Area Forecast Discussion
Issued by NWS Little Rock, AR

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FXUS64 KLZK 241053

Area Forecast Discussion
National Weather Service Little Rock AR
553 AM CDT Sat Mar 24 2018

Widespread MVFR conditions prevail this morning with brief IFR
conditions possible as low level moisture remains abundant. A weak
disturbance may bring a few showers to the TAF sites but confidence
is to low to include anything more than VCSH at this point. A
cold front will move through the area later today with a wind
shift to the north in its wake with VFR conditions returns.
Southwest winds will remain a little gusty until the frontal


.PREV DISCUSSION...(ISSUED 429 AM CDT Sat Mar 24 2018)

SHORT TERM...Today Through Sunday Night

Overall no significant changes have been made to the forecast as the
period looks to be a little unsettled with the main showing coming
in for the extended period. Upper flow has transitioned to zonal
versus this time last night with weak shortwave energy passing to
the north of the state. Pattern will continue to support a slight
chance of generally light showers over the northern and eastern
parts of the CWA and mainly this morning. Precipitation amounts are
expected to be light.

Latest surface analysis places a warm front to the north of the
state at this time with temperatures in the 60s thanks to southerly
flow. Low pressure is located over eastern kansas with a trailing
cold front slicing through central Oklahoma. The low is forecast to
move to the east/southeast today with the associated boundary moving
through the state and stalling somewhere over south Arkansas or
northern Louisiana. Front is dynamically challenged with only a wind
shift really marking its presence although a stray shower can not be
excluded, especially over the south.

Tight pressure gradient this morning will bring breezy conditions to
the area. These conditions will slacken somewhat this afternoon as
the low pressure moves away. High temperatures will remain well
above normal today but cooler air will spread back over the state
Sunday in the wake of the front.

Boundary will begin to lift back to the north on late Sunday as a
warm front and be located over western/southwestern Arkansas by the
end of the period. Upper level energy will approach from the west
Sunday night and into Monday with precipitation chances increasing
in response.

LONG TERM...Monday Through Friday Night

By Monday morning, southwesterly H500 flow will continue to amplify
as a large upper trough crosses the Intermountain West. Given
increasing dynamic forcing aloft and and an uptick in warm air
advection within the boundary layer, showers will become more
numerous, especially across northern AR, and then overspreading the
rest of the state through Tuesday morning. As the parent upper
trough closes invof the Four Corners region late Tuesday into
Wednesday, surface cyclogenesis in the lee of the Rockies will act
to lift a warm frontal boundary north through the state while an
attendant cold front will push across OK. Increasing southerly flow
from the Gulf will aid in significant large-scale moisture transport
ahead of the slowly-advancing cold front, and will set the stage for
a now well-advertised period of prolonged rainfall, with much of AR
seeing on the order of 2 to 4 plus inches total accumulation through
late next week. While there are no wholesale changes to the forecast
thinking for this event as of this morning, some interesting trends
have been observed in model guidance. That said, we remain in a
holding pattern until the system moves ashore Sunday morning and can
be more thoroughly sampled by the RAOB network.

Synoptic-scale consistency remains higher than average attm, outside
minor timing and strength differences, while mesoscale features
remain poorly resolved. 00Z operational guidance suggests the system
may be more progressive, which would imply overall rainfall totals
may be slightly less than previous forecasts. Additionally, the GFS
remains the most aggressive operational solution and continues
nudging the highest QPF axis south. Time will tell if this is a
reasonable trend, but GEFS means continue showing a large area of 3
to 4 plus inches of rainfall for most of the area, as has been the
case for several days now. The operational Canadian and ECMWF runs
have come into slightly better agreement with one another and have
somewhat decreased overall QPF through the event. Still, both
solutions have widespread 3 to 4 plus inches of rain as well, more
in line with the latest GEFS analysis. Respective ensemble means for
both models support this thinking. NAEFS analysis remains very
consistent, showing 1.25 to 1.50 inch PWAT values, which are +2
standard deviations above the climatological mean.

Even with the high degree of mesoscale uncertainty and run-to-run
inconsistencies among operational members, the overall pattern still
strongly supports a prolonged heavy rain event. Elected to not alter
the QPF forecast much until guidance shows better agreement.
Regardless, given the antecedent conditions from heavy rain in
February and early March, the flood threat is expected to increase
as early as Tuesday evening. All interests are strongly encouraged
to maintain awareness of the latest forecasts, especially over the
remainder of the weekend, as finer-scale details hopefully become
more clear. In addition to the persistent rainfall, some thunder is
possible, mainly across central and southern AR. Instability fields
remain unimpressive and the likelihood of significant capping is
high, so no real concerns with severe weather this go around. Did
include a mention of scattered thunder, however.

By late Wednesday into Thursday, the upper trough opens and ejects
northeast with the bulk of precipitation ending west-to-east. Some
lingering showers could be realized if enough moisture remains on
the back side of the departing system, but confidence is low.
Temperatures are expected to begin slightly above average and cool
to slightly below average from mid-week on. Initially gusty south
winds ahead of the front will gradually relax once the boundary
pushes through.



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


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