Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 11/21/19

Area Forecast Discussion...Updated

National Weather Service Phoenix AZ

 505 PM MST Wed Nov 20 2019

.UPDATE...Updated aviation discussion.

&&

.SYNOPSIS...
A low pressure system will continue to drift eastward through
southern California. Showers and thunderstorms ahead .of this system
will persist mainly across Arizona into this evening. After a likely
break in the rain tonight, scattered showers and thunderstorms will
return during the day Thursday, especially across the eastern half of
Arizona. Much cooler temperatures will persist over the next several
days with highs only in the 60s before some slight warming brings
temperatures back into the lower 70s this weekend.

&&

.DISCUSSION...
Another round of moderate rainfall moved through central Arizona
this morning, producing a few areas of flash flooding, particularly
across the West Valley. Storm totals have been highest across the
foothills and mountains north and east of Phoenix, where 1 to 2
inches of rainfall has been observed. Latest water vapor imagery
shows a well-defined closed low across southern California. Although
the upper levels have dried out somewhat since yesterday, latest
ACARS soundings indicate conditions remain quite moist below 700 mb.

Lift will be abundant this afternoon, both in the form of vorticity
and jet-forced ascent along with surface-based instability ahead of
an approaching frontal boundary. Shallow showers and thunderstorms
redeveloped quickly during the late morning, not surprising given the
insolation and SBCAPEs in excess of 1500 J/kg across central
Arizona.

Low-level shear was more conducive for rotating storms and an
isolated tornado a few hours ago. However, latest HREF indicates
depicts the higher 0-3 km SRHs moving well east of the area this
afternoon. Meanwhile, PWATs have decreased from yesterday,
though anomalies remain well above normal. Increasing
unidirectional flow this afternoon will support training cells and
repeating rounds of storms have the potential to produce flash
flooding. The Flash Flood Watch remains in effect through this
evening for central Arizona.

HREF simulated reflectivity appears quite reasonable, depicting
the current convection dissipating around sunset. A pre-frontal
north to south-oriented line of precipitation advertised in
previous models is still anticipated, though CAMs continues to
trend eastward. Consequently, heavy rain across Gila County will
present an increased threat of flash flooding near the Woodbury
burn scar this evening.

&&

.PREVIOUS DISCUSSION...
The upper level low will be slow to exit the region on Thursday
with the expansive cold core remaining across Arizona. Another
round of showers and low topped thunderstorms seems likely
Thursday afternoon, mainly focused from Phoenix and areas
eastward. This activity should be fairly scattered, but may bring
brief heavy downpours and small hail. Rain chances drop off
quickly after sunset Thursday with any lingering shower activity
mostly east of Phoenix. By Friday, the upper level trough will be
picking up speed, moving well northeast into Colorado with rain
chances coming to an end.

Temperatures over the next several days will be quite chilly with
highs roughly 15-20 degrees cooler than the last couple weeks.
Highs will only top out in the 60s across the lower deserts, or
around 5-10 degrees below normals. Overnight lows will start to
drop off beginning Thursday night with readings mostly dipping
into the 40s. The synoptic flow regime will remain fairly active
across the Western U.S. through the weekend into next week with
multiple shortwave troughs diving southeastward, but remaining to
our north through at least Monday. This will keep a fairly cool
air mass in place across the Desert Southwest with little chance
of seeing any meaningful warming, likely through the bulk of next
week. By Sunday night or Monday night, some locations may be
seeing overnight lows well into the 30s as drier air finally
seeps southward allowing for more efficient nighttime cooling.
Models are generally dry for our region through next week, but we
can`t rule out one of these upcoming systems dipping far enough
south to bring a return to rain/snow chances to Arizona.

&&

.AVIATION...Updated at 0005Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Scattered showers with heavier convective elements will continue to
form and move quickly north through the greater Phoenix area through
at least 03z this evening. Thunder less of a threat and as such only
mentioned SHRA in the TAFs this evening. Will need to amend should a
storm threaten or move into one of the airfields. After about 04z
guidance suggest that the most organized convection/bands of showers
will focus to the east of the Phoenix area with only isolated (at
best) showers in the metro for the rest of the night and through the
day on Thursday. Looks to be wet and unsettled through Thursday
afternoon, with potential for a shower pretty much at any time but
confidence in timing/location of any showers remains low. TAFs have
been somewhat broadbrushed in this respect with VCSH in the bulk of
the TAFs 24 hour time window. CIGs will also be an issue...deep
moisture is in place and will not go anywhere keeping CIGs generally
05-06k feet through Thursday afternoon. Opted to mention a SCT deck
around 3k feet reflecting potential for occasionally lowering CIGs
especially in association with showers/storms.

As far as winds, still expect gusty mainly south wind through about
01-02z with gusts over 25kt likely before the winds veer to the
southwest as a cold front moves through from the west. Potential is
there for brief spike in wind gusting over 30kt as showers cause
some mixing of stronger south winds (below 5k feet) to the surface.
After 02z winds should taper off with most gusts falling below 20kt.
Winds likely to return to the southeast after midnight but generally
below 12kt. Winds again to veer back to the southwest Thursday
afternoon mostly likely after 20-21z...with modest speeds at best.

Southeast California/Southwest Arizona including KIPL and KBLH:

Most of the significant convection has moved off to the east of the
lower Colorado River valley, but isolated showers remain possible
this evening and into Thursday morning. Will leave VCSH in the TAFs
through the evening; can`t rule out an isolated shower during the
day Thursday but confidence is low and local hi-res guidance is not
especially bullish on the idea. Also, thunder chances not zero but
too low to mention in the TAFs. CIGs should generally stay above 5k
feet...and in the 7-10k foot range for the rest of tonight and into
the day on Thursday. Gusty southwest to west winds should taper off
after 02z with speeds falling below 15kt...and by Thursday afternoon
light/variable winds should occur much of the time.

&&

.FIRE WEATHER...
Thursday through Monday:
Showers will continue to percolate through the eastern districts
Thursday while drier conditions work into the western districts.
More widespread dry weather will finally spread into the region over
the weekend. Regardless, given the pronounced wet weather preceding
this period, fuels across the majority of the area will be quite
moist for an extended period. Temperatures will cool substantially
Thursday, then only rebound near the seasonal average over the
weekend while humidity levels likely will not fall below 50% for an
extended period.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch until 11 PM MST this evening for AZZ534-
     537>563.

     Flash Flood Watch until 5 PM MST this afternoon for AZZ530-531-
     533-535-536.

CA...None.
&&

$$

DISCUSSION...Hirsch
PREVIOUS DISCUSSION...Kuhlman
AVIATION...CB
FIRE WEATHER...MO