Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus64 kmeg 261159 aaa 
afdmeg

Area forecast discussion...updated
National Weather Service Memphis Tennessee
659 am CDT Wed Apr 26 2017

Update...
updated for 12z aviation forecast discussion below.

&&

Previous discussion... /issued 506 am CDT Wed Apr 26 2017/

.Severe weather expected late this afternoon into tonight and
again Friday into the upcoming weekend...

Discussion...surface analysis places an area of low pressure
over eastern Iowa with a trailing cold front extending south into
west central Missouri and eastern Oklahoma early this morning.
Regional WSR-88D radar trends have indicated substantial weakening
of showers and thunderstorms associated with a mesoscale
convective system (mcs) over south central Missouri and northwest
Arkansas. Cloud cover associated with this upstream convective
activity has spread into areas mainly along and west of the
Mississippi River producing mostly cloudy skies with mostly clear
skies elsewhere as of forecast issuance. As of 4 am CDT,
temperatures across the mid-south remain mild in the 60s at most
locations. Severe thunderstorm/heavy rainfall potential later this
afternoon into tonight, Friday night, and again Saturday night
remains the primary concerns in this morning's forecast.

A tightening pressure gradient across the mid-south is expected to
produce sustained south winds between 20-25 kts with gusts up to
around 35 kts across portions of east Arkansas and northwest
Mississippi this afternoon into early evening. As a result, have
issued a Wind Advisory from 1 PM CDT to 7 PM CDT across the above
mentioned area. These winds will be monitored throughout the day
and headlines may be adjusted if short term trends warrant any
changes.

The primary concern through tonight will be the risk for severe
thunderstorms across the mid-south later this afternoon into
tonight. Surface dewpoints rising into the lower to middle 60s,
favorable upper level divergence produced by the right exit region
of a 300 mb 110 kt upper level jet streak, impressive 80-100 dm
500 mb mid level height falls, 0-6 km bulk shear values in excess
of 60 kts, very steep 700-500 mb layer mid-level lapse rates
around 8 c/km, and surface based cape values between 1500-2000
j/kg suggest a good potential for severe thunderstorms across the
mid-south beginning late this afternoon across portions of
east/northeast Arkansas, spreading east to areas along the
Mississippi River this evening, and across the remainder of the
forecast area late this evening into the overnight hours.

At this time, it appears the most likely Mode of convection
across the mid-south will be in the form of a line as upstream
discrete supercells congeal into a line as instability begins to
weaken with time tonight. Large hail and damaging winds will be
the main threats for any severe thunderstorms that reach severe
limits. 0-1 km storm relative helicity values between 300-400
m2/s2, and favorable lcl's may support a tornadic threat
especially west of the Mississippi River early in the event.
Precipitable water values around 1.8 inches will also support a
potential for heavy rainfall tonight.

Cooler and drier air will filter into the mid-south on Thursday
with highs in the upper 60s into the lower to middle 70s.
Temperatures are expected to moderate into the lower to middle
80s on Friday as a warm front moves back north across the mid-
south. A very warm, moist and increasingly unstable atmosphere may
result in the development of additional severe thunderstorms
across the forecast area, especially along and north of I-40.
Large hail, damaging winds, and tornadoes will be possible with
this warm front occurring predominantly Friday night.

Long term models indicate this front will become nearly quasi-
stationary across portions of Missouri and Arkansas on Saturday
then move through as a cold front on Sunday. Additional severe
thunderstorms may be possible during this time along with a
potential for heavy rainfall and flash flooding/river flooding
possible as models suggest rainfall accumulations up to 5-9
inches across portions of northeast Arkansas through the upcoming
weekend. Models indicate a break in unsettled weather early next
week with additional showers and thunderstorms possible towards
the middle of next week.

Cjc

&&

Aviation...
12z tafs

Primary short term challenge is MVFR ceiling chances. NAM bufr soundings
and GFS lamp guidance were too quick to bring in lower ceilings
early this morning, given overlying deck at fl040 and low level
moisture axis that was still over the western half of Arkansas. This
moisture axis will surge northeast through the morning, bringing
an MVFR ceiling chance for a few hours.

Next concern is thunderstorms and rain timing. Hrrr model has been fairly consistent
in the last several runs, speeding up timing slightly. NAM bufr
soundings suggest the pre-squall line airmass will be strongly
capped, limiting ts chances prior to arrival of the main convective
line. Expect impacts from gradient winds before storms arrive,
with cross winds exceeding 30kt on east/west runways.

The convective should be east of the mem tracon by the time the
late evening arrival push commences. Fair weather gradient winds
should gradually weaken through the overnight.

Pwb

&&

Meg watches/warnings/advisories...
Arkansas...Wind Advisory from 1 PM this afternoon to 7 PM CDT this evening
for cross-Lee AR-Phillips-St. Francis.

MO...none.
MS...Wind Advisory from 1 PM this afternoon to 7 PM CDT this evening
for Coahoma-Quitman-Tallahatchie-Tunica.

National Weather Service Glossary of Abbreviations