Forecast-Discussion

000
FXUS65 KPSR 212141
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
241 PM MST Sun Apr 21 2019


.SYNOPSIS...
Anticipate temperatures to remain near normal once again on Monday
before a warming trend begins over southeast California and
southwest Arizona on Tuesday. For south-central Arizona on Tuesday,
an upper level low will bring a chance of showers and
thunderstorms to the higher terrain. High pressure builds over
the region during the middle and latter part of the week leading
to a major warming trend. In the Thursday through Saturday time
frame, highs on the lower deserts will be in the upper 90s to
around 100. The warmest day looks to be Friday. Anticipate cooler
temperatures by late in the weekend and early next week.

&&

.DISCUSSION...
An upper trough is covering much of the Interior West this
afternoon. This trough lies within a southern branch of a split in
the Westerlies. There are variety of vort maxes within the system
as evident in the water vapor imagery. Perhaps the most
significant one of those is centered over western Utah. The models
are in good agreement on the evolution of the system as they show
it digging southward in response to upstream ridge amplification.
There will also be vort lobes continuing to rotate through. The
cooling from this system has largely already occurred. So,
high temperatures Monday will be quite similar to today and there
will even be warming on Tuesday (most noticeably over our western
areas). Anticipate renewed strong winds over far southwest
Imperial County this evening. Thus issued a Wind Advisory there
that runs from 01Z - 07Z.

The main effect from the trajectory of the system will be
a southward progression of rain and thunderstorm chances Monday
and Tuesday over Arizona. The northern half of AZ will be the main
focus - especially tonight and Monday - but east-central and
southeast Arizona will have an opportunity as well on Tuesday.
For our forecast area, that will mean southern Gila, far
northeast Maricopa, and north-central Pinal Counties. Of note,
there will be northeasterly steering flow. Some of the hi-res
models indicate debris showers reaching the valley floor of metro
Phoenix as well as some gustiness. For now, have held off
introducing PoPs for metro Phoenix. Before then, there might be
some isolated virga showers over southwest AZ on Monday but not
enough to go on to mention precip chances.

A ridge builds into the region Wednesday and Thursday before
beginning to shift eastward Friday. With H5 heights increasing
towards a 582-585dm range along with full late April insolation,
its a solid bet that most lower elevation locations will be
approaching or exceeding the 100F threshold. Ensemble and
guidance spread is unusually narrow for this 5-6 day forecast
period, so confidence in these hotter temperatures is very high.
The hottest day looks to be Friday.

Over the weekend, some measure of renewed troughing and height
falls will spread back into the SW Conus, though ensemble spread
grows tremendously leading to greater uncertainty whether cooling
will be rather modest or more abrupt and significant. Operational
models are favoring a late weekend/early next week arrival for the
trough axis. At this point, have hedged towards an ensemble mean
which tempers cooling only slowly towards late April normals.


&&

.AVIATION...Updated at 1730 UTC.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Outside of some breeziness this afternoon, expect skies to remain
clear with a few low clouds (around 10 kft) moving through late
tonight due to an approaching low pressure system. This is a dry
system and no precipitation is in the forecast. Typical westerly
wind shift has occurred this morning, and could see a few gusts up
to 15-20 kts out of the west during the afternoon. More typical
timing of diurnal wind transitions is expected overnight into
Monday.

Southeast California/Southwest Arizona including KIPL and KBLH:
Skies will be clear with a few passing low clouds expected
overnight due to an approaching weather system. This is a dry
system and no precipitation is in the forecast. Wind speeds have
subsided at both terminals and will remain this way through the
remainder of the morning. At both terminals, westerly winds will
persist through the TAF period with a few gusts up to 15-20 kts
possible this afternoon (especially at BLH).

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Wednesday through Sunday:
High pressure will build into the region Wed. and Thu. then shift
eastward Friday and Saturday. In the process, there will be a
significant warming trend with highs on the low deserts well into
the 90s on Wednesday and in the upper 90s to near 100 the rest of
the week. The warmest day looks to be Friday. Accordingly,
humidities decline with minimum readings on the lower deserts
hovering around 10% and overnight recovery declining to fair. Late
in the weekend and early next week, a low pressure system is
expected to bring cooler temperatures. As the system approaches,
anticipate more noticeable breeziness during the latter part of
the week. At this time, critical fire weather thresholds are not
anticipated.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter activation is not expected for the next 7 days.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...Wind Advisory from 6 PM this evening to midnight PDT tonight for
     CAZ562.

&&

$$

DISCUSSION...AJ/MO
AVIATION...Rogers/Hernandez
FIRE WEATHER...AJ

NWS PSR Office Area Forecast Discussion