Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 01/14/23

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 546 PM CST Fri Jan 13 2023

.SHORT TERM...
Issued at 149 PM CST Fri Jan 13 2023

Through Saturday night...

Disorganized lake effect snow showers will continue this
afternoon, primarily across parts of Lake and Porter counties in
northwest Indiana, with flurries expanding across parts of
northeast Illinois. Have seen a significant number of icing
PIREPs today, which is typically indicative of the loss or
significant reduction of cloud ice. Recent AMDAR soundings are
showing cloud tops hovering right around 5 kft and -11 C, and with
the subsidence inversion only continuing to build downwards this
afternoon, saturation will become increasingly marginal for
heterogeneous nucleation. While some flips in p-type to freezing
drizzle are possible, especially away from the "heavier"
convective elements, currently have not reflected this in the
gridded forecast given the pretty marginal environment (cloud
bases above about 2 kft with thicknesses under 3 kft), but will
continue to keep an eye on things. Have held onto lake effect
precip a bit longer into the evening across NW Indiana, but things
should be winding down through late this evening/early overnight.
Road temperatures are holding up well, generally in the mid 30s,
and while these will trickle downwards this evening, this should
continue to mitigate any travel impacts.

Cloud cover trends remain a pretty big wild card tonight and into
Saturday. Given how thin the stratus deck is, would have thought
we`d see a bit more in the way of clearing, but thus far that has
not happened (for the most part). Based on upstream satellite
trends, have bumped up cloud cover tonight and Saturday. There`s a
big temperature bust potential tonight (10-15+ degrees) if
clearing occurs suddenly. Am also not overly confident in sky
cover trends tomorrow afternoon, with aggressive guidance keeping
us pretty much socked in through the day. Have nudged highs down a
bit to get them more in line with this potential.

Carlaw

&&

.LONG TERM...
Issued at 257 PM CST Fri Jan 13 2023

Sunday through Friday...

Main forecast highlights and concerns:

* Breezy and mild Sunday

* Showers with isolated thunderstorms possible on Monday

* Potential for a colder storm system with accumulating wet snow
  and rain Wednesday night-Thursday

On Sunday, persistent pressure falls will help tap into a
strengthening low level jet, the extent of this depending on exact
mixing depths. The NAM is likely underdone given the lack of snow
cover and cloud cover signal being primarily mid and high level.
During the afternoon, gusts up to 30-35 mph, with some upside
potential in the western and southwestern CWA. Climatology of the
warmest 1/3 of progged 925 mb temps (given mild start, dry air
mass, and 0 snow cover) supports highs in the low-mid 40s to
locally upper 40s. Sunday night will be unseasonably mild for our
coldest time of year (solidly above normal highs) with gusty
southerly winds into or through the evening.

Later Sunday night through Monday, a neutrally tilted 500 mb
short-wave trough and associated surface low will eject northeast
from the southern/central High Plains to the upper Mississippi
Valley, and across southern Wisconsin Monday evening. Increasing
warm and moist advection and large scale ascent will foster
increasing shower coverage pre-dawn Monday through the day,
peaking mid day through early evening ahead of the dry slot. Cold
500 mb temps with the trough will steepen mid- level lapse rates
enough for minor MUCAPE and a threat for a few embedded
thunderstorms. Continued slight chance thunder mention roughly
along and south of I-90. Latest thinking continues to be that
MUCAPE is likely to be too meager for any stronger t-storm risk.

Muted cold advection and cyclonic flow aloft in the wake of the
system should keep a few spotty showers or sprinkles/drizzle
going into Tuesday, primarily near and north of I-80. It will
otherwise be a cloudy and mild day in the offing as highs reach
the 40s once again, following highs well into the 40s to around
50F on Monday/Monday evening. The next notable trough ejection
from the southwest will occur on Wednesday with broad low pressure
gradually consolidating and this low pressure then tracking
north-northeastward through Thursday. While this system is at a
range in which substantial run to run variability is common, the
overall trend has been a farther south trajectory of the synoptic
system (GFS in particular exhibited a several hundred mile shift).

While some guidance members in the 12z Friday 1/13 model cycle
shifted far enough south to keep meaningful precip much farther
south, a majority of operational and ensemble guidance affects the
entire area with precip, supporting maintenance of likely PoPs at
peak. A dry column with high pressure influence to start
Wednesday and early closing off of the 500 mb low continued to
suggest a slowed arrival of precip with this system. This entails
much of if not all daylight hours on Wednesday being primarily
dry, with PoPs adjusted downward to slight chance southwest 1/3.
arrival more likely.

Turning to Wednesday night and Thursday, farther south and east
starting point of the low pressure system and its track to the
north-northeast or northeast looks to be more supportive of
evaporative cooling and a marginal (temps in 30s) but sufficiently
cold column for wet snow roughly near and north of I-80, and
particularly I-55 and northwest. A strong enough synoptic system,
in addition to good large scale ascent, can also be expected to
draw a plume of well above normal PWATs northward. Thus regardless
of precipitation type, a solid precip. producer for this time of
year remains in the offing. Overall, the threat for impactful
winter weather in the form of heavy/wet accumulating snow for at
least portions of the area has inched up, again with important
caveats of this being 6 days out. Stay tuned to later forecast
updates for the Wednesday night-Thursday period. A colder and
active pattern looks to continue heading into late January per
solid ensemble consistency out in the medium-long range.

Castro

&&

.AVIATION...
For the 00Z TAFs...

Expansive area of MVFR CIGS blankets the Great Lakes and much of
the middle and upper Mississippi Valley. As inversion heights
lower tonight, anticipate CIGS will slowly build down to under
2000ft this evening and probably stay low end MVFR until they
clear out. Timing of clearing is lower confidence, but opted to
stick with more of a persistence forecast for now and will adjust
to early clearing when/if satellite trend support it. Not uncommon
for CIGS to lower immediately prior to clearing, so there is some
potential for a period of IFR CIGS later tomorrow or tomorrow
evening. Otherwise, no impactful weather expected.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...IL nearshore waters until 10 PM Friday.

     Small Craft Advisory...IN nearshore waters until 6 AM Saturday.

&&

$$

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