Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 282002

Area Forecast Discussion
National Weather Service Huntsville AL
302 PM CDT Sun May 28 2017

.NEAR TERM...(Tonight)
Issued at 300 PM CDT Sun May 28 2017

Well, some significant updates were necessary to most forecast
elements for today to match observations and latest trends. The
overnight rainfall/convection was enough to create a sufficiently
deep/stable cold pool to limit further convection today. In addition,
further convection today has been relegated far to our south in the
Gulf Coastal Plain region. So, mid and upper cloud development has
likewise been limited and suppressed farther to our south. The cold
front has become increasingly difficult to locate since the low-lvl
airmass in the region has become modified by the recent convection.
Nevertheless, the surface front still appears to linger to our west
over the mid-Miss Valley as noted by a narrow belt of slightly deeper
cu in satellite imagery. This meshes well with some streamline
convergence noted in regional observations. The front, albeit
increasingly non-descript, will likely move across the area tonight
but aided by limited upper level support. With that said, the region
will sit to the SE side of a upper level jet streak, which could help
to enhance some synoptic lift ahead of the front. Weak convergence
along/ahead of the front later tonight combined with some modest
mid/upper support and elevated instability may support the
development of isolated to scattered convection later tonight. Timing
with any convection is still difficult, but would probably be after
03-06Z. The latest 12Z model suite suggests development and coverage
tonight will be rather limited, so only going with 50 PoPs at best
across the area. Higher chances will be in the south, toward the area
of better instability. A few storms could produce some strong wind
gusts based on the thermodynamic profile, but the severe weather
threat has diminished.

.SHORT TERM...(Monday through Tuesday morning)
Issued at 300 PM CDT Sun May 28 2017

The surface front will essentially stall in the region early Monday,
although a broad upper trough will remain in the Grt Lakes region.
The TN Valley will lie along the base of the trough while ridging
centered just off the FL coast will keep the region essentially in a
quasi-zonal flow pattern from Monday into Tuesday. Generally weak
embedded short waves rotating generally W to E in the upper flow
pattern will manifest in periodic increased surface convergence along
the old stalled boundary and causing it to drift northward during the
period. The result will be continued chances for showers/storms
during the period, with the best chances in southern parts of the
area on Monday, closer to the better synoptic forcing. Thermodynamic
profiles are not overly impressive during the period, but suggest at
least a marginal threat for strong or severe storms.

.LONG TERM...(Tuesday through Saturday)
Issued at 300 PM CDT Sun May 28 2017

Tuesday morning most models develop an upper level wave that moves
northeast along the front and across northern Alabama and into
Georgia Tuesday night. This should keep mostly cloudy conditions in
place near and south of the Tennessee River with a 20 to 40 percent
chance of showers or thunderstorms in the forecast on Tuesday. This
should keep high temperatures a bit cooler south of the Tennessee
River (mid 70s to around 80 degrees) and in the lower 80s north of it
(where some clearing should occur at times). Drier air should push
south into Southern Middle Tennessee and near the AL/TN border
overnight. The passing of this upper level disturbance to the east
moves the front southeast overnight. With more clearing expected near
and north of the Tennessee River coupled with light winds, lows
should drop to around 62 degrees. Further south lows will be a bit

By Wednesday, models are not in good agreement concerning the
further evolution of this boundary (along with additional upper
level energy moving along it). NAM seems to be the main model
holding onto stronger convergence/forcing in this area. However, it
seems to be an outlier, so keeping closer to GFS and ECMWF
forecasts. 30 to 40 PoP at most Tuesday through Wednesday (even
though NAM is showing higher PoP) looks reasonable based on the
preferred guidance. Expect afternoon convective activity primarily
Wednesday through Thursday night as a result, with cumulus
dissipating later in the evening each night. Although moisture
advection will slowly increase, the gradient wind (developing
southwest flow) does not pick-up in earnest until Thursday night
into Friday. Thus expect lows to drop into the lower to mid 60s
through Thursday night.

By Friday morning, an upper level low pushes into Oklahoma and seems
to refocus a warm frontal boundary near or over the area.  As
dewpoints continue to rise into the mid to upper 60s, surface based
CAPE values rise into the 2000 to 3000 J/KG range. There looks to be
little bulk shear present along this boundary, as low/mid level
winds remain rather light. Thus, pulse storms look possible
producing gusty winds, frequent lightning, and heavy downpours. Most
of the strongest storms will be pulse in nature based on shear
values not extremely long lived in nature.

Models waffle this boundary into Tennessee and then back into
northern Alabama through the remainder of the weekend. Shear and
instability parameters do not change much. So the same threats look
possible with additional scattered strong storms possible.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1245 PM CDT Sun May 28 2017

Predominant VFR conditions currently at KHSV/KMSL TAF sites are
likely to continue through at least the next TAF issuance. A few SHRA
may develop in the area, with VCSH noted in TAFs beginning at 21Z,
but this is due to possible development in the warm sector ahead of
an approaching cold front. Confidence in any timing/impacts is low
attm and SHRA coverage is expected to be limited, thus the VCSH
mention. Winds will subside around 00-01Z and gradually veer to the
W-NW. Renewed convection is expected to take place later tonight,
with SHRA perhaps moving back into the area around 08-09Z. Timing
remains really uncertain, but TAFs will reflect latest consensus. A
TSRA is possible given elevated instability tonight, but coverage may
be limited so TAF only includes mention of SHRA currently.





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