Area Forecast Discussion
Issued by NWS Little Rock, AR

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931
FXUS64 KLZK 200604
AFDLZK

Area Forecast Discussion
National Weather Service Little Rock AR
1204 AM CST Tue Feb 20 2018

.AVIATION...

VFR prevails as of now, but MVFR likely central and south after
20/08Z. IFR becoming likely at KHRO/KBPK late in the period as a
cold front approaches. Meanwhile, low MVFR cigs expected to
develop central and south by 20/14Z, although confidence is low
concerning how long these cigs hold on. Southerly winds will
remain gusty with LLWS likely area wide through mid morning north
and afternoon south. As the cold front approaches from the
northwest late, winds begin veering to the SW.

Cooper

&&

.PREV DISCUSSION...(ISSUED 559 PM CST Mon Feb 19 2018)
DISCUSSION...

Updated for the 00z aviation discussion below.

AVIATION...

Expect widespread MVFR ceilings to develop by roughly 06z at all
terminals tonight, with -SHRA spreading across western and
northwestern parts of the state by that same time. Have SHRA and
even VCTS mentioned at KHRO and KBPK to account for this. However
suspect the cutoff between convection and lack of any
precipitation will be fairly sharp so did not extend convection to
any central terminals. That will likely occur near the end of the
00z TAF period. Will also have to deal with LLWS as a strong and
persistent low level jet will be in place by 06z tonight and last
through much of the day tomorrow.

PREV DISCUSSION...(ISSUED 245 PM CST Mon Feb 19 2018)
DISCUSSION...

Widespread and several periods of rain, heavy at times...will
lead to possible river and flash flooding beginning Tuesday
evening and much of this week...

Main concerns in this forecast cycle are of course rain chances,
periods of heavy rain and potential for river and some flash
flooding.

Currently, this mornings area of light convection has moved to MO
while another area was forming over eastern OK and also moving
northeast with the low and mid level wind steering flow. Have
seen some isolated thunder at times, but overall a rain and shower
areas. This was associated with upper energy as it moves over the
region. Clouds have held over northern and central areas, while
some thinning over the south. Very mild temperatures were seen
early this afternoon, with 60s north, 70s central and south. Winds
have been elevated from the south at 10 to 25 mph with some gusts
around 30 mph, as moisture levels have come up with dew point
temperatures in the 50s to mainly 60s.

SHORT TERM...Tonight Through Tuesday

A flood watch with some potential for flash flooding will be
issued covering most of western and central AR, in effect from
Tuesday evening through Thursday morning. A weather pattern will
set up with a deepening upper low pressure trough to the west,
while an upper high pressure ridge to the east. Over the plains, a
frontal boundary will become nearly stationary or waffle over AR,
and as upper level short wave energy moves through the region,
several periods of rain, heavy at times, will affect AR. Rain
amounts are forecast to reach 3 to 6 inches, with isolated higher
amounts, mainly from Tuesday through the end of the week. Although
the rain will fall over a 3 to 5 day period, and parts of AR are
in some level of drought, this amount of rain during the winter
with dormant vegetation conditions, will lead to some flash
flooding and river flooding. Hence a flood watch with some flash
potential will be issued. The heavier rain will start later
Tuesday and last through the week. See the hydrological section
below. Temperatures will be well above normal values, while as
the cold front sags southeast, temperatures will lower closer to
normal or a bit below normal values over northern AR.

LONG TERM...Wednesday Through Monday

Not much has changed in the extended period. It still looks very
unsettled, with periods of heavy/excessive rain.

An upper trough will be over the western U.S. to begin the long term
period with southwest flow over the eastern two thirds of the U.S.
This pattern will actually continue through Saturday. Friday night
an upper trough moves into the southern Plains and into the Midwest
Saturday night. This will bring drier air into Arkansas for Sunday
and Monday.

A cold front will be across central Arkansas Wednesday morning with
warm moist air to the south and cooler air to the north. Expect some
isolated thunderstorms near the front. Thursday, the front will be
in the southeast corner of the state, then lifts back to the north
on Friday. The front will finally be pushed east of the state as the
low pressure moves out of the Plains Sunday.

There is a potential for flash flooding and eventually river
flooding this week, with 4 to 6 inches of rain possible with
isolated higher amounts. Drought over the northern/western counties
may lessen high water problems a bit. But, runoff will be elevated
due to dormant vegetation and lack of consumption of ground water.
In the end, this much rain will create high water issues no matter
how you slice it.

Temperatures will vary through the period. It will be a cool start
as the front drifts southward through the area. Late in the week, it
will be mild again as the front lifts toward the Missouri border.

HYDROLOGY...

Rainfall forecast...

The initial wave of heavy rain will bring widespread 3 to 6 plus
inches of precip accumulation to most of the state. Some isolated
higher amounts are possible, particularly across western and
southwestern AR. As additional rainfall occurs beyond mid-week,
total precip accumulation will approach or exceed 7 inches in a
broad corridor roughly along and either side of the I-30/I-40
corridor.

Flash flooding...

The flash flood threat remains conditional given the broad drought
that continues across much of western and northern AR.
Additionally, while total rainfall accumulations appear impressive
in magnitude, the long duration over which these accumulations
will occur lowers some the flash flood risk as of now. 6-hour
flash flood guidance values remain in the 3 to 4 inch range, which
may be difficult to attain, at least outside of isolated areas.
Very dry soils in drought-stricken portions of western and
northern AR may also mitigate the flash flood risk some, but a
lack of vegetation and terrain could offset this. Will continue
advertising this more conditional threat with the understanding
flash flood potential may increase as time passes.

River flooding...

Lower flow levels continue on many rivers across the area, with a
few isolated exceptions along the Cache River in east-central AR.
This will initially be in our favor as basins should be able to
handle heavy rain early in the week. However, the widespread
nature of the heavy rain indicates most if not all basins will
receive significant rain through the next 7 days. The threat for
river flooding will increase by mid-week and should be maximized
late week into the weekend as additional rain is routed through
saturated basins. As of this morning, the highest threat for river
flooding should exist across roughly the southern and eastern
half of the state this week.

Cooper/59

&&

.LZK Watches/Warnings/Advisories...
Flood Watch from Tuesday evening through late Wednesday night
FOR Baxter-Boone-Clark-Cleburne-Conway-Dallas-Faulkner-Fulton-
Garland-Grant-Hot Spring-Independence-Izard-Jackson-Johnson-
Logan-Lonoke-Marion-Monroe-Montgomery-Newton-Ouachita-Perry-Pike-
Polk-Pope-Prairie-Pulaski-Saline-Scott-Searcy-Sharp-Stone-Van
Buren-White-Woodruff-Yell.

&&

$$


Aviation...COOPER



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