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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 181523

Area Forecast Discussion
National Weather Service Huntsville AL
1023 AM CDT Tue Sep 18 2018

.NEAR TERM...(Rest of Today)
Issued at 1023 AM CDT Tue Sep 18 2018

Earlier, patchy to areas of fog that mainly affected parts of
northeast Alabama (especially along the Tennessee River from
Pittsburg to Lake Guntersville // and U.S. Hwy 72) has finally 
dissipated, thanks to daytime heating and mixing. Earlier regional
soundings indicated a relatively moist profile with ~1.5" of
precipitable water and convective temperatures in the mid 80s. The
latter looks true, given "fair weather" cumulus were beginning to
form across parts of the greater Tennessee Valley. Further daytime 
heating, with high temperatures rising again into the lower 90s will 
lead to further destabilization and isolated shower and thunderstorms
forming this afternoon. 

Except for minor temperature trend adjustments and adding precip
amounts from updated WPC QPF, did not make any big changes to the
on-going forecast. 

.SHORT TERM...(Tonight through Thursday night)
Issued at 330 AM CDT Tue Sep 18 2018

Will add patchy fog again late tonight into early Wednesday morning 
given light flow and radiation potential beneath the ridge. More 
summer weather is ahead through mid week as the upper ridge now over 
southern Plains shifts eastward into the lower MS and TN valleys 
Wednesday, and into the Carolinas on Thursday. Northwest flow aloft 
on the northern and northeast side of the upper anticyclone may bring
convective clusters/MCSs into the OH Valley on Wednesday. The models
even show some small areas of precipitation developing in KY and 
into eastern TN Wednesday afternoon, but am expecting our area to 
remain hot and dry. Will stay above by a few degrees of suggested 
blended guidance for high temps both Wednesday and Thursday. As the 
ridge positions at all levels shift to the Carolinas/Georgia area, 
deep southerly flow will develop across AL into middle TN. A 
convergence line and showers/thunderstorms is indicated Thursday 
night across the central FL panhandle north into southeast AL. Will 
hold off from adding this into our forecast for now.

.LONG TERM...(Friday through Monday)
Issued at 330 AM CDT Tue Sep 18 2018

Overall, temperatures will tend to remain above normal in the TN 
Valley for the extended portion of the forecast (through next 
Monday). However, gradual SE movement and repositioning of the main 
hemispheric vortex over central and then eastern Canada, will result
in an intensification of the polar jet and tracks of mid-latitude 
cyclones, finally, over portions of the Great Plains and then the 
northern tier of the CONUS during the period. An initial front on the
weekend will likely cross the area, but without much temperature or
moisture advection, while another front around the middle of next
week appears more promising. More details follow. 

An initial cyclone will likely be tracking across the Great Lakes 
region at the start of Friday with an attendant, but weakening 
surface front moving into the Midwest and OH Valley regions. Our area
will still reside under the western flank of an upper level ridge 
centered near the Carolina coast. However, increasing influence from 
the upper trough to our NW combined with moisture convergence/pooling
ahead of the surface front should give rise to scattered convection 
in the region on Friday. The better chances generally favor northern
and western portions of the area closer to the better dynamic
forcing associated with the front. Chances for showers and 
thunderstorms will increase further for Saturday as the front crosses
the region. Thermodynamic profiles suggest just modest ML or SB CAPE
likely present, with values generally around 1000-1500 J/Kg, so 
mainly general thunderstorm activity is expected. Fairly weak deep 
layer flow an a tendency for some back-building cells could result in
a few locally heavy rainfall amounts where deeper convective cells 
do occur. The bulk of the drier air mass will certainly remain well 
to our north with the frontal passage across the area. However, some 
of the drier boundary layer air will gradually filter into the region
on Sunday and Monday, but do not look for significant decreases in 
moisture/humidity with dew point temperatures likely remaining in the
mid/upper 60s.

A look farther out indicates another frontal passage is possible
across the region by about Wednesday or Thursday, bringing somewhat
cooler/drier air to the region. The 00z operational GFS and ECMWF
show better agreement in this solution with regards to timing and
positioning of the air mass, but this is still outside the forecast
window. Nevertheless, wanted to mention this for those looking for
more autumn-like weather.  


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 605 AM CDT Tue Sep 18 2018

VFR flight weather conditions are expected for most of the next 24 
hours. Patchy river valley fog early this morning will dissipate by 
14-15Z. An isolated shower or thunderstorm is possible this 
afternoon, but the probability is too low to include in the TAFs at 
this time. However, br/fg is expected to develop by 09z along area 
rivers with visibility of 1-2sm or less after 09z.





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at weather.gov/huntsville.