Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/21/20

Area Forecast Discussion

National Weather Service Phoenix AZ

 502 PM MST Sun Sep 20 2020

.UPDATE...Updated Aviation discussion.

&&

.SYNOPSIS...
A weak slow moving disturbance will bring occasional scattered
clouds and very slight chances of isolated light showers and
sprinkles early this week. There is also a very slight chance of a
stray or isolated thunderstorm. The best chances will be in
southwest Arizona and some remote lower deserts outside of Phoenix.
Drier conditions return for the second half of the week. High
temperatures will fluctuate and also remain at least several degrees
above normal through the week.

&&

.DISCUSSION...
The upper air analysis depicts the anticipated weak, nearly
stationary closed low pressure disturbance centered over NW Baja
with a weak cold air trough over SW AZ at H5. Weak southerly
cyclonic flow dominated SW AZ ahead of the low with slightly stronger
southerly anticyclonic flow controlling SE-E AZ on the west flank of
the high to the east. The converging southerly flow was supporting
an axis of elevated/increasing PWs across SW AZ and the lower
Colorado river of 0.8-1.0" and some slightly elevated MUCAPE. The
latest midday ACARs sounding showed PW at 0.73" (near the same as
12Z) although with some elevated mid level moisture near 600-650 mb
and southeasterly flow through the lower BL. Low and mid-level GOES
WV and imagery showed an axis of slightly elevated moisture with sct
Cu across S-SE AZ and the borderland slowly drifting northwest in
advance of the low. Early afternoon radar had some isolated
light showers developing well south of Gila Bend.

The weak and relatively/mostly dry low pressure will linger over SW
and S-central AZ through Tuesday evening and bring a slight chance
of isolated, mostly light showers and sprinkles/virga, especially to
SW AZ and remote lower desert locations outside of Phoenix. GEFS PW
anomaly indicates moisture advection in advance of the low will
gradually increase PWs across SW and S-central AZ through Tuesday
somewhere near 0.9-1.0". There will be a minor increase in patchy
mid-level moisture and sct cloud cover near H6. Together with weak
vorticity and forecast weak CAPE/instability, a slight chance of a stray
or isolated thunderstorm can not be precluded--especially for the
aforementioned areas on Monday afternoon. The slightly more bullish
HRRR and ARW of the HREF family depict any isolated heavier showers
or an isolated storm as being very short-lived. Although most
members keep activity much more subdued. As such no significant
impacts are currently in the forecast with the exception of a slight
chance of dry lightning.

The day 3 and 4 NCEP/WPC 500 mb heights clusters and ensembles are
in good agreement on a progressive ridge of high pressure building
across S CA and the SW US through the Central Plains/US on Wednesday
and Thursday. By Friday the ridge flattens in response to a
progressive low and troughing zonal jet stream to the north. Drier
and slightly warmer conditions are on tap for the second half of the
week. With only a couple degrees of warming, highs will remain in
the low-mid 100s through the end of the week.

The weekend favors continued dry conditions with dry northwesterly
flow aloft and a chance of high pressure building across the West
Coast late in the period.

&&

.AVIATION...Updated at 0000Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Moisture will increase overnight ahead of a weak low pressure system
across the northern Baja Peninsula. Consequently, areas of
SCT-BKN mid clouds could persist into Monday. There is also a slight
chance of showers, though any cells that develop would be light and
brief. The focus for mid clouds and isolated showers will generally
shift into western Arizona Monday. Winds will also remain light and
diurnal at the TAF sites. Otherwise, areas of elevated haze
associated with distant wildfires will persist. Reductions in
surface visibilities are not anticipated, however slant range
visibilities may be affected around sunrise and sunset.

Southeast California/Southwest Arizona including KIPL and KBLH:
A corridor of deeper moisture will be transported into western
Arizona and extreme southeastern California through Monday. A brief
shower may be possible near KBLH Monday afternoon, however
probabilities are too low to include in the TAF. Otherwise, areas of
elevated haze associated with distant wildfires will persist.
Reductions in surface visibilities are not anticipated, however slant
range visibilities may be affected around sunrise and sunset.

&&

.FIRE WEATHER...
Wednesday through Sunday:
A lingering slight chance of afternoon convection will continue
outside of the forecast area (White Mountains) Wednesday. Highs will
be slightly warmer on Thursday and Friday although still remain at
least a few degrees above normal through the period. Min RH values
trend down slightly through Thursday but will generally remain
in the 10-15% range most places. Max RH values trend down through
Thursday as well, especially over southeast CA and southwest AZ with
decreases of as much as 30%. But readings are expected to generally
remain above 25% across the forecast area. Winds will be mostly
light and favor normal diurnal pattern with periods of typical
afternoon breeziness.

&&

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

&&

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

$$

DISCUSSION...Sawtelle
AVIATION...Hirsch
FIRE WEATHER...Sawtelle/AJ