Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Huntsville Alabama
1139 am CST Wednesday Dec 4 2013
morning forecast update and 18z tafs.
this morning lows in the upper 50s are comparable to that of our
normal highs for this time of year...which is 56f in hsv. Any fog
observed again this morning has since lifted but the mostly cloudy
skies will remain due to persistent isentropic upglide over north and
central Alabama. Even with the cloud cover temperatures will be in the lower
70s due to the warm air advection. Warm air advection is not strong enough to
create a big diurnal trend for today...so adjusted hourly temperature and
dewpoints accordingly. With the tightening pressure gradient out
west...gusts around 20kts are starting to appear across the valley
and could get as high as 25kts this afternoon. Otherwise forecast looks to
be on track...forecast products already sent.
for 18z tafs...low to middle level clouds will continue across the
region over the next 24hrs...with some fluctuations between few/broken MVFR
level clouds over the next several hours. Prevailing ceilings should be at or above
2500ft through much of the afternoon...deteriorating after sunset. Winds
will be periodically gusty to around 20kts through the afternoon and while surface
gusts drop off and sustained winds fall below 10kts after
sunset...speed around 30kts can still be expected just off the surface.
This will lead to low level wind shear conditions overnight as ceilings also fall below
1kft. A cold front will then approach the terminals nearing the end
of the period...increasing rain chances and shifting winds to be out
of the south-southwest. A strong shift to more northerly winds will be just west of
kmsl by around 18z tomorrow.
/issued 311 am CST Wednesday Dec 4 2013/
an active weather pattern is setting up across the Continental U.S. This
morning, with several weather events set to unfold through this forecast
period. A deepening 500 mb trough was noted over the intermountain west
this morning, with the convergence of the polar and subtropical jets
at the base of this feature. As this trough deepens, an upper ridge
will take up residence southeast of the County warning forecast area. The Tennessee Valley will find
itself sandwiched between these two systems, and their vastly
different airmasses, making for a tricky and rather difficult
forecast for the next 7 days.
8z surface analysis indicated a warm front sprawled across the County warning forecast area this
morning, with dewpoints rising steadily since yesterday evening
(currently in the upper 50s to lower 60s). Isentropic lift aloft is
generating another round of light rain as the front wobbles slowly
northward, making for another soupy and dreary day across the area.
Though more dense fog (~1sm or so) was noted at the beginning of the
shift, increasing winds have kept the lower levels a bit more mixed
than this time yesterday, so 3-5sm visibilities are expected to improve
through the middle-morning hours (if not sooner). Despite the cloud
cover today, afternoon highs will top in the lower 70s in many areas (well
above the middle 50s for norms today) as warm air advection strengthens.
The surface low currently over the Central Plains (and its eventual track
NE into the great lakes) will be the main forecast feature of the
next several days. The front will be approaching northwest Alabama overnight
tonight and by sunrise tomorrow. Have retained chance probability of precipitation tonight to
account for warm sector showers that will likely be ongoing across
the area ahead of the frontal activity itself. Soundings again this
morning continue to show instability/shear sufficient to mention
strong storms with gusty winds for tomorrow. Precipitable waters ~1.5-1.6" are
climatologically high enough for the formal inclusion of heavy
rainfall to the grids for tomorrow night as well. GFS/ECMWF/NAM show
the front stalling out over the County warning forecast area into Friday, with several waves
of low pressure riding newrd along the boundary. A temperature
gradient of 15-20 degrees is expected Friday (colder temperatures NW, warmer
se). Rainfall totals on the order of 2-3" appear likely (higher
northwest/lower se), but flooding concerns with this round are relatively
low due to the generally dry soil conditions in place.
The forecast fun begins Friday night into Saturday as the shallow
Arctic air begins to seep into northwest Alabama. As temperatures fall, rain will
change to freezing rain as temperature profiles plunge rapidly in the
lowest levels of the column. Temperatures aloft will be warm (50-60f) while
surface temperatures in northwest Alabama will be in the lower 30s (per the NAM/GFS
soundings). As temperatures warm into Saturday, the freezing rain will change back to
a cold rain, with afternoon highs struggling to reach 40 degrees there
(with 40-45f expected elsewhere along/east of I-65). Temperatures won't
change much Saturday night into Sunday as another wave of low
pressure develops in the western Gulf and tracks northeastward directly
over the Tennessee Valley, surging warmer air back northward as this occurs.
This will spread another round of heavy rainfall over the area, with
an additional 1-3" possible (making for a 5-day total approaching 6"
in some places). This round of heavy rain may cause river flooding
with saturated grounds converting most (if not all) additional rain
Sunday into Monday to runoff. This will need to be monitored, and an
esf may need to be issued later this week. As the cold front pushes
through the area early Monday morning, temperatures will fall through
the day. As this happens, rain will mix with snow by Monday evening,
changing over to all snow Monday night, and ending as flurries (if
any moisture is left) by Tuesday morning across the area. Little if
any accumulations will be light in nature, but a lot can change this
far out in the forecast. Either way, the first half of next week will
be downright cold, with morning lows in the lower 20s (and windchill
values in the lower teens) Tuesday morning.
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