Area Forecast Discussion
Issued by NWS Little Rock, AR

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FXUS64 KLZK 160545

Area Forecast Discussion
National Weather Service Little Rock AR
1145 PM CST Fri Dec 15 2017

Light winds will be seen overnight...but pick up from the south
and SW on Sat with some gusts over 15 kts possible. VFR conditions
should persist through the period...with rain chances returning
to the forecast Sat night (primarily after 06z Sun).


.PREV DISCUSSION...(ISSUED 521 PM CST Fri Dec 15 2017)

Updated to include the 00Z aviation discussion below...


Light and variable winds will be seen overnight...but pick up from
the south and SW on Sat with some gusts over 15 kts possible. VFR
conditions should persist through the period...with rain chances
returning to the forecast Sat night.


SHORT TERM...Tonight through Sunday Night...

Shortwave energy and an accompanying surface trough/remnant cold
front will move through the state today. No clouds or precipitation
will be seen, but winds will veer to a more west or northwesterly
direction as these features move through.

The biggest story of the short term is rain. Yes, you heard
that correctly, rain. A cutoff low over the Baja will eject
northeastward tomorrow, bringing Gulf moisture into Arkansas from
the south. This would spread rain across much of the state beginning
late Saturday evening and lasting into early Sunday morning, with
precipitation coming to an end quickly from southwest to northeast
during the morning hours Sunday. Rainfall amounts over northern
parts of the state look modest at best, perhaps a quarter of an inch
or so for places in northwest and north Arkansas. However, areas
across the southeastern half of the state, especially southeast of
Camden to Pine Bluff to De Witt line, could see over an inch of rain.

Don`t expect a big cool down behind the weekend system however as
another cutoff low will replace the aforementioned Baja low by
Sunday night, leaving Arkansas beneath west to southwesterly mid and
upper level flow and generally in seasonal temperatures.

LONG TERM...Monday Through Saturday

The period starts off with southwest flow aloft ahead of a
progressive trough cut off from the northern stream. Notable timing
differences lie within the models regarding this trough. The NAM is
considerably faster and shallower, while the ECMWF is slower and
deeper. While not perfect, the *latest* (12z) GFS and Canadian
solutions advertise the trough passing through the region sometime
on Tuesday. Depending on exact placement, there should be
substantial lift from differential positive vorticity advection.
However, very dry air (< 0.75" pwat) and displacement with
surface low (well ahead) and boundary will likely inhibit much, if
any, precipitation. There is fairly high uncertainty given the
various model solutions. As far as the grids are concerned, I`ve
selected to go much more in line with the GFS as it`s closer to
the mean, discarding the ECMWF and NAM as outliers.

Moving on to the interesting part of this discussion...Thursday
night and onward through the weekend.

The main concern for this period will be the potential for a
hazardous winter weather scenario. However, at this time, there is
significant uncertainty regarding this scenario. For the last
several days, the main global models and their
ensembles/perturbations have shown two very different upper flow
patterns resulting in significantly different outcomes. some of
which are concerning. Both basic/quasi-mean solutions advertise a
trough beginning to dig across the western CONUS when models begin
to diverge.

*Basic* Solution 1: Upper trough digs southward and cuts off from
the northern stream, becoming a deep, closed upper low across the
southwest CONUS eventually retrograding into the pacific. In this
solution, the remaining trough associated with the upper stream
moves across the north central plains into the great lakes region
with a cold front pushing through the southern plains and mid-south
region. Aloft, comparatively warm and moist air begins to ascend
over the cold air that has just penetrated the region. With enough
isentropic lift and some jet-level divergence, precipitation will be
generated along and north of the cold (likely now stationary) front.
What type of precipitation? This will depend on the depth of the
cold air, which is highly uncertain at this time. But given strong
warm air advection overriding the cold surface air, it certainly
appears a freezing rain or sleet event would be favored. Even if the
event begins with snow, as many seem to be "wish-casting," the
precip type could easily transition to sleet and then freezing rain
as the warm nose increases in depth. So for those wanting to snow:
"Be careful what you wish for."

*Basic* Solution 2: Upper trough digs southward across the northwest
CONUS, but does NOT cut off from the upper stream. This results in a
much stronger upper trough and associated surface low moving quickly
across the north-central CONUS. A cold front would push through the
region with possibly little precipitation... mostly likely to be
rain. Something wintry on the back side can`t and won`t be ruled out
at this time with this event, but deep dry air filtering in quickly
behind this front would likely preclude much of anything frozen from

For those pondering which models are showing which solution, the
answer is simply: All. The ECMWF, Canadian, and GFS (ensembles
included) have depicted either of these solutions at some run-time
over the last couple days. Even the experimental/non-operational
FV3! Confidence is somewhat high on one of these two general
solutions panning out, however, determining which solution is
correct is beyond the current skill of numerical weather
prediction... and this forecaster. It should also be noted that
there are many variations of each solution... e.g. depth of cold air
during precip in solution 1.

In a scenario like this, it would not be wise to spend such time
chasing small differences with each of the models runs. As concluded
from Hamill 2003 in regards to dprog/dt, "extrapolation of forecast
trends was shown to have little forecast value." Instead, one should
look for general systematic model biases if possible when it comes
to determining how the model has trended with time. Lack of
consistency run to run may instead be an indicator of "flow
stability." Shafer (2017, email) states: "What I mean by this is...
If model run after model run produces similar results for a given
flow type/pattern, it suggests that small changes in that flow type
don`t mean a whole lot to the overall pattern.  But errors within
that duration as simulated by the model may become meaningful
(large), to the point where the flow type is changed -- which tends
to result in a "surprise" major shift in the model output."

So at some point (hopefully) soon, these errors will effectively be
scaled out of significance for the overall solution, and in due
time, we`ll know what we`re dealing with for Arkansas. Until then,
don`t buy into the hype. And again, be careful what you wish for.


.LZK Watches/Warnings/Advisories...NONE.


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