Forecast-Discussion

000
FXUS65 KPSR 261449
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
750 AM MST Wed Apr 26 2017

.SYNOPSIS...
A series of mainly dry Pacific weather disturbances will move across
the western states through early next week. These systems will keep
afternoon temperatures near or slightly below normal, along breezy
afternoons and evenings. One stronger disturbance on Friday will
likely bring windy conditions and has the potential to produce a
slight chance of afternoon showers over the eastern Arizona
mountains.

&&

.DISCUSSION...

Dry northwest flow aloft providing mostly clear skies and warmer
afternoons are forecast today and Thursday in the aftermath of
Tuesdays windy cold front. Another windy episode is expected Friday,
details below. Otherwise current forecasts look ok. No short term
updates necessary.


.PREVIOUS DISCUSSION...555 AM MST...

An upper level trough axis has shifted well into New Mexico early
this morning with increasing subsidence and modest warming aloft
over SE California and Arizona. The warming aloft will lead to
highs increasing a few degrees over Tuesday`s readings with the
Phoenix area reaching near 90 degrees and the SE California and SW
Arizona lower desert locations reaching into the lower 90s. Winds
will again be breezy today, but gusts should mostly remain below
25 mph. Similar conditions are expected for Thursday as dry
northwest flow continues.

Much better model agreement is now shown for the next low
pressure trough bringing a closed low southward through eastern
Utah into the Four Corners area Friday night. This will bring a
cold frontal boundary southward through Arizona Friday afternoon
through early Friday night. Some limited mid level moisture ahead
of the front will likely result in some scattered showers and a
very remote chance of a thunderstorm over the high terrain north
and east of Phoenix. Can`t completely rule out a shower or two
making it off the high terrain into the south-central Arizona
deserts, but a very dry sub-cloud layer should only result in
virga or sprinkles. Very dry air will usher in behind the cold
front with surface dew points likely crashing into the single
digits late Friday afternoon and evening. Another windy day is
likely on Friday with a high likelihood of some areas being under
Wind Advisory. The strong winds and lowering humidities on Friday
will likely bring dangerous fire weather conditions once again.

Ahead of the cold front on Friday, highs should reach into the
upper 80s across the deserts. Cooler air ushering in behind the
cold front will allow for a fairly mild but breezy day on
Saturday as highs should only top out between 80-85 degrees.
Rapidly building heights and warming aloft will take place
Saturday night into Sunday as the upper level trough shifts
eastward into the Southern Plains. This will return temperatures
back to near normals on Sunday and a few degrees above normal for
Monday.

Considerable model spread exists after Monday with potential for
yet another upper level trough digging southward through the Great
Basin and Four Corners area. The operational GFS is on the
extreme side bringing a closed low through eastern Arizona while
the operational European keeps it much farther north and east,
essentially showing ridging over the Desert Southwest. The GFS
and European ensemble means are somewhere in between the
operational solutions. For now we are keeping our forecast mainly
based on the ensemble means calling for near normal temperatures
with continued dry and breezy conditions.

&&

.AVIATION...
South-Central Arizona Including KPHX, KIWA, and KSDL:

We can expect another breezy day today, but we should see much less
wind that what occurred yesterday. Still, we can expect afternoon
and early evening breezes favoring the west at the TAF sites. Winds
at KPHX may have a hard time returning to the east for any
significant time this morning, possibly just going light/variable
for a few hours before picking up again from the southwest to west.
Winds likely to remain out of the west (or a westerly component)
past midnight tonight before exhibiting some typical diurnal trends.
No aviation concerns for sky, just some variable mainly thin high
clouds with SCT to BKN decks at times.

Southeast California/Southwest Arizona Including KIPL and KBLH:

Less wind expected today as compared to yesterday with speeds mostly
below 12kt next 24 hours. Winds to favor the north at KBLH this
morning before swinging back to the south/southwest by early
afternoon. Winds rather light/variable at KIPL this morning then
likely turning southeast during the afternoon. Expect winds to
become breezy from the west again after 01z this evening and
possibly gusty at times for much of the night with peak gusts over
25kt possible. No sky issues, just variable amounts of mainly thin
cirrus at times.

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Friday through Tuesday:

A sharp cooling trend will settle into the region as a mostly dry
storm system sweeps through the Great Basin and down through the
Southwest Friday into Saturday. Very strong and gusty winds will
develop across the area Friday with gusts over 40 mph likely over
much of the area, especially south-central Arizona. Humidity levels
will fall sharply during the day as a cold front sweeps across the
area and given the high fire danger, we will likely see widespread
dangerous fire weather conditions across most of the area by
afternoon. Critical thresholds may well be reached from the far
southeast California deserts into the higher terrain of southern
Gila County. Less wind is expected Saturday but breezy conditions
from the north will persist over the western deserts and with
minimum RH values to stay mostly below 10 percent area-wide, we can
expect continued dangerous fire weather conditions out west during
the afternoon hours.

Dry and warmer conditions are expected Sunday into Monday as high
pressure builds into the area from the west. By MOnday high
temperatures over the deserts will climb into the mid 90s for the
most part with minimum RH values to stay mostly in the single
digits. Less wind is expected although we should see continued
afternoon and early evening breezes across much of the area,
favoring the west. It will continue to be dry and warm Tuesday
although there is the chance for an increase in wind especially
across south central Arizona where gusts in excess of 30 mph are
likely for much of the afternoon.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter activation will not be needed through the middle of next
week.

&&

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

$$

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DISCUSSION...Vasquez/Kuhlman
AVIATION...CB
FIRE WEATHER...Hernandez/CB

NWS PSR Office Area Forecast Discussion