Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 221454
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
755 AM MST Wed Feb 22 2017

.SYNOPSIS...
The above normal temperatures will abruptly come to an end on
Thursday as a dry cold front ushers in a substantially cooler air.
The dry and cool conditions will persist through much of the
weekend before an approaching Pacific low pressure system brings
increasing clouds and a return to unsettled weather for early next
week.

&&

.DISCUSSION...

A relatively dry cold front was approaching AZ from the west this
morning. Considerable high clouds were seen spreading into our
forecast area, southeast CA to south central AZ ahead of the front
and should continue throughout the day. Since the dry cold front
will move through the area during the late afternoon hours, it will
be another relatively warm day with above normal temperatures by 2-5
degrees. Ten degrees of cooling are expected Thursday and Friday
behind the front.

Additionally, gusty west to southwest winds will develop this
afternoon mainly over portions of southeast CA and the Colorado
River area, with lighter winds toward Phoenix. And, boundary layer
moisture left over from last weekends rain, and causing annoying
morning dew on everything, including car windshields every morning,
will finally be scoured out Thursday and Friday behind the front.

Current dry forecasts look ok through Friday. No short term updates
planned.


.PREVIOUS DISCUSSION...521 AM MST...

Split flow across the eastern Pacific will bring dry zonal flow
to the Desert Southwest through the early part of this weekend.
The southern jet branch will remain nearly zonal during this time
while the northern branch will be quite active with a couple of
waves breaking over a highly amplified ridge situated across the
Northern Pacific. An upper level trough currently situated just
off the Pacific Northwest down through the Great Basin will slide
eastward into Thursday with the main PV anomaly passing through
southern Nevada and southern Colorado. This will act to lower our
heights aloft substantially while also bringing a significant dry
cold front through Arizona tonight.

As this trough passes by to our north, it will bring some
increasing upper level clouds today, but we will remain dry below
15K feet. The cold front will drop surface temperatures at least
10 degrees with highs only reaching into the mid to upper 60s for
Thursday and Friday. This will also result in quite chilly
overnight lows for Thursday night and Friday night with readings
into the upper 30s to lower 40s across the lower deserts. Air mass
modification and a slight boost in upper level heights will lead
to some modest warming for the weekend, but temperatures should
remain a few degrees below normals.

Model guidance continues to point toward a more unsettled pattern
for the Desert Southwest starting late Sunday through early
Tuesday, though timing has been pushed back some over the past few
model runs. The general model consensus shows a strong and compact
trough dropping southward along the Canadian and Pacific
Northwest coast Friday into this weekend, but then getting pulled
back to the southwest as it interacts with the southern jet branch
off the California coast. Models then seem to handle a
complicated scenario involving this cold trough merging with a
cut-off low well off the California coast. As this happens, the
southern jet branch strengthens and eventually phases together
with the northern branch along the west coast early next week. In
a nutshell, this phasing will result in some modest Pacific
moisture to be drawn into the Desert Southwest Sunday night into
Monday with the best rain chances over the region likely on Monday
and mainly focused in upslope favored locations in Arizona.
Forecast moisture levels for now don`t look all that impressive,
so not looking at any significant rainfall amounts. Winds will
also likely be an issue on Monday, especially across southeast
California where a Wind Advisory may end up being needed.

Temperatures for next week will start out quite cool with highs
only in the 60s for Monday and Tuesday. Should see some moderating
of temperatures through the rest of next week, but seems likely
temperatures will remain below normal.

&&

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

A band of high clouds is gradually shifting eastward across
southern California and northern Arizona, and should reach south-
central Arizona later this morning. Expect a period of overcast
conditions, with cigs AOA 20k ft. Light easterly winds are
expected until 19-21Z before a westerly shift occurs, with speeds
increasing to around 10 kt. West winds are forecast to persist
overnight into early Thursday morning ahead of a cold front.

Southeast California/Southwest Arizona including KIPL and KBLH:

Thick high-level cloudiness will persist into the afternoon, with
cigs AOA 20k feet expected. Strong west winds will occur at both
KIPL and KBLH this afternoon and evening with gusts approaching 30
kt. Gusts should subside somewhat early Thursday morning, but a
few to 15-20 kt will still be possible. A cold front will approach
the region and may result in a shift out of the north at KBLH
around 12Z.

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Friday through Tuesday...Dry conditions are expected for most
areas through early Sunday. Near-normal temperatures are expected
on Saturday and Sunday with humidity values bottoming out in the
15-25 percent range each day. Winds will be light amidst high
pressure over the region. Strengthening of westerly winds will
occur Sunday across southeast California ahead of a storm system.
Precipitation chances will also increase late Sunday into Monday
across the forecast area, especially over the higher terrain of
south-central Arizona, but precipitation totals are forecast to be
light. Winds will strengthen again on Monday especially across
southeast California and southwest Arizona, but this will also
coincide with an increase in humidity values, which should
mitigate fire weather concerns.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

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

$$

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DISCUSSION...Vasquez/Kuhlman
AVIATION...Rogers
FIRE WEATHER...Rogers


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