Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 290359
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
900 PM MST Tue Mar 28 2017

.SYNOPSIS...
A Pacific storm system will pull away from the southwest states
leaving tranquil warming conditions in its wake Wednesday and
Thursday. However, another weather system will move into Arizona
later this week bringing another round of gusty winds and cooler
temperatures.

&&

.DISCUSSION...
At 830 pm this evening, the core of the upper low that had moved
into Arizona had shifted well to the east of the state, leaving
behind a drier subsident northwest to north flow aloft. Some
lingering low level moisture remained across the eastern third of
the state, and partly cloudy skies lingered over areas to the east
of Phoenix with a few dissipating light showers still present across
portions of southern Gila County. By midnight, as subsidence
continues to work its way east across the state behind the low,
these showers and the associated low clouds should continue to
dissipate leaving behind mostly clear skies by early Wednesday
morning. Of course as the low exited and gradients weakened, winds
tapered off and now we are left with some local breezes favoring the
northwest to north over the lower deserts. No weather issues are
expected for the rest of the night as the dry northwest flow
continues across the area; no updates are needed to the forecasts.

.PREVIOUS DISCUSSION...
The main core of a stout PV anomaly and deep trough axis has now
translated towards central New Mexico this afternoon with an
objectively analyzed -24C H5 cold core situated very near far SE AZ.
In fact, 12Z KTWC sounding sampled impressive 12dm H5 height falls
while -23C fell within the lowest 10th percentile for the end of
March (quite interesting as just a week ago we were constantly
talking about temperatures aloft near record levels). Meanwhile,
regional radar and satellite imagery indicated another weaker
vorticity center along eastern AZ/western New Mexico border
descending into the trough base. This feature has tapped residual
midlevel moisture and pockets of SBCape through srn Gila County
invigorating sct showers/storms. This shower activity will tend to
dissipate and advect eastward with the loss of daytime heating and
the further downstream movement of the associated vorticity center.

Through far western AZ and southeast CA, though the sfc pressure
gradient was slowly weakening, sufficient vertical mixing has
partially tapped a 30-35kt H8 LLJ. This gradient and lower level jet
core will more rapidly evaporate after sunset allowing wind gusts to
promptly abate. Otherwise, a transient shortwave ridge will build
into the SW Conus Wednesday and Thursday pushing temperatures back
into a slightly above normal category.

Models show yet another trough diving from the Pacific Northwest
into the forecast area Thursday night and Friday. This will cause a
rather sharp cooling trend Friday with highs 10F-15F cooler than
Thursday. This transition and approaching deep cold core trough will
increase pressure gradients and drive another round of windy
conditions, especially in SE California and the Colorado River
Valley. Although at this point, winds may not be quite as strong and
gusty as experienced in the past 24 hours though it may be close. As
with the current trough, the best chance of precipitation will
remain in Gila County with amounts remaining rather light.

All major global circulation models agree that this trough will
linger through Saturday but major differences arise beyond that.
Because of this, there is very low confidence in the forecast for
early next week as each one of the models has their own unique
solution. For now, have gone with a blended approach and have kept
temperatures near seasonal normals.

&&

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

Upper low continues to shift east allowing gradients to relax;
locally breezy west/northwest winds should continue to taper off
overnight with winds tending to favor the north to northeast by
early Wednesday as north-south surface gradients set up. Wind speeds
to be mostly below 10kt rest of tonight into Wed morning. Expect
winds to favor the north to northwest by Wednesday afternoon with a
few gusts over 15kt possible. Skies to be mostly clear; just a few
mid cloud decks with bases around 9-11kt feet to linger past
midnight and into the morning hours Wed.

Southeast California/Southwest Arizona including KIPL and KBLH:

Less wind expected over the next 24 hours compared to what we have
seen earlier today as gradients gradually relax. Light west winds at
KIPL will pick up a bit Wed midday and turn towards the north.
Somewhat stronger winds to persist at KBLH as northerly flow aloft
behind the exiting low persists; we could see a few gusts over 20kt
overnight tonight and thru the afternoon on Wednesday. Best
potential for gusts though will be after 15z Wed into the afternoon
hours. Skies to be genly clear through Wed afternoon at the TAF
sites.

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Thursday through Monday:
The weather pattern will remain relatively active across the Desert
Southwest through early next week, though in general precipitation
is not expected. Only exception will be in the higher terrain well
north and east of Phoenix. Main weather impact will be breezy to
windy conditions, which are expected to redevelop Thursday and
Friday as the next low pressure system moves through the region.
With RHs generally remaining below 15 percent, elevated fire danger
will again be a possibility for a few hours across southeastern
California, particularly Thursday afternoon. Thereafter, the low
pressure system will likely move through Arizona Saturday, bringing
below normal temperatures along with a general decrease in wind and
an increase in moisture. High pressure will follow for Sunday and
Monday, resulting in a warming trend and a return to above normal
temperatures. Otherwise, little to no changes were needed in the
morning update to capture hourly trends.

&&

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

&&

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

CA...None.

&&

$$

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DISCUSSION...CB
PREVIOUS DISCUSSION...MO/Wilson
AVIATION...CB
FIRE WEATHER...Hirsch


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