Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/22/20
Area Forecast Discussion
National Weather Service Phoenix AZ
615 PM MST Mon Sep 21 2020
.UPDATE...Updated Aviation discussion.
A weak slow moving disturbance will bring occasional scattered to
broken clouds and a slight chances of widely isolated light showers
and sprinkles through Tuesday. Drier conditions return for the
second half of the week. High temperatures will fluctuate slightly,
but remain at least several degrees above normal through the week.
The modest UL trough axis remains negatively tilted across far NW MX
and the AZ borderland with a cold air trough now centered over NE AZ
at H5. Slightly stronger cyclonic flow ahead of the trough was
shifting into SW AZ as it slowly shifts east. A thin zonal H3 60 kt
jet streak was slicing across S-central AZ just ahead of a mid-level
vort max. Early afternoon/midday ACARS soundings maintained modest
mid-level CAPE, BL CIN and PWs near 0.95-1.0". Models continue to
maintain the elevated PW anomaly and very weak and spotty mean CAPE
through Tuesday. Low and mid-level GOES WV and IR continued to show
the slightly elevated moisture axis with sct-bkn ACCAS across S-S-
Central AZ and the borderland slowly drifting northwest in advance
of the low. Early afternoon local radar shows the resulting isolated
light showers and sprinkles continuing mostly across S-Central-SW AZ
including the western half of Maricopa Cty.
The weak and relatively/mostly dry low pressure will shift slightly
east as it loiters over SW and S-central AZ through Tuesday evening
and continue to bring slight chances of widely isolated, mostly
light showers and sprinkles/virga and isolated moderate showers
across the lower deserts and Phoenix. For tonight and Tuesday the
best slight chances will be for Maricopa county westward, including
Phoenix. The only precipitation-related impacts might be some
locally breezy conditions near a heavier shower, due to some locally
elevated forecast DCAPE, or the very slightest chance of a lightning
strike from a very isolated renegade thunderstorm. Downpours and
flooding are currently not expected with QPF generally remaining at
little to none.
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 a broad troughing-zonal jet stream combo 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. A
gradual cooling trend for overnight lows begins in the middle of
this week with overnight reading bottoming out in the upper 60s for
the outskirts of Phoenix and lower deserts and the mid 70s for the
The weekend favors continued dry conditions with dry northwesterly
flow aloft and a fair chance of high pressure building across the
West Coast Sunday and Monday. Another stint of briefly warmer
temperature are possible starting late this weekend, although
some 7-day H5 height cluster uncertainty remains on the east-west
position of the ridge.
.AVIATION...Updated at 0115Z.
South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Mid clouds are expected to thicken once again tonight ahead of a
weak low pressure across southern California. Isolated showers will
likely develop across central Arizona around 07Z-09Z with the threat
continuing through mid-morning. If a shower does materialize at
any of the TAF sites, it may be strong enough to briefly wet the
runways. Meanwhile, diurnal winds will generally prevail at all TAF
sites. Improving conditions are expected Tuesday afternoon as the
aforementioned low pressure system lifts out into northern Arizona.
Southeast California/Southwest Arizona including KIPL and KBLH:
Isolated showers near KBLH will dissipate early this evening, though
some mid clouds may persist overnight. Otherwise, diurnal winds with
mostly clear skies can be expected through Tuesday.
Thursday through Monday:
A dry weather pattern will dominate the period with highs
remaining several degrees above normal in the low to mid 100s. Min
RH values near 10-14% will fall to near 10% on Sunday and Monday.
Max RH near 25-40% most places will gradually dry to the low to
mid 20s by Sunday across the lower deserts and 25-40% for the
higher elevations. Winds will be mostly light and favor normal
diurnal patterns with typical afternoon breeziness.
.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.