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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

                            
000
FXUS64 KHUN 201220
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
620 AM CST Wed Feb 20 2019

.UPDATE...
For 12Z TAFS.

&&

.NEAR TERM...(Today)
Issued at 335 AM CST Wed Feb 20 2019

There's a little bit of a break now in the rainfall/showers across
most of the cntrl TN Valley this early Wed morning, after some very
heavy rainfall/tstms impacted most of the area earlier this evening.
Rainfall totals from yesterday afternoon/evening look to be in the 
1-3 inch range, with some locally higher amounts. This brief reprieve
in the moderate/heavy rainfall is only xpcted to last shortly past
daybreak, as another wave of showers/tstms translates east across
west TN/north MS. This activity looks to be developing/occurring
along/just ahead of a cold front that is just now crossing the MS
River. Latest model suites all have this sfc boundary moving into the
mid TN Valley later today, with showers/embedded sct tstms continuing
along the advancing front.

Given the lack of any substantial buoyant energy/instability ahead 
of the front, most of the tstm activity will likely remain elevated, 
given the presence of a persistent low level weak inversion. However,
a strengthening H85 jet out of the SW near 50kt may result in a few 
strong tstms capable of brief wind gusts near 40 MPH or higher. The 
more pressing issue will be additional rainfall totals later today 
with the oncoming moderate/heavy precip, which looks to be around 
0.50-1.00 inch, with locally higher amounts in/near any of the 
stronger tstms. This additional rainfall coupled with the earlier 
precip will certainly maintain a continued risk for areal flooding, 
along with elevated river stages, in some locations across the area 
thru the afternoon hrs. As such, no change in the ongoing Flood Watch
is needed at this point. Rainfall may begin to taper off some late 
this afternoon from the west, as the cold front continues to move 
east across the cntrl TN Valley.

.SHORT TERM...(Tonight through Friday night)
Issued at 335 AM CST Wed Feb 20 2019

The front will begin to exit NE AL to the east this evening, with 
rainfall continuing to gradually taper off. With some elevated CAPE 
lingering across the area, a few embedded tstms will remain possible 
this evening, before any convective activity tapers off with the 
onset of the overnight period. Rain chances though will continue into
the morning hrs Thu, given an active SW flow pattern remaining in 
place over the region. Even with the front south of the area, rain 
chances will continue thru the day Thu, as an upper wave drifts north
into the mid TN Valley. However, given the lack of more sig 
mid/upper forcing, rainfall amounts are generally xpcted to be rather
light/less than a quarter inch into Thu evening.

Rain chances will then increase going into early Fri, as an H85 
front moves north into the gen area. Rain chances are a little more 
uncertain heading into the day Fri, as the stalled sfc front to the 
south begins to double back to the north as a warm front. However, 
pending how far north the warm front moves will likely determine the 
extent to which showers spread back into the area, with the latest 
model runs somewhat inconsistent with the position of the sfc 
boundary. However, with the passage of the H85 front to the north, 
some elevated instability looks to move back into the area and 
likewise may translate into a few embedded tstms within increasing 
showers thru the day Fri. This overall trend will continue into the 
evening hrs Fri, with rainfall/embedded tstms dependent on the 
location of the front. Additional rainfall amounts from tonight into 
Fri night look to be in the 1.00-1.50 inch range, with again locally 
higher amounts pending any convective development. The lack of more 
substantial rainfall totals thru Fri night may help to tempo 
alleviate much in the way of areal flooding. However, rivers stages 
that are in/near flood will likely remain fairly stagnant and 
likewise, the Flood Watch will be retained thru at least Fri. The 
watch may very well need to be extended into the weekend period, as 
another potent synoptic/storm system out of the srn Plains begins to 
lift into the Midwest states, with an attendant cold front stretching
south into the extreme NW Gulf. 

.LONG TERM...(Saturday through Tuesday)
Issued at 335 AM CST Wed Feb 20 2019

An upper low will dig into the Southern Plains early Saturday
morning, with an upper ridge remaining anchored over the central
Atlantic. At the surface, cyclogensis will occur near the TX/OK 
Panhandles in response to the deepening upper low, while high 
pressure persists along the Gulf. Given this set-up, deep southerly 
flow will quickly filter in a more unstable airmass northward and 
into the TN Valley. The area will see no shortage of moisture well 
ahead of this, given the prolonged period of rainfall leading into 
Saturday and this moisture will linger through the day. As the upper 
trough moves eastward, the surface low will take a more northeastward
trajectory, reaching the Mid Mississippi Valley Saturday afternoon. 
A cold front will trail south of this low, with showers and 
thunderstorms moving eastward ahead of the cold front. Forecast 
models are in fair agreement with the overall timing of this system,
though there are some minor discrepancies with regard to the 
strength and placement. Based on the latest forecast, the cold front 
will move through the TN Valley late Saturday night, with showers and
thunderstorms moving through ahead of the front during the afternoon
and evening hours. Ahead of the precip, strong WAA will allow highs 
to warm into the lower 70s, despite mostly cloudy skies during the 
day. 

In terms of impacts with this system, it will certainly be something
to watch closely over the next couple of days. The trough is expected
to take on a negative tilt as it crosses the Plains and moves into
the Mississippi Valley. Meanwhile, the surface low is expected to
deepen quite a bit as it races into the Ohio Valley Saturday 
afternoon/evening. All of this will give way to fairly strong wind 
shear which will be enhanced by the low level jet reaching upwards of
60 kts over the TN Valley. The limiting factor is going to be the 
more northward track of the low, but if it does deepen as much as 
models are indicating, there will be enough wind shear over the TN 
Valley to at least help generate organized thunderstorms. In terms of
instability, the southerly flow will send dewpoints into the upper 
50s to lower 60s by Saturday afternoon. However, mid level lapse 
rates may struggle to steepen as the trough moves into the Great 
Lakes keeping the heights falls minimum as far south as the TN 
Valley. This will limit the surface based instability despite those 
warmer dewpoints forecast to filter into the region. If we are able 
to see more sunshine and warm a bit more than forecast, low level 
lapse rates will be able to steepen and allow us to tap into some 
surface based instability. With all of that said, there looks to be a
window during the late afternoon through early evening hours when 
the ingredients will line up and lead to a chance for organized 
thunderstorms, a few of which could become severe. This is still a 
lower confidence forecast due the reasons above, however the strength
of the system may be enough to overcome the limitations. If the 
trough and surface low move a little further south, a greater chance 
for severe thunderstorms would be possible. Given the quick movement 
of the expected precip, rain accumulations will be less than an inch.
This may keep the river flooding ongoing for a little bit longer, 
however flash flooding doesn't appear likely outside of any intense 
rainfall rates. 

The cold front will pass through the TN Valley between midnight and 6
am on Sunday, with high pressure building in its wake. Dry conditions
will persist through Monday, as the high slides east across the Ohio
Valley. Temperatures will be fairly mild for late February, with
highs in the upper 50s to lower 60s and lows in the upper 30s to
lower 40s. Winds will shift to the south late Monday night, with a
return of moisture filtering into the region. Several shortwaves may
move through the region in a zonal flow pattern aloft and combine
with the increase in moisture to produce isolated to scattered
showers on Tuesday. Rainfall amounts will generally remain light.
Temperatures on Tuesday will be similar to Monday, with highs warming
into the upper 50s to lower 60s. 

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 620 AM CST Wed Feb 20 2019

IFR cigs/conds are in place at the two main airports this early Wed
morning, as a broken line of shra/embedded tsra begins to move into
NW AL. This precip is ahead of a cold front that is now beginning to
cross into west TN/north MS. This line of shra/tsra will continue to
move ewd thru the area this morning, with vrb winds 30kt gusting near
40kt, coupled with reduced vis, in/near some of the stronger tsra.
Little change in IFR cigs is xpcted thru most of the day, even with
the passage of the shra/tsra to the east. Sustained sfc winds out of
the SSE near 20kt with gusts 30kt will begin to turn more WSW later
today with the frontal passage. Cigs may then lift more into the 1-2K
ft range heading into the overnight hrs, as low level flow diminishes
and turns more north.

&&

.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...09
SHORT TERM...09
LONG TERM...73
AVIATION...09


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