565 
FXUS64 KLZK 272132
AFDLZK

Area Forecast Discussion
National Weather Service Little Rock AR
332 PM CST Fri Dec 27 2019

.DISCUSSION...

Water vapor satellite imagery shows a strong upper level trough
over the southwestern CONUS, moving east towards the southern
Plains. Subjective surface analysis showed a warm front in place
across far southern Arkansas where temperatures and dew points
were observed in the lower 60s. The 12Z RAOB showed a very shallow
saturated layer this morning, extending from the surface to only
100 feet or so above ground level. This low-level saturated layer
was associated with the widespread dense fog that developed from
central Arkansas to the east and held in place for much of the
morning hours today. As of 20Z, the fog had dissipated across the
state as daytime heating was able to mix some of the drier air
right above the fog/shallow inversion layer towards the surface.

Tonight through Saturday night...

The large upper trough over the southwestern CONUS will lift
northeast over the next 24 hours with two significant shortwave
troughs moving through the larger trough axis during that time.
The first of these shortwave troughs looks to spread large scale
forcing for ascent over western Arkansas late Saturday afternoon
or in the 23/00Z timeframe. As the large scale and shortwave
troughs approach during the day on Saturday, they will induce
surface cyclogenesis over the central high plains, bringing the
warm front north of Arkansas Saturday morning. As the warm front
lifts north, low-level warm and humid air advection will likely
result in areas of drizzle across eastern Arkansas and may result
in scattered rain showers across western Arkansas where some weak
lift associated with the approaching trough supplements low-level
lift from warm air advection. 

Throughout the day on Saturday persistent low-level warm air
advection should be enough to cause rain showers to increase in
coverage until rain showers transition from scattered to numerous
during the afternoon hours of the day. Thunderstorms will begin
developing within the rain showers as the upper level trough
approaches and deeper forcing for ascent helps to lift and cool a
low-level inversion. The upper trough is expected to approach
western arkansas late Saturday afternoon or early evening, likely
causing a focused line of showers and thunderstorms to develop and
move east. This line of showers and storms will move east across
Arkansas after sunset Saturday evening/night and this period
represents the most likely window where we could see some strong
to severe thunderstorms move across the state.

The severe weather threat remains largely conditional on the
amount of warm and humid air advection combined with the impact of
the lift associated with the upper trough on the low-level
inversion. Forecast soundings indicate that before the upper
trough arrives, all showers and thunderstorms will remain 
elevated above an inversion located around the 850 mb level.
Stronger low-level warm and humid air advection would help weaken
the inversion by heating the air below the inversion, and large
scale forcing for ascent associated with the upper trough will
weaken the inversion by lifting and cooling the warm air at the
850 mb level. These two forces combine over Arkansas in the 23-08Z
timeframe with a line of strong storms essentially developing
across western Arkansas closer to 23Z and moving east out of the
CWA towards the Mississippi river after 08Z. 

The GFS/NAM/ECMWF were all a little low/cool on temperatures and
dew point values across southern Arkansas this afternoon. This
doesn't necessarily mean that Arkansas will see warmer or more
humid air move north ahead of the upper trough, but it's certainly
worth keeping an eye on over the next 24 hours. Even if warmer and
more humid air makes its way across the state, that's only half of
the equation as it relates to the severe weather threat. The
amount of lift/cooling of the inversion remains a significant
forecast question and will likely be the deciding factor on how
much, if any, severe weather develops and moves across the state
Saturday evening. All forecast soundings show a long clockwise
looping hodograph with the inversion in place, but any isolated
storms out ahead of the eventual squall line that is expected to
develop should remain elevated and unable to take advantage of the
low-level helicity forecast in the 0-3km hodographs. This should
prevent any thunderstorms from becoming severe out ahead of the
line of storms/squall line that will likely develop as the upper
trough approaches. 

As the squall line approaches, there is a small window of
opportunity of an hour or two where forecast soundings maintain 
2-300 j/kg of helicity in the vicinity of the squall line where 
surface based convection is possible. Surface and most-unstable 
CAPE values are only forecast to be on the order of 500 j/kg, so 
in general, the threat of supercells...especially isolated 
supercells, appears to be very low at this time. The main thing 
to keep an eye on will be the strength of the squall line that may
develop, and how organized that becomes relative to the low-level
wind field. 

A well developed squall line will likely be able to drive the 
low-level jet down towards the surface, and all model guidance 
consistently advertises a 50 kt low-level jet in the 925 to 850 mb
level. Due to the strong forcing for ascent and brief window of 
opportunity for surface based convection, think that damaging 
winds will be the primary threat with any strong to severe storms 
that move across the state. There is a small threat for a tornado 
embedded within a squall line, but with the very brief window of 
opportunity for surface based convection, the current thinking is 
that the tornado threat will remain low, although non-zero, as the
line of storms moves across the state. The threat of 1 inch or 
larger hail appears to be very small due to the lack of stronger 
cooling aloft.

Behind the squall line a second shortwave trough is expected to
move northeast over Arkansas on Sunday, bringing a second round of
rain showers over the state. This second round of rain showers is
not expected to have any threat of severe weather, and will
primarily affect the central to eastern portions of the state.
This second shortwave trough will send a Pacific-type cold front
across the state Sunday afternoon, bringing cooler and drier air
across the state by Monday evening and pushing all precipitation
east of the state.

Tuesday and Wednesday...Surface high pressure and dry air in place
over the state will lead to cooler and drier conditions, with no
rain chances in the forecast as a result. Temperatures will remain
seasonal with highs in the 50s and overnight lows in the 30s for
most locations.

Thursday through Friday...Another large upper level trough is
expected to move east towards Arkansas, but this one is expected
to move much farther south than tomorrow's trough. Due to the
southerly track of this upper level storm system, low-level
cyclogenesis will likely develop along the Texas Gulf coast before
lifting northeast and remaining just east of the Mississippi River
Thursday night into Friday. For this time of year, this is a very
favorable upper level pattern that has traditionally brought 
winter weather across Arkansas. The big missing ingredient this 
time around will be the presence of cold air which will not be in 
place ahead of this upper trough. As a result, just have rain 
showers in the forecast with no threat of severe or winter weather
in the forecast at this time.

Saturday and Sunday...Behind the Thursday/Friday upper trough,
drier air will move back in across the state and we should have a
dry weekend for the first weekend of 2020.

Cavanaugh

&&

.PRELIMINARY POINT TEMPS/POPS...
Batesville AR     60  53  64  52 /  10  20  70  90 
Camden AR         69  58  69  54 /  20  20  70  90 
Harrison AR       57  51  63  45 /  10  40  90  90 
Hot Springs AR    66  58  66  50 /  20  30  80  90 
Little Rock   AR  65  56  67  53 /  10  20  70  90 
Monticello AR     67  58  69  57 /  20  20  60  90 
Mount Ida AR      67  59  67  47 /  10  40  90  90 
Mountain Home AR  59  52  62  48 /  10  40  90  90 
Newport AR        62  55  66  54 /  10  20  70  90 
Pine Bluff AR     66  56  68  55 /  20  20  60  90 
Russellville AR   65  57  65  49 /  10  30  90 100 
Searcy AR         64  54  67  53 /  10  20  70  80 
Stuttgart AR      66  56  68  55 /  10  20  60  90 
&&

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

$$

Short / Long Term...CAVANAUGH