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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 181253

Area Forecast Discussion
National Weather Service Huntsville AL
753 AM CDT Wed Jul 18 2018

.NEAR TERM...(Rest of Today)
Issued at 753 AM CDT Wed Jul 18 2018

Dense fog areas continue mainly east of I-65, tucked in many of our
eastern valleys. Have extended this an hour in the weather grids. 
This should dissipate fairly quickly over the next hour or two. The 
cold front is still situated over northern TN (i.e. Nashville dew 
point is 64). This will gradually shift southward today. There is 
still a very low chance of a thunderstorm this afternoon in our 
southwest counties given the boundary and some support from the NAM. 
However, the latest HRRR run does not indicate development at this 
time for our counties. 

.SHORT TERM...(Tonight through Friday Night)
Issued at 450 AM CDT Wed Jul 18 2018

Tonight will be dry and have the coolest temps of the forecast 
period with values in the mid to upper 60s.

The sfc high across the Great Lakes will start to shift eastward on 
Thursday and be along the east coast by Friday. This will veer winds 
to the southeast and start returning moisture to the area. A few 
storms along the southern tier of counties is possible Thursday 
afternoon, and in NE AL towards the evening as a wedge front starts 
to push in. Still stuck in northwesterly flow aloft, attention will 
also be on the active weather occurring in the Midwest. This activity
could slide an MCS into the region on Friday. Models still do not 
agree on exactly what time the MCS will arrive, however they do agree
a round or two of storms will push into the TN Valley on Friday. 
Kept with an increase in clouds as we head into Friday with POPs also
increasing from Thursday night into Friday night. Will not rule out 
a strong storm or two on Friday with forecast soundings bringing 
PWATS back towards 2 inches, a plentiful amount of CAPE in place and 
bulk shear around 30kts combined with diffluent flow aloft. As models
come to a better consensus, these factors along with timing could 
change so be sure to keep an eye on further updates.

.LONG TERM...(Saturday through Tuesday)
Issued at 450 AM CDT Wed Jul 18 2018

Medium range guidance suggests that the TN Valley will be embedded 
within an unseasonably strong northwest flow regime at the beginning
of the long term period, with the region between an amplified mid- 
level trough over the Great Lakes and a subtropical high centered 
across the southern high Plains. The trough is predicted to drift 
slowly southeastward into the upper OH Valley over the course of the 
weekend, while digging as far southward as the southern Appalachians 
by 00Z Monday. Flow at the 500-mb level will likely peak in the 45-55
knot range at some point Saturday afternoon/evening, which will 
support the threat for perhaps a couple of highly-organized clusters 
of thunderstorms to drop south-southeastward into the region from 
adjacent portions of middle TN. Based on the thermodynamic/kinematic 
environments projected by forecast soundings, conditions during the 
day on Saturday will be quite favorable for organized severe 
thunderstorms producing perhaps some significant winds. We have 
indicated the greatest coverage of storms to occur during the day 
Saturday, with conditions improving from west-to-east Saturday 
evening as a southeastward-moving cold front begins to advect drier 
air into the region.

The northwest flow regime is predicted to subside beginning Sunday 
night/Monday morning, as the trough to our east weakens and the mid- 
level subtropical high to our west retrogrades further into the 
southern Rockies. Although a few diurnal showers and thunderstorms 
will be possible in the higher terrain of southern middle TN and 
northeast AL on Sunday/Monday, a drier boundary layer will limit 
coverage compared to previous days. However, a subtle increase in 
low-level moisture should result in a few more afternoon convective 
cells by Tuesday. High temperatures in the drier airmass will still 
range from the m-u 80s east to lower 90s elsewhere, with lows mainly 
in the u60s-l70s.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 628 AM CDT Wed Jul 18 2018

Fog could create down to IFR conditions at KHSV and MVFR conditions 
at KMSL until 14Z. Once the fog dissipates, VFR conditions will
prevail with light northerly winds with mid-high clouds in place. A 
shower or storm may impact KMSL this afternoon, however due to the 
uncertainty of storms directly impacting the terminal, kept the 
mention of it out of this TAF issuance. 





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at weather.gov/huntsville.