Area Forecast Discussion
Issued by NWS Memphis, TN
FXUS64 KMEG 280535
Area Forecast Discussion
National Weather Service Memphis TN
1135 PM CST Mon Feb 27 2017
Updated for 06Z Aviation Discussion.
.PREV DISCUSSION... /issued 908 PM CST Mon Feb 27 2017/
Update to increase pops areawide and add mention of severe storms
Latest observational and regional sounding data suggests
increasing elevated instability across much of the region with
increasing elevated CAPE after midnight as a surface warm front
across the Gulf Coast surges northward. Deep layer shear around 40
kts along with increasing ascent and moisture advection has
resulted in the development of scattered showers and thunderstorms
this evening...mainly north of Interstate 40. A few storms have
approached severe limits with the potential for large hail.
Latest high resolution and global model solutions suggest
additional convective development after midnight focused across
the central and southern portions of the Mid South. As elevated
CAPE values around 1300 j/kg...as observed on the 00z SHV sounding
advects northeast on increasing low level jet winds...expect the
potential for elevated severe storms with a threat of large hail
Temperatures are currently bottomed out at overnight lows and will
gradually rise through the night. This is currently handled well
by the latest forecast.
Updated forecast and grids will be sent shortly along with an
updated HWO to add severe potential.
.PREV DISCUSSION... /issued 329 PM CST Mon Feb 27 2017/
A complex convective scenario in store for Tuesday and Wednesday.
A maritime warm front will lift into the Midsouth late tonight
enhancing moist isentropic lift mainly along and north of the I-40
corridor. 18Z NAM Bufr soundings depicted a weakly-capped
unstable layer around 925mb. This isentropic lift in this layer
may be sufficient to overcome weak convective inhibition, allowing
deep updrafts to form and persist into the morning daylight hours
along and north of I-40.
For Tuesday, NAM Bufr soundings depict an elevated mixed layer
above 850mb late morning into the afternoon. The EML is shown to
weaken during the mid to late afternoon, but likely sufficient to
cap deep convection. Should storms manage to break this weaker cap,
quick intensification to severe levels would be possible given
CAPE in excess of 1500 J/KG and near 7C/KM midlevel lapse rates.
Given the expected limited coverage of storms, have reduced Tuesday
afternoon rain chances by 10 percent over most areas.
Short range models begin to diverge by the Tuesday evening period,
with the NAM showing a significant EML redevelopment above 900mb,
capping deep convection. GFS soundings showed no such capping
mechanism and as as such, more thunderstorm coverage late evening
and overnight. Tend to favor the GFS in this scenario, given the
midlevel height falls. Instability won`t be as impressive east of
the MS River overnight, but still adequate, given midlevel lapse
rates near 7.25 C/KM. A marginal overnight tornado threat would
be associated with any storms that form out a head of a developing
line and with any of the stronger QLCS line segments.
Storms should exit to middle TN by midmorning Wednesday. A secondary
enhanced risk of severe storms will develop over northeast MS,
where a secondary Enhanced Risk exists early in the SPC Day 3
Following Wednesday morning`s cold frontal passage, a dry and
cool midcontinental airmass will spread into the Midsouth.
Thursday will see afternoon relative humidity levels fall below 30
percent east of the MS River. A reinforcing dry cold front will
bring enough cold advection to moderate afternoon humidity levels
slightly, but may be offset by stronger north winds.
Return flow will develop behind a departing surface high pressure
late Saturday. A few showers will be possible late in the weekend,
associated with a maritime warm front lifting through the lower
MS River valley. This warm and humid airmass will prevail until a
cold frontal passage on Tuesday.
AVIATION...06Z TAF Set
VFR to start...with the exception of IFR at TUP. Scattered showers
and thunderstorms will persist during the first half of the
period...with cigs lowering to IFR/MVFR at the the remaining
sites. Break in the convection for most of the second half of the
period...though a few isolated storms may develop after sunset.
South winds will be another aviation concern...with a slow
increase throughout the period. Highest gusts will be seen at JBR
and MEM. Low level wind shear likely late tonight at all sites.