Area Forecast Discussion

FXUS65 KPSR 222030
Area Forecast Discussion
National Weather Service Phoenix AZ
130 PM MST Thu Mar 22 2018
.UPDATE...Updated Aviation Discussion.
A weak weather system will move in from the west tonight bringing
a slight chance of showers through early Friday morning. 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.
An abundant amount of high and mid level clouds continue to spill
over into the desert SW this afternoon as a trough of low 
pressure works its way onshore. These high level clouds have kept
temperatures at bay in SE California this afternoon with upper 70s
reported across most stations in the area. However, out in south-
central Arizona, fewer clouds this morning and a warmer airmass 
has pushed temperatures into the low/mid 80s already this 
afternoon. The warmest temperatures in the state are actually
being observed in SE Arizona where clouds have been slow to move
As this trough progresses eastward, the chance of showers will 
increase across south-central AZ this evening with most activity 
staying north of I-10. Since the best dynamics and subsequently 
synoptic lift will pass to our north, we are not expecting much 
in terms of rainfall with only a few hundredths accumulating for 
locations that actually do see activity. Come Friday morning, a 
cold front will race through the Phoenix area bringing cooler and 
drier air to our region. This will effectively shut off any chance
of showers over Phoenix around the 15Z timeframe with rapidly 
clearing skies thereafter. With cooler air rushing in behind the 
front, highs will struggle to reach 80 degrees in the lower 
deserts on Friday.
Heights will stay suppressed through the weekend as yet another
trough slowly drops down from the Pacific Northwest. This will
bring breezy conditions through the weekend with the strongest
winds confined to SE California. As it stands right now, winds in
California do not look to be of advisory strength but gusts up to
30-35 mph will be possible each afternoon. In addition to the
winds, cooler air will slowly advect over our area as the trough 
inches closer this weekend allowing temperatures to drop a few
degrees each day. 
Model agreement remains good through Monday with some increasing
spread for next Tuesday and Wednesday as this 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 showers for the higher
elevations as the closed low moves into Arizona on Monday. It is
not entirely out of the question for lower elevations to see some
shower activity but it does look unlikely at this point. Forecast
highs have again been lowered a few degrees Monday through 
Wednesday as this closed low solution is looking more likely.
South-Central Arizona including KPHX, KIWA, and KSDL:
Increasing cloud coverage will continue through this evening. CIGs
will gradually lower to near 10k feet by late afternoon and 
approach 6k feet by tonight. There is a slight chance for a few 
showers in the Phoenix area, but the best threat for showers will 
be north of the terminals. In the meantime, westerly winds will 
prevail with some stronger afternoon gusts up to 25kts. The gusts 
should subside by sunset. The wind direction may become a bit more
tricky after 6Z as the winds may shift back from the east; 
however, there is some uncertainty as whether or not that shift 
will occur.
Southeast California/Southwest Arizona including KIPL and KBLH:
Thick cloud cover will continue to filter through the area with
CIGs lowering to near 8-10k feet. There may be a few showers
developing this afternoon but any shower activity should taper 
off by this evening. Meanwhile, the winds will maintain a mostly 
westerly component with some strong gusts up to 25kts through late
Aviation Discussion not updated for amended TAFs.
Saturday through Wednesday...Dry southwest flow aloft and partly 
sunny skies can be expected Saturday, with high temperatures 
hovering near seasonal normals and minimum RH values mostly in the 
teens across the deserts. Sunday into the early part of next week, a 
large area of low pressure will build across the desert southwest. 
This system will be dry for the most part but will usher in a 
cooling trend with high temperatures falling into the upper 60s to 
low 70s across the deserts by Monday. Slight warming will then take 
place through Wednesday but highs will stay below normal and in the 
70s. Some moisture will sneak into south-central Arizona and by 
Tuesday afternoon there will be a slight chance of light showers 
across higher terrain areas north and east of Phoenix. A slight 
chance of thunder is also possible during the afternoon on Tuesday. 
For the most part minimum RH values will stay a bit low and mostly 
in the teens each day, reaching into the mid 20s over the higher 
terrain areas east of Phoenix. We can expect breezy conditions each 
day during the afternoon hours, favoring the west to northwest over 
the deserts with slightly higher values across ridgetops and over 
favored higher terrain locations.
Spotters should follow standard reporting procedures.