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afdhun

Area forecast discussion
National Weather Service Huntsville Alabama
626 am CDT Thu Mar 30 2017

Update...
for 12z tafs.

&&

Near term...(today and tonight)
issued at 259 am CDT Thu Mar 30 2017

Ongoing convective line moving across central MS may arrive across
western Alabama, but is showing signs of weakening before its
arrival. The northern portion of the line has all but dissipated
across northern MS. Hi-res model output shows this activity moving
east, but in a weakened state before its arrival. This is mainly due
to a lack of a sustainable lifting mechanism ahead of the deepening
mid-level deepening cyclone slowly moving east towards MO/AR.
Further west, more convective storms have formed over Arkansas near a
vorticity lobe on the southeast flank of the upper low.

The progression of this deepening mid-level cyclone eastward, the
southern flank shortwaves rotating around the cyclone, and the
whether the induced environment will be enough to support severe
weather are just some of the questions with this scenario. For
purposes of this discussion, will describe what current thinking of
this scenario is followed by the limitations of severe weather
occurring.

As a result of its approach the southern flank of this cyclone has
caused and is expected to continue a mesoscale convective system or linear convective feature
moving along the deep south/Gulf Coast area this morning into the
afternoon. To the north over MS, additional convective showers/storms
may develop within the warm sector as a result of a couple of
mesoscale shortwave troughs moving within the southeast flank of the
cyclone. Due to the potential for these storms to initiate and the
growing conditionally unstable environment throughout the
morning/afternoon, have kept thunderstorms in the forecast this
morning. However, reduced the pops to reflect an overall lower trend
in confidence. As we approach the afternoon, forecast soundings show
a cap around 750 mb that is expected to erode with daytime heating.
If that indeed happens, could see sbcapes of between 1200-2200 j/kg
especially over north MS/northwest Alabama. With modestly curved hodographs and deep
layer bulk shear values of 45-60 kts, could see discrete supercells
and bowing segments along any linear features. In addition, diffluent
flow aloft just to the northeast over the Appalachians would also be
favorable for severe storms. Impacts could be very large hail,
damaging winds, and tornadoes. The hail threat is mainly due to a
large amount of instability in the hail growth zone and wet bulb zero
heights between 8500-9500 feet (depending on model). To compound the
discrete cell threat, there is also expected to be another qlcs
forming just ahead of the cold front late this evening until
midnight. The main threats with this qlcs are damaging wind and
tornadoes.

Now, the limitations:

1) the cirrus canopy from the ongoing linear convection and even
southern convective activity (along Gulf coast) could limit
destabilization. Have adjusted the daytime highs a couple of degrees
to account for this possibility.
2) if above occurs, the Tennessee Valley will be capped with limited
discrete cell activity occurring. However, if any discrete cells
break the cap due to some mesoscale outflow boundaries or shortwave,
these storms within this environment would at least briefly reach
severe thresholds.
3) the extent of the mesoscale convective system activity along the Gulf Coast could limit
moisture/warm air advection. If this occurs, our severe threat will
be greatly diminished, especially the tornado threat.
4) the models have had great difficulty resolving any of the above
limitation and are displaying great variability on the environment
and quantitative precipitation forecast representations.
5) if severe discrete cells do not form and the Gulf Coast mesoscale convective system
continues its trajectory directly to the south of the Tennessee Valley,
there is great uncertainty on whether the late evening qlcs will be
severe.

So having mentioned all of that information, will continue to
mention the possibility of severe weather this afternoon and evening
especially over northwest Alabama where the greatest forcing,
stronger shear, and instability (at least initially) are expected.
However, realize that there may be quick changes to this forecast
based on how this very complex scenario evolves.

Short term...(friday through Saturday night)
issued at 259 am CDT Thu Mar 30 2017

As the shortwave trough moves off to the northeast and cold front
finally passes over the region a quiet and pleasant weather
environment on Friday is expected. Daytime highs should be in the low
70s/upper 60s with overnight lows in the upper 40s to low 50s on
Saturday morning. Very nice Spring conditions should continue through
Saturday into Saturday night as a upper level ridge builds over the
southeast.

Long term...(sunday through wednesday)
issued at 259 am CDT Thu Mar 30 2017

By Sunday, a trough will be exiting The Four Corners into the
Southern Plains. The GFS and European model (ecmwf) are showing distinct timing
issues (about 12 hours apart with the 500 mb wave). However, the
onset of precipitation may not be too far off, with blended pop
guidance looking good at this point. The lead shortwave that is
indicated will be ejecting northeast through the lower MS valley on
Sunday. However, this wave will become starved for moisture as it
encounters the southeast U.S. Ridge. Thus, the chance of
precipitation will not begin until Sunday night as the main wave
tracks east into through MS and Alabama. The track is far enough south
that our forecast area will remain in the cool sector at the
surface. The best low level inflow will likely be directed into a
vigorous mesoscale convective system that will track along the Gulf Coast. Again, timing is
different, but the overall scenario appears similar amongst the GFS
and European model (ecmwf). Elevated showers and thunderstorms will remain possible
over the Tennessee Valley Sunday night into Monday as an 850 mb jet of 50-
60kt enhances convergence northeast of the 850 mb low track. Then, a
distinct deformation zone will produce rainfall across Tennessee and MS
that pivots across north Alabama into Georgia on Monday.

The synoptic pattern will remain quite progressive and active later
next week. After a brief break late Tuesday, the next trough digs
southeast quickly behind, but with a more northerly track through
the central and eastern corn belt. A cold frontal passage will occur
on Wednesday or Wednesday night bringing the next shot of
thunderstorms. It's far too early to say what hazard/impacts we will
see with this one, but initially looks like a high-shear/Low Cape
(hslc) environment. A noticeable cooling trend should follow on day 7
and just beyond.

&&

Aviation...(for the 12z tafs through 12z Friday morning)
issued at 627 am CDT Thu Mar 30 2017

MVFR cigs have arrived this morning as remnants of a line of showers
moves across far northwest Alabama. These cigs will continue to overspread the
region through 18z-19z before -tsra form later this afternoon and
evening ahead of a strong cold front. Some of these thunderstorms and rain could be
severe with hail and damaging wind gusts greater than 35 kts
possible. Timing of the severe storms in the vicinity of the
terminals would likely be between 30/19z-31/03z. IFR and possibly
lower category will be possible with this activity, but have not
placed in tafs yet. VFR conditions should then return after
31/06z(kmsl)-31/08z(khsv).

&&

Hun watches/warnings/advisories...
Alabama...none.
Tennessee...none.
&&

$$

Near term...sl.77
short term...sl.77
long term...17

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