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335
FXUS64 KLZK 172339
AFDLZK

Area Forecast Discussion
National Weather Service Little Rock AR
639 PM CDT Thu Aug 17 2017

.AVIATION...
High pressure is building over the area this evening with high
clouds dissipating. Expect skies to become clear overnight and
some patchy to develop in the south...with MVFR conditions. The
rest of the area will be VFR. Winds will be light.

&&

.PREV DISCUSSION...(ISSUED 259 PM CDT Thu Aug 17 2017
)

SHORT TERM...Tonight Through Saturday

An area of showers and storms made its way into western portions of
the state this morning and slowly diminished as it made its way into
central and eastern areas. With widespread cloud cover initially,
temperatures struggled. But, through the afternoon hours they were
starting to rebound in most places as cloud cover continued to
break.

At the surface, a weak cold front was situated over northwestern
portions of the state and some drier air will make its way into
northern counties. With clearing sky conditions expected tonight,
temps will be cooler than the last couple nights across the north
and near normal elsewhere. Temperatures on Friday will be warm(aka
summerlike) and with dewpoint values in the lower to mid 70s across
central and southern areas, heat index values will likely top 100 in
several spots.

Aloft, ridging will remain in place along and south of the region
through much of the short term, providing a mostly zonal flow across
the area. A weak shortwave will approach the state on Friday, with
some rain chances mentioned in the forecast for primarily western
portions of the state. Friday night into Saturday a better chance of
rain and thunderstorms exists as a strengthening trough advances
from the Great Plains through the Ohio River Valley, aiding in
thunderstorm development to our northwest. This will provide more of
a northwesterly flow over Arkansas, increasing the chances of a
thunderstorm complex making its way into the state by Saturday
morning.

LONG TERM...Saturday Night Through Monday

Early afternoon water vapor satellite imagery showed a band of deep
moisture extending from northeast Texas to the northeast across
Arkansas towards Indiana. Upper level ridging/anticyclonic flow was
noted over the AL/GA/FL border and a tropical upper-tropospheric
trough (TUTT) cyclonic circulation was observed southeast of
Florida. The band of deep moisture was associated with overnight
heavy rainfall and thunderstorms and will likely move south through
the weekend. This deep moisture will likely remain in place over
Louisiana through the weekend and into early next week. Through the
weekend...a strong upper level trough will move over the eastern
CONUS coast while the TUTT low pressure circulation moves west over
the Gulf of Mexico. These two systems will effectively weaken and
deamplify the upper level ridge allowing for diurnally driven
showers and thunderstorms each afternoon within the axis of deepest
moisture.

As a result...have thunderstorm chances in the forecast from Sunday
through Monday...with southern Arkansas having the better chance at
seeing a storm with the moisture axis centered on Louisiana during
this time. Rain chances and cloud cover in general are a big
forecast concern on Monday as much of Arkansas will be interested in
viewing the eclipse Monday afternoon. The time of maximum eclipse
will vary from about 1:10 to 1:20 PM Central Daylight Time across
the state. The start and end times of the partial phase of the
eclipse will range from around 11:45 AM through 2:45 PM. There will
likely be some cloud cover over the state during this period...but
there are some good indicators in model guidance that there will be
breaks in the cloud cover that will hopefully allow people enough
clearance to (safely) view the eclipse. The current cloud forecast
during the eclipse calls for 40-50 percent cloud cover across the
region...however it is important to note that it is still way too
early to have a truly deterministic cloud cover forecast. For those
interested in looking directly at model forecast output for the
eclipse...here are some pointers:

There is not an obvious large scale (synoptic) lifting mechanism
that will be in place across the state. With no obvious large scale
feature to provide lift (e.g. like a front, large upper level
trough, etc) confidence in rain chances will remain inherently
small. The TUTT low is expected to be moving slowly west over the TX
Gulf coast while a shortwave trough is moving over the northern
plains/western Great Lakes region. This will cause the flow aloft to
be westerly to the north of Arkansas but easterly to the south of
the state. This leaves Arkansas in a saddle point (col) in the flow
field which likely won`t favor any organized rainfall or
thunderstorm activity during the time of the eclipse. It is still
possible that remnants of thunderstorm activity from the weekend may
leave something in place that increases rain chances across Arkansas
on Monday...but this far out it`s best to look at the large scale
pattern and determine what factors will support rain versus a dry
forecast. It`s even more important to consider the large scale
pattern when considering raw model output cloud and rainfall
forecasts on Monday.

Monday afternoon sits in the 102 to 108 hour forecast period for
weather models that were initialized this morning (12Z). For people
taking a casual glance at two of the more popular models that
forecast out this far (the GFS and ECMWF), it`s easy to note that
both of these models advertise widespread light rainfall amounts and
cloud cover. For instance both the GFS and ECMWF forecast that
between 0.01" and 0.10" of rain will fall across almost the entire
state between 18Z and 00Z. It`s easy to misinterpret this output as
a forecast for light rain over the entire state for 6 hours...but
that is not going to happen...so don`t panic. If you would like to
better understand why the models do this read on...

Medium range models (4-7 days out) forecast thunderstorms using a
method called convective parametrization. All that means is that
when certain conditions are met in each model`s forecast fields, the
models will flip a switch and turn rainfall accumulation "on" for
the purposes of raw model output. This is a common error that occurs
in regions where warm and humid air resides at the surface with weak
forcing (e.g. no strong lift or subsidence) in place. To help
determine when a model is forecasting true deep convection, it can
be helpful to look at mid to upper tropospheric humidity forecasts
within the model fields. A good example of this is from the ECMWF
which forecasts 0.05" of rain over Little Rock Monday afternoon, but
at the same time forecasts a 500 mb relative humidity of 25%. It is
unlikely that rainfall will occur while it`s very dry aloft, so
humidity aloft is something you can look at within the model fields
to help separate what is more likely to be "real" rainfall and what
may be an error in one of the parametrization schemes.

As we get closer to Monday...forecast models with higher resolution
and more complex physics capabilities will become available to help
build a higher confidence forecast during the eclipse. Shorter range
models (24-48 hours out) attempt to model and forecast individual
storms as opposed to turning thunderstorm processes on and off.
These type of models are called convection allowing models and offer
forecasters a much better look at where storms are most likely to
wind up across Arkansas (if anywhere) during the eclipse. Any
individual convection allowing model is unlikely to have the right
answer...but when we look at all of the model output in an ensemble
we can have a much higher confidence forecast for rainfall and cloud
cover for early Monday afternoon. This is nothing new for the
eclipse...this is generally how and why forecast confidence
increases as we get closer to ANY weather event.

For now...the forecast during the eclipse across Arkansas calls for
partly cloudy skies and a 20-30 percent chance of showers and
thunderstorms. We will be refining this forecast carefully as Monday
approaches.

Tuesday through Thursday...

The medium range guidance agrees that a
strong shortwave trough will move southeast over the Great Lakes
region extending south through the Ohio and Tennessee valleys from
Tuesday through Tuesday night. This will send a cold front that will
likely be accompanied by scattered to numerous thunderstorms as it
moves towards Arkansas. The front may stall out across Arkansas for
Wednesday and Thursday which would likely provide enough lift for
scattered thunderstorms all along the front each day. The actual
position of the front will vary depending on thunderstorm activity
each day with more storms pushing the front more towards Louisiana
and less thunderstorm activity causing the front to hang farther
north. A larger upper trough will push southeast over the eastern
CONUS on Thursday which will finally send the front south out of the
state for Friday. Continue to have some low chance POPs in the
forecast on Thursday without much confidence in the timing of the
front. Friday and next weekend look pretty nice though with a much
drier and cooler air mass moving in across the state.

&&

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

$$


Aviation...51