Area Forecast Discussion
Issued by NWS Little Rock, AR

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FXUS64 KLZK 242351 AAA

Area Forecast Discussion...Update
National Weather Service Little Rock AR
645 PM CDT Sat Mar 2018


Overall VFR flight conditions, with some ceilings, will be seen. A
few light showers may be seen this evening to early tonight, as a
weak cold front sags through AR. Winds will be south to southwest at
5 to 15 mph ahead of the cold front, then shift to the west to
northwest behind the cold front. Some lower MVFR ceilings may be
seen toward Sunday morning over northern AR. Winds on Sunday will be
northwest becoming northeast to east at 5 to 10 mph. (59)


.Prev Discussion.../ Issued 339 PM CDT Sat Mar 24 2018/
.SHORT TERM...Tonight through Monday...

Latest GOES 16 data and subjective surface
analysis indicates a pre-frontal trough from near Walnut Ridge, to
near De Queen, with a cold front approaching Benton county.
Compressional flow ahead of the cold front has allowed for
temperatures to reach the upper 70s across northwest sections of the
forecast area.

The cold front will reach southern Arkansas late this evening before
stalling.  Forcing along the boundary will be weak, any coverage of
convection near the boundary will be small.

The boundary will lift slowly north through on Sunday, and will
provide some focus for convective activity.

The overall upper flow across the U.S. will undergo amplification
during the second half of this period, with southwesterly flow
developing across the mid south by late in the period. Well above
normal precipitable water values are forecast to be in place across
the region, for increasing convective coverage to act on.


.LONG TERM...Monday Night Through Sunday

A favorable pattern will set up across the CONUS for heavy
precipitation across much of Arkansas and neighboring states.
Flooding - both river and flash flooding - will be the highest
concern over the next week as areas will receive 2 to 5 inches of
rainfall w/ locally higher (and lower) amounts. There will be a
limited severe potential across portions of Southern Arkansas... but
nothing overly impressive.

Tuesday...The pattern begins with deep upper trough across the SW
CONUS with upstream ridging. At this time, the subtropical jet is
becoming or has become superimposed with the polar jet. Southerly
winds ahead of the high plains surface low will continue to advect
very warm, moist air into Arkansas. Temperatures will be quite warm
Monday Night as values are unlikely to fall below 60 degrees
Fahrenheit with near 100% RH. The surface low will eject out towards
the Upper Mississippi Valley with a frontal boundary extending down
through much of the Mississippi valley and "curling" back to Texas.
Showers and thunderstorms will likely be stretching along this front
Monday evening as it approaches Arkansas.

Moisture will be more than sufficient thanks to this split-
atmospheric river type flow with boundary level moisture
streaming in from the Gulf of Mexico and low-to-mid moisture
originating from the East Pacific / Baja peninsula (i think a good
climatology study followed by a name for this synoptic setup
would be an excellent idea). Models advertise integrated water
vapor transport (IVT) values between 400 and 1000 kg m^-1 s^-1
into Arkansas for much of the period. This all will result in near
or above record precipitable water values between 1.25 and 1.75
inches through the event.

Given decent lapse rates, abundant lift from the aforementioned
superimposed jets, and moderate deep layer shear, thunderstorms
will be ongoing through much of the time as the front slowly moves
southward. Parallel flow in the mid to upper levels to the
orientation of the slowly moving front will support storms
training one another, thus flash flooding potential is certainly a
concern at the moment. It`s unclear at this time what, if any,
other severe threat there may be.

By Tuesday night, the GFS begins to diverge from the rest of the
global/medium-range models. The GFS is advertising an upstream
shortwave cascading down the intermountain west, helping open and
ejecting the deep SW low into the main flow. Other models (Euro,
CMC, DWD, etc) don`t show this feature (or as strong), which slows
the ejection of the deep upper low. As a result, the forecast
package is much strays from the GFS as far as timing. Precip
accumulations run- to- run are still varying, but it does look
like the very highest amounts will fall across southeast Arkansas
or even farther southeast. Amounts will be 2 to 5 inches across
much of the CWA, with higher amounts possible... especially in the
southeast. This is supported by most previous runs as well as
the gfs/ecm/cmc ensemble members. As the the system comes onshore
(which it is at the time of this AFD), we`ll be able to sample it
much better...that said, models are already in agreement with the
general synoptic pattern. It`s the mesoscale and propagation of the
storms / frontal boundary that will determine where the very highest
totals fall. We probably won`t know that until the system is in
range of the convective allowing models. Interesting-DWD`s
Icosahedral Nonhydrostatic model, which is one of the few
nonhydrostatic global and medium- range models, advertises similar
synoptic flow to that of the ECMWF, but keeps the highest rainfall
bands a bit farther north.

Even as the front does clear the state of Arkansas (or stall near
the border) some time on Wednesday, isentropic lift and DPVA from
the ejecting low will continue to support showers and
thunderstorms into Thursday. The euro advertises surface
cyclogenesis in Texas as the upper low ejects Thursday. This low
will likely move across Southern Arkansas or Northern Louisiana
Thursday and into MS overnight. The system will move out by Friday
with high pressure building in afterward. This won`t last long as
return flow begins on Saturday with rain chances increasing


Batesville AR     76  44  59  47 /  20  10  20  50
Camden AR         76  56  70  59 /  20  20  30  20
Harrison AR       74  39  60  47 /  10  10  30  60
Hot Springs AR    77  50  65  54 /  10  10  30  40
Little Rock   AR  76  50  63  53 /  20  10  30  40
Monticello AR     75  56  67  59 /  20  20  20  20
Mount Ida AR      77  49  65  54 /  10  10  40  50
Mountain Home AR  75  39  59  44 /  10  10  20  60
Newport AR        76  45  57  49 /  20  10  10  50
Pine Bluff AR     76  52  63  55 /  20  20  20  30
Russellville AR   78  47  63  51 /  10  10  30  60
Searcy AR         76  45  60  49 /  20  10  20  50
Stuttgart AR      75  52  62  55 /  20  20  20  40

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


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