Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Phoenix Arizona
1018 am MST Sat Jul 4 2015
relatively high monsoon humidity and unsettled southerly flow aloft
will continue the chance of showers and thunderstorms over portions
of southwest and south central Arizona today. The threat for showers
on Sunday will shift into central and eastern Arizona...with a
more drier and stable airmass next week.
there are really no changes in forecast thinking from overnight...as
the 700-400mb upper level flow has many small features and embedded
waves that will continue to Foster precipitation chances for south-
central Arizona today and tonight. Visible satellite imagery has
quite a bit of middle to high level cloud cover that continues to
stream to the north-northwest across the state this morning...and
could definitely reduce insolation and surface heating...thus the
possibility of coming up short of the 103-107 maximum temperature forecast for
the lower deserts. In Tow with the cloud cover...isolated showers
and thunderstorms are also developing/persisting across portions of
Pima and southwest Maricopa counties. Another aggravating feature
is an mesoscale convective vortex noted in the satellite imagery lurking over Sonora Mexico.
All these factors/features continue to complicate the forecast...
especially in deciding the spatial and temporal trends for storms
within the next 12 hours that could impact outdoor Holiday
Based on the current radar and mesoscale analysis...elected to spread
slight chance probability of precipitation across west central deserts...including the
Phoenix metropolitan area for the rest of the morning and afternoon.
By later this evening...close to 7 PM...would anticipate that storms
will persist and therefore nudged probability of precipitation up to chance category. Any
thunderstorm that forms will be capable of brief heavy rainfall and
gusty winds. Other than changes to pop/wx/sky...forecast in the
near/short term is on track.
for today...the 300/250 mb anti-cyclone center will shift south from
the 4 corners area Friday afternoon to near El Paso by late today.
As a result...upper level deformation that has existed in northeast
Arizona will shift into the southeast quarter of Arizona. Additionally...
cyclonic influence from the offshore trough will begin to back 850/700
mb winds from the Gulf of California where copious amounts of moisture is
located. In other words...moisture advection along with a warmer
afternoon...and deformation aloft should focus the best threat of
afternoon/evening showers and thunderstorms over the southeast
quarter of the state...including parts Maricopa...Pinal...and
southern Gila counties.
The flow aloft on Sunday is forecast to become more southwesterly
and laminar a day earlier than previous model runs suggested...
resulting in a low grade slight chance of shower forecast in south
central Arizona...with slightly higher precipitation probabilities in the
mountains of southern Gila County.
Monday through Friday...
For the most part upper level flow becomes more west south-westerly
aloft...with the deepest monsoon moisture slowly shifting into
eastern Arizona. This pattern is quasi-stable...however some uncertainty
will exist around midweek...namely the eastern Pacific trough is
forecast to shift inland creating broad difluent flow aloft over the
southwestern states and maybe some potential for renewed storminess
Warmer air aloft will shift into Arizona from the southwest direction
Thursday and Friday and likely stabilize the airmass again.
Saturday and Sunday...
As the West Coast trough weakens...much stronger southeasterly
middle/upper level winds from Mexico are forecast shift back into the
southeast quarter of Arizona...increasing monsoon moisture along with a
few hard to time inverted troughs. In other words...an increasing
threat for showers and thunderstorms over Southeast Arizona...including
our portions of south central Arizona from Phoenix eastward.
Aviation... south-central Arizona including kphx...kiwa...ksdl...
After a rather quiet morning...ample moisture and favorable
southerly flow will combine to produce more thunderstorm activity
over south-central Arizona this afternoon and evening. Although
confidence is fairly high that one...or more of the terminals will
be affected by convective activity...there is less confidence on the
timing of this activity. Thus...decided to just go with thunderstorms in the vicinity for now
in the tafs through the and afternoon and early evening hours until
a better handle on the timing can be established. The main impacts
from the storms will be gusty winds and brief heavy downpours...with
significant blowing dust being less likely. Ceilings to remain mainly
at or above 10k feet through the taf period...except during
thunderstorms...when few-scattered ceilings as low as 8k feet are possible.
Winds to mainly follow typical diurnal trends...except during
Southeast California and southwest Arizona including kipl and kblh...
Mainly clear skies...with just some few-scattered cirrus layers passing
overhead from time-to-time through the taf period...with
thunderstorm activity remaining well to the east of kblh. Winds to
mainly follow typical diurnal trends at kipl...and to remain mainly
out of a southerly direction at kblh...with some afternoon
breeziness possible at kblh as well.
Aviation discussion not updated for amended tafs.
Monday through Friday...
flow aloft will become more southwesterly next week...allowing the
deepest moisture to be re-positioned to the North/East of Phoenix.
This will result in a decreased chance of desert showers and
thunderstorms next week...with fairly typical afternoon activity
across southern Gila County. Humidities will fall into the 10-15
percent range across the deserts with fair to good overnight
recoveries anticipated all week. Higher values will be seen in the
mountains. As for winds...light/diurnally driven speeds/directions
will dominate all week.
Spotter information statement...
spotters are encouraged to report any significant weather conditions
according to Standard operating procedures.
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