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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 211115

Area Forecast Discussion
National Weather Service Huntsville AL
615 AM CDT Wed Mar 21 2018

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 340 AM CDT Wed Mar 21 2018

Synoptically this morning an upper low is centered over the Ohio
Valley with a weak surface low stacked below it. The lingering low
level moisture behind this low is keeping a stratus deck over areas
around Huntsville and east. This deck has been slowly eroding from
the west as the upper low moves off to the east and the drier air 
moves in. With plenty of dry air and mid level subsidence, skies 
should be mostly sunny by the end of the afternoon. 

However, that doesn't mean warm! One more shortwave along the base of
the upper trough will move in this afternoon bringing a reinforcing
shot of cold air and shifting the mostly westerly winds to northwest.
This will keep highs in the lower 50s to upper 40s in the northeast

This shortwave will be enhanced by an upper level jet streak with 
30-40kts at the low levels. With sufficient mixing in the dry lower 
levels, we should be able to tap into some of this wind. Sustained 
winds this afternoon could be around 20 MPH with gusts up to 30 MPH 

.SHORT TERM...(Tonight through Friday Night)
Issued at 340 AM CDT Wed Mar 21 2018

Winds should diminish fairly quickly tonight as we cool off after 
sunset and surface high pressure starts to build in from the west. 
Although we won't see calm winds tonight, the combination of clear 
skies, winds less than 5 mph and dew points dropping into the mid 
20s, will allow low temps to drop to below freezing, upper 20s to 
around 30. Therefore, and in coordination with the neighbors, will 
issue a freeze warning from 7z/2am to 13z/8am for the entire area. 
Some areas may not see a full three hours below freezing but it is 
widespread enough that the freeze warning is warranted. 

With the high pressure in place Thursday and a developing mid level 
ridge pressing in from the west, we should see sunny skies and highs 
in the mid to upper 50s with winds less than 10 mph. 

A mid and upper level ridge strengthens significantly over the
intermountain west Thursday night as a low deepens over the
northeast. This will create a pretty strong upper level jet oriented
more north to south stretching from MN down through GA. This lift 
combined with a slight increase in mid level moisture has allowed 
guidance to show some light QPF tracking into our area between 6-12z 
Friday morning. Guidance has backed off in the last two days with 
this, leaving only the NAM with any QPF. Will pull back the pops from
the last forecast and leave it only in northeastern areas since the 
mid level lift isn't too strong and there really isn't much moisture.
Lows will be warming into the upper 30s/lower 40s so it should be 
all rain and maybe a few snow flurries in S. middle TN, if anything.

Guidance is also coming in drier for Friday and Friday night likely
due to the lack of any appreciable lift and only a mid level layer of
moisture, so just enough for a mostly cloud day. Removed any pops
from Friday but will leave them in for Friday night as a surface low
starts to develop over KS/MO. This will start to extend a frontal
boundary to our area by 9-12z Friday night/Saturday morning. With
lows Friday night in the 40s, it will be all rain. Highs on Friday 
will warm into the lower 60s. 

.LONG TERM...(Saturday through Tuesday)
Issued at 340 AM CDT Wed Mar 21 2018

Most guidance moves the surface low from Missouri southeast into 
northwestern Tennessee on Saturday. A boundary extends southwest from
the surface low into northeastern Texas. Good shear and low level 
helicity will be in place over the area ahead of this boundary, as 
the surface lows swings towards northwestern Tennessee. However, very
meager moisture advection occurs on Saturday and a strong 
temperature inversion is shown by models around 700 mb. These two 
factors will likely keep shower activity very light and keep any 
isolated thunderstorms from developing. As the surface low continues 
to slide southeast Saturday night, the models try to break the cap. 
They do not however really do this, but come close. If models do 
break it though, there may be just enough moisture and good enough 
low level lapse rates for a few strong storms capable of producing 
some small hail, but for now left out of the forecast given the 
strong cap and limited moisture expected. High chances of showers 
look like a good bet though Saturday and Saturday night. It should be
fairly warm over the weekend with highs in the 60s to lower 70s and 
lows in the upper 40s to mid 50s. 

Models diverge quite a bit by Sunday into Sunday night concerning 
how far south the cold front associated with the departed surface low
moves. Regardless though Sunday and Monday will be cooler. GFS does 
not move it to the florida panhandle (like the ECMWF), but moves in 
into southern/central AL, before lifting it further north late Monday
night as a warm front. If models are correct it should be a bit more
unstable as this warm front moves northeast through the area Sunday 
night into Monday. Overall forecast atmospheric soundings do not look
very impressive. Some rumbles of thunder and brief heavy rainfall 
may be possible with these storms, but not much else. Mainly 
scattered showers and few thunderstorms (none too strong) look 
possible Sunday night into Monday.

The next surface low and associated storm system develops over the 
Rockies/Oklahoma on Sunday night into Monday morning. However, once 
the warm front pushes north of the area by Tuesday evening, a drier 
atmospheric profile returns, as upper level ridging builds into 
Mississippi and Alabama. This strengthens on Tuesday, keeping a 
strengthening storm system over Arkansas and Texas. This gives us a 
break from precipitation Monday night through Tuesday. This period 
could be a bit warmer than models are currently indicating, based on 
ridging over the area. Overall models are similar, but do hold off 
higher rain and possibly thunderstorms chances until Wednesday 
afternoon evening of next week, given the strong upper level ridge 
the system is trying to move into.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 615 AM CDT Wed Mar 21 2018

Clouds are lingering near KHSV as an upper system lifts east but
should stay few or sct for the rest of the morning. Another batch of
clouds are moving in from the northwest and may impact KMSL, maybe
KHSV, in the next few hours. Although they may go bkn or ovc, heights
will stay VFR. Both terminals should be seeing mostly clear skies by
the afternoon. Winds will also be gusty today out of the northwest. 
They should increase quickly this morning with gusts around 30kts 
possible by this afternoon. They will diminish with sunset 
this evening. 


AL...Freeze Warning from 2 AM to 8 AM CDT Thursday for ALZ001>010-016.

TN...Freeze Warning from 2 AM to 8 AM CDT Thursday for TNZ076-096-097.




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