Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Memphis Tennessee
341 am CST Thursday Nov 27 2014
Currently...deep upper level troughing exists across eastern Continental U.S.
With strong coastal wave beginning to exit New England. Across the
middle south...secondary shortwave embedded in cyclonic flow is
pushing east and has been producing some very light snow showers
across portions of middle Tennessee. For this reason...did
introduce some light flurries this morning.
For Thanksgiving day...cold cyclonic flow will dominate the middle
south. Some low clouds will linger through the morning which is
represented in nearly all hi-res guidance especially across middle
Tennesee in closer proximity to the upper level low pressure.
Through the day...low cloud deck should erode and give way to
partly cloudy skies. The cold airmass is likely to offset much in
the way of solar heating so highs near 40 across middle Tennessee
to middle 40s across portions of northern Mississippi are likely.
A gradual is warmup is shown in model guidance beginning Friday
as meridional flow gives way to a more zonal pattern through the
weekend. 850mb temperatures increase into the low teens by
Sunday...the warmest day of the period with highs in the middle to
upper 60s forecast.
Models have also been consistent in depicting fairly rapid
moistening of the boundary through Sunday which is a
climatologically favorable setup for periods of drizzle. The GFS
actually depicts this scenario beginning Friday but did not
introduce this into the forecast at this time. However...with
continued low level moistening and very weak/slow low level
vertical motions...expect drizzle to develop Saturday as moisture
streams north. Whether this will be measurable is yet to be
determined but enough of a signal exists to introduce into the
forecast at this juncture. This scenario looks to continue through
Sunday morning with light rain becoming more widspread by late
afternoon as forcing gradually increases.
A more synoptic signal exists early next week for more widespread
precipitation as a strong baroclinic zone impinges on a moistening
airmass. Even though dewpoints increase into the middle and upper
50s...lapse rates are so weak that instability...even elevated...remains
negligible. Have therefore left thunder out of the forecast at
this time. Light to moderate rain is expected along and just north
of this boundary as moisture is lifted up and over the frontal
zone. Given flow nearly perpendicular to the boundary...it will
likely meander through the area very slowly so an extended period
of precipitation appears possible.
For the longer term...later in the week...models suggest that the
front washes out as the thermal gradient relaxes and frontal
circulation slows. Precipitation will gradually end as another period of
warming begins late in the week.
06z taf set
Clouds currently located over Missouri...behind the
departing cold front...will spread into the midsouth overnight
into Thanksgiving morning. Ceilings could drop to MVFR for several
hours. By 18z-21z...expect VFR conditions at all taf sites. Northwest
winds will be around 10 kts overnight. Winds will become northerly
during the day and speeds should begin to decrease during the
afternoon hours as high pressure begins to settle into the area.
Light and variable winds expected after 00z.