Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Memphis Tennessee
638 am CDT sun Apr 19 2015
updated to add 12z aviation discussion.
Previous discussion... /issued 611 am CDT sun Apr 19 2015/
Update to increase probability of precipitation to likely along and east of the
Mississippi River this morning.
A convective squall line moving northeast from the arklatex
overnight has persisted more than models advertised. This line of
thunderstorms will likely move northeast across the area and bring
likely rainfall to much of these areas this morning.
Therefore...have updated probability of precipitation to likely along and east of the
Mississippi River this morning.
Updated zones and grids sent.
Previous discussion... /issued 357 am CDT sun Apr 19 2015/
An upper level low pressure system over western Kansas this
morning will continue to lift to the northeast today. A series of
shortwave disturbances will rotate around the southern periphery
of this upper low and bring showers and thunderstorms to the middle
south today and tonight. The first shortwave of interest and
associated weak surface low is currently over the area and
forcing the large rain shield currently impacting mainly areas
along and east of the Mississippi River. This should move
northeast with rain ending by middle morning over northeast sections
of the forecast area. A second shortwave now over the arklatex is
forcing a line of convection over this area which continues to
lift northeast. After careful review of water vapor imagery...it
appears that this shortwave will outrun the current convection
with the currently solid line breaking up a bit as it moves into
our region around sunrise. Plan to carry high end chance probability of precipitation for
the remainder of the forecast area this morning as this shortwave
feature traverses the region.
There should be a break in the convection around midday which
should carry through middle afternoon across most locations as a middle
level dry slot rotates into the region. This should allow for
breaks in the cloud cover and also support decent heating and
atmospheric destabilization. Considering the degree of heating
expected combined with low to middle 60s dewpoints...expecting surface
based cape values to range from 1500-2500 j/kg by later this
afternoon. A third and more potent shortwave should rotate across
the region later this evening. As this wave approaches...low and
middle level wind fields should strengthen and middle level lapse rates
should steepen. This wave will act upon the existing favorable
thermodynamic environment in place to initiate new thunderstorm
development across the arklatex by middle afternoon which will
quickly spread northeastward into the middle south by late afternoon
and evening. Severe weather will be possible especially
considering the degree of expected shear and instability. Storm
Mode initially may support supercells and short bowing line
segments across our western areas which should transition to a
squall line as it moves across the remainder of the middle south
overnight. The primary severe weather threats appear to be
damaging winds and large hail...but an isolated tornado or two
will be possible as well...especially early in the storm evolution
when supercell thunderstorm potential is greatest. The Storm
Prediction Center has outlooked eastern Arkansas...far southwest
Tennessee...and much of north Mississippi in an enhanced risk for
severe storms...with the remainder of the region in a slight risk.
This seems reasonable and plan to carry the outlook in local
forecast products. Showers and thunderstorms should move east of
the area by Monday morning as a cold front sweeps through the
By later Monday morning...most areas will dry out with cooler
temperatures expected as high pressure builds over the region. Dry
and cooler than average temperatures can be expected through middle
week as high pressure moves east.
The end of next week appears more unsettled again with increased
chances of showers and thunderstorms and continued cooler than
12z taf cycle
Low pressure currently moving across lower Mississippi Valley.
Several shortwaves moving through the region are producing rain
showers along with mainly IFR conditions prevailing at terminals.
IFR/MVFR conditions will gradually improve to VFR conditions by
this afternoon. A strong cold front will move through the region
tonight and have included tempos at all sites as stronger
convective activity moves through the area. S/SW winds will
increase to around 10 kts today and veer west with frontal passage