Area Forecast Discussion
Issued by NWS Little Rock, AR

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FXUS64 KLZK 251143 AAA
AFDLZK

Area Forecast Discussion...Update
National Weather Service Little Rock AR
640 AM CDT Sat Mar 25 2017

.Aviation...

The surface and upper low pressure systems will move across AR today
and continue to develop scattered showers and isolated
thunderstorms. Used VCSH or VCTS in Tafs due to uncertainty in
location of convection. Ceilings will be VFR and MVFR with patchy
IFR ceilings to start, then through the day, mainly VFR ceilings will
mainly be seen. Winds will be southeast to southwest at 10 to 20
mph, then become southwest to west behind the cold front as it moves
through AR later in the day. (59)

&&

.Prev Discussion.../ Issued 433 AM CDT Sat Mar 25 2017

Main concerns in this forecast cycle remain chances of convection
for Arkansas, as a progressive west to east upper pattern sets up
that will bring several storm systems through the region. Each
storm system will have showers and thunderstorms, with some strong
to severe storm potential.

Overnight, the large area of convection had pushed across AR and
will continue to push east of AR by Saturday sunrise. Severe
storm reports were noted with mainly damaging winds in several
locations. Behind the main line, isolated to scattered showers
with some thunder will still be seen until the cold front and
surface/upper lows move east of the region. Temperatures were
mostly in the 50s to lower 60s, while dew point temperatures were
only a few degrees lower. Some areas of low clouds and fog will be
seen before the cold front moves through. Rain amounts were from
a few tenths to well over an inch. Winds remained elevated from
the southeast to east a head of the low and front, then will shift
to the southwest to west to northwest behind it.

&&

.SHORT TERM...Today Through Sunday Night

Forecast will start today with the lowest chance of convection in
the west, while the highest in the east. As forecast, the surface
and upper lows will gradually move east and take the rain chances
with them into Saturday afternoon. Highs today will mainly be in
the 60s to 70s. A dry forecast tonight with a bit cooler air as
lows will be mostly in the 50s. Sunday will be dry and see
increasing clouds through the day with highs in the 70s to low
80s. In the upper pattern, the next shortwave system will develop
convection as it moves through AR late Sunday to Monday. A slight
chance of isolated strong to severe storms will be possible. Winds
and hail will be the primary threats. Lows Sunday night will be in
the 50s to 60s.

&&

.LONG TERM...Monday Through Saturday

Monday...

Early morning water vapor satellite imagery shows a strong shortwave
trough over central California moving east. The consensus of model
guidance is that this trough will be moving over Arkansas Monday
morning between 12 and 18Z. As the strong upper trough moves over
the state...the GFS/NAM/ECMWF all show a warm front racing north
across the state...pushing north over Missouri by 15Z Monday with a
relatively strong surface low pressure system located over far
northeast Oklahoma. As the warm front is lifting north across
Arkansas...the GFS and NAM both indicate that large scale forcing
for ascent from differential positive vorticity advection (DPVA)
will move across the state from west to east. These same models
indicate the presence of 1500 J/kg of CAPE present along the warm
front with deep layer bulk shear on the order of 40 kts. Assuming
forcing is strong enough to initiate convection near the warm front
early Monday morning...the thermodynamic and kinematic environments
appear favorable for supercells with a reasonable tornado threat
near the warm front. The primary limiting factor with regards to a
tornado threat is the climatological rarity of tornadoes near
sunrise. It is likely that with the warm front moving across the
state during the typical coolest time of the day...that the tornado
threat will be somewhat dampened. However the synoptic environment
looks favorable...so cannot rule out the potential for an early
morning bout of severe storms across the state on Monday.

Once the warm front lifts north...Arkansas resides in the low-level
warm sector through Monday afternoon. GFS and NAM forecast CAPE
values increase to 2000 to 2500 J/kg with dew points forecast to
climb into the mid 60s while the cold air of the upper trough
resides over the state. Both models advertise QPF associated with
high relative humidity through the troposphere which is a good
indication that the models have entered their respective deep
convective schemes. What is a bit puzzling is that convection
initiation is activated despite apparent large scale forcing for
subsidence on the back side of the shortwave trough. It is possible
that significant mesoscale forcing for ascent resides in the low-
level warm sector but there is no obvious synoptic lift ongoing at
that time. Assuming convection does develop behind the upper
trough...there is certainly enough CAPE to support a severe weather
threat...however the tornado threat would seem to be reduced by the
veered low-level wind profile. As a result...would expect that any
storms that develop Monday afternoon across the state will primarily
be a large hail and straight line wind threat despite having more
available potential energy for storms.

A cold front will push southeast across the state Monday evening and
bring much drier air across the state leading to a dry forecast for
Monday night and Tuesday.

Wednesday and Thursday...The GFS and ECMWF both indicate another
powerful upper trough will dig southeast and move over the southern
plains on Wednesday. Where the models start to diverge is with
regards to their solution on the evolution of the upper trough as it
moves east over the southern plains Wednesday afternoon.

The GFS shows the upper trough breaking into two significant pieces
causing the reservoir of cold air aloft to take on a strong positive
tilt with the northern piece of the trough moving over Arkansas
Wednesday evening. If this solution verifies...then the chances for
showers and thunderstorms across the state are extremely high
Wednesday evening and Wednesday night...but the chances for severe
weather are relatively low as this upper trough evolution keeps the
warm sector south across eastern Texas and into Louisiana. Heavy
rainfall and flash flooding would be the primary threats in this
scenario. Rain chances would continue on Thursday...but primarily in
the form of rain showers with isolated thunderstorms.

The ECMWF keeps the upper trough consolidated into one large piece
of energy Wednesday afternoon...bringing it towards Arkansas
Wednesday evening while maintaining more of a neutral tilt. If this
solution verifies...rain chances remain high Wednesday evening and
overnight...however convection may tend to be more scattered versus
widespread during this time. There also appears to be a much higher
chance of realizing severe thunderstorms as the low-level warm
sector surges north across the state. At the same time deep layer
shear would be quite high with a jet streak moving over the state
Wednesday evening. Strong to severe thunderstorm chances would
continue on Thursday as the cold core of the upper trough is
forecast to be directly over the state as the synoptic cold front
pushes east Thursday afternoon.

At this time with the upper trough over the northern Pacific
Ocean... there`s little evidence to support one solution over the
other. Will keep a close eye on this storm system as it moves over
the CONUS Monday afternoon.

It is possible that another significant upper level storm system
will approach Arkansas next weekend. Being 7 days out simply went
ahead with chance POPs in the forecast to account for the potential
for showers and thunderstorms across the state. The trough certainly
appears to be strong enough to support some chance of severe
thunderstorms across the region.

&&

.PRELIMINARY POINT TEMPS/POPS...
Batesville AR     69  50  74  60 /  50  10   0  50
Camden AR         75  50  80  63 /  30   0   0  40
Harrison AR       60  47  71  55 /  40  10   0  60
Hot Springs AR    71  49  77  61 /  10   0   0  60
Little Rock   AR  72  51  77  62 /  30   0   0  50
Monticello AR     75  53  79  64 /  40   0   0  30
Mount Ida AR      69  47  76  59 /  10   0  10  60
Mountain Home AR  65  48  72  57 /  50  10   0  60
Newport AR        71  50  73  60 /  70  20   0  50
Pine Bluff AR     74  52  77  62 /  40   0   0  40
Russellville AR   66  47  76  59 /  20   0  10  60
Searcy AR         71  49  75  59 /  50  10   0  50
Stuttgart AR      73  52  76  63 /  50  10   0  40
&&

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

$$

Short Term...59 / Long Term...CAVANAUGH



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