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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

                            
000
FXUS64 KHUN 192347
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
547 PM CST Tue Feb 19 2019

.UPDATE...
For 00Z TAFS.

&&

.NEAR TERM...(Tonight)
Issued at 332 PM CST Tue Feb 19 2019

An unusual event is unfolding across the TN Valley today. We started
the day with a mix of rain and sleet as a result of dry/cold sub 
cloud layer. As rain moved in it was able to saturate this layer 
closer to noon converting everything over to all rain. Soundings via 
UAH as part of VORTEX SE operations at 18z showed strong WAA 
occurring aloft. Strong lift in this region coupled with cold/steep 
lapse rates was resulting in numerous thunderstorms. Due to the 
colder nature of the atmospheric profile we have had reports of pea 
to dime size hail with some of the stronger storms. What makes this 
so unusual is the fact that temps have remained below 40 degrees so 
far. The main surface based warm front is still anchored along the 
Gulf Coast and much of these storms are developing along the elevated
frontal boundary moving through the area. Strong W/SW winds aloft 
oriented almost parallel to this boundary are resulting in some 
training of storms across MS and into NW AL. 

MRMS rainfall estimates with the band of storms indicate totals 
closer to 2 inches. Saturated soils were resulting in almost all of
this rain being converted over to all runoff. This is something we
will have to watch closely as some of the storms moving out of MS
move into the area. Any training of storms over a particular area
could warrant Flash Flood Warnings, especially in many urban
locations. 

Now in terms of what will occur for the rest of the evening, an upper
level system was shifting out of the desert SW and into the Central
Plains. This was causing pressure falls over the TX Gulf Coast with a
surface low expected to develop and track N/NE along the MS River 
basin tonight. A strong 50-60kt LLJ tied to this surface low is 
forecast to move N/NE towards the area overnight. This coupled with a
stronger jet aloft will be able to transport near record moisture 
into the TN Valley. As it stands now, the latest guidance continues 
to show that best lift/forcing associated with system moving into the
Plains will stay to our west with the heaviest rainfall axis being 
centered along the MS River tonight. However, current radar/satellite
imagery indicate a narrow band of showers/storms moving north out of
New Orleans area and into central MS. Moving that forward in time 
would bring it up over the area later this evening, which models are 
not quite accounting for. The current forecast will show a slight 
decreasing trend overnight as the warm frontal boundary aloft moves 
north into TN. But as mentioned above we'll have to watch the narrow 
band of precip in MS. 

Rainfall totals are likely too low and will be higher where we see
any training of precip. We will continue to monitor and make
adjustments to the forecast this afternoon/evening. The Flood Watch
was updated this morning and went into effect at Noon and will run
through Friday. The one other concern that we might be
underforecasting the winds tonight. Model soundings indicate a
sizable inversion over the area and the stronger winds may not quite
be able to mix down to the surface. Some of the higher winds might 
be possible in the terrain in NE AL and southern middle TN and have 
tried to show some of those gusts there. 

.SHORT TERM...(Wednesday through Thursday night)
Issued at 332 PM CST Tue Feb 19 2019

The surface low will be shifting into western TN Wednesday morning
and the heavier rainfall axis will be centered near the low. As the
low moves into western TN and the pressure gradient tightens over the
area, it looks like a CAD situation might develop in the Lee of the
Appalachians. Some of that is going to spill into the TN Valley and
likely could help to reinforce our low level inversion through the
morning. The other impact this might have is that it could really
enhance the convergence over the area and increase forcing. 

As the surface low shifts into the OH Valley it will push a cold
front into the area later in the day. Latest guidance is probably a
bit too quick with the frontal passage tomorrow and move it east of
the area by Wednesday evening. Think that even though the CAD is
projected to weaken through the day this may end up slowing the
progression of the front and it might stall over the area for a brief
period of time tomorrow. When analyzing forecast soundings, there is
upwards of 500 J/kg of MUCAPE expected along and ahead of the front.
With near record PWATs, strong convergence, and a rather unstable
environment for late February could result in intense rainfall rates
of 1 to 2 inches per hour. Winds from 850mb to through 250mb would 
be paralleling the frontal boundaries and we could see training of 
storms. When taking all of that into consideration was a bit reserved
to use the lower QPF amounts guidance was providing. So, have tried
to show higher amounts during the afternoon hours as the front moves
into the area and where we might see training of storms. 

High pressure shifting E/NE into the OH Valley will help to push the
lingering frontal boundary south of the area Thursday morning. While
the core of the heavier rain will be south we should still see some
light rain continue. River flooding should be ongoing at this point
and the concern that we have is the models trending lower with
rainfall amounts has brought forecasted river levels below where 
they might actually end up after all this rainfall. Taking into
consideration where QPF might be, flooding could be comparable to the
Christmas 2015 flood at the low end. 

While this front is moving across the area a trough will be digging
into the desert SW and should result in a continuation of the strong
S/SW flow over the area. Shortwaves ejecting out of this trough could
bring the front back into the area Thursday evening. Some guidance is
showing a few bands of heavier rain moving across the area as these
waves cross the area. Rainfall amounts won't be nearly as high due
some slightly drier air aloft. But any rain will continue to impact
river flooding. 

Thursday night into Friday could end up similar to what we had today,
a warm front lifts back to the north with strong convergence focused
along the boundary. Light to moderate rainfall moves back into the 
area as we head into Friday. 

.LONG TERM...(Friday through Monday)
Issued at 332 PM CST Tue Feb 19 2019

The extended period begins Friday morning with a warm front nearly
overhead/slightly displaced just N of the CWFA, with light to
moderate rain likely ongoing by this time. Friday overall will be a
wet day, thanks to the stalled frontal boundary and several
disturbances riding NE along it through the day. This round of
rainfall will exacerbate any ongoing/lingering flooding concerns from
heavy rainfall received earlier in the week. Rainfall totals for the
Friday/Friday night timeframe look upwards of 1.5-2", with isolated
higher amounts from any thunderstorms that occur during this time. It
goes without saying that this will add to flooding issues ongoing
across Northern AL/Srn Middle TN. 

The bigger story in the extended revolves around the forecast during
the day/evening timeframe Saturday. A potent upper level system 
takes shape across the Intermountain West later this week, spawning 
cyclogenesis Friday night into Saturday over the TX/OK Panhandles 
region, and ejecting NE into the Ern KS/MO through the day
Saturday/Saturday evening. This part of the forecast will definitely
require some fine-tuning in the next couple of days. The pattern is
favorable for a QLCS for our area, with perhaps QLCS/supercells to
the N/W over portions of Wrn TN. Any shift of the upper level pattern
would increase/decrease our threat for severe weather during this
time, and this will need to be watched closely. For now, the 
ingredients are there for at least strong to possibly severe wind 
gusts along the line of storms that march across the area Saturday 
evening/night. Daytime high temperatures Saturday will top out in the
lower/middle 70s, which is unseasonably warm for this time of year.
Dewpoints will creep up into the lower 60s as well with stout Srly flow
ahead of the front. An 850mb jet of 50-60kts, 300-400 m2/s2 SRH, and
0-1km shear of 40kts (with 0-3km shear of 50kts) will lead to the 
potential for 50-60mph wind gusts along the line, with a lesser 
chance for a brief spin-up tornado. Add to that PWat values around 
1.6", which are record values for both BMX/OHX SPC PWat Climo values 
for this time of year, and you're looking at a mixed bag of weather 
on tap for Saturday. In addition to the severe threat, moderate/heavy
rainfall will accompany this fropa through early Sunday, adding 
another 1-2" (with isolated higher amounts possible). Again, it 
sounds like a broken record, but this will further exacerbate the 
ongoing flooding conditions across the area. For this reason, the 
Flood Watch may need to be extended further in the weekend (as it
currently runs through 00Z Saturday. This is in the 'fine tuning' 
that will happen over the next couple of days, so stay tuned. The 
fairly active pattern isn't quite done yet.

There is an end in sight, however brief, that will occur in the wake
of this fropa this weekend early Sunday. DRY WEATHER WILL BE EXPECTED
FOR ALMOST A FULL 36 HOURS, so get out there and enjoy it! Dry 
conditions are expected mid-morning Sunday and all day Monday, 
before more rainfall develops and overspreads the region on Tuesday. 
Depending on your model of choice that far out in the extended, 
another 1" (at least) is on the horizon (you guessed it: 
further adding to the lingering flooding concerns across the area). A
lot can change between now and then, though, so stay tuned for
further forecast updates between now and then.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 547 PM CST Tue Feb 19 2019

Inclement flying weather will continue across the area for the TAF
period. Reduced CIG/VIS values into the MVFR/IFR category will 
continue, with possible reductions to LIFR in the heavier showers 
and/or tstorms. ESE winds in the 10-15kt range early, will increase 
into the 20-30kt range with some higher gusts later this evening, 
continuing to around daybreak Wed. Forecast models indicated stronger
winds developing aloft, enough to include a Low-Level Wind Shear for
the mid evening through after midnight, with ESE winds at FL 2kft to
60kt. Those winds should subside somewhat in the overnight, but 
widespread showers and embedded tstorms will continue into Wed 
afternoon. 

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Flood Watch through Friday afternoon for ALZ001>010-016.

TN...Flood Watch through Friday afternoon for TNZ076-096-097.

&&

$$

NEAR TERM...Stumpf
SHORT TERM...Stumpf
LONG TERM...12
AVIATION...RSB

For more information please visit our website 
at weather.gov/huntsville.