Area Forecast Discussion
Issued by NWS Little Rock, AR

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FXUS64 KLZK 162335 AAA
AFDLZK

Area Forecast Discussion...UPDATED
National Weather Service Little Rock AR
535 PM CST Sat Dec 16 2017

.DISCUSSION...

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

&&

.AVIATION...

VFR conditions will be seen to start off the period...but expect
conditions to decrease overnight as moisture/rain lifts NE across
AR. Expect MVFR to IFR CIGs overnight...with some reduced VIS.
Expect the rainfall to end from SW to NE by just after sunrise
Sun...though flight rules may only improve slightly by Sun
afternoon.

&&

.PREV DISCUSSION...(ISSUED 258 PM CST Sat Dec 16 2017)

SHORT TERM...Tonight Through Monday

Brace yourselves...but rain is currently spreading north-
northeastward through central Texas and will move into the
forecast area later tonight. Before you get too excited however,
while any rain is better than none, only locations in the
southeast will see meaningful accumulations. Still expect to see
somewhere between 0.75" to 1.25" of rain south and east of areas
like Camden, Pine Bluff, and De Witt. The further north and west
you go, the lesser the rains. Expect northern and northwestern
Arkansas will see a quarter inch or less.

Otherwise, no significant weather impacts to talk about in the
short term. The incoming rainfall will largely be out of the state
by 9-10am tomorrow morning, with dry conditions rounding out the
remaining part of the period.

LONG TERM...Monday Night Through Friday

The period starts off with southwest flow aloft ahead of a strong,
fast moving shortwave trough. The models have trended a bit towards
the ECMWF solution, which would allow a longer period of return flow
across Arkansas. Exact timing and placement of cyclogenesis across
Texas will remain to be seen, but will likely move along or just
south of the AR/LA border. Lift from DPVA and upper level divergence
will be more than sufficient for rainfall across much of the state.
The highest totals will be across the south, co-located in the
higher moisture region ahead of the system. Rainfall totals of
anywhere from near 2 inches across the south to just a few
hundredths across the northern Arkansas will be possible.

Thursday through the weekend...

Anything after mid-week is a very complex forecast. Lee Carlaw at
FWD wrote a fantastic discussion this morning -- this upper level
scenario couldn`t be explained better, so I`ll just quote it.

"We know that exceptional amplification to the +PNA ridge, which has
been a staple of the large-scale weather pattern of late, should
occur as the ridge axis builds through the Arctic Circle, perhaps
coming close to scraping 85 degrees north by next weekend. We`re
also pretty confident that a vort max associated with energy
currently nearing the Bering Sea should crest this building ridge
axis during the middle of next week. What happens after this is up
in the air, and ultimately boils down to where this shortwave is in
relation to the ridge axis as an anticyclonic wave break occurs. Too
far east--like the most recent 00z ECMWF and last night`s GFS--and
this shortwave trough simply slips eastward, remaining an open wave
as it quickly transits Texas sans much fanfare at all. If this
system hangs back just a few hundred miles farther west as the 2PVU
surface pinches off, and you`ve got a developing closed upper-low
near Baja/Sonora."

These two scenarios are strongly supported by the nearly bimodal
solutions advertised by the main global model ensembles (GEFS, GEPS,
ECMWF Ensemble). There is slightly less ensemble spread than
yesterday, but not enough to gain confidence in either solution.
This uncertainty is due to limitations of numerical weather
prediction at this day in time. Small sampling errors in initial
conditions and/or limitations to model physics tend to grow with
increased run-time. The yo-yoing between deterministic solutions of
one model is likely indicative of the sensitivity of this
particular pattern. In many instances, models may "smooth out"
errors if they are not significant in scale. However, this is not
one of those instances.

Quick note to those weather enthusiasts looking at these ensembles:
With the bimodal solutions advertised in the ensembles, make use of
the spread products rather than the mean. The latter may provide
little value until the spread is reduced.

The closed low scenario is most concerning. A cold front will push
through Arkansas Thursday night into Friday, ushering in much colder
low to mid level air. Eventually, return flow in the middle levels
is expected. This isentropic lift precipitation scenario is
potentially hazardous for areas along and north of the surface
frontal boundary. In this scenario, there is considerable
probability that a N-S gradient of precipitation types
(snow/sleet/freezing rain/rain) would exist. Not only that, but this
transition gradient would likely move northward as a warm nose above
the surface deepens. So if areas see snow, they`ll likely see other
precip types afterwards. It`s also possible that all precipitation
falls as rain (or less likely: snow).

Determining where this gradient of precip types would be, if this
scenario even played out, is even further beyond the skill of
numerical weather prediction and this forecaster at this time. By
early to mid week, we should have a good handle on the overall
pattern (closed low vs. open wave). After that is determined, the
next step will be forecasting the depth of cold air and strength of
warm nose. With small temperatures differences making for large
variations in precip type, it`s important not to buy into 1 single
deterministic model run.

Be careful what you wish for.

If this other scenario (open wave) comes to fruition. Still expect
much colder air to filter into the region, but with little upstream
lifting mechanisms to generate precipitation in the subsequent
days.

&&

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

$$

Aviation...62


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