FXUS64 KLZK 191708 AAA

Area Forecast Discussion...Update
National Weather Service Little Rock AR
1100 AM CST Mon Feb 19 2018


Overall forecast on track. Main area of convection will be over
northwest to northern AR today, as upper energy moves over the
region. Some thunderstorms have been noted over northwestern AR and
may affect the far northwest area. Clouds will hold over the area
with some thinning over southern AR. Temperatures will be warm with
60s north, 70s central and south. Afternoon update will fine tune
elements but no significant changes expected. (59)


.Prev Discussion.../ Issued 550 AM CST Mon Feb 19 2018/

Low clouds will persist most areas with MVFR likely through much
of the period. Best chances for SHRA remains at northern terminals
with TSRA possible after 20/00Z, and S winds will be breezy all
terminals with gusts above 20 kts expected. LLWS also likely area
wide with strong SW flow aloft. Confidence in prevailing weather
and timing remains low, although high confidence in MVFR
categories through most of the period.



.PREV DISCUSSION...(ISSUED 522 AM CST Mon Feb 19 2018)
..Widespread and prolonged heavy rain event with river and flash
flooding possible beginning Tuesday evening...

SHORT TERM...Today Through Tuesday Night

Southerly return flow continues pumping in rich Gulf moisture near
the surface with showers and some isolated thunder observed within
a broad area of WAA across western and northern AR. With saturated
low-level profiles, patchy fog has also been observed, although
the fog is less dense than yesterday and should dissipate through
the morning. Low clouds will persist with showers and isolated
thunder continuing where WAA and dynamic lift is maximized in
western and northern zones. Winds will also remain elevated today
and through much of the period, with sustained winds between 15 to
20 mph and gusts between 25 to 30 mph across much of the area. The
breeziest conditions will occur in higher terrain and across the

By late tonight into Tuesday, rainfall chances will increase
northwest- to- southeast as WAA overspreads the area and the LLJ
increases, augmented by transient weak shortwave troughs embedded
in southwest flow aloft. It is also worth noting the anomalous
warmth expected with afternoon highs in the upper 70s to low 80s
this afternoon and Tuesday afternoon across southern AR. Mid- to-
upper 70s are likely elsewhere with 60s in NW AR. These readings
are upwards of +2 standard deviations above climatological
averages based on NAEFS data.

As the upper longwave pattern continues to amplify through the
period, a digging trough across the Intermountain West will drive
a sharp cold front south from the Plains late Tuesday into
Wednesday. An anomalously moist air mass, coupled with deep
along-front flow and dynamic forcing, will produce widespread
rainfall with heavy rain becoming likely in NW AR as early as
Tuesday afternoon. Outside any elevated convection -- which
remains a possibility given modest MUCAPE values up to 1000 J/kg
-- the maximum precip rates should be observed with rainfall along
and near the front itself. QPF within this frontal rain band will
range from 1 to possibly 2 inches per 6 hours, with isolated higher
totals. Flash flooding cannot be ruled out, although the threat
remains conditional. See the hydrology discussion below for more
details on potential flooding.


LONG TERM...Wednesday Through Sunday

Extended period still looks unsettled with several periods of rain,
possibly heavy, expected. The good news is the models are showing a
definite drying trend late in the period but until then, high water
problems will remain a concern. Models still exhibiting some spread
regarding the speed of the front and as a result, the heaviest QPF
axis. That being said, a blend of forecast solutions will be used.

The period initiates with high pressure sitting over the western
Atlantic and troughing over the western CONUS. This will result in a
pronounced, southwest flow aloft. A cold front will be bisecting the
state from southwest to northeast but crawling through as the
boundary will be parallel to the persistent meridonal flow. Plenty
of gulf moisture will remain in place with the low level jet still
advecting in PWAT values around 1.75 inches along and proceeding the
front. The result will be a heavy swath of rain continuing with the
focus over the central and northeast parts of the state. As the
front slowly sags to the south, the focus for the heavy rain will
shift with it later Wednesday and into Wednesday night with a second
surge of deep moisture coming through late Wednesday evening/early
Thursday morning.

Models remain consistent in eventually moving the boundary through
and stalling it briefly south of us before it lifts back to the
north Friday and Saturday as western trough essentially reloads.
Another round of widespread rain is expected until the trough
finally moves through and takes the cold front with it Saturday

Little in the way of severe weather is expected but occasional
thunder can not be ruled out but stability will be elevated. Also,
it does not look cold enough behind the front for any wintry
precipitation and all winter weather has been pulled from the

The main and ongoing story with this event will be the potential of
flash flooding and eventual river flooding. Widespread four to six
inches of rain is feasible by the end of period. There could be
locally more than half a foot. Ongoing drought over the northern and
western counties may lessen high water problems a bit. But, runoff
will be elevated due to dormant vegetation and a lack of consumption
of ground water. In the end, this much rain will create high water
issues no matter how you slice it. That is, of course, if this much
rain actually materializes.

Temperatures will waver 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.


A prolonged heavy rain event is likely across much of Arkansas
with heavy rain pushing in from the northwest beginning late
Tuesday. Heavy rainfall will continue through Wednesday and
additional heavy rainfall is possible later this week as
additional upper disturbances affect the region.


Well above-average meridional flow continues advecting very rich
moisture from multiple source regions, including the Pacific and
Gulf, northward into the area. NAEFS guidance has increased the
magnitude of forecast PWAT within the column, with values
reaching or exceeding +3 standard deviations above climatological
averages through the week, most notably Tuesday into Wednesday.
Model guidance continues advertising a slow-moving cold front
crossing the state late Tuesday through Wednesday, although there
are still some timing differences. This boundary will serve as a
focusing mechanism for the heaviest rain Tuesday through Wednesday
as it interacts with the very moist air mass in place. As the
Western CONUS upper trough reorganizes later this week, additional
heavy rain is possible with vigorous upper impulses crossing
aloft atop a still-saturated air mass in the lower troposphere.

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

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 precludes significant 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 continues 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 precip 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.



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


Back to Main Forecast Discussion Page