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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 181110

Area Forecast Discussion
National Weather Service Huntsville AL
610 AM CDT Mon Jun 18 2018

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 217 AM CDT Mon Jun 18 2018

Latest water vapor imagery shows an upper low over the Pacific
Northwest and a ridge encompassing much of the eastern US, with a 
weak upper low spinning from northwest Alabama and into northeast 
Mississippi. At the surface, winds have become light across most of
the area, with temperatures in the upper 60s to lower 70s. As
temperatures continue to fall by another few degrees, dewpoint
depression will approach 0 and patchy fog is expected to develop. In
fact, GOES 16 Nighttime Microphysics composite shows some fog
beginning to develop in valley locations across northeast Alabama and
northwest Georgia. So, we might see a bit more prevalence of fog 
this morning than previous mornings, but most of the area will see 
visibilities remaining above dense fog advisory criteria. 

The aforementioned upper low currently over northwest Alabama will
continue to move to the northwest over the next few hours before it
weakens altogether. Meanwhile, the subtropical ridge will begin to
build westward across the area. Although surface flow will remain
relatively light through the day, we will see a more divergent flow
pattern across the area. All of this will combine to create a less
favorable environment for thunderstorm development this afternoon.
However, still cannot rule out widely scattered thunderstorms, as
ample daytime heating may combine with lingering outflow boundaries.
We may see two separate areas of storms, one over the west close to
the upper low and the other over the east, where terrain effects may
enhance the lift. Even still, did not go higher than a 30 percent
coverage given the uncertainty in the mesoscale details. Given steep
low level lapse rates and ample instability, combined with PWs 
around 1.75, strong winds up to 50 mph and locally heavy rainfall 
are possible with any storm that develops. 

Outside of thunderstorm areas, expecting another hot day across the 
TN Valley, with highs rising into the upper 80s to lower 90s and heat
indices in approaching 100 degrees for some locations. 

.SHORT TERM...(Tonight through Wednesday night)
Issued at 217 AM CDT Mon Jun 18 2018

The subtropical ridge will continue to build northwestward through
the overnight hours and become established across the TN Valley by
Tuesday. Another mild night is expected tonight, with overnight lows
only falling into the lower 70s. Patchy fog is possible once again,
though the coverage of fog will depend on if and where we see
thunderstorms today. Tuesday is looking to be mostly dry across the
region, as subsidence becomes more pronounced. For consistencies
sake, will maintain a slight chance for thunderstorms Tuesday
afternoon, as the ridge isn't overly strong and any mesoscale
boundaries could provide the lift needed to generate an isolated
thunderstorm. In addition to it being the driest day of the period,
Tuesday will also be the warmest given the upper level high rises,
with temperatures warming into the lower 90s.

The ridge will begin to break down across the area Tuesday night as a
trough ejects east of the Rockies and into the Plains. Meanwhile,
models are showing the potential for tropical waves moving around the
northwest periphery of the ridge on Wednesday, clipping the TN 
Valley to the northwest. These weaknesses in the ridge will help to 
enhance the diurnal convection across the area, with the highest 
coverage expected across northwest Alabama. We will also see 
thunderstorm chances continuing into the overnight hours as models 
bring a wave through the Lower/Mid Mississippi Valley. Temperatures 
will be a degree or two cooler on Wednesday, with highs in the upper 
80s to lower 90s. 

.LONG TERM...(Thursday through Sunday) 
Issued at 217 AM CDT Mon Jun 18 2018

The medium range models are in fair agreement on the pattern 
evolution late this week (days 4-5), then differ on amplification 
next weekend (days 6-7). The amplification of the Plains upper 
low/trough will bring shortwaves northeast from the previous Gulf 
upper low and through lower OH/TN valleys on Thursday into Friday. 
This should bring a good chance of one or more MCS clusters to the 
region. Will side with the less amplified ECMWF vs the GFS next 
weekend, although the blended PoP guidance doesn't appear too 
unreasonable at this point. With this in mind, the upper ridge 
broadens west to east across the Gulf Coastal region. This will 
likely bring an uptick to the heat with suggested blends yielding 
lower 90s for highs both days 6-7. Thunderstorms will become more 
diurnally heat driven, and corresponding PoPs both days will be in 
the lower chance range. 


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 610 AM CDT Mon Jun 18 2018

VFR conditions will prevail at the KMSL and KHSV terminals through
the next 24 hours. Winds will shift to the west later this morning
and increase to around 6 kts. Thunderstorms are possible again this
afternoon, however the coverage and confidence is too low to add
thunder to the forecast. 





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