Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/24/23

Area Forecast Discussion...UPDATED

National Weather Service Indianapolis IN

 634 PM EST Sat Dec 23 2023

.Key Messages...

- Areas of dense fog through early Sunday
- Damp and Dreary through Tonight with occasional drizzle
- Temperatures 10 to 20 degrees above normal through Tuesday
- Widespread light/moderate rainfall expected Christmas day/evening
- Occasional light precipitation through the week...with a trend
  to snow flurries/few snow showers for the late week

&&

.Forecast Update...
Issued at 634 PM EST Sat Dec 23 2023

Latest ACARS sounding shows sufficient depth for some light drizzle.
The larger scale pattern is supporting slightly diverging 10-m
streamlines and only modest ascent at best in the saturated layer.
But, there is a shallow layer of weak instability below the capping
subsident layer, so some patches of briefly light/moderate drizzle
will probably occur through the evening and into the night.
Measurable amounts are unlikely. We have switched KIND radar to VCP
31 for more sensitivity to see very light hydrometeors tonight.

We issued a Dense Fog Advisory earlier through early Sunday.
Southern portions of this Advisory may experience enough dry
advection to cause improving visibility later in the night and
adjustments to cancel a portion of the Advisory early may become
necessary. We think that widespread persistent one quarter mile
visibility or less is less likely than periodic one quarter mile
with brief improvements. This is the nature of a neutral
temperature/moisture advection scenario with weak MSLP gradient and
mechanical mixing, and without a radiative component.

&&

.Short Term...(This evening through Sunday)
Issued at 300 PM EST Sat Dec 23 2023

IR satellite imagery depicts deeper cloud cover pushing east of the
Ohio Valley this afternoon, with some agitation in the mid-lvls to
the top of the stratus layer, suggestive of some omega presence to
parcels. Upstream of this feature in Southern IL there is some
improvement to the thick stratus deck where some warm air advection
is able to provide marginal mixing to the stratus deck. Until this
arrives across Central Indiana, expect continued thick solar
shielding to occur and limit mixing due to the decoupled lower
levels. Some fog has developed as a result of the very moist
stagnant airmass; however, as mentioned earlier about the agitation
to the clouds aloft over Central Indiana the VSBYs have been able to
improve slightly.

The main concern for tonight will be on potential for redevelopment
of fog and drizzle. With the continued moist shallow layer and lack
of omega or vertical movement to parcels, expecting fog to redevelop
around sunset or shortly after. Guidance has been consistent with
indicating several points along and north of the I-74 corridor in
Central IN to see VSBYs reduced to around 1/2 mile, and could easily
see some patchy dense fog develop due to the recent rainfall and a
saturated environment that is poised to not observe any mechanical
mixing. Further supporting this is that we are trapped between
systems with heights on the rise. Confidence is not high enough yet
that we would need a dense fog advisory, so instead went with an SPS
for the areas of greatest concern to see reduced VSBYs this evening.

Sunday should start with the continued stratus deck overhead, but
southerly flow does appear to be strengthening. Guidance has been
consistent with this setup, and should help to steadily erode the
cloud cover. But given the clouds are still present well upstream,
have taken a conservative approach to the clouds to depart and held
onto mostly cloudy conditions until midday. Then with the modest
warm air advection and increasing southerly flow aloft, expect
surface parcels to realize this and easily push well into the upper
50s for most locations, and likely even the lower 60s along and
south of I-74.

&&

.Long Term...(Sunday night through Saturday)
Issued at 304 PM EST Sat Dec 23 2023

Sunday Night and Christmas Day...

Guidance is continuing to show the potential for another moderate
rainfall event through Christmas` daytime and evening hours...with
still some disagreement on probability of widespread 0.50 inches of
rain, dependent on how much heavier convective rains to our
southeast may zap this potential.  Into a strengthening/occluding
surface low over the Middle Missouri Valley will be an elongated
ribbon of Gulf and Atlantic deep moisture...whose fetch may struggle
to provide prolonged steadier rain north/west of the Ohio Valley.
Nonetheless, expect 0.25-0.50 inches for most of the region, with
the potential for 0.50-0.75 inches south of Interstate 74 should
steadier rainfall rates continue into Christmas Afternoon.

Additional light rainfall Christmas night should total less than
0.10 inches with best forcing spinning north-northeast of the
region.  Fog may become a concern following the main slug of rain
with south-southwesterly winds progged to drop off later Monday
night into Tuesday morning.  A warm-advective flow will persist
through early portions of the long term with temperatures progged to
drop to only the upper 40s for most locations Christmas Eve...with
readings rebounding to mainly the upper 50s Christmas Day.

The record high minimum for December 26 is a relatively-low 47F,
with the overnight low early on the 26th likely staying above this
level...before late evening readings likely falling closer to 40F.

Tuesday through Saturday...

Despite varying model solutions over the past few days regarding the
rate of progression/departure of the broad central US upper trough
through the late week...decent agreement currently exists on a chunk
of northern energy plunging from interior Canada through the Upper
Midwest around the Thursday timeframe.  This evolution should be the
fulcrum that lobes the lingering cut-off east of Indiana...with the
arrival of seasonably cold Canadian air promoting at least a day or
two of widespread flurries and perhaps a few low-impact snow showers.

Yet prior to this return to more typical winter conditions...rather
brisk southwesterly winds Tuesday amid the wide system`s dry
conveyor will push organized rain our east while allowing one last
day of well above normal temperatures.  Lower heights and H850
temperatures under the belly of the stacked low will return readings
to only slightly above normal through the rest of the mid-week...
while embedded vort maxs bring occasional chances for light and
widely scattered precipitation...which will then trend from rain to
mixed to snow p-type while potential for measurable precipitation
decreases.

The late week will be more typical early-winter courtesy of a
pronounced, if not broad upper ridge occupying nearly all of western
North America...promotes a lingering troughy/zonal or northwest flow
into the Midwest.  Broad Canadian high pressure centered over the
northern Plains will continue to infiltrate into the region...with
Lake moisture trapped in this somewhat cyclonic flow continuing to
support flurries.  Considerable cloudiness and wide open lakes will
guide lower diurnal spreads amid the 30s/20s.

&&

.Aviation...(00Z TAF Issuance)
Issued at 626 PM EST Sat Dec 23 2023

Impacts:

* LIFR to VLIFR conditions common through Sunday morning

Discussion:

A moist environment with neutral moist advection, and long nocturnal
period will support steady conditions through early Sunday and
potentially longer than TAFs indicate. It`s difficult this time of
year to recover from low ceilings and visibility hoping for help
from diabatic surface heating. Nevertheless, slightly drier air
upstream may eventually improve conditions across at least southern
portions of the forecast area. Later updates may have to extend poor
flying conditions longer than current TAFs indicate.

Forecast confidence is moderate to high with the main
caveat/uncertainty being when conditions will improve. 1/4SM or
lower conditions may not be persistent and may bounce to 3/4SM at
times. This is the nature of a neutral temperature/moisture
advection scenario with enough MSLP gradient for mechanical mixing.


&&

.IND WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 11 AM EST Sunday for INZ021-028>031-
035>049-051>057-062>065.

&&

$$

Update...BRB
Short Term...Beach
Long Term...AGM
Aviation...BRB



Area Forecast Discussion

National Weather Service Phoenix AZ

 452 PM MST Sat Dec 23 2023

.UPDATE...Updated 00Z Aviation Discussion.

&&

.SYNOPSIS...
A gradual warming and drying trend is expected this week in the
wake of the low pressure system that produced widespread rainfall
across the Desert Southwest. A return to slightly above normal
temperatures is expected late in the week as high pressure
predominates across the intermountain West.

&&

.DISCUSSION...
Latest RAP streamline analysis indicates the trough axis has
pushed eastward into New Mexico. Meanwhile, water vapor imagery
reveals a sharp meridional delineation between moist air across
eastern Arizona and a northwesterly flow transporting drier air
across the Desert Southwest. However, conditions remain quite
moist in the lower levels and surface dewpoints have dropped only
slightly into the upper 40s and lower 50s. Dense fog was observed
this morning, mainly across the Imperial Valley though spots as
far east as Phoenix saw reduced visibilities. Clouds have been
prevalent for most of the day across Arizona, where several
smaller vortices promoted the early development of mainly closed-
cell cumulus with bases generally below 3500 ft.

Latest ACARS soundings from Phoenix continue to indicate a
building subsidence inversion and with little to no instability,
showers have struggled to form, except where lift has been
enhanced across isolated portions of the higher terrain north and
east of Phoenix. PoPs have consequently been lowered below the NBM
through Sunday morning, which has been somewhat slow to respond
to this trend. The abundant moisture will instead translate into a
threat of low clouds and fog across portions of the area early
Sunday morning.

The development of fog Sunday morning will largely hinge on to
what extent skies clear and the subsequent radiational cooling.
Latest HREF does in fact indicate widespread potential across
portions of the lower deserts. Raw probabilities are generally
around 10-30 percent, though actual odds are in reality a bit
higher, given the favorable antecedent conditions. Any fog that
develops Sunday morning will have the potential to produce
visibilities less than one quarter of a mile along with hazardous
driving conditions.

Model ensembles continue to show little variability through next
week, depicting gradual height rises and a persistent
northwesterly flow with a drying trend. A high amplitude ridge
will likely stretch from the Desert Southwest well northward into
far northwestern Canada by midweek. However, height anomalies will
only peak slightly above normal yielding temperatures initially
near normal followed by a warming trend later in the week. Light
winds and mainly clear skies associated with the ridge will
promote areas of strong radiational cooling, along with the
potential for freezing conditions each morning across portions of
Pinal County, particularly between Arizona City and Casa Grande.

&&

.AVIATION...Updated at 2350Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
The primary weather concerns through Sunday morning will be
periods of upper end MVFR cigs over the next couple hours, then
the potential for dense fog around the Phoenix metro Sunday
morning. Current west winds are very light (and nearly calm in
some cases), and may become variable at times as ceilings start to
lift and break up over the next couple of hours. Ceilings will
gradually become more scattered through the evening and
potentially clear overnight with the exception of locations on the
far east side of town where very moist air is drawn over higher
terrain.

A greater concern is the potential for areas of dense fog Sunday
morning with at least a 30% chance of IFR or lower flight categories
around the metro. If clouds hold more steady through the night, fog
would be more localized. However, similar historic scenarios along
with hints from numerical guidance suggest more widespread dense fog
with VLIFR flight categories around the metro (particularly at
KIWA). At this time, confidence is moderate with respect to this IFR
or lower outcome, and have continued with the variety of Tempo
groupings with this package.

Southeast California/Southwest Arizona including KIPL and KBLH:
Light and variable winds are expected through the TAF period for
KIPL whereas they will develop a more defined NW component early
Sunday morning at KBLH. Clear skies are expected at both sites
through the TAF period. While there is another chance for shallow
fog to develop Sunday morning, odds of direct impacts to any
given terminal is less than 25% and not included in this TAF
package.

&&

.FIRE WEATHER...
An unsettled and moist weather pattern will exist over the region
through the weekend, and temperatures are expected to remain
slightly below normal levels through the first half of next week.
Additional scattered showers are likely Saturday across the
eastern districts, with accumulations focused along higher terrain
areas. Nearly saturated conditions will be common over the weekend
with humidity levels generally not falling below 60%. Drier weather
returns next week, however humidity levels should remain above a 25-
35% range. Winds should largely remain below 15 mph through the
period.

&&

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

$$

DISCUSSION...Hirsch
AVIATION...Berislavich/18
FIRE WEATHER...Whittock