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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 170442

Area Forecast Discussion
National Weather Service Huntsville AL
1042 PM CST Sun Dec 16 2018

For 06Z TAFS.


.NEAR TERM...(Tonight)
Issued at 748 PM CST Sun Dec 16 2018

RGB imagery indicates valley fog developing in Franklin TN/Jackson AL
with observations at KTHA, KBGF, KGAD and KJFX reporting fog
developing in or near our forecast area. T-Td depressions are near or
at zero, and fog tools all suggest at least areas of fog, some of it
dense. Surface winds continue at 3-7 mph at a few spots, although
many sheltered valleys have dropped to calm. We will continue to 
monitor during the next hour or so, but may need to issue a Dense Fog
Advisory at some point tonight. Would like to see a bit more 
coverage to develop before issuing this evening.

.SHORT TERM...(Monday through Wednesday night) 
Issued at 234 PM CST Sun Dec 16 2018

Monday and Tuesday will be near-repeats of each other, with
slightly-above-seasonable temperatures topping out in the 
middle/upper 50s each day under a few clouds here/there. High 
pressure will build into the area at the sfc and aloft, with CAA the 
strongest on Monday night. As such, lows Tuesday morning will start 
out around the freezing mark, and will slowly rebound ahead of the 
next weathermaker set to affect the area as early as late in the day 
Wednesday, but more so for Thursday and Friday. 

An upper low will deepen over the Intermountain West during this
time, and will become a cutoff low by Thursday into Friday over our
area. While little in the way of cloud cover will affect the CWFA on
Monday/Tuesday, strong southerly flow ahead of the deepening upper
low will translate to an increase in cloud cover through the day
Wednesday. While the bulk of the rainfall will hold off until between
06-12Z Thursday, some WAA showers may sneak into areas W of I-65 late
Wednesday afternoon. Have kept this mention isolated with this
forecast package, and coverage will be very spotty (at best) prior to

The real surge of moisture/showers arrives after 00Z as the better
support/lift associated with this system nears the region closer to
Thursday morning (mainly after 06-12Z Thu). 

Temperatures Wednesday night will be the warmest of the short-term
period, only falling into the middle/upper 40s for much of the area.
A few isolated/sheltered valleys in far NE AL/Srn Middle TN may
bottom out in the lower 40s, but again, this is the mildest morning
from now through Wednesday. The mild trend continues as the system
nears for Thursday. More details on that below...

.LONG TERM...(Wednesday night through Saturday) 
Issued at 234 PM CST Sun Dec 16 2018

Guidance continues to show a surface low developing over Arkansas and
Missouri Thursday evening and overnight as an amplifying longwave 
trough axis stretches further south from the western Great Lakes 
region. A significant increase in moisture and isentropic lift
(oriented in a southeast to northwest area of convergence from 
central/southern Alabama into northwestern Alabama/northern
Mississippi) is shown by most models. Models show this surface low 
occluding over northeastern Arkansas/western Tennessee overnight 
Wednesday into Thursday morning. The combination of lift and a good 
moisture profile produce fairly widespread (50-60 percent coverage of
rain) over most areas near and west of I-65. This higher coverage 
should shift east overnight into northeastern Alabama.  

Models do differ slightly concerning the track of the surface low
with this system. GFS moves it more northeast, while ECMWF moves it
directly east with time into northern Alabama. The former GFS
solution could enable cold air aloft to be drawn into the
surface/upper low after midnight. If this occurs, with the strong
lift near the surface low and lagging upper low, could see some a mix
of rain or snow develop during the morning hours on Friday. ECMWF 
looks a bit too warm though for snow on Friday with the more easterly
movement of the low. Will have to watch this period, but even if 
snow occurred, right now the surface temperatures should not allow 
wintry precipitation to accumulate. Liquid precipitation totals in 
most guidance are around 1 inch, so some more quick rises on area 
streams are possible (especially where isolated heavier amounts of 
rainfall occur).

Models linger some light rain or flurries in extreme northeastern
Alabama and southern middle Tennessee Friday evening, but think this
may be overdone. Expect clearing and strong cold air advection
overnight Friday to drop temperatures into the upper 20s to lower 
30s by daybreak of Saturday. High pressure moves into the area behind
the departed system through the day on Saturday. Highs in the upper 
40s to lower 50s return, with lows back in the lower to mid 30s. 

By Sunday evening, both synoptic models are showing precipitation
pushing back into the area along an inverted trough axis from a
developing surface low over the northwestern Gulf of Mexico. This 
may produce some wintry precipitation, but very uncertain and far in
the forecast to say much more than that. 


.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1042 PM CST Sun Dec 16 2018

Areas of dense fog will continue to develop across north AL and
southern TN through the morning hours. Fog should also settle in at
KHSV and KMSL with LIFR conditions possible from 09-13Z with
visbility of 1/2sm or less expected. The fog may be slow to dissipate
Monday morning, so have kept MVFR visibility through 16Z.





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