Area Forecast Discussion
Issued by NWS Little Rock, AR

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FXUS64 KLZK 221729 AAA

Area Forecast Discussion...UPDATED
National Weather Service Little Rock AR
1229 PM CDT Thu Mar 22 2018


Updated to include 00z aviation discussion below.



VFR conditions expected across the area through the TAF period.
Relatively light winds will be in place through Friday morning
before S-SW winds increase across the area. Clouds will increase
through Friday as well, with ceilings remaining at or above 5kft.


.PREV DISCUSSION...(ISSUED 616 AM CDT Thu Mar 22 2018)
VFR conditions will continue during this period. A southeasterly
near surface flow of 4-7 kts will prevail between 15 and 23z.


.Confidence increasing in another prolonged rainfall event
next week with flooding possible...

SHORT TERM...Today Through Friday Night

08z subjective surface analysis indicates a ridge axis from
southeast Arkansas, to western Illinois. This feature will continue
to slide eastward during the day. A low level warm air advection
pattern will develop during the rest of the period across the mid
south. A few light rain showers will be possible late in the period,
weakly focused by a developing warm front across northern Arkansas.
Above normal temperatures are expected for Friday.

LONG TERM...Saturday Through Wednesday Night

An increasingly amplified upper flow pattern is expected to develop
through the extended period, dominated by significant upper
troughing across the western CONUS and western Atlantic, with upper
ridging building into the eastern CONUS. Of most concern locally
will be a large, cutoff upper low migrating into the Desert
Southwest through the early part of next week. This will have
significant impacts across the state, including the likelihood of
prolonged rainfall and an increasing flood threat.

Through this weekend, a relatively flat upper ridge, punctuated by
shortwave troughing across the northern/central Plains, will drive a
surface cyclone southeastward across the Mid-Mississippi Valley by
Sunday morning. An associated cold front will sweep through north-to-
south during the pre-dawn hours Sunday. Modest boundary layer
moisture will exist, with dewpoints in the upper 50s to low 60s F,
and a 50+ kt low-level jet will offer some dynamic support with this
impulse. However, the best upper-level dynamics are expected to be
out of phase with the best low-level moisture. Add to that
significant capping based on forecast sounding analysis, as well as
unimpressive instability fields, and the probability of rainfall or
thunder look very limited as of now. Expect a brief cool down in the
wake of the front Sunday afternoon before southerly return flow and
warm air advection become re-established by Monday.

Meanwhile, a significant upper trough will continue migrating
southeastward during the early week time frame, with a closed upper
low developing near the Four Corners region by late Tuesday.
Comparison of operational model runs and ensemble guidance continue
showing uncharacteristic agreement with synoptic features and their
evolution, despite uncertainty with mesoscale details, particularly
the placement and behavior of a frontal boundary that may become
nearly stationary over some portion of the state next week. Early
thinking is this boundary will serve as a primary focus for the
heaviest rain, although it`s eventual placement and impact on
overall rainfall accumulation remain in question.

Regardless, NAEFS guidance shows an anomalously moist atmospheric
profile through much of next week, with PWAT values 2 sigma above
the climatological mean, much like was observed with the late
February into early March heavy rain event. Given the discrepancies
in mesoscale features and some synoptic timing differences, it
remains too early to make comparisons to the previous event or
delineate specific axes of maximum rainfall accumulation. Suffice to
say, confidence continues increasing that we will see another
prolonged heavy rain event with all modes of flooding (flash, areal,
and river) possible. Details concerning the magnitude of the flood
threat and associated timing will be ironed out in subsequent
forecasts, but wet antecedent conditions from previous rainfall have
primed numerous basins and soils already.

Decided to forego a strongly-held forecasting rule of thumb for the
extended period and increase PoP to likely all the way though day 7
given the very unsettled pattern in play. Thunder will also be
possible, mainly across central/southern AR where the best
instability is expected to reside. Even with persistent cloud cover
and rainfall, daytime temperatures through much of next week will
remain in the upper 60s to upper 70s F with 50s and 60s F during the
nighttime hours. A gradual cooling trend is expected by and just
after the end of the day 7 period.



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


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