Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Memphis Tennessee
620 am CST Wednesday Nov 26 2014
Updated for the 12z aviation discussion.
Previous discussion... /issued 431 am CST Wednesday Nov 26 2014/
Main challenge with this forecast is timing the precipitation from a
rather potent shortwave rotating southeast...eventually affecting
the middle south region today.
A complex upper level flow structure exists currently with a very
strong jet streak located across the southeastern United States
aiding in the development of the northeastern coastal storm. This
trough axis passed through the midsouth overnight and shortwave
ridging is currently overhead. Another shortwave upstream is
rotating southeast rapidly and will continue to do so early this
morning. Temperatures fell rapidly last night but leveled off
after midnight as forecast but still remain in the upper 20s to
low 30s areawide. A precipitation shield has developed on a convergence
axis associated with the surface low pressure and is diving
quickly southeast. Although low reflectivity echoes are present
across southern MO...low levels are too dry to support precipitation
reaching the ground. Isentropic analysis correctly depicts the
smallest condensation pressure deficits relegated to a narrow band
colocated with very strong local isentropic upglide. This will be
the main precipitation generating mechanism and was used to time the
precipitation through the region today.
This area of lift should begin to affect areas of northeast
Arkansas around 15z then points east and southeast along the
missiissppi river by 18z. Temperature profiles across northeast
Arkansas are marginally favorable for some mixed precipitation prior to
14z but precipitation will not arrive early enough and current thinking
is that low levels will warm sufficiently supporting only liquid
precipitation. There remains some uncertainty with respect to how far
southwest precipitation will develop however forcing is limited further
southwest...trends will need to be monitored through the morning.
Precipitation should vacate the area by 00z with only a few showers
remaining across middle Tennessee at this time.
This shortwave briefly reinforces the longwave trough across the
eastern Continental U.S. And cyclonic northwest flow will prevail through the
Thanksgiving Holiday. A reinforcing shot of cold air works
southeast from Canada which supports below average temperatures
through Friday. Zonal flow attempts to establish itself late in
the day Friday...fully doing so over the weekend. A nice warmup is
on tap during this timeframe with temperatures in the 60s expected
as a warm plume of 850 mb air overspreads the midsouth in west
By early next week...models have been consistent in stalling a
baroclinic zone across the midsouth region which could result in a
large temperature gradient waffling around the region. General
warm advection showers are likely as well as long as the boundary
remains near the region. Have kept broad probability of precipitation in the forecast for
an extended period of time through early next week to represent
this. The forecast will undoubtedly change but the takeaway here
is that the forecast during this timeframe will likely be quite
changeable and forecasting the position of this boundary at this
range is difficult.
12z taf cycle
VFR ceilings and visibilities will prevail at all sites through this morning.
There should be some temporary MVFR reductions of ceilings at kjbr and
kmkl along with periods of light rain. The remainder of the sites
to include kmem and ktup should remain VFR and dry. Brisk
southerly winds early today ahead of a front will abruptly turn
west northwesterly and remain brisk around 10 kts. These winds
should be maintained at all sites through tonight.
Skies may temporarily scatter out at kmem...kjbr...and ktup but
may fill back in with MVFR ceilings late at kjbr. MVFR ceilings should
prevail through tonight at kmkl but lower late tonight.