Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 171028

Area Forecast Discussion
National Weather Service Phoenix AZ
328 AM MST Thu Aug 17 2017

Relatively dry conditions will persist across the Desert Southwest
through Saturday. Moist monsoonal flow will return to Arizona
over the weekend bringing an increase in thunderstorm chances.
Afternoon and evening storms will remain in the forecast through
the middle of next week along with near normal temperatures.


Latest streamline analysis shows a weakening trough across Nevada
with relatively dry southwesterly flow across the Desert
Southwest. Surface dewpoints continue to run well below normal,
resulting in favorable conditions for radiational cooling. After a
relatively cool morning, near normal temperatures are expected
this afternoon under sunny skies. Meanwhile, models indicate the
aforementioned upper trough will continue to shear out to our
north, with no impact across the region.

Height/thicknesses will continue to increase steadily through
Friday, resulting in a subtle warming trend. Near normal
temperatures are expected thereafter as the subtropical ridge in
the Gulf of Mexico migrates westward, but remains generally well
east of area.

The approach of another weak trough in the eastern Pacific will
ultimately herald a southwesterly flow and a return of the
monsoon moisture to the region. Models have also come into better
agreement with regard to the timing of the moisture. Activity
will mainly be confined to the higher terrain Saturday, per the
latest SREF, while the GFS and ECMWF have converged on Sunday as
the most likely day to initially see isolated convection in the
lower deserts.

Extended forecast is weighted more towards the latest ECMWF, which
has been more consistent but has also trended somewhat drier. Forecast
PoPs represent roughly the 70th percentile of the entire model
suite through early next week, though this only equates to
isolated to scattered coverage of storms each day. Nevertheless,
the synoptic flow remains favorable for storms capable of
producing strong to severe wind gusts along with the possibility
of blowing dust each afternoon/evening.


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

Very dry air in place will keep skies clear into early Friday with
no aviation concerns. Winds will remain on the light side and
generally follow diurnal patterns. A few hour window of southerly
winds are again possible early this afternoon, but with little to no

Southeast California/Southwest Arizona including KIPL and KBLH:

Predominately westerly winds will prevail through tonight for KIPL
while southerly winds dominate for KBLH. Very dry air will persist
providing clear skies during the period.

Aviation discussion not updated for amended TAFs.


Saturday through Wednesday:
Better moisture will finally begin to seep into the Arizona
districts this weekend where rain chances will start increasing
again. Higher terrain areas of Gila County will have the best chance
of storms over the weekend, with modest chances working into lower
elevations of Arizona early next week. With the increasing moisture,
minimum afternoon humidity levels will increase back to a 15-30%
range with good overnight recovery. Except near the influence of
thunderstorms, wind speeds will be rather light.


Spotters should follow standard reporting procedures.




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