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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

                            
000
FXUS64 KHUN 170359
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
959 PM CST Sat Feb 16 2019

.NEAR TERM...(Rest of tonight)
Issued at 959 PM CST Sat Feb 16 2019

Low temperatures in most locations across northern Alabama and
southern middle Tennessee hit their mark early this evening. 
Temperatures have remained steady or slightly increased in most 
areas since then. As we approach midnight, these temperatures will 
climb higher, as the front that pushed into southern Alabama this 
evening moves back to the north overnight. As it does so, a strong 
southwesterly low level jet develops over the southeast in response 
to upper level energy that spreads east from Nebraska and Kansas into
the Ohio Valley around daybreak on Sunday. This creates strong low 
level lift over northern Mississippi and western Tennessee after
midnight. This lift moves eastward into eastern Tennessee by the 
daybreak hours on Sunday.

Newer guidance continues to show elevated CAPE between 700 mb and 
500 mb after 2 or 3 in the morning over portions of northern Alabama 
and southern middle Tennessee. Given the strong low level lift around
700 mb shown by omega values, think that this elevated instability 
could end being a bit higher than models are currently showing (mainly
due to some enhanced cooling aloft the strong lift could cause in 
that layer). Thus for the update, increased thunderstorm chances in 
northwestern ALabama and southern middle Tennessee into the chance 
category overnight. Showers will likely begin to become more 
widespread around 3 am as the stronger forcing associated with the 
developing low level jet moves into the area. 60 percent or better 
chances of rain looks reasonable over the area overnight and into 
Sunday morning. With abundant cloud cover, strong warm air advection,
and winds increasing into the 5 to 10 mph range into Sunday morning,
temperatures around daybreak should climb into the upper 40s to 
lower 50s. 

.SHORT TERM...(Sunday through Monday)
Issued at 343 PM CST Sat Feb 16 2019

Showers should continue during the day on Sunday and Sunday night, 
as the system swings across the region. This system should being 
another 1-2 inches of rainfall across the area. Keep in mind that 
soil moisture content across the Tennessee Valley is already very wet
to saturated; thus runoff from excessive rainfall will produce rises
in area rivers and streams. Could not rule out some of them 
approaching minor flood stage on Monday. Despite the rain, 
temperatures to start a new week should warm into the low/mid 60s, 
thanks to a southerly flow preceding the approach of the parent low. 

Winds will become NW-N Sunday night and on Monday, as colder air west
of the system sweeps across the area. This cooler and drier air will
allow some sun return for Washington's Birthday holiday on Monday.
Despite a brief return of the sun, high temperatures from cold air
advection will rise into the low/mid 50s --- close to seasonable 
norms of around 56. This "dry spell" will be short lived, as the mid
and latter half of the new week will be a very wet and possibly
stormy one with serious flooding a distinct possibility. 

.LONG TERM...(Monday night through Friday)
Issued at 343 PM CST Sat Feb 16 2019

The confidence continues to increase that a higher end flood event 
will unfold across the TN Valley next week.

Latest guidance is showing a slightly slower trend in how quickly
the warm front lifts back north on Tuesday. We may not start seeing
the heavier rain along the warm front until later in the day Tuesday. 
There is also some growing discrepancies between the GFS and ECMWF 
and where they track the heaviest precip axis Tuesday into Wednesday. 
The ECMWF has been trending slightly deeper with the western US trough
which would result in a more westward shift in the heavy rainfall
Tue/Wed. The GFS is a bit flatter and more broad with the trough and
has been consistent in showing the heaviest rain over the area. However,
the past few runs of the operational GFS have been at the top of the
GFS Ensemble members for QPF from Tuesday through Sunday. Those amounts
would put us in the 9 to 10 inches of rainfall. The ensemble mean has
been holding pretty consistent at around 7 inches for the TN Valley. 
Also believe the GFS might be under forecasting the impact the ridge
will have on the shortwaves that move out of the western trough. 

Now with that said though, the ECMWF/FV3 kick the western trough east
on Wednesday and end up stalling out a frontal boundary over the area
from Wednesday through Friday as another trough digs into the western
US. This is larger shift in the models as they were initially 
indicating that we might end up getting a break in the rain sometime 
Thu/Fri. We may still see some additional changes in rainfall totals
and timing of the heaviest rainfall as models adjust the position of
where the frontal boundary stalls out. While they may differ on the
exact timing of the heaviest rain, storm total rainfall from all of
the models through Fri/Sat puts the TN Valley under a swath of 8+ 
inches of rain and in some cases 10+ inches. Early indications would
suggest river levels may be close to above what we saw with the
Christmas 2015 flood event. If rainfall totals do end up on the
higher side of guidance, river levels could even approach that of the
2003 flood. 

All of this is tied to a persistent trough over the western US and a
strong ridge over the Caribbean. The TN Valley will be stuck in the 
middle of those features and a very strong S/SW orientated upper 
level jet will advect deep tropical moisture into the area. 
Precipitable water via all models would suggest near record values 
for February (1.6-1.7 inches). 

There is surprising consistency in guidance that we could see a final
round of showers and storms on Saturday into Sunday. This system will
have stronger forcing and will be able to advect warmer temperatures
into the area. While it's still far out, forecast soundings would
suggest that stronger storms are certainly possible Sat/Sun as the 
system moves across the region. Only reason to bring this up is that 
we could be dealing with strong storms in the middle of an active 
flood event. 

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 542 PM CST Sat Feb 16 2019

Both terminals are experiencing MVFR CIGS at this time (mainly
between 1000 and 1500 feet). This should continue at KMSL, but KHSV 
may see some brief dips in CIGS below 1000 feet (IFR) through 03Z. 
Showers and isolated thunderstorms over eastern Arkansas are pushing 
east towards the area ahead of an approaching shortwave. Showers 
ahead of this activity could arrive as early as 3Z at KMSL and 5Z at 
KHSV. At this time, due to low confidence that this initial activity 
will directly impact either terminal, only included VCSH in the 
forecast starting at these times. As the stronger forcing and 
arrival of deeper moisture occurs ahead of this system after midnight
(especially after 08Z), included a definite period of -RA with a 
tempo group of -TSRA between 08Z and 15Z. During this period, IFR or 
lower CIGS or VSBYS are expected. By 15Z at both terminals expect 
only drizzle to occur with CIGS remaining below 1000 feet. Expect 
this to continue through 18Z with -RA returning to both terminals by 
then.


&&

.HYDROLOGY...
Issued at 343 PM CST Sat Feb 16 2019

Rainfall totals from multiple rounds of shower activity this weekend
and next week will add up, bringing a potential of significant
flooding across the Tennessee Valley. Another episode of showers and
embedded thunderstorms are forecast to impact the region tonight, as
a weak system moves across the area. Another 1-2 inches of rain is 
possible with this system. Given that soil moisture content is very 
wet to saturated, runoff from excessive rainfall will produce rises 
in area rivers, streams, and creeks. Minor flooding cannot be ruled 
out on Monday. 

Chances for much heavier rain looks possible during the middle and 
latter half of the week. Another 5-7 inches of rain could fall across
portions of the Tennessee Valley from Tuesday into early Saturday, 
with rainfall totals to the end of next week potentially ranging 
from 6-9 inches. The heaviest amounts of this rain apparently will 
fall over north central and northeast Alabama, and adjacent southern 
middle Tennessee. Flash flooding, areal flooding, and substantial 
rises in levels of rivers all remain possible. The flooding from the 
runoff could rival past flooding events like on Christmas 2015, or 
the spring flood of 2003. 

Persons across the Tennessee Valley should remain up to date with the
latest information regarding a possibility of widespread, damaging
flooding. Those in flood prone areas should begin plans to minimize
damage should flooding affect your property. 

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

NEAR TERM...KTW
SHORT TERM...RSB
LONG TERM...Stumpf
AVIATION...KTW
HYDROLOGY...RSB


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