Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 181444 AAB

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
944 AM CDT Sun Mar 18 2018

.NEAR TERM...(Rest of Today)
Issued at 935 AM CDT Sun Mar 18 2018

Stable layer around 900-850 mb noted on model soundings continues to
support/hold on to low stratus this morning over much of the
Tennessee Valley. Northerly winds will try to reinforce these clouds
for most of the day. However, some drier air aloft may be able to
mix down initially this morning, helping to create some breaks and
allow for some intermittent sunshine/heating to take place. In
response, temperatures should gradually warm into the 65 to 70 degree
range by late this afternoon, but it will certainly feel cooler than
yesterday thanks to this cloud cover.

As a surface low deepens over the Plains this afternoon a warm front
will slowly begin to work it`s way north. Isentropic lift ahead of
this boundary will help generate some weak lift and allow some
isolated to scattered showers to work their way in this afternoon
(mainly areas along the south of the Tennessee River). Some very weak
instability may permit an isolated rumble of thunder across far
southern portions of the HUN CWA. However, with the boundary and the
best lift/moisture to the south, coverage will be limited and QPF
very light. In other words, most locations will remain dry today.
Very minor tweaks needed to the forecast.

.SHORT TERM...(Tonight through Monday Night)
Issued at 455 AM CDT Sun Mar 18 2018

As previously mentioned, shower/isolated thunderstorm coverage will
increase as the warm front moves into the forecast area ~00-03Z. Precip
will taper off when the warm front exits the forecast area ~09-12Z.
Overnight lows will range from the upper 50s to the upper 60s.

Continue to be weather aware for Monday`s forecast.

A shortwave will quickly swing from the Plains on Monday morning to
the TN Valley by Monday evening while the sfc low will glide into TN,
dragging a dry line into the area by afternoon.

After the warm front lifts through in the morning, southerly winds
will be in place, returning moisture. Model soundings show a cap
~850mb to help suppress convection ahead of the dry line despite
elevated instability being present. If convection from the warm front
lingers, or if there is any late morning convection, this would have
some effect on the available instability and moisture for the
afternoon/evening storms.

As the shortwave approaches, midlevel temps cool, wind profiles
strengthen and the 850mb cap erodes. The developing low-level jet
will help speed moisture recovery with dewpoints reaching the lower
60s. Not to mention it will provide deep layer shear of 60-70kts and
~30kts of 0-1km shear. Mixed-layer CAPE values should easily exceed
500 J/kg with a few short-range models suggesting higher values. This
combination will likely yield severe storms between 18-00Z Monday.
Hazards include all modes, especially large hail. Uncertainty still
exists for the tornado threat, as a wedge ridge along the
Appalachians could create backed surface winds which would enhance
the low-level shear. There is also still some uncertainty about where
storms will initiate, since any delay could push the threat area
further to the east. In short, this resembles almost more of a
Plains-type severe pattern than a typical Southeast pattern. Will
look for storms to develop across NE Miss and NW Alabama around
midday and push east into Georgia by late evening.

.LONG TERM...(Tuesday through Saturday)
Issued at 455 AM CDT Sun Mar 18 2018

Another robust shortwave trough will swing across the TN Valley
again on Tuesday. It will have no instability to work with but
sufficient moisture for more scattered showers. Decent cold air
advection (CAA) will push temperatures down into the mid 50s (about
10 degrees below normal). 00Z runs indicate that the moisture will
exit before the coldest air arrives (lows will flirt with freezing),
so any mention of snow or flurries has been removed.

Another reinforcing shortwave will move across the area Wednesday,
but moisture will have pushed eastward. A few showers cannot be
ruled out in NE Alabama and the lower Plateau, but the forecast will
be largely dry. CAA will weaken but persist enough to keep temps in
the 50s despite increasing sunshine. Lows Wednesday night will fall
to near freezing with a good radiational cooling scenario. Thursday
will be the quietest day of the extended with ridging building behind
the departing trough, but it will remain cool.

Models diverge sharply after 00Z Friday and turn into quite a mess
for the weekend. The GFS is typically more progressive with another
Pacific longwave trough and grazes the area with warm frontal QPF as
early as Thursday night. It also quickly drags a front through the
area Saturday, with a rapid warm frontal retreat in response to yet
another Pacific system for Sunday. Conversely, the ECMWF and Canadian
are about 24 hours slower with the first system and slower with the
subsequent frontal passage. The effect on sensible weather ends up
having the three model groups wet and dry at different times. So the
forecast will reflect a middle-ground solution which resembles the
ECMWF: less aggressive with initial PoPs (which look to be deflected
north anyway) and a longer period of mid-range PoPs for the weekend.
Temperatures will go with blended guidance, which washes out the
wildly different front timings and ends up with the area staying
slightly above normal.


.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 652 AM CDT Sun Mar 18 2018

TAF sites are currently VFR however, MVFR CIGS ~1500ft are still
expected to spread southward over the next 2-3 hours. Conditions
should improve to VFR shortly after. Light rain will spread into the
TAF sites around 00-02Z and increase in coverage through the night.
The greatest confidence in timing MVFR CIGS and VIS is between
04-08Z. As the rain will move out towards the end of the TAF period,
CIGS will lower back to MVFR.





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