Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 300445

Area Forecast Discussion
National Weather Service Phoenix AZ
945 PM MST Mon May 29 2017

Warm conditions with temperatures roughly five degrees above
normal will affect the area through Tuesday. Scattered mountain
storms across far eastern Arizona will be possible, mostly for
Tuesday and Wednesday. Temperatures cool somewhat into the end of
this week with overall readings ending up near seasonal normals.
Another minor warming trend this weekend into early next week will
push lower desert highs closer to 105 degrees by next Monday.


Southern branch troughing centered just off the West Coast brought
thick cirrus today and that will continue overnight before
starting to thin from west to east during the day Tuesday. This
feature will weaken as it moves eastward due to the ridge over
the interior western CONUS. However, a larger trough off the PacNW
coast will nudge the ridge eastward allowing troughing to advance
inland. In the process...there will be some moist advection into
portions of central and eastern Arizona starting tonight. Thus, we
should expect to see more thunderstorm coverage over the higher
terrain Tuesday. A number of the CAMs depict at least isolated
coverage over our higher terrain zones east of Phoenix. This is
handled in current forecasts. No changes at this time.


Upper level ridge axis will slowly move across Arizona today with
fairly weak flow aloft. Today should end up being the warmest day
with a slow decline back toward normals into the middle of the
week. Weak mid level southeasterly flow has already brought a
modest moisture increase into extreme southeastern Arizona from
New Mexico. This has allowed for a few isolated showers and
thunderstorms to develop already early this afternoon, but
coverage has mostly remained east of our CWA. Any storms will be
high based and fairly low topped, but dry sub-cloud layers may
promote a few gusty outflows into southern Gila County. Elsewhere,
a weak trough moving into California has spread some high cirrus
across the Desert Southwest which may hold down high temperatures
down a degree or so from previously forecast.

Tuesday and Wednesday look to be the best days for elevated
thunderstorms. Increased southeasterly winds in the lowest half of
the atmosphere will continue to draw additional moisture into the
area. GFS and NAM model soundings indicate a range of 200 to 600
j/kg of CAPE over the higher terrain including the Rim, White
Mountains, and the Pinal Mountains. Any thunderstorms that do
develop will have the potential for lightning strikes and
producing outflow winds that may pose fire weather concerns for
ongoing fire suppression operations and potential for new fires.
Unfortunately, precipitation amounts with any storms leaves much
to be desired with most model outputs indicating 0.01 of an inch
or less of rain. Additionally, the surface pressure gradient will
tighten as the trough progresses through California which will
lead to breezy conditions for Wednesday afternoon. Gusts 15-25 mph
will be possible, especially along the Lower Colorado River area
and over the higher elevations in Gila County.

Weak troughing moving through the Western United States Wednesday
and Thursday will allow for temperatures to drop to near seasonal
normals for the middle part of the week before gradually warming
as H5 heights recover. The GEFS indicates H5 heights will return
to the 582-586dm range by the weekend. As such, the temperature
forecast was weighted more towards the upper half of the available


South-Central Arizona including KPHX, KIWA, and KSDL, and
Southeast California/Southwest Arizona including KIPL and KBLH:

Light surface winds will continue overnight and through the day
Tuesday favoring familiar diurnal patterns. However, there will be
a period from about 15Z-19Z over south-central AZ (including metro
Phoenix) where stronger than average southeasterly winds are
anticipated before slowly becoming southwesterly in the afternoon.
Thick cirrus will continue overnight before starting to thin from
west to east during the day Tuesday.

Aviation discussion not updated for amended TAFs.


Thursday through Monday:
Near normal temperatures will last into early next week under weak
high pressure. Drier air will move back over the districts
starting Thursday ending the thunderstorm/lightning threat. Winds
will obtain the typical late afternoon breeziness on Thursday
with somewhat lighter winds through the weekend. Minimum humidity
levels will fall to around 10% except in a 15-20% range over
higher terrain of Gila County, as well as lower elevations of
southeast California. Overnight recovery will generally be fair.


Spotters should follow standard reporting procedures.




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