Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 191005

Area Forecast Discussion
National Weather Service Phoenix AZ
305 AM MST Wed Sep 19 2018

An upper level disturbance will move through the region today with
relatively deep tropical moisture drawn northward into Arizona.
This will result in scattered to widespread showers and
thunderstorms across much of southern, and especially eastern
Arizona starting this afternoon and possibly lingering into the
early overnight hours. Mostly cloudy skies today will also keep
temperatures below seasonal normals across southern Arizona.
Slight rain chances will persist into Thursday before much drier
conditions and warmer temperatures return for this weekend and
early next week.


Dramatic moisture advection is currently underway across much of
Arizona. This is quite evident on water vapor imagery as a plume
of moisture moved into southern Arizona by early evening and now
has pushed northward through the entire state. This moisture
advection is the result of increased southerly flow ahead of a
Pacific trough moving through California and Nevada and a
weakening inverted trough moving northward over the Baja peninsula
bringing moisture northward out of Mexico. Current GOES-16 Total PWAT
imagery shows 1.60" over southern Arizona with upwards of 2"
south of the border. This deeper moisture will continue to advect
northward today, but it will also lead to quite a bit of clouds
and much cooler temperatures, especially across Arizona.

The increased moisture has temporarily increased MUCAPE values
over the area to over 1000 J/kg, but significant CIN is keeping
any showers or thunderstorms from developing. Warming aloft will
eat away at some of the available CAPE through the day along with
the expected extensive cloud cover. At this point it seems most
of the activity today will hold off until this afternoon, but we
could see a few isolated showers and thunderstorms form later this
morning across La Paz County and southeast and east of Phoenix.

A strong north to south oriented upper level jet max centered over
the Great Basin will gradually shift eastward throughout the day
providing broad upper level divergence to much of Arizona with
the strongest divergence focused across southeast Arizona. The
placement of this divergence aloft over southeast Arizona in
combination with a northeast extension of the weakening inverted
trough over northern Mexico should give rise to numerous showers
and thunderstorms this afternoon and evening. This widespread
activity should remain mostly southeast and east of the greater
Phoenix area, but may affect Pinal and Gila County. Our Flash
Flood Watch for this area still looks okay, but the likelihood of
extensive cloud cover and struggles with lowering instability and
shear may keep activity more subdued. The rest of southern
Arizona, which is outside of the stronger upper level divergence,
will likely see mostly isolated to scattered showers and a few

Shower and thunderstorm activity will begin to diminish by late
evening, but we may see some lingering over eastern Arizona
through Thursday morning as moisture will remain plentiful and
upper level divergence will still be present. Considerable drying
will begin above 700mb Thursday morning once the Pacific trough
exits well to the northeast. This drying will lead to much better
insolation on Thursday while enough low level moisture should
still be present for some isolated high terrain showers and
thunderstorms Thursday afternoon.

The drier air from the north will shut off rain chances by
Thursday night while a high pressure ridge gradually rebuilds
across northern Mexico. The Desert Southwest will remain under the
influence of this ridge through at least early next week bringing
above normal temperatures beginning Friday. A deep Pacific low off
the California coast will help to enhance the ridge over our area
while also providing several days of blocking flow. Forecast highs
have been raised some this weekend and early next week with lower
desert highs topping out between 100-105 most days.


South-Central Arizona including KPHX, KIWA, and KSDL:
There are no major aviation weather concerns for tonight through
the morning push hours. During this time, winds will be fairly
light and should follow typical diurnal trends. However, showers
and isolated thunderstorms should develop around midday Wednesday
and will impact the terminal area. Some impacts include gusty
winds, abrupt wind changes due to outflows, lower CIGS and heavy
rains. Isolated shower and storm activity may carry through the
afternoon and well into the evening.

Southeast California/Southwest Arizona including KIPL and KBLH:
There are no major aviation weather concerns for tonight through
the morning hours. However, there is a slight chance for isolated
thunderstorms Wednesday afternoon, particularly near KBLH and the
Colorado River area. Any storms that develop will have the
potential for producing strong outflow winds and heavy rains.

Aviation Discussion not updated for amended TAFs.


Friday through Tuesday:
Slightly below normal temperatures will linger into Friday before
rebounding over the weekend into a near normal range. Showers and
storms will likely have moved east of the districts with a drying
trend through early next week. Minimum humidity levels in a 15-30%
range late this week will fall more solidly into the teens next
week. Overnight recovery will vary widely from fair to excellent.
Winds will remain mostly light and quite typical for late
summer/early autumn.


Spotters reports may be needed later this week in Arizona.


AZ...Flash Flood Watch from 11 AM MST this morning through this
     evening for AZZ548>555-558>563.



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