Area Forecast Discussion
Issued by NWS Little Rock, AR

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FXUS64 KLZK 072302 AAA
AFDLZK

Area Forecast Discussion...UPDATED
National Weather Service Little Rock AR
602 PM CDT Tue Apr 7 2020

.DISCUSSION...

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

&&

.AVIATION...

Some patchy MVFR CIGs may be seen overnight...with the best
chances for seeing low CIGs near sunrise. Otherwise...VFR
conditions will be dominant through this TAF period. Chances for
SHRA will be limited through 00Z Thu...though an isolated
SHRA/TSRA could be seen Wed afternoon. Better chances for precip
come beyond this TAF period.

&&

.PREV DISCUSSION...(ISSUED 304 PM CDT Tue Apr 7 2020)

SHORT TERM...Tonight Through Thursday

Early afternoon visible satellite data and surface observations
showed mostly cloudy skies over much of Arkansas, with clouds
clearing out from the southwest. Where clouds were breaking up
across the state, temperatures rose quickly from the lower to mid
70s to the lower 80s. Dew point values ranged from the mid 60s
across northern Arkansas to the low 70s in southern Arkansas.
Early afternoon water vapor satellite imagery showed some weak
ridging over the southeastern CONUS with a large upper low over
the west coast and a strong shortwave trough moving southeast
towards the northern plain out of Canada.

Tonight and Wednesday...

The Canadian shortwave trough is expected to move southeast over
the Midwestern CONUS breaking down the upper level ridge over the
southeastern CONUS by Wednesday afternoon. The strongest large
scale forcing for ascent associated with this trough will remain
north of Arkansas, however, as the trough breaks down the weak
upper ridging over the southeastern CONUS it will advect some
cooler air aloft in over the state. The net effect of this trough
on the flow aloft will be to change the flow aloft from southwest
to westerly by Wednesday morning. With the large upper low
anchored over the southwestern CONUS and the westerly flow aloft
over the central CONUS, surface cyclogenesis is expected to set
up over central Oklahoma by Wednesday afternoon.

This surface cyclone is not expected to become overly strong with
upper level support and lift holding steady over the southwest
CONUS or over the Midwest, downstream of each upper level
low/trough. The westerly flow aloft combined with the position of
the surface cyclone will lead to westerly flow aloft over Arkansas
from the 850 mb level to the 500 mb level. The 850 mb westerly
flow will bring hot and dry air off the southern plains to the
east, bringing a strong dryline at the surface into central
Oklahoma. As the westerly flow continues east it will increase in
altitude with cooler air over Arkansas acting like a bubble in
isentropic coordinates, causing the warm and dry air to rise up
from the near surface in Central Oklahoma to the 850 mb level over
Arkansas. This will lead to lead to a rare elevated mixed layer in
the lower to mid troposphere over Arkansas which will serve as a
cap to surface based convection through much of the day on
Wednesday, and will lead to steep mid-level lapse rates in the 850
to 500 mb layer.

The elevated mixed layer building east over Arkansas represents
low-level warm air advection and the westerly flow aloft due to
the Canadian upper trough will lead to cold air advection in the
500 to 300 mb layer leading to differential temperature advection
further steepening lapse rates over the state. Between the
advection of the elevated mixed layer over the state and cooling
aloft, steep lapse rates over the state will likely lead to
copious amounts of surface/mixed layer CAPE by Wednesday afternoon
with CAPE values climbing up above 3000 J/Kg by 21Z/mid-
afternoon. With mainly zonal flow aloft, most of the wind shear
profile will be unidirectional in nature, however even slightly
backed near surface flow (e.g. winds out of the southwest) will
provide some low-level directional shear, looping in a clockwise
fashion on a hodograph, consistent with warm air advection from
the surface to the 850 mb level. This will leave much of the state
with relatively strong deep layer bulk shear and the low-level
veering with height will provide some low-level helicity for
thunderstorms to ingest and stretch if thunderstorms are able to
develop.

The key forecast question for Wednesday is: Will thunderstorms be
able to develop Wednesday late afternoon or evening? The elevated
mixed layer consistently provides a convection initiation problem
over the southern Plains, but it is usually advected high enough
off the ground over Arkansas that it makes little difference
whether we get storms or not because the warm air aloft is lifted
and cooled as it moves east. That is not expected to be the case
tomorrow, so the elevated mixed layer will do what it commonly
does in the plains; it will allow CAPE values to climb up to
abnormally high levels over the state but it will also place a
fairly stout cap in place. This will allow temperatures and
humidity to increase near the surface without much of a chance at
convection initiation from heating or small scale lifting features
that may move across the state (e.g. gravity waves won`t be enough
to set off storms).

There will be some consistent lift over the state Wednesday, but
the primary lifting mechanism will be low-level isentropic lift,
which may not be sufficient to lift and cool the elevated mixed
layer enough to allow thunderstorms to develop. Thunderstorms are
most likely to develop Wednesday afternoon when a strong synoptic
scale cold front moves south across the state providing strong
mesoscale forcing for ascent as it moves across the state. Even in
this scenario, storms will only develop where the cold front can
lift the elevated mixed layer enough to cool off the capping
inversion and initiate convection. This is most likely to occur
over northeast and east Arkansas where the elevated mixed layer
will start off higher (around the 800 mb level) due to the
relatively cooler air in place over the state at this time.

To account for this fairly complex forecast, went ahead and placed
30 to 40 POPs over the eastern half of the state where the
elevated mixed layer is highest and most likely to be lifted out
of the way for convection to develop Wednesday evening. Assuming
convection does not develop until the front arrives, anticipate
that the primary storm mode would be multicellular to linear in
nature with large hail and damaging winds posing the primary
threats. With abnormally steep mid level lapse rates in place,
hail could be quite large in some areas, with baseball sized hail
possible in the stronger storms. If warm air advection/isentropic
lift is sufficient to allow isolated convection to develop out
ahead of the cold front, then we would probably be looking at
isolated supercell thunderstorms as the dominating storm mode. If
isolated supercells are able to develop out ahead of the front,
large hail will remain a significant threat, but the tornado
threat would be much higher as these storms would be capable of
taking advantage of the low-level directional shear increasing
low-level helicity powered by 3000 J/Kg or greater of CAPE.

Overall, the threat for tornadoes appears to be tied to the
chances for isolated supercells to develop out ahead of the cold
front on Wednesday. The chance of storms developing through the
elevated mixed layer cap look to be low, but not zero, as the 3 km
NAM was advertising this afternoon. Aside from the 3 km NAM, other
convective allowing models were not showing convection initiation,
and the convection parameterized GFS and NAM advertise very
different values of CAPE tomorrow afternoon. The NAM is
advertising almost 5,000 J/Kg of CAPE while the GFS holds
thunderstorm potential energy back in the 1500 to 2000 J/Kg range.
The difference here appears to be tied to how much dry air is
entrained from the elevated mixed layer down to the surface
tomorrow afternoon. The reality of available potential energy for
tomorrow is likely somewhere in the middle of these two models,
but we won`t know for sure until the elevated mixed layer gets
here and we can sample it with weather balloons and watch surface
observations below it for signs of entrainment.

Either way tomorrow...the elevated mixed layer over the state will
lead to little cloud cover in the morning through early afternoon
hours. Mostly sunny skies combined with persistent warm air
advection should allow highs tomorrow to climb near or above
record highs for the day with most locations topping out in the
mid to upper 80s.

Behind the cold front on Thursday...expect drier air to quickly
build south across the state bringing a quick end to precipitation
behind the front Wednesday evening or Wednesday night. Kept the
forecast dry on Thursday with temperatures holding 20 degrees or
more cooler than on Wednesday.

LONG TERM...Thursday Night Through Tuesday

Not a lot of value added changes will be forth coming this afternoon
as overall thinking has not varied much from this morning. Not to
say there are not some differences between solutions but using a
blend of models seems prudent this afternoon.

Models all agree in a rather progressive pattern through the period
characterized by broad, synoptic scale troughing over the nations
mid section. This will lead to temperatures well below normal for
early to mid April with freezing temperatures not out of the realm
of possibility.

Period initiates with an old frontal boundary sitting somewhere over
the northern gulf and high pressure dropping down from the central
plains and over the state. While some clouds from a passing wave to
the south are expected, Thursday night through Friday night will be
dry with near seasonal temperatures.

Meanwhile, a strong upper low will be located over the southern
Great Basin. This low will drop southeast and then eventually kick
out to the east. As it does, the aforementioned front will be
shunted back to the north and a wave of low pressure will develop on
the boundary. This will result in numerous showers and thunderstorms
for late Saturday through Sunday with the heaviest amounts likely
over the south.

Severe weather can not be discounted entirely with this system and
will be dependent upon where the warm sector ends up. ECMWF is most
negatively tilted with the trough as it swings through and with the
amount of cold air aloft, it will bear watching. A new front will
drop down from the north behind the departing wave Sunday night and
early Monday. It will likely stall to the south of the state which
will require at least keeping low end rain chances in the forecast.

Much colder air will sweep in behind this front as upper pattern
feature a broad upper trough covering the majority of the central
and eastern CONUS. Depending on how things play out with the final
position of this Canadian high, low temperatures could drop into the
30s late in the period.

&&

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

$$

Aviation...62


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