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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 132352

Area Forecast Discussion
National Weather Service Huntsville AL
552 PM CST Thu Dec 13 2018

For 00Z TAFS.


.NEAR TERM...(Tonight)
Issued at 313 PM CST Thu Dec 13 2018

Southeasterly winds and fairly strong warm air advection through the
day aided by peaks of sunshine, have pushed temperatures into the 
lower 60s in portions of northwestern Alabama. Likely will be as warm
as it will get this afternoon, with thicker cloud cover building 
back into the area from the south over the next few hours. Low/Mid 
level moisture will quickly build north as well into the evening
hours, but dry air (dewpoints around 30 to 40 degrees) east of I-65 
will need to be modified significantly before rainfall aloft can make
it to the ground. More moist air west of I-65 should be able to 
modify more quickly as the southerly/southeasterly low level jet 
(around 925 mb) increases to between 30 and 40 knots tonight. This 
should allow isolated to scattered rain to push into northwestern 
Alabama early this evening and become widespread in coverage after 10
PM or so. Further east, expect scattered rainfall to develop around 
midnight and become widespread overnight. This increasing moisture/lift
is all courtesy of stronger isentropic lift generated north of a 
warm front extending from central Arkansas southeast into extreme 
eastern portions of Lousiana. Lows should be tempered by cloud cover 
across the area, but temperatures should fall a bit late through the 
early evening hours into the mid 40s to around 50 degrees overnight 
east of I-65 (where thicker cloud cover may take a bit longer to 
develop). Rainfall this evening should be generally light, but up to 
a quarter looks possible west of I-65. The best lift looks like it 
will shift quickly into northeastern Alabama around and after 

.SHORT TERM...(Friday through Saturday)
Issued at 313 PM CST Thu Dec 13 2018

This will likely produce heavier rainfall east of I-65 as the warm 
front moves into those locations before pushing into Georgia/eastern 
Tennessee just after daybreak on Friday. A strong tongue of warm air
advection in the boundary layer is shown along and behind that warm 
front Friday morning. This could allow temperatures to climb into the
mid 60s, but at this time only including highs in the lower 60s in 
the forecast. This may need to be raised in future updates. Instability
is not shown in guidance until Friday afternoon, as the warm air 
advection continues to bring more moist and possibly slightly 
unstable air into the area. However, there is a large degree of 
uncertainty still in forecast data whether this will materialize. 
Bulk shear values remain very strong during the afternoon/early 
evening hours on Friday. If surface based instability can develop 
(GFS is more strongly hinting at that), some strong to severe storms 
would be capable of producing damaging winds as the surface low and
phased upper low pushes east into northwestern Alabama. Either way, 
elevated instability and low wet-bulb zero heights around 7000 feet 
set the stage for hail up to the size of quarters as this occurs
in the mid afternoon hours. This threat could linger into the early 
evening hours, as the surface and phasing upper low move through 
northern Alabama. 

After midnight, rainfall will continue through Saturday on the back 
side of the surface/upper low. In all, through Saturday, between 1 
and 1.5 inches of rainfall looks possible. This could cause some 
quick rises on area river/streams, but is not expected to produce 
widespread flooding conditions. Cooler air will move in behind the
front/surface low late Friday night into Saturday. Cloud cover and
precipitation also help to keep highs from climbing above the lower
50s. Cold air advection Saturday night should drop lows into the
lower 40s in most locations.

.LONG TERM...(Saturday night through Wednesday)
Issued at 313 PM CST Thu Dec 13 2018

Models suggest that strong north-northwest flow aloft will be in 
place over the TN Valley on Sunday, in the wake of an upper low 
shifting east-northeastward across the southern Appalachians. As the 
deep-layer cyclone advances further eastward and a shortwave ridge 
aloft translates across the region, winds throughout the vertical 
column will subside even further during the period Sunday night- 
Tuesday, before assuming a more pronounced southwesterly component on
Tuesday night and Wednesday. At the surface, we anticipate any 
lingering light rain across northeastern AL/southern TN to end during
the morning hours on Sunday, with the southeastward advection of 
drier air and lack of large scale forcing for ascent resulting in dry
conditions through at least the first half of the work week. Low 
stratus clouds in the wake of the departing upper low should persist 
well into the late afternoon hours on Sunday before clearing, which 
along with northwest winds/weak CAA will make for a cool day as temps
should only reach the lower 50s. Due to the predominately north 
Pacific origin of the incoming airmass and based on expectations of 
mostly clear skies, highs on Monday-Wednesday should easily reach the
m-u 50s, with lows cooling into the 30-35 degree range each morning.


.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 552 PM CST Thu Dec 13 2018

Conditions will start out as VFR however as rainfall approaches, 
CIGs will lower into MVFR by 01-02Z. Coverage of rain will increase 
around midnight and drop CIGS into low-end MVFR around 1,500ft with
IFR CIGs possible. A dry slot and lighter winds are expected early 
Friday morning which will diminish rain at HSV but light rain could 
linger across MSL into the afternoon. To account for uncertainty, 
included a PROB30 at MSL 15-20Z. CIGs do not seem to improve by the 
end of the TAF period. 





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