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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

                            
162 
FXUS64 KHUN 201135
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
635 AM CDT Fri Oct 20 2017

.UPDATE...
For 12Z TAFS.

&&

.NEAR TERM...(Today)
Issued at 338 AM CDT Fri Oct 20 2017

A powerful system that came ashore the Alaska Panhandle and British
Columbia coast yesterday has resulted in troughing digging along 
the western CONUS. This system was being pushed eastward by a strong 
zonal flow over the North Pacific, with winds greater than 150kt in 
parts of 200-300mb region between 40 and 50N. A deepening trough to 
our west usually means amplifying ridging to the east - which is the 
case this go around. This will translate to warmer than normal 
temperatures to end the week. The trough advancing eastward along 
with a cold front and possible developing low over the southern
CONUS, will bring unsettled weather to the Tennessee Valley 
starting Sunday.

In the short term, surface high pressure was in place just to our 
ENE over the southern Appalachians, resulting in light winds locally. 
The light winds, mostly clear skies and close temperature/ dewpoint 
convergence has led to patchy fog development across parts of the 
Valley. Per a night micro-physics view from experimental GOES- 16, 
this fog as has been the case the last few nights has been confined 
to sheltered valleys and near large bodies of water. Any fog should 
dissipate shortly after sunrise, with another nice day expected 
weather-wise. Highs should warm into the upper 70s, with 80-81 in the 
warmest spots. Normal high temperatures for today are around 73 
degrees. 

.SHORT TERM...(Tonight through Sunday)
Issued at 338 AM CDT Fri Oct 20 2017

A not as clear night, as additional moisture filtering across the
region brings few to scattered clouds. The clouds and a slight 
uptick in winds (from light/variable to SE 2-4kt) should be enough to
keep fog formation minimized. Lows should be warmer, into the lower 
50s - with seasonable normal lows around 49 degrees.

The last day of the week will be warm and dry, with highs warming
into the upper 70s to lower 80s in a few more spots. Warmer highs 
approaching the mid 80s is possible in a few spots if cloud cover 
holds off more than expected. 

In the meanwhile, the system to our west will edge closer, with more 
clouds especially during Saturday night. Even warmer with lows only 
in the mid 50s east to lower 60s west. Winds will also increase into 
the 5-15 mph range. Have kept a chance of showers and thunderstorms 
in on Sunday, especially in the afternoon as showers preceding the 
cold front make their way to the forecast area. Only "general" storms
intensity wise are expected, with minimal instability/shear values 
needed for more substantial convection. More clouds and showers 
should keep high temperatures on Sunday held into the mid/upper 70s. 

.LONG TERM...(Sunday night through Thursday)
Issued at 338 AM CDT Fri Oct 20 2017

Cold front should be just W of the cntrl TN Valley by the start of
the extended forecast period, as an upper ridge axis slowly makes 
its way E of the mid/srn Atlantic seaboard. The moisture column 
should be rapidly increasing Sun evening as the T/Td spread 
diminishes, given an almost stacked S/SSW flow thru the air mass 
layer. Upper trough pattern is still xpcted to move out of the Plains
states and into the ern Conus Sun night/early Mon, helping the 
attendant front push into the SE states. Clusters of showers, along 
with embedded tstms, should be developing along/just ahead of the 
advancing front, with most of the precip spreading into the mid TN 
Valley from the W starting Sun night. PW content looks to climb as 
high as 1.75 inches by early Mon morning, and then perhaps continue 
climbing closer to 2 inches by the middle of the day, in closer 
proximity to the frontal passage. This certainly suggests the 
potential for locally heavy rainfall, within total rainfall amounts 
around 2-3 inches. Buoyancy/shear continue to look modest at best, 
with SBCAPE values perhaps reaching 1500 J/kg Sun night into Mon and 
Bulk Shear numbers hovering just below 40KT. This may translate into 
a couple of strong tstms, but the prob remains fairly low attm.

Going with a slightly faster 00Z ECMWF solution, the cold front is
pretty much E of the local area by Mon evening, with just a trailing 
band of showers extending along the passing upper trough axis. The 
blended guidance seems to support this thinking as well, with rain 
chances quickly ending/tapering off from the W early Tue, as a 
reinforcing front sweeps ewd thru the region. A cooler/drier thermal 
advection stream, evidenced by thickness values diving swd well into 
the cntrl Gulf region, should then spread well into the region by the
middle of the week, with overall temp trends falling around 10-15F. 

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 635 AM CDT Fri Oct 20 2017

Scattered low clouds in the vicinity of KMSL could cause brief LIFR
<500 ft AGL ceilings for the start of the TAF. Those clouds should
scatter out shortly after sunrise this morning. VFR weather otherwise
is expected to continue over the KHSV terminal. Light/variable winds
this morning will become SE-S generally less than 5kt this afternoon.
Winds again should become light/variable after dusk. A presence of 
more clouds and a slightly higher wind late in the TAF should help 
minimize fog formation before daybreak Sat AM. 

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

NEAR TERM...RSB
SHORT TERM...RSB
LONG TERM...09
AVIATION...RSB


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at weather.gov/huntsville.