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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

                            
000
FXUS64 KHUN 211719
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
1119 AM CST Thu Feb 21 2019

.UPDATE...
For 18Z TAFS.

&&

.NEAR TERM...(Rest of Today)
Issued at 832 AM CST Thu Feb 21 2019

The band of showers and thunderstorms along the 850 mb front over
central and northeast AL has shifting east-northeast or dissipated.
There is some lingering drizzle or very light rain in DeKalb County.
The band further southwest into western AL and central MS is progged
by mesoscale models to lift north-northeast and expand through the
day. This may bring moderate to locally heavy downpours at times, but
appears that it will make northward progress that will hopefully
limit residence time. Nonetheless, will have to watch precip rates
closely for flash flood potential. Elevated CAPE on the BMX sounding
suggests a few embedded thunderstorms will remain possible today. In
far northwest AL, there is actually some temporary clearing of the
low clouds, but that will not last long.

.SHORT TERM...(Tonight through Saturday night)
Issued at 251 AM CST Thu Feb 21 2019

The upper trough over the Intermountain West will continue to
progress eastward overnight, with the weak ridge holding over the
southeastern CONUS. It's almost a rinse and repeat pattern, as more
rounds of rainfall are expected to move through the TN Valley. The
most intense rainfall looks to occur late tonight, as an 850 mb warm
front begins to lift northward and stalls just to our north.
Meanwhile, the nocturnal jet will intensify and help aid in strong 
lift to generate bands of heavy rainfall. The axis of the heaviest 
rain looks to set up right over our northwestern counties, with as 
much as 1.5 inches of additional rainfall possible between midnight 
and sunrise. This is a bit on the pessimistic side and confidence is 
not overly high that this much rain will be realized. However, given 
the potential impacts, would like to err on the side of caution. So, 
with that said, significant flooding is possible across areas north 
of the TN river and west of I-65, where the heaviest rain has already
occurred. To make matters worse, this will occur at night, when it's
much harder to see flooded roadways. So extreme caution should be 
used late tonight if travel is necessary. Again, confidence is on the
lower side but guidance is fairly consistent on bringing that heavy 
band over our area. 

Numerous showers will remain possible through the morning hours on
Friday as the 850 mb front remains stalled. So the moderate to heavy
rain will continue through 18Z, with an additional half an inch
expected. So all in all, we could see upwards of 2 to 3 inches of
rain in the 12 hour period from midnight to noon on Friday. It can't
really be stated enough that all of this will fall as runoff and will
exacerbate the already swollen rivers and streams. A surface warm
front will lift northward during the afternoon, pushing the bulk of
the heavy rain to our north by the early evening hours. Given the
persistent moisture and weak waves moving through the area, numerous
showers are still possible overnight Friday, but the rainfall amounts
will remain on the lighter side.  

Focus then turns to the aforementioned trough and the impacts it will
bring to the TN Valley. As the trough ejects into the Plains Friday
night, surface cyclogensis will quickly occur and a cold front will
trail to its south. This system will slowly pivot to the east-
northeast through the day, taking on a negative tilt as it lifts into
the Mid Mississippi Valley. Ahead of this, southerly flow will
strengthen over the local area as the TN Valley becomes established
within the warm sector of this approaching system. Dewpoints in the
lower 60s will surge northward, reaching the area by the afternoon
hours. Meanwhile, temperatures will climb into the lower 70s despite
mostly cloudy skies. A line of storms will move ahead of the front,
reaching the TN Valley by the late afternoon-early evening hours and
then quickly pushing east of the area by the predawn hours on Sunday
as the cold front crosses the area. Forecast models are in decent
agreement, however the run to run consistency with regard to timing
and strength is still on the lower side (which is typical at this
point in the period). 

So, what are the potential impacts? Well, it's still tough to
determine with high confidence just how strong thunderstorms will
become over our area. The main upper level dynamics will push well
north of the area as the storms move through. However, the low and
mid level jets will strengthen quite a bit, with 850 mb winds
approaching 60 kts. This will give way to plenty of shear, as 0-1 km
bulk shear approaches the 30 to 35 kt range. Meanwhile, the surge of
lower 60 dewpoints will combine with sufficient daytime heating to
lend some destabilization over the area. The limiting factor, though,
will be the northward movement of the upper low and thus the weaker
height falls this far south. This will keep lapse rates on the 
weaker side and may limit the coverage and severity of thunderstorms.
But, as I stated in the discussion yesterday, any breaks in the 
cloud cover and/or warmer temps in the afternoon will help overcome 
the weak mid level lapse rates and allow us to destabilize enough to 
give us an organized severe threat. The main coverage will come with 
the line of thunderstorms currently expected to move through late in 
the afternoon and into the evening hours. All threats are on the 
table (damaging wind gusts, hail and tornadoes), with the damaging 
wind gusts the highest threat. However, we will have to watch for any
cells that develop ahead of this line, as these would provide the 
greater risk of severe thunderstorms, especially tornadoes. Heavy 
rainfall is possible within the strongest storms, which may lead to 
localized flooding. But given the quick movement of the line, 
widespread flooding is not anticipated. Even still, with the 
saturated soils, the flood watch may need to be extended through 
Saturday. 

The cold front will move east of the area between 06Z and 09Z Sunday,
bringing an end to showers/thunderstorms. Dry air will quickly push
into the area, with skies clearing through the early morning hours.
Temperatures through the short term will be mild, with highs on 
Friday in the upper 50s to mid 60s and lows in the upper 50s. Temps 
on Saturday will depend on the extent of the cloud cover, but for 
now, lower 70s look reasonable. Overnight lows will fall into the 
upper 40s behind the front. 

.LONG TERM...(Sunday through Wednesday) 
Issued at 251 AM CST Thu Feb 21 2019

The next storm system and attendant cold front should be east of the
cntrl TN Valley heading into the second half of the weekend period. 
Any rainfall associated with the frontal passage should also be clr
of NE AL based on the latest blended guidance, as a strong ridge of
high pressure out of the Plains states gradually builds southeast
into much of the region. Cloud cover should also be diminishing over
the area during the day Sun, finally providing a break in all of the
rainfall over the previous week. Quiet and tranquil wx conditions
look to prevail for the second half of the weekend, as temps Sun 
afternoon trend more seasonal, with highs mainly around 60F.

The quiet wx pattern should continue into the new work week, as the
strong sfc high settles more into the Midwest/OH Valley states. 
Drier air at/below H5 looks to spread east into much of the region, 
as a broad WSW flow regime becomes established over the srn Plains 
into the SE states. After a seasonably cool night, with early 
morning lows Mon falling more into the mid 30s, temps look to be 
quite mild later in the day, as highs climb predom into the upper 
50s. Little change in the overall pattern will continue thru Tue/Tue 
night, as the sfc high translates more into the nrn/mid Atlantic 
Basins. Temps late Mon night look to trend more in the upper 30s/near
40F, before rebounding into the lower/mid 60s Tue afternoon.

Rain chances may then return to the forecast Tue night into Wed, as a
return flow pattern developing over much of the region ushers Gulf
moisture back into the mid TN Valley. The latter half of the global
model runs are hinting at another low pressure system out of the mid
Plains lifting into the Great Lakes by Wed, with the attendant cold
front stretching south into the Lower MS Valley region. As this next
sfc boundary moves east, sct showers look to develop along/ahead of
the front, as they spread more into the local area by mid week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1118 AM CST Thu Feb 21 2019

Moderate rain will spread into north AL over the next few hours. The
rain will lower visibility to around 3-4SM (MVFR) to along with the
already MVFR ceilings of 015-025agl. Tonight, showers and a few
thunderstorms are expected to continue, mainly over northwest/north
central AL. Ceilings will likely drop back below 010agl (IFR) with
moderate to occasionally heavy rain expected. The greatest chance of
thunderstorms with heavy rain will be at KMSL after 12Z (mainly
14-18Z with a TEMPO group), dropping visibility to 2sm or less. This
is also possible at KHSV, so have included VCTS.

&&

.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...17
SHORT TERM...73
LONG TERM...09
AVIATION...17


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