NWS Forecast Discussion

FXUS64 KLZK 230218 AAB

Area Forecast Discussion...Update
National Weather Service Little Rock AR
815 PM CST Thu Feb 22 2018


Overall forecast on track. The rain keeps coming. The threat for
flooding remains elevated over all of AR. The upper level southwest
to northeast flow has brought plenty of short wave energy to the
region, and some continued focus over AR. This has developed the
rain and showers, with heavy rain, and some isolated thunder. 00Z
LZK sounding had a precip water value of 1.49 inches, usually a
spring time value when we have had a prolong south flow to moisten
the atmosphere. This time the repeated days of rain have raised the
value well above normal values. The threat of flooding and flash
flooding continue across AR, with another half to near 2 inches
expected through Friday morning. No change in the tonight forecast
with continued high chances of rain and isolated thunder. Rain
chances will lower a bit over northwest AR, while holding high over
the rest of the state, except the far southeast with lower chances.
The surface boundary was over northern LA, and models gradually lift
it northward overnight to Friday into southern AR, again helping
focus rain chances along the I30 to 67/167 route. Temperatures will
stay above freezing, and no winter weather threat is present. Lows
from the 40s north to 50s south. (59)


.Prev Discussion.../ Issued 550 PM CST Thu Feb 22 2018

Widespread MVFR conditions will be seen across AR, with patchy VFR
ceilings away from convection, while IFR conditions with much of the
heavier rain and convection. The upper system moving across AR
continues to develop rain, showers and isolated thunderstorms. Heavy
rain with lower ceilings and visibilities, as well as fog, will also
be seen. This pattern will hold much of this evening, while a bit
lower convection chances later tonight. Winds are mostly out of the
northeast, east to southeast at 5 to 15 mph, while variable and gusty
winds near convection. Low level wind shear will be seen with
convection, as well as around Taf sites in the first 6 to 10 hours
with a southwest jet around 2K feet. (59)


.Prev Discussion.../ Issued 249 PM CST Thu Feb 22 2018/
.SHORT TERM...Tonight Through Friday Night

Heavy rainfall...and the threat for flash and river flooding will
remain the primary concerns in the short term period...

The next round of SHRA and a few TSRA is lifting NE over SRN and WRN
AR this afternoon as the next upper level disturbance lifts NE over
the state to the NW of the stalled SFC front to the SE of the state.
Initially the most widespread precip will be across the WRN portion
of the CWA...but the focus for rainfall will shift the the SE over
the I-30/US-67 corridor this evening and overnight. The front the SE
of the state will shift slightly NW...allowing for this more focused
area of convection through Fri morning before wobbling back SE Fri
evening. Rainfall amounts will range from 1 to 3 inches along this I-
30/US-67 corridor through Fri morning...with locally higher amounts
expected closer to 4 inches. Given the already very saturated
soils...expect the potential for flash and river flooding to
continue for most areas...with the most short term impacts possible
along this heavy rain axis.

By Fri afternoon/evening...the area of convection across the I-30/US-
67 corridor will weaken...but another area of convection will
develop over NWRN portions of the state from a new upper
disturbance. Additional rainfall amounts for Fri night/Sat morning
will range from only a few tenths across the SE half of AR...to 0.5
to around 1 inch across the NWRN portion of AR. This will result in
storm total rainfall amounts from now until 12Z Sat 1 to nearly 4
inches for all of the CWA...with that heaviest rainfall along that I-
30/US-67 corridor. Locally higher amounts remain possible. Have kept
the Flood Watch going through the period into Sat evening as the
threat for flash and river flooding will persist.


.LONG TERM...Saturday Through Thursday

The long term period (Saturday onward) begins in southwest flow
aloft as a deep upper level low is centered over the rockies, with
downstream ridging. Upper level divergence will be near optimal with
southern stream jet divergence coupled with a supergeostrophic
northern stream right entrance region. Developing surface cyclone
will experience quick intensification across the central plains and
move off to the northeast as another deep upper level trough rolls
down the western CONUS.

At the surface, a boundary/warm front will lift across Arkansas to
somewhere near southern Missouri. Exact timing and placement is
still a bit uncertain, but should clear the state by sometime Friday
night. A strong low level jet will develop across the CWA as the
strengthening cyclone moves from Kansas into the Mid-Mississippi
Valley. Organized multi-cell convection will likely develop in
Oklahoma and Texas in the morning hours along the cold front and
then move into Arkansas during the afternoon.

Moderate isentropic lift in Arkansas will result in the possibility
of showers and thunderstorms Saturday morning. With overcast skies,
lack of insolation (not insUlation) will hinder surface instability.
925 mb winds will be in excess of 40 knots, with 850mb in excess of
50 knots. Rich moisture advection will result in surface dewpoints
greater than 60 for much of Arkansas, with the Ozarks being the
likely exception. Given the low level shear (0-1 SRH >300 m^2/s^-2)
and (nearly) ground-level LCLs, it`s going to be one of those
situations in which embedded supercells in the convective line will
certainly be something to watch for, along with high and potentially
damaging winds mixing down to the surface.

A major concern Saturday will be flooding. Any rainfall at this
point will be excess given (likely) antecedent conditions. Model
census advertises widespread pwats between 1.4 and 1.7 inches for
the event. To put this in perspective, the "moving" maximum for this
time is around 1.3. The advertised values will be near the February
precipitable water record. Given this available moisture, organized
convection will be rather efficient rain-producers. Being the
"caboose" of this multi-day heavy rain event, this is especially
concerning. A flash flood watch has already been issued for the next
several days. Please stay weather-aware.

Sunday through Friday...

Luckily for this forecaster, models don`t diverge until after
Sunday. A transient high will keep things mostly dry Sunday through
Monday night. Below normal temperatures will begin this period
before return flow quickly warms us back above average. The GFS
becomes more progressive with the upper stream than the ECMWF at
this point. Additionally, the former is deeper which results in a
nearly identical meteorological set-up to Thursday. The slower, more
shallow ECMWF doesn`t produce nearly the rainfall. Forecast
confidence is rather low at this point... details will becoming more
clear this weekend. In any case, stay alert to any forecast changes
as any additional rainfall will likely aggravate any ongoing
flooding situations.


Batesville AR     48  59  53  69 / 100  80  60  90
Camden AR         51  67  60  74 /  90  70  40  70
Harrison AR       46  55  50  65 /  90  60  90  90
Hot Springs AR    48  61  57  68 / 100 100  50  90
Little Rock   AR  49  61  58  72 / 100  90  50  90
Monticello AR     54  72  64  76 /  60  50  50  70
Mount Ida AR      48  60  56  68 /  90  90  50  90
Mountain Home AR  45  57  51  65 /  90  60  80  90
Newport AR        50  59  55  69 / 100  90  60  90
Pine Bluff AR     50  65  60  74 /  90  70  40  80
Russellville AR   48  58  54  67 / 100  80  70  90
Searcy AR         49  60  55  71 / 100  90  50  90
Stuttgart AR      50  63  60  72 /  90  80  50  80

.LZK Watches/Warnings/Advisories...
Flood Watch through Saturday evening FOR Arkansas-Baxter-Boone-
Drew-Faulkner-Fulton-Garland-Grant-Hot Spring-Independence-Izard-
Saline-Scott-Searcy-Sharp-Stone-Van Buren-White-Woodruff-Yell.



Short Term...62 / Long Term...BROWN

NWS LZK Office Area Forecast Discussion