539
FXUS65 KPSR 171055
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
355 AM MST Tue Sep 17 2024

.UPDATE...Updated Aviation Discussion...

&&

.SYNOPSIS...
A weather system will pass northeast of the region today, yielding
rain chances over the high terrain of eastern Arizona through this
morning. Behind this weather system, a cooler, drier air mass is
settling over the region. Temperatures through the end of the work
week will be seasonably mild, hovering around 5 to 10 degrees below
normal. Readings should moderate closer to normal over the weekend.

&&

.DISCUSSION...
Satellite imagery at this hour reveals an upper level trough
centered over Nevada, with a jet streak wrapping around its base and
500 mb southwesterly flow analyzed upwards of 70 kts over
Northwestern AZ. Moist low level SSW`rly flow continues to stream
over the high terrain east of Phoenix, and with MUCAPEs analyzed
between 100-500 J/kg (higher values to the south), scattered fast-
moving showers have developed over the foothills of northern Pinal
County and portions of Southern Gila County early this morning. The
best synoptic forcing over AZ is still expected to occur this
morning as the trough ejects to the northeast over the course of the
day, but with dwindling moisture, rain chances drop below 25% after
sunrise and become essentially zero midday onwards.

In the wake of the upper level trough, our region will be positioned
within a considerably drier and cooler air mass, as deep, longwave
troughing persists across the western US. NAEFS mean 850 mb
temperatures hover between 16-20 deg C through the end of the work
week, mostly below the 10th percentile of climatology. As such,
forecast highs through Friday will be seasonably mild, mainly in the
low to mid 90s across the lower deserts, which is around 5 to 10
degrees below normal for this time of year. Ensembles show a
shortwave sinking south along the west coast and eventually moving
onshore Thursday over Central CA before it eventually ejects off to
the northeast late this week. However, the dry environment remaining
in place will make for near zero rain chances as the shortwave
passes. The main sensible impact of the shortwave passing to the
north late this week will be some daytime breeziness.

This weekend into early next week, the overall pattern becomes less
certain. However, global ensemble H5 heights in almost all solutions
begin to increase over the Desert Southwest, with mean values up to
584-587 dam late Saturday. With this, temperatures are expected to
begin trending back up to near normal over the weekend, possibly
reaching an above normal range early next week. Probabilistic NBM
output shows an 82% chance of the high temperature at Phoenix Sky
Harbor Airport reaching or exceeding 100 deg F on Sunday and a
95% chance by Monday (25% or lower chances each day before
Sunday). WPC cluster analysis reveals some key uncertainties
Sunday onward. Mean troughing over the western US weakens and
shifts eastward in all the clusters by Saturday, allowing heights
aloft to increase and ridging to set up just off the West Coast.
However, there exists large differences in the amplitude of the
ridging and its eastward extent early next week. About 10% of the
ensemble members comprise a cluster that shows a negative height
anomaly dipping down into the Intermountain West, which would
likely continue to moderate temperatures in a near normal range.
In any case, after this morning, rain chances remain essentially
zero across the CWA through the next 7 days, and our below normal
temperatures are likely to come to an end over the weekend.

&&

.AVIATION...Updated at 1055Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No aviation concerns are expected through the TAF period. Winds
this morning will predominantly be out of the south southwest, but
there may be brief periods of southeast winds through mid morning.
By early afternoon, winds will shift more directly out of the
west, lasting through the evening hours. FEW to maybe SCT cloud
decks around 8-10k ft are expected into the afternoon hours before
clearing out.

Southeast California/Southwest Arizona including KIPL and KBLH:
Clear skies and light winds will present no aviation concerns
through the period. Expect westerly winds to prevail at KIPL,
while KBLH will have a period of variable winds before
predominantly being west southwest by this evening.

&&

.FIRE WEATHER...
Rain chances over the high terrain of the eastern districts will
come to an end sometime later this morning. Some of these scattered
convective showers may contain lightning. Chances for wetting
rainfall are not expected to return through at least this weekend,
as a drier and cooler airmass encompasses the region. Temperatures 5-
10F below normal can be expected through the end of the week. Winds
will return to generally light and diurnal patterns today, with
typical upslope gusts to 15-25 mph during the afternoon each day.
Afternoon minRHs in the teens will be common in lower elevations and
closer to a 20-30% range across higher terrain. Overnight recovery
will range widely from fair to good at 30-70%. As temperatures warm
closer to normal this weekend, humidity levels will decrease further
with single digit readings more likely in lower desert communities.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

DISCUSSION...Whittock
AVIATION...Kuhlman
FIRE WEATHER...Whittock/18