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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 251520

Area Forecast Discussion
National Weather Service Huntsville AL
920 AM CST Sun Feb 25 2018

.NEAR TERM...(Rest of Today)
Issued at 920 AM CST Sun Feb 25 2018

The line of storms that moved through overnight is now well south of
the area, along a prefrontal trough axis. The main cold front extends
from the eastern Great Lakes, south-southwestward through Middle 
Tennessee and into Louisiana. This front has just passed east of the 
Muscle Shoals airport, where dewpoints have already drop around 5 
degrees. Expect this front to continue its eastward progression 
through the day before it stalls to our south this afternoon. Ahead 
of the front, isolated to scattered light showers may continue, 
mainly east of I-65 through the remainder of the morning, though 
rainfall amounts aren't expected to amount to much. Additionally, 
extensive cloud cover will linger through the day and keep our 
temperatures from warming too much. Despite very weak cold air 
advection behind the front, temps will still only warm into the lower

An upper level trough will move eastward through the day, with
southwest flow aloft strengthening as it inches closer to the region.
Despite a light northerly flow at the surface, plentiful moisture in
the low and mid levels will persist. A wave will begin to move 
northeast late this afternoon, with light rain increasing from south 
to north, mainly south of the TN River during the daylight hours. 

.SHORT TERM...(Tonight through Tuesday)
Issued at 423 AM CST Sun Feb 25 2018

A secondary mid-level vort max within the base of the broader 
longwave trough is predicted to shift eastward across the southern 
Rockies and into the adjacent high Plains today. Although this 
feature will become highly sheared as it translates into the mid-MS 
Valley tonight, weak synoptic scale ascent should promote development
of a frontal wave of low pressure along the northwest Gulf coast by 
late this afternoon. As this wave lifts east-northeastward into AL/GA
by 12Z, isentropic ascent above the shallow cool airmass at the 
surface will promote renewed development of light rain--which should 
impact the entire region overnight. The rain will end from west-to- 
east rather quickly Monday morning, with increasingly zonal flow 
aloft promoting clearing skies during the late morning/early 
afternoon, and allowing temps to climb well into the mid 60s. 
Conditions will be favorable for radiational cooling Monday night, as
a surface ridge to our north spreads across the central Appalachians
and winds become light/variable, and we have forecasted lows to drop
into the u30s/l40s. Fairly widespread fog will also be possible late
Monday evening/early Tuesday morning, although we have not included 
in the official forecast at this time. In spite of an increase in 
cloud cover on Tuesday, strengthening southeasterly return flow will 
force temps into the u60s/l70s during the afternoon.

.LONG TERM...(Tuesday night through Saturday)
Issued at 423 AM CST Sun Feb 25 2018

Extended range guidance suggests that deep-layer forcing for ascent 
will begin to increase once again by Tuesday night, downstream from 
another amplifying longwave trough over the western CONUS. Elevated 
warm/moist advection will begin to strengthen during the evening 
hours, but should peak early Wednesday morning when the low-level jet
will be maximized across our region. This should favorably coincide
with the northward movement of a warm front to bring widespread 
rainfall to the region overnight. Although forecast soundings 
suggests adequate elevated CAPE for embedded thunderstorms, it 
appears as if the main threat with this initial wave of 
precipitation will be locally heavy rainfall and flooding.

Several episodes of moderate-locally heavy rainfall could 
potentially occur in the region from Wednesday through Wednesday 
night, as smaller scale disturbances eject northeastward out of the 
longwave trough migrating across the central Plains. The threat for 
heavy/flooding rainfall will also be enhanced by diffluent flow 
aloft, based on the predicted orientation of the trough and position 
of the subtropical ridge across the Bay of Campeche. Regardless of 
these factors and the potential for elevated convection within an 
environment characterized by PWATs of 1.4-1.6 inches, the threat for 
flooding will ultimately depend on where the warm front resides and 
it is too early to pinpoint location at this time.

A deepening surface low across the MS Valley should send a fairly 
strong cold front southeastward on Thursday, bringing an end to 
widespread rainfall. However, clouds and a few showers will persist 
Thursday night into Friday as we will remain along the southern rim 
of a closed upper low shifting eastward across the lower Great Lakes.
Northwest flow aloft will develop in the wake of this feature, 
allowing a cooler/drier Canadian airmass to settle into the TN Valley
next weekend.


.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 524 AM CST Sun Feb 25 2018

VFR conditions are prevailing across most of the forecast area as
lingering rainfall from early cold front passage moves east--with 
the exception of northeast and central AL where MVFR conditions 
exist. In addition, an area of MVFR cigs exists over southern TN/NE
MS. These cigs should gradually move east behind the area of rainfall
currently over the TN Valley. Thus, have added a TEMPO group for the
12-14Z time frame for potential for MVFR cigs. There should be a
break in precipitation between 14-02Z before the next round of
rainfall arrives as a warm front shifts northward. VFR cigs should
prevail after 02Z with light rain prevailing. 





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