure expands and builds over
AZ and the Western US. By late Thu. H5 heights reach near 583-584
dam. The associated subsidence and WAA will allow temperatures to
climb into to the upper 90s to low 100s late in the week across the
lower deserts. Localized areas of moderate heat risk will spread
from SE CA on Wednesday east into the lower deserts and Phoenix on
Thursday and Friday. The hottest day in Phoenix is currently
expected on Friday when highs will reach well above normal in the
low 100s.

By late in the weekend and early next week, expect lower
temperatures closer to seasonal norms ahead of and with a compact
Pacific low pressure system that is expected to move through S CA
and into AZ around Monday.

&&

.AVIATION...Updated at 2340 UTC.

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

Upper level low pressure system south of Arizona continues to wrap
mid/upper level moisture westward and into south-central Arizona
late this afternoon. Modest mid level instability resulted in a few
high-based thunderstorms over high terrain east and southeast of
central Phoenix but as of 4 pm the storms had pretty much ended with
trends suggesting storms mostly over for the day. Given the very dry
sub-cloud layer, with high amounts of DCAPE, potential still exists
for gusty outflow winds over 30kt, but feel the chances of winds
that strong are slim for the duration of the evening and have been
removed from the TAF package. An isolated light shower or virga
might still impact the TAF sites and VCSH will remain in the TAFs
thru about 03z.

For the rest of the evening, north/northeast winds 10-20kt will
transition to mainly east winds less than 10kt by around 04z with
east wind persisting into the morning on Wednesday. BKN-OVC decks
aoa 14k feet will persist much of the evening, with SCT decks in the
9-10k foot range. After 06z clouds will thin as the main low shifts
off further to the east and by 12z Wednesday look for generally
clear skies. Winds will return to the southwest/west after 19z
Wednesday with speeds mostly below 8kt.

Southeast California/Southwest Arizona including KIPL and KBLH:

Weather impacts associated with the passing low should remain well
to the east of the western deserts and no impacts to the TAFs are
expected over the next 24 hours. Winds continue to favor the north
at KBLH, but should fall below 15kt by 02z, and overnight west to
northwest winds less than 10kt are expected. Winds should mostly
favor the west at KIPL next 24 hours with speeds below 12kt. Look
for generally clear skies through the TAF period at both KIPL and
KBLH. No significant aviation impacts expected for at least the next
24 hours.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Friday through Tuesday:

Strong high pressure over the region to bring very warm temperatures
to the region on Friday, with highs on the low deserts rising well
into the upper 90s and lower 100`s. Accordingly, humidities are
expected to decline with minimum readings on the lower deserts
hovering around 10% and overnight recovery declining to fair. Late
in the weekend and early next week, a low pressure system is
expected to bring cooler temperatures. As the system approaches,
anticipate more noticeable breeziness during the latter part of the
week. At this time, critical fire weather thresholds are not
anticipated.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter activation is not expected for the next 7 days.

&&

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

$$

DISCUSSION...Sawtelle
AVIATION...CB
FIRE WEATHER...Percha/Rogers/AJ


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