Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 282341

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

For 00Z TAFS and near term.


.NEAR TERM...(Tonight)
Issued at 641 PM CDT Sun May 28 2017

A modified cold pool remains across the TN Valley this evening which
has acted to stabilize the surface based environment ahead of a weak
cold front approaching. Though elevated instability remains a
concern, without any evidence of convective lift to overcome the cap,
have lowered POPs to isolated and changed the prevailing weather to
shower activity for the rest of the evening until midnight. Around
midnight hi res model output does show weak undulations within the
SW flow moving atop the approaching cold front which would mean a
higher potential for elevated showers and storms. These models are
also showing convective development between the 05Z-09Z (midnight-
3AM) time frame, from west to east. Have incorporated these thinking
into the forecast and made minor adjustments to the sky cover to
reflect approaching low cloud cover from the west, and air
temperature/dewpoint temperature trends.

.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 00Z TAFS through 00Z Monday evening)
Issued at 641 PM CDT Sun May 28 2017

VFR conditions are expected to prevail through 29/13Z-14Z as a weak
cold front approaches. There is the potential for scattered SHRA and
isolated -TSRA during the next 24 hours as a cold front gradually
moves southeast towards the area. Though difficult to predict the
timing of these periods of SHRA/TSRA have included the -SHRA mainly
after 29/08Z-09Z with further drops in cigs to MVFR by 29/13Z-14Z.
Then, included VCTS after 29/18Z as the front stalls over north AL.
Fog may also occur at times if the clouds scatter enough and may need
to amend the TAFs to include a drop in VIS if that occurs.





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