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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 191518

Area Forecast Discussion
National Weather Service Huntsville AL
918 AM CST Mon Feb 19 2018

.NEAR TERM...(Rest of Today)
Issued at 918 AM CST Mon Feb 19 2018

A dry morning is ongoing across the TN valley with widespread mid 
level clouds remaining. A strong mid and upper level ridge is 
starting to build over the Atlantic, off the SE coast, permitting a 
trough to deepen significantly into the the western states. The 
southwest winds locally are ushering in sustained low level moisture
across the area as well as waves of low pressure along a front well 
to our west. 

Latest model soundings show strong mixing due to the warm advection
which may mix high enough to mix out the low level moisture.
According to the NAM, we could be almost completely sunny by the
afternoon. However, other short term guidance, HRRR, CONShort, etc 
are showing almost 90-100% cloudiness today. This is making the range
of potential highs anywhere from 69 to 80 today. We already have a 
few breaks in the clouds but there is a more overcast deck sneaking 
it's way north. Either way, it seems like in these situations in the 
past we do get enough warming and mixing to break the clouds just a 
bit. I won't jump on the 80 degree highs but I did bump highs a deg 
or two. This puts both Muscle Shoals and Huntsville near record 
highs. Huntsville's record is 75 from 1939 and Muscle Shoals is 76 
from 1986. Depending on the cloud cover, we could tie or break these.

Winds are still expected to be gusty, thanks to mixing that will tap
into some 25-30kt winds along with a bit of a pressure gradient.
Gusts of 25-30 mph will be possible. 

.SHORT TERM...(Tonight through Wednesday Night)
Issued at 312 AM CST Mon Feb 19 2018

It will be quite mild tonight as southerly flow persists. Low clouds
may tend to redevelop as well, but should again mix out on Tuesday as
the pressure gradient increases even further. Once again, gusts
should reach 15-20kt at most locations by midday into the afternoon
hours of Tuesday. Near record or record highs are likely again. There
will be a weak lobe of vorticity rotating northwest-northward around
the ridge from the eastern Gulf of Mexico and into the TN valley. 
This may be enough to produce isolated to scattered light showers by 
Tuesday evening.

Then the forecast dilemma begins on whether the frontal system and
heavy band of showers and thunderstorms across the middle MS valley
into the ArkLaTex region on Tuesday will make progress into the
middle TN valley on Wednesday or Wednesday night. The SREF/NAM and
now even the new ECMWF are more convinced that the large ridge will
be strong enough to delay progress of this rainfall, at least into
Wednesday night for most of the area. The NAM is just about
completely dry through Wednesday and and Wednesday night (keeping the
rain band to our west and northwest). The ECMWF is trending more this
direction from it's past run. The GFS continues to bring things
forward quicker. We will go with this trend, backing down our PoP at
least a category Wednesday and Wednesday night. We may need to
continue to delay the rain arrival if the NAM is correct. Otherwise,
Wednesday could near or break record highs once again. 

Record Highs for Tuesday and Wednesday:

Feb 20th
KHSV: 79 in 1986
KMSL: 79 in 1917

Feb 21st
KHSV: 76 in 1922
KMSL: 76 in 1993

.LONG TERM...(Thursday through Sunday)
Issued at 312 AM CST Mon Feb 19 2018

Through the extended the main weather story will be the lingering 
unsettled, wet pattern with warmer than normal temperatures. 

An upper ridge from the Gulf of Mexico to the Western Atlantic will 
remain anchored in place as we head into the weekend. This will 
inhibit the progress of a frontal system of easily moving through the
forecast area this week. As the frontal system is kept to our NW, a 
round of disturbances will swing out from the upper-level trough and 
create heavy rounds of rainfall. These disturbances will help weaken 
the ridge by the weekend. 

Models agree on the persistent upper ridge, but not on the 
progression and timing of the front. The GFS stands out as the one 
who pushes the front through faster, while the NAM is hesitant to 
bring the front in on Thursday and the latest ECMWF is almost in the 
middle but more towards the NAM. With the amount of uncertainty and 
with the front on our doorstep to the NW, will lean towards the ECMWF
at this time. I pulled back a little bit on POPs for Thursday but 
kept in the isolated chance for some thunderstorms. With the front 
fluctuating and disturbances moving across the area, cannot say which
timeframe will have the highest opportunity for rain so have chance 
POPs lingering through the weekend. Another opportunity for 
thunderstorms will be on Saturday, Saturday night and maybe on 
Sunday. Precip chances will decrease as the front finally exits 
Sunday into Monday. 

Temperatures during this time will be unseasonably warm. Thursday 
will start off with highs in the lower 70s and lows around 60 
degrees. Breezy southerly flow Friday and Saturday will help temps 
overcome those cloudy skies and warm them up into the mid 70s Friday 
and lower 70s on Saturday with lows again hovering around 60 degrees.
Saturday's highs will be just a few degrees shy of their records, so
will have to keep an eye on those. Sunday temps will be a little 
cooler with highs in the upper 60s and lows around 50 degrees. 


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 513 AM CST Mon Feb 19 2018

VFR conditions may transition quickly to IFR at times through 15Z
with low ceilings below 010agl developing. The ceilings will lift
into the 010-015agl range (MVFR), and eventually ~030agl around 18Z.
The clouds should scatter out by 21Z. Southerly flow of 10-15kt with
gusts around 20kt are expected this afternoon. The winds should drop
below 10kt after 00Z. The moist flow will produce more low clouds
in the 015-020agl range (MVFR) by ~05Z.





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