Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Phoenix Arizona
455 am MST Friday Feb 27 2015
Update...updated aviation and fire weather discussions...
dry weather with above normal temperatures are expected through the
end of the week. However...low pressure will approach the area this
weekend bringing gusty winds and widespread rainfall to the area
along with much cooler temperatures through early next week. Cool
and unsettled weather is likely to continue into the middle of the
dry northwesterly flow persists across the region this morning with
only a few high clouds moving through Arizona as of 0945z. These
clouds have had a negligible impact on temperatures with most desert
locales in the middle 40s to lower 50s. Should be able to see several
more degrees of cooling before daybreak with light winds across the
area. In the short term through this afternoon...the forecast
remains largely unchanged for today...mostly sunny with temperatures in the
Forecast concerns are centered on a potent winter storm that is
expected to make its way through the Pacific northwest and off the
Southern California coast this weekend. The system is located west
of Washington state per water vapor imagery...it will gradually move
southward today and will eventually carve out a rather large upper
level circulation by Saturday evening. Once the low deepens it will
advect a substantial amount of moisture into the area. With precipitable waters
expected to rise from roughly 0.3 inches today to 0.8-0.9 inches and
integrated water vapor transport values approaching the 99th
percentile at times this weekend...confidence continues to increase
that this will be a prolonged wet winter weather event across the
Trends in the probabilistic and deterministic guidance over the past
few days have suggested the system will slow down a bit as it
approaches the area. Therefore...confidence in exactly when the
highest probability of precipitation should be drawn in for this weekend/early next week is
low. Conceptually...much of Saturday should be spent moistening the
atmosphere with only a few scattered showers possible mainly
North/East of Phoenix in the higher terrain locations. This idea is
supported by this morning's 00z deterministic models as a number of
bufr soundings across the forecast area. Also Worth noting...winds
will increase quite a bit on Saturday /especially across southeast
California/ and with many desert locations going on a month since
our last measurable rain event...could be some patchy blowing dust
in the normally susceptible areas near open fields. May also need to
address the necessity of a Wind Advisory for portions of southeast
California as well.
Bl moisture will continue to deepen Saturday night and the 00z
naefs/gefs indicate most of Sunday will be a wash.
However...quasi-stationary upper lows off the California coast can
be particularly tricky in terms of timing precipitation moving into the
area and my confidence in exactly how Sunday will play out is
low...especially across the Phoenix area several hundred miles away
from the circulation center. As the ensembles continue to indicate
70-90 probability of precipitation across the deserts...I will hang onto similar values.
00z NAM/GFS/ECMWF all indicate the middle level circulation will move
through the area Monday morning with a frontal band of showers
affecting the lower deserts around 12z Monday. This is a bit slower
than the models were showing yesterday and I increased probability of precipitation quite a
bit for Monday to account for this slower solution. The best q
convergence and isentropic ascent is indicated during this period
and the ensembles also indicate elevated probability of precipitation during this timeframe.
Also Drew in the highest quantitative precipitation forecast during the day Monday with precipitation
rapidly shifting east of Phoenix by Monday evening. Meanwhile...snow
levels will fall immediately behind the frontal band and there
should be a rain/snow mix east of Phoenix late Monday night. With
snow levels expected to fall to around 6000ft Monday night...only
the highest peaks would see accumulating snowfall.
Still some uncertainty with respect to a Post-frontal shortwave
trough and its impacts on Tuesday. This one will be much less
organized than the previous upper low but if enough moisture lingers
across the area...there could certainly be additional showers mainly
North/East of Phoenix Tuesday/Tuesday night. With snow levels around
5500ft by this time...could be light accums as well although
confidence is way too low to pin down exact snowfall amounts at this
time. Made some slight upward adjustments to probability of precipitation from the Colorado
River eastward. Precipitation chances should begin to wane quickly on
Wednesday as dry northwesterly flow takes hold across the western
As for temperatures...below normal temperatures are expected from
Saturday through much of the upcoming week. Sunday and Monday will
be particularly cool with highs barely getting out of the lower 60s
in most locations. Some moderation will take place by middle week as
drier air moves in from the northwest.
south-central Arizona including kphx...kiwa and ksdl...
southeast California and southwest Arizona including kipl and kblh...
A low pressure system approaching from the northwest will bring
increasing high clouds today and into tonight. Low level moisture
will start increasing late tonight which should allow for some lower
few to scattered clouds at around 6k feet just before sunrise Saturday.
Southwesterly winds will become gusty at times this afternoon and
especially across southeast California. Winds at kipl and kblh will
continue to increase through the overnight hours with some gusts up
to around 25 kts possible.
Aviation discussion not updated for amended tafs.
Sunday through Thursday...
cool and wet conditions will be present through the first part of
the period with the best chances for widespread rains on Sunday and
Monday. There will be a chance of a few thunderstorms on Monday and
potentially Tuesday. Humidities will be well above typical values
through Tuesday before minimum values fall back to around 20 percent
on Thursday afternoon. Periods of breezy southwesterly winds can be
expected on Monday and Tuesday. High temperatures across the deserts
will only reach into the 60s Sunday through Wednesday. High pressure
and drier weather should move in starting later Wednesday and should
persist through the end of the week.
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