Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Memphis Tennessee
511 PM CDT Thursday Mar 26 2015
Updated the sky cover grids for tonight showing gradually clearing
from the northwest as the cold front pushes southeast of the
midsouth. Clouds will then increase again from the northwest
beginning after midnight as an upper level disturbance approaches
in northwest flow aloft. Latest models indicate that rain
associated with this feature will not begin to move into northeast
Arkansas until late...so adjusted the timing on this precipitation to
be a little later. Updated zone forecasts and grids have already
Discussion... /issued 307 PM CDT Thursday Mar 26 2015/
Main forecast concerns with this package is the extended period
of below normal temperatures which could lead to a damaging freeze
across portions of the middle-south late Friday night/Saturday
morning and again Saturday night/Sunday morning.
Currently...a cold frontal boundary was surging southeast and has
almost cleared the middle-south entirely. A non-diurnal temperature
trend was observed today with much of the region experiencing
falling temperatures from the late morning through early
afternoon. The upper level flow pattern was continuing to amplify
with pronounced ridging through the western Continental U.S. And a digging
trough across the east.
Late this afternoon through the overnight...expect the lingering
drizzle and showers to gradually diminish from west to east.
Temperatures will continue to drop...however...cloud cover should
prevent them from bottoming out tonight...although middle 30s appear
possible across middle Tennessee.
On Friday...a secondary jet streak will begin to round the base of
the longwave trough thus reinforcing it across the eastern US.
This will also provide broad-scale forcing for ascent over the
middle-south on Friday. Upper level temperatures look as though they
would support mixed precipitation...however...low level thermal profiles
should warm sufficiently to keep precipitation liquid. This will need to
Brief shortwave ridging will move across the middle-south in the
wake of the departing shortwave Friday night with enough
subsidence to sufficiently clear skies across the region...as
noted in model cross-sections. An anomalously cold 850mb airmass
will be entrenched across much of the middle-south setting the stage
for a potential hard freeze. Middle Tennessee and northeast
Mississippi appear most at risk for a damaging freeze as these
locations will experience sub-freezing temperatures the longest.
For this reason...have issued a freeze watch for late Friday night
through Saturday morning.
Another shortwave moves through the region on Saturday but
confidence remains low at this time with regard to precipitation amount
and type. Given the anomalously cold temperature profiles...it
appears that a mix of rain and snow is possible for areas of
middle Tennessee during the morning hours on Saturday. May have to
extend this mix further southwest in subsequent packages...but
confidence is simply not high enough given climatology this late
in the season. Another potential freeze across the same areas is
possible Saturday night/Sunday morning as the cold regime at 850
mb persists. Plan to address these concerns in later
forecasts...but the possibility is there for a second consecutive
hard freeze across the same areas.
For the long-term...a general moderating trend should commence on
Sunday lasting through the early portions of next week back to
near seasonal averages. Several storm systems look to impact the
region but the timing of each is uncertain at this time.
Post frontal IFR ceilings and occasional -dz will gradually transition to
MVFR ceilings during the middle/late afternoon...as drier Continental
air near the surface spreads in and deepens. Transition to MVFR
and eventually VFR will arrive latest at tup...likely well after
Light north winds and VFR should prevail for the late evening mem
cargo arrival push.
MS...freeze watch from late Friday night through Saturday morning for
Tennessee...freeze watch from late Friday night through Saturday morning for