Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Phoenix Arizona
840 PM MST Tuesday Mar 11 2014
a weather system and weak front will move into the region today
before slowly exiting later in the week but little to no
precipitation is expected. Winds will be on the
increase...especially along the lower Colorado River valley and
across eastern Arizona during this time. Temperatures will remain
slightly above normal through the end of the week...despite the
passing weak front...with highs in the lower 80s across the deserts.
Even warmer temperatures are expected by the weekend.
early this evening an upper low continued to develop to the
north/northwest of Arizona...slowly dropping down the front side of
an upper ridge located along the West Coast. Upper plots at 00z
indicated minor 30m height falls across Arizona ahead of the developing
low...with rather dry westerly flow in place across southern Arizona and southeastern
California. Infrared imagery showed variable high clouds spreading east across the
state ahead of the low...while the area radiosonde observations indicated dry
conditions overall with precipitable water values below one third of a inch.
Surface dewpoints over the deserts were a mixed bag with values
ranging from low teens along the lower Colorado River valley to low 30s in
portions of the central Arizona deserts. Surface gradients remain tight
over southeastern California...especially along the lower Colorado River valley...and
locally breezy north winds will likely persist there overnight.
Otherwise winds will be generally below 15 miles per hour most places...favoring
normal diurnal tendencies. For the rest of tonight the low will sort
of stall as it continues to develop...leaving dry west flow aloft in
place across our area. Except for some high clouds at times...mostly
clear to partly cloudy skies will be the rule overnight. Forecasts
look to be in good shape and no updates are needed at this time.
rest of today through Friday...
a weak cold front has been slowly moving through the deserts of
southeast California and southwest Arizona. Cooling has been
modest...though Joshua Tree National Park is a good 10 degrees
cooler than yesterday at the same time of day. Windy conditions over
portions of southeast California and southwest Arizona...especially
along the lower Colorado River valley...have weakened at most places
and will continue to do so the rest of the afternoon. To our
north...the western lobe of an upper trough will break away
and develop into a closed low centered over southern Nevada by
tomorrow morning. This is being aided by a positively tilted
upstream ridge. In the process...the low level pressure gradient
tightens with 925 mb winds getting quite strong over southeast
California and southwest Arizona. With radiative cooling
overnight...mixing should be substantially decreased thereby
limiting surface wind potential. However...some short-lived isolated
strong gusts are quite possible tonight west of Maricopa
County...especially near the lower Colorado River valley and above
valley floors. Otherwise...breeziness should be moderate. The 12z
models indicated that Wednesday would be a repeat of todays wind
pattern though the 18z NAM is not quite as strong at 15z and 18z as
the previous run. At this time...conditions look to be too marginal
for Wind Advisory or red flag issuance. Wednesday night and
Thursday...locally windy/breezy conditions shift to our eastern
areas. As the upper low spins over the Cal-Nev-Ari area Wednesday
into Thursday...humidities begin to increase as the system tries to
draw in moisture...plus dynamical lift leading to some cooling and
associated relative humidity increase. Looks like moisture will be too meager too
mention probability of precipitation for our forecast area. A proviso is that on Thursday
the low begins shifting eastward and a decent batch of q forcing
brushes northeastern portions of our forecast area where there is
higher terrain. For now anticipate only a variety of cumulus clouds.
Further north and east outside of our forecast area over the rim
country and White Mountains there will be a slight chance of showers
Thursday. The system exits Arizona Friday...GFS faster than European model (ecmwf)
and Gem. Anticipate only minor fluctuations in high temperatures
Wed-Fri. Low temperatures will be more variable due to occasional nocturnal
Saturday through Tuesday...
this weekend...high amplitude Pacific ridging begins to build. 500mb
heights push into the upper 580dms...supporting a decent warm-up for
the coming weekend. Middle to upper 80s are possible throughout the
Lower Desert elevations...with the warmest spots like Blythe...el
centro and even Yuma seeing near 90f readings. A short wave/dry
front sliding down the front side of the ridge will lead to breezy
conditions later Saturday into Sunday. Forecast model solutions
begin to widely differ for next Monday and beyond as they resolve a
large trough moving into the northeast Pacific and how it will /or
will not/ break through the ridging over portions of the West Coast.
south-central Arizona including kphx...kiwa...and ksdl...
periodic cirrus will spread across terminals through Wednesday
afternoon. Timing of the typical wind shifts is somewhat lower than
normal given recent frontal passage and pressure gradient. Still
expect winds to turn to the southeast after 08z...and increase to
12k or greater by middle morning Wednesday as backing intensifies ahead of
the developing upper low. Of note...a period of southerly cross
winds approaching 15kt may be possible after 18z Wednesday...
particularly at kphx...continuing into the late afternoon hours.
Southeast California and southwest Arizona including kipl and kblh...
gusty surface winds will be the primary aviation concern through
Wednesday afternoon. North winds may decouple tonight...limiting
overall gustiness...but will rapidly redevelop late Wednesday
morning. Have not included any low level wind shear mention...feeling overall speeds
and shear measure will fall a bit short of criteria...though Georgia
pilots should be prepared for stronger winds in the 1k-3k feet level
this evening through sunrise. Confidence in wind gusts is greatest
at kblh where gusts may once again exceed 25kt for extended periods
Wednesday. Otherwise expect mostly clear skies with just a bit of
high cirrus at times.
Aviation discussion not updated for amended tafs.
Thursday through Monday...
a weather disturbance will move through northern Arizona on Thursday
bringing locally gusty winds. High pressure building eastward into
the Desert Southwest will bring a warming trend to the region from
Friday into the upcoming weekend...with temperatures approaching 90
at a few Lower Desert locations by Sunday. Dry air will remain in
place with minimum humidities generally in the single digits across
the lower deserts each day...with fair to good overnight recoveries.
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