Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/16/23

Area Forecast Discussion

National Weather Service Indianapolis IN

 942 PM EST Fri Dec 15 2023

.Key Messages...

- Chances for rain with amounts likely less than one half inch this
weekend.

- Cold and windy Monday with light lake-effect snow mainly
over northeast sections. Wind gusts may reach 40 mph.

- Drier with moderating temperatures mid to late next week. Drought
conditions will continue.

&&

.Forecast Update...
Issued at 942 PM EST Fri Dec 15 2023

Cloud cover has thickened some, slowing the temperature fall. Still
had to adjust a few areas cooler for low temperatures though.
Readings will vary early on as thin spots in cloud cover move
across, but believe that readings will still become steady or rise a
bit overnight.

&&

.Short Term...(This Evening through Saturday)
Issued at 326 PM EST Fri Dec 15 2023

.This Evening and Tonight...

Latest observations and satellite imagery show a surface high
pressure just east of Indiana, with associated high clouds streaming
northward off an approaching system to the west. Increasing
southwesterly upper level flow and moisture advection ahead of this
feature will result in the thickening of these high level clouds
overnight. This is captured well in recent Hi-Res model soundings
and in upstream upper air observations. Here at IND, ACARs soundings
continue to depict a very dry atmospheric column, and the
approaching system must drastically modify moisture parameters if it
is to produce any kind of meaningful precipitation. As such, dry
conditions are expected to continue into the overnight hours, with
temperatures slowly falling into the mid 30s to lower 40s. Western
Indiana is likely to be a few degrees warmer than Eastern Indiana
tomorrow morning, in accordance with the aforementioned gradient. As
the elongated surface low slowly pushes eastward tonight, winds will
back towards the southeast and remain around 4-8 mph in magnitude.
This further supports the notion that temperatures will not fall too
dramatically overnight.

.Saturday...

The upper trough over the Plains will shift northeast towards Iowa
and phase with a secondary northern jet stream feature dropping
southwest out of Canada. An upper level jet streak ahead of the
trough should intensify over the Ohio Valley and Indiana, placing
the region in an area of enhanced lift and increasing upper level
moisture advection. A lower level jet also increases ahead of the
deepening trough, with a 40 kt southerly 850mb jet overhead by
Saturday evening. However, this low level jet is not connected to
the Gulf of Mexico.

The developing low down in the Gulf will likely inhibit direct
moisture transport from the Gulf northward into Indiana.
Southeasterly low level flow off the Atlantic ahead of that Gulf low
will pull moisture northwestward over the Appalachians and into
Indiana ahead of the system. This is not a very efficient moisture
advection set-up for the region, especially due to the downsloping
of the winds off the Appalachians and the associated adiabatic
warming/drying of the air in the lowest 5,000ft agl. Confidence is
high that models may be overdoing moisture advection into the region
and thus over-inflating PoPs and QPF amounts. As mentioned above,
the very dry 0-9km agl layer in place here at IND will take a while
to saturate to a magnitude at which any rainfall could reach the
ground. With all this in mind, went well under guidance for PoPs
Saturday, keeping much of Central Indiana dry until the evening.
Western portions of Indiana, especially locations right along the IL
border, may see very light rain earlier Saturday afternoon. Rainfall
should overspread the entire region towards the end of the short
term period Saturday evening from west to east, despite the less
than ideal set up for moisture advection in the region.

In short, expect a drier day Saturday, with warm air advection
producing above normal highs in the upper 40s to mid 50s across the
state once again.

&&

.Long Term...(Saturday night through Friday)
Issued at 326 PM EST Fri Dec 15 2023

Active start to the extended as the track of an intensifying low
pressure moving up the East Coast will lead to indirect impacts
across central Indiana this weekend and into early next week. Once
the system moves by and a brief but notable shot of Arctic air
advects into the region for early next week...we will essentially
resort to the mean for the middle and latter part of next week as
zonal flow aloft resumes spreading mild Pacific origin air across
the bulk of the county.

The primary focus for the extended period comes in two parts and
sets up late weekend into the early part of the upcoming week. Model
guidance has aligned well with the track and progression of the
intensifying Nor`Easter style system that will track up the East
Coast through Monday. The core of the direct impacts from the
surface low will remain well to our east...but the presence of a
trailing upper level wave and associated surface low and front set
to move across the Ohio Valley Saturday night and Sunday. Questions
have abounded for the last few days as to how much moisture the
northern stream system may be able to draw into the region from the
southeast part of the country as the main low pressure organizes.

Trends have supported a subtle bump in moisture advection into the
Ohio Valley over the last day or two...but there remains a vital
concern that the models yet again are overdoing precip potential
locally because they are not correctly diagnosing the antecedent
dry conditions present over the region. Have seen multiple systems
over the last several weeks where qpf values have been overdone
versus reality and that is a direct result of just how dry
conditions are across the region. In addition...not seeing a strong
southerly flow present through the boundary layer that would justify
a robust surge of deeper moisture into the Ohio Valley Saturday
night. If anything...the low level flow is largely from the S/SE or
SE which would mean the moisture would be coming overtop of the
Appalachians and susceptible to a downslope component that would dry
the airmass out. Basically in a nutshell...not overly impressed with
the moisture advection component and have cut down on qpf totals
through Sunday night.

The second part of the primary focus follows right on the heels of
the rain through Sunday and quite frankly...is likely to be the more
impactful portion of the forecast. The northern stream energy will
become absorbed by the intensifying coastal system and serve to
enhance the intensification process as it lifts through the Mid
Atlantic and into New England into Monday. This will promote an
increasingly tight surface pressure gradient that will be only
further strengthened by the passage of an Arctic cold front early
Monday followed by an amplifying upper level wave that moves across
the region for Monday. This will aid in the development of scattered
snow showers and flurries with a lake enhancement component as
boundary level flow aligns out of the northwest.

The expected orientation of the mean wind would support northeast
portions of the forecast area being impacted the most with perhaps
even light accums of a couple tenths of an inch generally northeast
of a Kokomo to New Castle line. Add in the potential for occasional
wind gusts nearing 40mph and temperatures remaining in the 30s and
you have the components for a cold...blustery and overall unpleasant
day. Will begin messaging the potential for these conditions on
Monday and will highlight the possibility for slick spots developing
on roads and other minor travel impacts as confidence increases
through the weekend.

In the wake of the upper wave passage Monday...surface high pressure
will expand into the region followed by a return to zonal flow aloft
resulting in the dry and mild conditions experienced at length over
recent weeks. Temperatures will recover into the mid and upper 40s
late next week as return southerly flow develops on the back side of
the broad surface high. Hints remain of unsettled conditions
returning heading into the Christmas holiday weekend but at this
early stage...all signs point to rain as the precip type. Much more
on this as we move into next week.

&&

.Aviation...(00Z TAF Issuance)
Issued at 554 PM EST Fri Dec 15 2023

Impacts:

- MVFR conditions developing toward the end of the period

Discussion:

Clouds will gradually thicken and lower into Saturday, with MVFR
ceilings moving in from west to east after 20Z Saturday. Rain will
accompany the MVFR ceilings, eventually lowering into the MVFR
category themselves.

Winds will generally remain out of the southeast, up to around 10kt
at times.

Confidence is only medium on arrival of MVFR conditions due to the
very dry low levels of the atmosphere.


&&

.IND WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

Update...50
Short Term...CM/Marcus
Long Term...Ryan
Aviation...50