Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 191102

Area Forecast Discussion
National Weather Service Huntsville AL
602 AM CDT Mon Mar 19 2018

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 300 AM CDT Mon Mar 19 2018

Confidence is increasing on a severe weather outbreak this afternoon
with the greatest threat being very large hail with tornadoes and
damaging winds also possible. (Warning: long discussion below)

To start the day, an upper trough will be deepening into southern MO
with a surface low developing to its south. A warm front will be
advancing northward with its 3AM position still in central AL.
Southwesterly winds aloft will strengthen after 9-10z/3-4AM and that
should help push the front northward. Dew points are already in the
lower 50s and they will continue to increase as the front moves
north. There will be scattered showers along this frontal boundary
and in the warm sector this morning, maybe a few rumbles of thunder
even. As a lead shortwave ejects ahead of the main system, scattered
showers and thunderstorms, will continue, especially east of I-65
into the early afternoon. This is not the severe weather threat; that
will be coming later.

The area should be well into the warm sector by this afternoon.
Soundings do show a modest cap through about 18z so there will be a
lingering stratus layer along with the showers. However, guidance is
in very good agreement that the cap will break after 18-19z,
starting in the west, as temps warm into the lower 70s and a mid
level dry layer moves in from the west. Once this cap breaks, a very
volatile environment will be in place for severe storms. If the
clouds break out sooner and more than expected, temps could get into
the mid to upper 70s. Guidance, including the hires, is also in
surprisingly good agreement in regards to the timing of the line of
storms. The main line of severe storms looks to initiate in NE MS/NW
AL 19-21z (2-4PM), strengthening as it gets near the I-65 border 22-
01Z (4PM-7PM) and NE AL by 01z-03z (7PM-9PM). There are still some
slight discrepancies in this and it could be off +/- an hour or two
with the NAM continuing to be the latest choice which is about an
hour later than the previously mentioned window. The latest HRRR
runs are also leaning more towards a NAM/later arrival time solution.

Around the time of storm initiation this afternoon, a low level jet,
the left exit region to be exact, will be pushing into NW AL and
moving east. This will help erode the cap and enhance the shear and
be a primary focus of storm initiation. Although the upper support
is back to the north and west, mid level lift, with a surface cold
front and this LLJ should be plenty for rapid intensification of
supercells this afternoon.

Now, lets talk about threats. As expected, all modes of severe
weather are possible this afternoon, including tornadoes. As the day
heats up, MUCAPE values rise to 500-1500 J/KG with the NAM, as
usual, much higher than any other model and predicting highs near
80. The most significant part of the instability is a deep -10C to
-30C layer of CAPE, coupled with 60+kts of bulk shear, WBZ heights
near 8500ft; all which indicate a very good probability of large to
very large hail. Golfball to tennis ball size hail cannot be ruled
out. This will be especially true if we get near the 2000 J/kG mark.
The LLJ, as mentioned will increase the 0-1km shear to 25-30kts by
00z, west to east. 0-1KM SRH could be as high as 300-400 m2/s2 (with
the NAM being the highest once again). Low and mid level lapse rates
will be near or above 7C/KM across the area. Given the strength of
the shear and the mid level dry slot, damaging wind in severe storms
will also be likely.

The tornado potential will be maximized approximately around I-65
and east due to the timing of certain parameters coming together. By
the time the low level jet moves in and breaks the cap, this eastern
area will have more time to destabilize, the jet location and
strength will be more favorable and all guidance is showing high
instability and extremely classic tornadic hodographs. Although a
tornado cannot be ruled out areawide, including NW AL, hires guidance
is very consistent showing reflectivity/updraft helicity increasing
rapidly as it nears the I-65 corridor. LCL heights will also be
below 500m in most locations, another favorable value for tornadoes.
Although the threat for "significant" tornadoes is not too high, if
we maximize the parameters, especially if the NAM is realized, a few
strong tornadoes would not be out of the question.

Obviously with any event, there are uncertainties. The biggest today
will be the destabilization of the warm sector. The warm front has
been somewhat stuck overnight and if this lingers to the south, we
may not warm or destabilize enough. This could lead to the cap not
breaking or breaking much later than anticipated. The only change to
the forecast if the cap didn`t break would be a lessening of the
severe threat in the west. As already mentioned above, the models are
differing in the strength of the environmental parameters which
would also impact the strength of the hazards. Stay tuned as the
forecast will quickly change today. Although dew points will rise
with the warm front, right now they are only forecast to get in the
lower 60s. This is still high enough for severe storms but usually,
we like to see them in the upper 60s for our significant tornado

.SHORT TERM...(Tonight through Wednesday Night)
Issued at 300 AM CDT Mon Mar 19 2018

Behind the line of storms, clouds likely will not clear out much as
quick cooling at the surface traps the clouds under an inversion but
the rain should quickly end. Rain could be done in NW AL by 00z,
central N AL by 1-3z and out of NE AL by 4-6z. Lows will be mild, in
the upper 40s.

Models are slow to move the surface low east once it moves into
eastern Tennessee and the Carolinas. However, good cold air advection
is in place behind the cold front by Tuesday morning. Also, a strong
deformation band looks to develop just to the west and northwest of
the surface low further east over eastern TN and western North
Carolina. Luckily, most models have the low far enough east that this
strong deformation band will likely setup over the north
Carolina/eastern TN border and extend southwest to near Chattanooga.
This would produce a band of heavier and more persistent
precipitation over those areas and keep rain chances lower over
northern Alabama and towards Moore county, TN. However, will still
see some higher chance pops in our southern middle Tennessee and
northeastern counties Tuesday afternoon into Tuesday evening. The
strong cold air advection mentioned earlier will drop temperatures
from the lower 50s /west of I-65/ to near 60 degrees /east of I-65/
around daybreak into the lower 40s/upper 30s by the early evening
hours. It will be fairly breezy and cloudy with a stiff northwest
wind sustained at around 15 mph with gusts to between 20 and 30 mph,
given a strong 30 to 35 knot 850 mb jet aloft Tuesday and Tuesday

Precipitation chances should lower a bit more around and
after midnight. However, as temperatures drop into the mid 30s around
midnight, this precipitation looks to be in the form of a mix of
very light rain and snow (but surface temperatures stay at least a
few degrees above freezing which should keep any accumulation from
occurring). As precipitation chances become more isolated before
moving east into Georgia around daybreak on Wednesday, this could
briefly change to all snow as temperatures drop to near freezing.
However, this period of all snow looks to be too brief and isolated
in nature for any snowfall accumulation. However some slick conditons
may develop around daybreak as temperatures drop near or just below
32 degrees east of I-65.

A return to cold temperatures and clear skies are expected on
Wednesday, as high pressure builds toward the area. Highs in the mid
40s to lower 50s are expected.

.LONG TERM...(Thursday through Sunday)
Issued at 300 AM CDT Mon Mar 19 2018

A gradual warm up will commence on Thursday as a trough builds
initially into the western CONUS and a ridge with deepening S-SW
flow develops in the Plains States and moves eastward. Some
discrepancies exist as expected between the next upr wave and cold
front expected to impact the region. But, the global models indicate
a rather quick transition from a developing ridge to a near zonal
flow pattern by the early weekend, with low pressure moving rapidly
across the OH Valley region on Saturday. What is appearing likely is
that at least a part of the west CONUS trough will be ejected into
the Plains and Midwest, but what is not easy to determine yet is the
southern extent of its influence, including on the TN Valley.
Nevertheless, this has been a persistent large scale feature and thus
some POPs are retained for the weekend period. Weak instability during
the period may produce a few thunderstorms, but only slight chances
were mentioned owing to the generally weak instability and forcing
over our region.


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 602 AM CDT Mon Mar 19 2018

CIGS around 1500-2000ft will linger this morning until we warm up
and mix out the lower clouds. We should remain bkn/ovc around 3-4kft
most of the day outside of rain/storms which will lower the cigs/vis.
Scattered showers and a few thunderstorms will develop this afternoon
and indicated that with a VCSH/VCTS in the TAFS. Winds will also be
gusty with gusts around 20kts this afternoon, outside of
thunderstorms. A line of strong to severe thunderstorms will move
through the area this afternoon; probably through KMSL by 20/21z and
KHSV by 22/23z and have included a TEMPO for that with a little
padding on the side for timing uncertainties. Once the line moves
through, rain should quickly end and CIGS will rise slightly. Toward
the very end of the TAF period, CIGS could lower again but
confidence was too low to add another group.





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