Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 171128

Area Forecast Discussion
National Weather Service Phoenix AZ
428 AM MST Mon Dec 17 2018

.UPDATE...Updated Aviation and Fire Weather Discussions...


A weak weather system passes through the area later today and
tonight with continued mild conditions and periods of thicker high
clouds. Expect a very slight chance of showers late this afternoon
and tonight mainly over the higher terrain east and north of
Phoenix. A warming trend begins in the middle of the week as strong
high pressure builds across the region. High temperatures will
potentially approach record highs late in the week.


Nighttime satellite imagery is showing the trailing edge of some
upper-mid level clouds associated with a departing low pressure
system east of Phoenix. Another band of high and upper-mid level
clouds is moving across S CA ahead of an open wave trough moving
into the west coast. Upstream upper air analysis shows a trough
axis positioned offshore from S CA and a spit-flow H5-H3 jet
pattern near SW AZ and SE CA. Overnight radar only showed virga
and/or light sprinkle return signatures over over S Pinal and Gila

Today the open wave trough moves east into the Intermountain W. The
southern end of its axis moves through S CA today and digs further
southeastward into S-central AZ during late afternoon and tonight.
This relatively weak system brings a modest amount of mid-level
moisture with it with a bump in PW near the 0.5-0.6in range this
afternoon and evening. Some models also show a sliver of very weak
mid-level CAPE later today. However, lacking substantial moisture
there is only a very slight single digit chance of showers in the
deserts this afternoon and evening with only slightly better
chances across the high terrain areas north and mostly east of
Phoenix tonight. No significant rainfall is expected with this
system. Some high-res models only suggest that some virga and
isolated sprinkles will drift across the deserts this afternoon
and evening. This system will bring some fleeting colder air with
it keeping highs in the 60s through tomorrow. Then the coldest
morning this week is expected on Wednesday morning with lows in
the upper 30s to mid 40s across the lower deserts.

On Tuesday evening a ridge axis slides east into the Intermountain W
and SW US.  On Wednesday a warming trend begins as the ridge then
reinforces its presence and builds in amplitude and expands over the
W US. On Thursday the strong high pressure will climax and the ridge
axis will extend from AZ through the N Rockies as H5 heights are
forecast to peak near 585-587dam over the area--near the 99th
percentile for climatology. Forecast high temperatures for Thursday
and Friday are well into the 70s, or at least 10 degrees above
normal, and could come fairly close to the record highs for those

On Friday the fair weather pattern continues as the ridge axis
shifts east and a dry zonal/west-northwesterly pattern develops. The
fair weather should continue during the weekend as a couple of weak
waves pass uneventfully across the region.


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

An approaching system today will allow for mid/high decks to
redevelop with SCT to BKN decks forming later today, again mostly
AOA 14k feet. Winds will continue to be very light, weakly
following typical diurnal patterns. West winds never developed
yesterday at KPHX, but there is a little better support for west
winds to develop after 23z today. No significant aviation
concerns for at least 24 hours.

Southeast California/Southwest Arizona including KIPL and KBLH:

Expect FEW-SCT decks mostly 15k feet and above today and tonight.
Winds to remain very light and mostly variable. No aviation
concerns for at least the next 24 hours.

Aviation Discussion not updated for amended TAFs.


Wednesday through Sunday:

Strong high pressure will affect the region during the first part
of the period with weak troughing moving in by the weekend. Well
above normal temperatures will last through the bulk of the period
with Thursday seeing the highest readings with gradual cooling
into the weekend. Dry conditions will persist through the period
under generally light winds. Min RH values will mostly fall in a
20-25% range each day with little variation. Overnight recoveries
will be good to excellent throughout the period.

Spotters should follow standard reporting procedures.




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