Scientific Forecaster Discussion
fxus64 kmeg 151755 aaa
Area forecast discussion
National Weather Service Memphis Tennessee
1154 am CST sun Dec 15 2019
update for 18z taf issuance.
Previous discussion... /issued 936 am CST sun Dec 15 2019/
Quasi-zonal flow aloft continues across the mid-south in advance of
a digging trough near The Four Corners. A strong upper-tropospheric
jet approaching 150 kts is located just north of the County Warning Area with
strengthening southwesterly flow above the relatively shallow
frontal zone. Yesterday's cold front has stalled south of the County Warning Area
and will make a return north today and tonight setting the stage for
increasing rain chances.
Isentropic ascent has already commenced but condensation pressure
deficits between in the 285-300k layer are still on the order of 50-
100 mb. It will still take a few hours to saturate so rainfall
trends were delayed a few hours from the previous forecast,
increasing from west to east after 18z. Rainfall today will be
favored along/north of I-40 but amounts will remain light through
sunset. Rain chances will increase overnight and Monday.
Widespread cloud cover and potential wet bulb effects should limit
today's diurnal temperature range across most of the County Warning Area. Max
temperatures were cut by a few degrees in many areas with highs
ranging from near 40f in the bootheel and extreme northeast Arkansas to
around 60f in north MS.
Previous discussion.../issued 400 am CST sun Dec 15 2019/
Currently...surface high pressure extends from the upper MS valley
into the mid-south with light north-northeast winds. Mostly cloudy skies
prevail across the region with temps in the upper 30s to mid 40s.
Today...an upper level trough will eject out of the SW U.S. Today
as surface high pressure over the mid-south moves east. In
response, winds turn southerly across the lower Mississippi
Valley as a developing warm front lifts north into the mid-
south. Expect mainly dry weather this morning but rain chances
will begin to increase this afternoon especially along and north
of I-40. Temps will range from the lower 60s across north MS to
the lower to mid 40s over NE Arkansas and the MO bootheel.
Tonight...the upper trough will push into the Southern Plains
tonight with deep, moist SW flow aloft across the mid-south. 00z
guidance has shown a trend toward a more southerly track of the
upper trough. As a result the associated surface low will track a
little further south across southern Arkansas/northern la and the warm
front across the mid-south will sag a little south overnight to
more a Clarksdale-Holly Springs-Parsons Tennessee line by 12z Monday.
Expect precipitation to continue to develop overnight, especially
after midnight as lift increases across the region with the
approach of the upper trough. Will start to see some embedded
thunderstorms mainly along and north of the warm front as some
elevated instability moves into the area.
Monday...as the upper trough pushes into the mid-MS valley the
surface low will track along the warm front and through the mid-
south. Expect the track to be south of I-40 now and as a result
the severe potential will shift slightly to the the southeast. There will
be a wide range of temps across the mid-south with highs in the
lower 70s over parts of NE MS to only the lower 40s across NE Arkansas
and the MO bootheel. Expect surface dewpoints to climb into the
lower 60s in the warm sector with sbcapes of 250-750 j/kg. Strong
vertical wind shear is expected as a 80-90 kt mid level jet pushes
into the area. The surface low should serve to back low level
winds a bit and model soundings are indicating this and producing
0-1km helicity values over 200 m2/s2, suggesting rotating storms
are possible. As the surface low tracks through the region Monday
afternoon and the trailing cold front pushes across north MS
expect storms to intensify. The positively titled upper trough
and flow parallel to the front will produce a messy storm Mode
with lines and clusters likely. Expect a few of these storms will
become severe across the warm sector with damaging winds the main
threat. A tornado is also possible. Hail could occur with any
cells that remain semi-discrete. Best chances for severe storms
will be across north-central and NE MS during the afternoon and
Monday night...severe threat will continue until about 02z before
the surface low and front track east. Rain will taper off overnight.
Any changeover to snow threat remains minimal. By morning temps
will drop into the upper 20s north to upper 30s over parts of NE
MS on gusty northwest winds.
Tuesday and Wednesday...back to winter. A stiff northwest breeze and
strong low level cold air advection will bring back the chills. Highs will only
be in the lower 40s Tuesday. Tuesday night will see temps back in
the 20s followed by another day in the 40s on Wednesday.
Thursday through Saturday...high pressure moves east and southerly
flow and slightly milder temps will prevail for Thursday. A weak
moisture starved system moving through Friday and Friday night
could bring some light precip but does not look significant attm.
Weekend looks decent at first blush with perhaps a significant
warming trend into early next week as upper level high pressure
builds into the region.
An active period is in store for the area taf sites. As upper system
approaches from the west, warm front draped over the mid-south will
move slowly north. Moisture will move north into the area, so any
improvement in cigs this afternoon should be short-lived. Short-term
guidance suggests rain showers developing by mid-late afternoon affecting
all but ktup.
Moisture/instability to increase overnight, continuing the MVFR/IFR
cigs and increasing precip. Instability should increase after 06z so
added thunderstorms in the vicinity to all but ktup. Strong low-level jet /45-50knots/ over
kmem/kmkl, so continued low level wind shear wording overnight. Expect thunderstorms and rain chances
to generally increase during the day Monday, but kept thunderstorms in the vicinity wording
at all sites as some timing disagreements continue. More thunderstorms and rain
emphasis at kmem the latter part of the forecast period.