Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 221026

Area Forecast Discussion
National Weather Service Phoenix AZ
326 AM MST Thu Mar 22 2018

High pressure will begin to shift to the east today, but highs
will again easily reach into the 80s, even under mostly cloudy
skies. A weak weather system will then move in from the west later
today bringing a slight chance of showers mainly tonight. A
gradual cool down is in store starting Friday with highs dipping
into the lower 70s by Sunday. A fairly dry weather system will
bring another chance of showers early next week while keeping
temperatures below normal.


Strong moisture transport out of the southwest continues into
central California with upper level moisture beginning to shift
eastward into southern California and western Arizona. The large
trough off the West Coast has tapped into an atmospheric river,
but the majority of this moisture is heading into Nevada and
Utah. Cloud cover will become more widespread and thicken as the
day progresses, but modest warm air advection out of the south
will still allow for very warm start of spring. Quite a bit of
moisture in the mid-levels will shift into southern California by
this evening, eventually moving into Arizona tonight, but the low
levels will again mostly remain quite dry. A cold front will
shift through the area tonight, but the weakening shortwave trough
will move northeastward into northern Arizona. The lack of
support aloft and dryer air in the low levels will keep much of
shower activity associated with the cold front as virga or
sprinkles over the deserts. Even the higher terrain north of
Phoenix will struggle to see measurable rainfall tonight.

Behind the exiting cold front on Friday, skies should quickly
become mostly sunny. A modestly cooler airmass behind the front
will allow for highs around seasonal normals with some lower
desert spots around 80 degrees. The main part of the large trough
will then shift southeastward into the Great Basin this weekend,
further lowering heights over the Desert Southwest. Highs by
Sunday will likely only reach into the lower 70s across the
deserts as the base of the trough nears the region. The inland
track of the low will result in very little system moisture with
any rain chances remaining north of our area likely through
Monday. The system will bring an increased pressure gradient and
result in breezy to at times windy conditions Saturday through
Monday, but likely below Wind Advisory levels.

Model agreement remains good through Monday with some increasing
spread for next Tuesday and Wednesday as the trough slowly moves
through the Desert Southwest. Confidence is now fairly high that
this trough will result in a closed low over the Desert Southwest
early next week, but confidence in how long it will affect the
region is still somewhat low. Though system moisture will be low,
we should see at least a slight chance of rain as the closed low
moves into Arizona on Tuesday. We may also see a chance of
thunderstorms under the cold core as mid level lapse rates will
become favorable. Forecast highs have been lowered a few degrees
Monday through Wednesday as the closed low is looking more
likely, but we are leaning a bit higher then median guidance until
model spread decreases.


South-Central Arizona including KPHX, KIWA, and KSDL:
Strong southwest and westerly flow aloft will lead to a slow
thickening of cloud cover from the top down overnight and during
the day Thursday. There is potential for scattered light showers
Thursday afternoon and evening but that looks to be mainly west
and north of metro Phoenix - probability too low to reflect in
TAFs. Instead, look for a lot of virga with ceilings remaining
AOA FL100. Surface winds will favor familiar diurnal patterns
overnight and through the morning before a period of stronger than
normal southerly winds develops midday/early afternoon. Then,
southwest/west directions develop by mid afternoon with peak gusts
of 20-25 kts during late afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:
Strong southwest and westerly flow aloft will lead to a slow
thickening of cloud cover from the top down overnight and during
the day Thursday. Expect isolated light showers/sprinkles Thursday
along with a lot of virga. Anticipate ceilings remaining AOA
FL100 through 18Z before trending down to FL080-100 by late
afternoon. Surface winds will favor familiar diurnal patterns
overnight and through the morning. Winds strengthen in the
afternoon favoring southwest and west (southerly in the Lower
Colorado River Valley) with gusts of 20-30 kts during the

Aviation Discussion not updated for amended TAFs.


Friday through Tuesday...A fast moving weather disturbance will
bring cooler high temperatures Friday along with isolated to
scattered showers east of the lower Colorado River Valley. Best rain
chances will be over higher terrain areas north and east of Phoenix.
Expect high temperatures to drop to near seasonal normals. Humidity
will be a bit elevated with minimum RH values over the deserts
mostly 20 percent and higher. Dry southwest flow aloft on Saturday
will keep skies partly sunny and temperatures will remain near
seasonal normals. A large area of low pressure will gradually
develop over the southwestern states Sunday into early next week;
little if any rain is expected but temperatures will cool with highs
falling into the low to mid 70s each day over the deserts. Humidity
levels stay mostly in the teens each day across the deserts.
Relatively light west wind are expected Friday afternoon; after that
winds will increase and become rather breezy during afternoon and
early evening hours each day, favoring the west to northwest.


Spotters should follow standard reporting procedures.




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