Absolute Vorticity

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At this juncture, we shift our focus to Friday night into the day on 
Saturday -- a setup that continues to be concerning for the 
region. There are several items to break down as we discuss the 
expected evolution and potential impacts locally, so an attempt 
to do so follows here:

We have just reached the time window for which we can analyze
some of the various CAM solutions. And one thing that continues
to stick out, from a CAM /and/ a synoptic-based deterministic 
and ensemble perspective, is the depiction of a stronger/deeper 
system, which in turn would make the overall progression of it 
eastward... a bit slower. This trend is significant for several
reasons, most notably a corresponding depiction of slightly 
stronger LL wind fields (and corresponding LLJ maxes), as well 
as an indication that the threat for strong to severe storms 
will linger past daybreak on Saturday. 

This is a climatologically-favorable track that is /generally/ 
supportive of a severe threat across parts of the OH Vly. A 
slowly-deepening sfc low is fcst to track from N MO to Lake 
Michigan from Friday afternoon into Friday night. Deep lows on 
this track have a rather long history of being supportive for at
least some severe weather across the OH Vly. And although the 
low will be pulling away/N through MI by the time the FROPA 
actually occurs locally late Saturday morning, it will continue 
to deepen as it does so. Both deterministic and ensemble 
solutions have trended toward a deeper system, with the prospect
of a sub-985mb low in MI Saturday morning seeming more and more
likely. The overall track and deepening of the sfc low as it 
progresses from MO to MI puts the region squarely in a favorable
zone for robust isallobaric and LL mass field response, aiding 
in enhanced convergence and large-scale ascent, especially after
06z. And with this track and a very favorable dynamic and 
kinematic environment, traditional thermodynamic evolution, such
as instby, tends to become slightly less important as the mass 
transport/response can often be sufficient to produce strongly- 
forced vertical ascent amidst an increasingly-frontogenetic 
response. 

Looking more closely at some of the ingredients... the effective
shear of 50+ kts will be /more/ than supportive of storm 
organization, likely manifesting itself in a SSW-NNE linear 
convective structure for the overnight period, especially 
considering the /largely/ unidirectional profile with favorable 
vertical speed shear in place. A /very/ robust H9 LLJ of 
45-50kts and H8 LLJ of 60-65+ kts will overspread the ILN FA 
toward midnight and beyond, supporting tremendous LL mass 
response and ascent. It does appear there will be at least 
/some/ cross component of the LL bulk shear vector to the front 
itself, helping enhance LL shear and SRH, supporting 
circulations within any QLCS structure that is able to develop
(likely closer to the front very late in the night). Anticipate
that the magnitude of the LL shear (45-50kts) may be sufficient
to overcome the larger angular overlap with the initiating 
boundary. As for the instby... this may be one of the main 
limiting factors, despite the impressive theta-e advection/LL 
moisture transport north into the area ahead of the front (with 
sfc DPs in the upper 50s and lower 60s and PWATS approaching 
250-300% of seasonal norms). It is likely that one or more 
convective lines may develop along a pre-frontal trough or LL 
confluence axis, initially to the west of the ILN FA, with the 
front itself hanging back even further to the west across MO 
after midnight. Synoptic-based deterministic and ensemble 
guidance continues to depict a narrow ribbon of several hundred 
J/kg SBCAPE nosing north into IN and parts of N KY and W OH 
after 06z. This narrow ribbon of slightly more favorable 
thermodynamics will likely overlap, at least in a small spatial 
and temporal capacity, with the most favorable kinematic 
environment in place as the core of the strongest H8/H9 LLJs 
translates NE through the region between 06z-12z. 

Regarding storm evolution, there remains some uncertainty at
this juncture regarding the coverage /and intensity/ of
prefrontal activity in the 06z-12z time frame. Sounding analysis
from a variety of synoptic and CAM guidance suggests that a LL
isothermal layer or cap may remain in place across parts of the
area through at least 08z (and perhaps even several hours
longer). Eventually, the strong forcing and large-scale ascent
will be enough to overcome this, allowing for convection to
become more surface-based as we progress later into the night 
and beyond. If /and this remains a BIG IF at this juncture/ the 
prefrontal activity is able to become more sfc-based between 
08z-12z, the severe threat would likely be maximized a bit 
earlier (opposed to just along/ahead of the front itself). 

So taking this all in from strictly a setup and ingredients-
based fcst perspective, there is certainly some concern that the
OH Vly may be subjected to one or more slightly elevated (or 
even surface based) rotating clusters before daybreak and morning
QLCS (around/after daybreak). At this juncture, strong 
straight- line winds will remain the primary concern, but any 
convectively- driven sfc wave development could allow (with some
favorable LL shear orientation as well as some brief backing of
the winds), for circulations to develop with any activity that 
moves through. This would indicate a tornado potential in the 
ILN FA, at least in a small spatiotemporal manner. Will continue
to highlight this threat in the HWO, and could very well see 
/additional/ upgrades/expansion of SPC convective outlooks to 
encompass more of the ILN FA as we get closer to Friday night 
and we have a better look at some of the CAM guidance and can
further hone in on the timing and overlap of parameter spaces.

The other item, and it is not an insignificant one, will be the 
robust synoptic-based winds Friday night and especially during 
the day on Saturday. With the trend of a stronger system, it is
appearing more and more likely that a Wind Advisory will
eventually be needed for parts of the ILN FA, mainly for the
daytime Saturday. In the WAA advection Friday night ahead of 
the FROPA, could very well see sustained winds of 20-25kts, 
especially near/N of I-70, which in-and-of itself would be close
to advisory-level criteria. Behind the front during the
day/afternoon on Saturday, within the increasingly-well mixed 
BL environment, we could see gusts 30-40+ kts at times, again 
favored for locations further north. A stronger and slightly 
slower system evolution would naturally favor more enhanced 
synoptic-scale wind/gusts on Saturday, with indications for 40+ 
kts very possible across parts of the area. Additionally, with a
very moist and strongly- forced environment, any training 
convective activity may yield localized flooding, despite the 
progressive nature of the system as a whole. With /some/ overlap
between the steering- layer flow and the initiating boundary, 
some briefing training of activity will be possible, even with 
individual elements moving quickly within the overall evolution.
However, anticipate the overall flood/flash flood threat will 
be limited and isolated in nature.

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...SIGNIFICANT SEVERE WEATHER THREAT LATE FRIDAY NIGHT/EARLY
SATURDAY...

The overarching focus for the short term however is on the potential
for a classic high shear/low CAPE severe weather setup for the
region that likely does not get going until close to midnight Friday
night over the Ohio Valley. The aforementioned surface wave will
track across the central Plains Friday before lifting as a Lakes
Cutter style system into lower Michigan by daybreak Saturday. The
amount of mid and upper level energy diving into the trailing upper
trough is substantial and will promote steady strengthening to the
surface low to a sub-990mb level...and possibly a sub-985mb
level...by early Saturday. Ahead of the trailing cold front...strong
southwest flow aided by an 850mb jet poised to maximize at 70kts
across the area Friday night will transport rich low level moisture
into the region at levels much greater than normal for this time of
year. Temperatures will rise steadily through the course of the day
Friday before peaking Friday night in the 60s with dewpoints likely
rising as high as the low to mid 60s from north to south across the
forecast area as well.

For late Friday afternoon and most of the evening...expect showers
to steadily increase in coverage over much of the area. The arrival
of a sharp..diffluent 100kt+ 500mb jet rounding the base of the
trough to the west will however open the floodgates for convective
development as it interacts with the moist...highly sheared and
weakly unstable environment out ahead of it across the lower Ohio
and Tennessee Valleys. Model sounding data showing Tds above 60
suggestive of MLCAPE values that will peak at 500-1000 j/kg across
the forecast area from north to south after 05Z which will be more
than sufficient to generate convection.

Storm mode initially is likely to be a mixture of discrete cells and
small bowing lines/QLCSs that will carry a risk for damaging winds
and a few tornadoes in particular considering the amount of speed
shear and 0-1km SRH values over the region. The primary focus for
the greatest risk for tornadoes/rotating storms appears aligned from
the lower Wabash Valley southwest into western portions of Kentucky
and Tennessee...but the potential for spin ups will exist over most
if not all of central Indiana. While the directional shear is not
quite as extensive through the boundary layer...it is most prevalent
in the lowest 1-2kft with LCLs progged to be low as well...both of
which support the tornado risk. Remain most concerned about the
damaging wind threat with 50-60kts noted at just about 2kft off
ground level. The combination of stronger cells that can tap into
that along with a forward speed to showers and storms that could
approach 60mph raise the ceiling on the wind risk the convection
will carry. Most of the severe threat discussed above will focus in
the 05-09Z timeframe with a secondary risk from 09-13Z Saturday as
the cold front plows across the forecast area. Storm mode at this
point will transition to a predominant QLCS with bowing structures
and potential brief spin ups...but damaging winds will be the
predominant risk by this point.

From a historical analog standpoint...it is not easy to find this
type of a setup in December that peaks in the middle of the night
across the Ohio Valley and central Indiana. Many of these types of
events tend to occur during the afternoon or evening. One prominent
analog that keeps popping up via the CIPS is 11/6/2005 which is
being mentioned here strictly to highlight the inherent risk in play
Friday night. That produced widespread wind damage across central
Indiana and several tornadoes across the lower Ohio Valley...
including the significant Evansville tornado. One notable difference
however is that 925-850mb flow in that event was southerly where as
it looks to be more southwest for Friday night. This may serve as a
subtle limiting factor to the tornado threat and instead promote the
damaging wind potential.

So to summarize...confidence in a potentially significant severe
weather event for Friday night continues to grow. Messaging will be
critical locally for four main reasons:

1) it is a nocturnal threat
2) it is mid December
3) it has been a while since widespread severe weather has occurred
4) convection will be moving potentially at 60mph or greater

Concern is high because most people are not thinking about the risks
for severe weather in the middle of the night in the middle of
December. Be considering your safety plans now...and stay weather
aware.

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...SIGNIFICANT NIGHTTIME SEVERE WEATHER EPISODE EXPECTED LATE FRIDAY
AND SATURDAY MORNING...

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Synoptic Overview
===================================

A strong/strengthening upper level trough axis will move eastward
from the Plains through the Ohio Valley at the beginning of the
period.  Associated with the upper level trough axis, a strengthening
mid-level jet is expected to intensify to near 100-105kt as it passes
through our region accompanied by an intense low-level jet.  This
feature combined with a surface cold front will produce a round of
strong to severe thunderstorms with damaging winds and possible
tornadoes across much of the Ohio and Tennessee Valleys.  In the
wake of the system, the weather pattern is expected to become a bit
more quiet as a large upper level ridge builds across the southern
US and eventually into the eastern US. This will result in a
continued mild pattern for much of next week.  The next weather
system of note will approach the region toward the end of the
forecast period.

===================================
Model Discussion/Trends/Preferences
===================================

Overall, the 09/12Z models seem to have initialized well with no
noticeable errors.  Model continuity is fairly stable within the
high resolution guidance as well as the global guidance.  There has
a been a noted trend on slowing the Friday night severe event down
slightly and delaying it further into the night.  The NAM-WRF
solutions want to delay the frontal passage until after sunrise
Saturday.  The 09/12Z HRRR was slower as well, but a little faster
than the NAM runs.  The high res FV3 was similar to the NAM with the
frontal passage.

There remains a bit of spread regarding the convective development
across the suite of high resolution models.  The NAM is a little
more aggressive, while the HRRR is a bit less.  The FV3 core seems a
bit hot throughout, so we`re leaning more toward a NAM/HRRR
combination here.

Beyond the weekend, generally good continuity and agreement
continues with the global models.  So a GFS/Euro blend will be used
here.  The NBM guidance may be a bit underdone in the longer range
(around Wednesday/Thursday of next week).  However, I suspect that
we`ll probably have to adjust temps upward with successive forecasts
here if upper level ridging is as strong as advertised.

===================================
Sensible Weather and Impacts
===================================

As we go into Friday night, we expected to be well within the warm
sector of the approaching system.  A large wedge of warm air aloft
will be in place which will act as cap to supress much in the way of
convection for the evening hours.  Above this cap there will be
plenty of elevated instability.  This instability is expected to
increase a bit and erosion of the aforementioned cap is likely due to
1) increased moisture advection aloft and 2) mid-level cooling
overspreading the region.  Just how much erosion of this mid-level
cap remains in question as of this writing.  Additionally, marginal
low-level lapse rates and the lack of appreciable surface forcing
across the warm section lead me to believe that things probably will
remain rather quiet Friday evening, with an increase in convection
later in the night.

Deeper into the night, after 06Z, we expect isolated/scattered
convection to develop across the region.  Given the shear profiles,
some of this could be discrete and decently organized in nature with
wind damage and tornadoes being a threat.  The low-level forcing
here is not all that great and low-level lapse rates are marginal.
The convective models are more suggestive of stronger slabbed forced
ascent along the front out to our west.  A decent squall line looks
to form and then quickly move east across the region.  Throughout
the night, deep layer shear is expected to be strong and will easily
favor rotation with storms ahead and within the squall line.  0-3KM
bulk shear vectors are still not forecast to be orthogonal to the
convective line, but the wind speeds will be well in access of 30kts
yielding plenty of line normal bulk shear.  Therefore, any bowing
segment within the squall line will be capable of producing swaths
of damaging winds with embedded squall line tornadoes.  Outside of
storms, gradient winds will be an issue throughout the night.  Wind
gusts of 30-35 MPH will be common across the region outside of
storms.  Heavy rainfall will be possible with the convection given
that PWATs will be near record levels for mid-December.  However,
given the progressive nature of the system, short residence time of
heavy rainfall over the area should limit the flooding threat.
Nonetheless, a few isolated flooding issues will be possible.  Some
minor river flooding at our quick rising flood points on area river
is possible.

Given the high shear, low instability set up, we agree with this
being outlined as an enhanced risk by the SPC.  Historically,
overnight wind damage/tornado outbreaks are relatively uncommon in
the Ohio Valley in the month of December.  However, we have had some
recent severe weather outbreaks in mid-late December (12/21/2013 and
12/23/2015).  The current NAM guidance reminds a little bit of the
11/5/2006 outbreak that produced a squall line overnight and
tornadoes out over SW Indiana.  The coupled jet structure from the
NAM also reminds me a bit of the 2/5-6/2008 outbreak that produced a
nasty squall line across KY with quite a few mesovortices. However,
I want to make clear that analogs are just similar types of setups,
but do not necessarily result in the same outcome.