Area Forecast Discussion
Issued by NWS Huntsville, AL

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641
FXUS64 KHUN 281542 AAA
AFDHUN

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
1042 AM CDT Fri Apr 28 2017

.NEAR TERM...(Rest of Today)
Issued at 1010 AM CDT Fri Apr 28 2017

Based on current observational data and visible satellite imagery,
a weak warm front extends from near Rome Georgia into northeastern
Alabama and to near the Memphis area. There is some weak convergence
shown by models near this boundary, but primarily west of I-65 near
the Alabama and Mississippi border. The question is will this spur
very isolated shower and thunderstorm development later this morning
into the late afternoon hours, before it moves to near the Nashville
area or further north.

Looking at early morning soundings, there is already a very strong
cap in place between 850 mb and 700 mb aloft (850 temperatures ~ 20
degrees Celsius). This cap along with cloud cover will provide
additional challenges for convection initiation today. CAPE values
still climb to over 1000 J/KG in mesoscale models this morning and
increase further (to between 2000 and 4000 J/KG) this afternoon,
where bulk shear remains between 40 and 50 knots in northwestern
Alabama and Southern Middle Tennessee. If the cap can be broken,
these areas could see at least isolated showers and thunderstorms
that could be strong to severe. At this point damaging winds and
large hail look like the main threat with any severe storms that
develop. Some newer mesoscale model guidance does show more coverage
in these areas around noon into the early afternoon hours. This will
need to monitored over the next few hours to see if shower and
thunderstorm chances need to be increased.

Expect clouds to erode a bit between 3 and 7 pm in our southeastern
counties. This should allow temperatures to warm a bit more into the
mid to upper 80s east of I-65. Further west, more prevalent cloud
cover should keep highs in the upper 70s to lower 80s. Raised
dewpoints significantly given pooling of lower 70 degree dewpoints
south of the warm front moving northward.

.SHORT TERM...(Tonight through Sunday Night)
Issued at 335 AM CDT Fri Apr 28 2017

The mid level ridge continues to build in overnight with strong
southerly flow continuing. Lows will be well above normal and remain
in the middle to upper 60s. Should be dry overnight as any storms
will diminish with the loss of day time heating.

The ridge starts to weaken slightly on Saturday with the cap
noticeably weaker, albeit still there. The upper low also starts to
get its act together over west Texas with a surface boundary
extending from SE Texas up through the Great Lakes. Given the weaker
cap, continued moisture/high dew points and potentially a weak short
wave ahead of the low, diurnally driven thunderstorms are once again
possible. Could have "slightly" more coverage than today but would
expect that would be in the higher terrain areas.

Cloud cover is really going to be the question in terms of high
temperatures for Saturday and even if we break the cap or not. Looks
like there may be at least a broken layer of cirrus for the first
part of the day until drier air aloft builds in for the afternoon.
Given the continued warm advection, will go slightly above previous
forecasts with highs in the upper 80s, probably 90 in a spot or two.

Convection ends again after sunset with the cold front starting to
produce a line of thunderstorms somewhere between E Texas up through
SW Illinois. Mid to upper level winds will be south-southwesterly so
storms will be moving quickly in that direction but given the
departing surface high pressure the line will be overall slow to
build east. Guidance continues to slow down this line and current
thinking is it may just be reaching NW Al by 00z, with the GFS being
the faster outlier. The LLJ will be kicking in around this time as
well and there will be no lack of shear/helicity with the line. Once
we break through the cap Sunday, there will be limited instability
ahead of the line but should be enough to sustain at least strong
storms as it moves into our NW counties. The threat will lessen as it
moves east and instability wanes. There is also a "hint" that
stronger convection could form to the south a bit earlier in response
to the LLJ which could limit our instability even further. It is
also noted with these model runs the upper lift with the low is
lagging behind the initial line compared to previous runs. This may
also hurt chances for deep convection.

Timing will be everything with this line so we will continue to
monitor that in terms of possible hazards. Damaging winds and large
hail are both possibilities and a tornado or two cannot ruled out,
especially in the NW, if the line can arrive early enough to tap into
the better severe parameters.

The line will continue to move through overnight bringing slightly
cooler lows, in the middle to upper 50s. Although the line will be
slow on Sunday, with the upper jet dynamics moving in overnight, it
will likely pick up some speed.

.LONG TERM...(Monday through Thursday)
Issued at 335 AM CDT Fri Apr 28 2017

The previously mentioned system should be exiting between 12-18z on
Monday with timing differences still evident in the global models.
The GFS is the quicker solution although individual ensemble members
are lagging slightly behind the operational GFS so would imagine how
quickly the front exits may match closer to the ECMWF. This would
leave lingering thunderstorms, mostly in the east, through the
morning with front moving through by 18z. Relief in the temps will
come Monday with highs in the lower 70s.

The upper low moves northeast into the Great lakes by Tuesday with
near zonal flow aloft and weak surface high pressure building in
from the south over the local area. The return of southerly flow
will push highs back into the mid to upper 70s.

Another potent system will be tracking through the area Wednesday
into Friday with the timing and overall syntopic set up still
uncertain. Overall, it looks like more of a rain producer with any
stronger storms confined to the coast but it`s too far out for many
more details than that. Will leave the blended pops for now which
does bring likely pops in for Wed night. I will say that it will get
cooler after this system with highs Thu/Fri possibly down to the
upper 60s.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 640 AM CDT Fri Apr 28 2017

A low stratus deck with bases in MVFR range per regional obs and
multi-spectral satellite imagery is moving northward towards
KHSV/KMSL TAF sites this morning. Ceiling impacts expected shortly
after 12Z, with broken ceilings likely continuing for a few hours.
Clouds are expected to become scattered in coverage as is evident in
satellite imagery and with the development of deeper mixing later in
the morning. Winds will become gusty as well during the late morning
and will continue into the early evening until the loss of deep
mixing. Broken ceilings in MVFR range may once again develop in the
region around 06Z.


&&

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

$$

NEAR TERM...KTW
SHORT TERM...LN
LONG TERM...LN
AVIATION...KDW


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