Area Forecast Discussion
Issued by NWS Little Rock, AR

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FXUS64 KLZK 252103

Area Forecast Discussion
National Weather Service Little Rock AR
303 PM CST Sun Feb 25 2018

.SHORT TERM...Tonight Through Tuesday

A mix of sun and clouds were noted acrs AR this aftn, with the bulk
of the sunshine seen acrs NW AR. Patchy -RA was confined to the far
SE part of the state, as an upper lvl impulse tracks along the
frontal bndry that moved thru the FA on Sat. Mid aftn temps were
mainly in the 50s.

The impulse tracking over SE AR wl shift E of the region later tngt.
Kept PoPs confined to the SE part of the FA, with assocd rainfall
moving out of the region before daybreak on Mon. Sfc high pres wl
again strengthen ovr AR, pushing the front further to the S of AR.
Abundant sunshine wl return for Mon, with high temps topping out in
the 60s at most locations.

The respite fm the wet weather wl be brief though. Heading into Tue,
the aforementioned high pres system wl have shifted E of AR. Clouds
wl again be in the increase as the old front starts to lift N as a
warm front. Rain chances are back in the fcst Tue aftn, acrs the SW
half of the FA, as WAA pattern/lift ramps up. Highs on Tue wl be a
bit cooler due to clouds/rain. Unsettled conds wl cont into the
middle of the week.

.LONG TERM...Tuesday Night Through Sunday

The long term period begins with the CWA in WSW upper flow on the
northwest periphery of a Gulf of Mexico high and a deep upper low
cascading down the CONUS West Coast. The subtropical and polar jets
become superimposed Wednesday in the southwest to central CONUS. As
the deep low ejects into the plains, surface cyclogenesis will occur
in the west-central plains, quickly ejecting to the northeast.

By Saturday morning, an extended period of moisture transport is
already occurring. Models are in agreement with an atmospheric river
originating from the Baja region coupled with lowest level flow from
the gulf of Mexico resulting in a plume of 400 to 1000 kg/m/s of
integrated water vapor transport (IVT). At the surface, a warm front
boundary will slowly be lifting northward across TX/LA/MS to near
Arkansas. Much of the state will likely be in drizzle or fog from
isentropic lift above the colder surface stable layer. Have kept fog
out of the grids for now since details will depend on meso-
microscale conditions once overall certainty increases. Otherwise,
light rain will be likely at first which is represented in the
forecast initially.

Model spread increases Wednesday with the depth of the approaching
trough, but most are in fairly good agreement with the overall
pattern. The exact depth and placement of the warm front boundary is
still too far out to determine unfortunately since the system in
question is still located in quick moving flow of northwest north
america. The upper levels will be sampled much better overnight and
tomorrow morning, thus forecast confidence should greatly improve
after this forecast.

That said, most models are in agreement with the warm front axis
getting somewhere into Arkansas. The warm front will likely be the
main focus for convection in Arkansas. PWATS are expected to range
from 1.0 to 1.5 inches over the event... which is anywhere from the
90th percentile for late February to record-breaking. Instability
will be marginal (but more in the case the warm front lifts farther
north than I expect), but should be more than sufficient with plenty
of upper level lift. Rainfall should increase throughout the day
Wednesday as the warm front slowly lifts northward. Later Wednesday
into Wednesday night, the cold front will push through the state.
There is currently high uncertainty regarding the strength of the
placement/strength/severeness of this convection. A slight shift
south in everything will mean no severe weather for Arkansas, while
a slight shift northward may result in a large part of the state
with the potential for strong storms. These details will be worked
out likely in the next forecast as forecast confidence is likely to
increase significantly in the next package for aforementioned
reasons. Regardless, an axis occurring somewhere near or south of
the east-of-Little Rock I40 corridor will likely receive several
inches of rain which will only aggravate any ongoing flood issues
from this previous event.

After the cold front moves through, temperatures will return to
below seasonal. It does appear that we`ll have a few days of dry
conditions afterward-- good news for those who need it! At this
point, the models diverge such that forecast confidence is generally
low at this point. Upper flow takes on split flow while subsidence
behind the cold front should result in clear skies for a bit. Low
level return flow begins on Sunday as high pressure slides eastward.
Rain chances will likely return Sunday or Monday but details are
unclear at this point.

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


.Short Term...44 / Long Term...Brown is the U.S. government's official web portal to all federal, state and local government web resources and services.