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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 251124

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

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 423 AM CST Sun Feb 25 2018

Within a broad longwave trough encompassing much of the Rockies and 
central Plains, a negatively-tilted shortwave disturbance will 
continue to lift northeastward over the upper MS Valley this morning,
before closing off into a low over eastern Ontario later today. 
Models suggest that southwesterly flow aloft across the TN Valley 
will be maintained between this feature, and an elongated 500-mb 
ridge extending from the Bay of Campeche northeastward to the FL 

At the surface, a cold front attached to the deepening surface low 
related to the northern stream trough will continue to decelerate as
it pushes to the southeast of the CWFA later this morning. Although 
the threat for thunderstorms will diminish with the passage of the 
front, a 2-4 hour period of light-moderate rain will persist before 
the axis of postfrontal precip drifts further to the south. Although 
low-level clouds will tend to dissipate as the lingering rain ends, 
overcast decks of altostratus/cirrostratus will persist through the 
day with afternoon temps only warming a few degrees into the 

.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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