Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 191748

Area Forecast Discussion
National Weather Service Huntsville AL
1248 PM CDT Tue Sep 19 2017

For 18Z TAFS.


.NEAR TERM...(Rest of Today)
Issued at 949 AM CDT Tue Sep 19 2017

What patchy dense fog was in place across the sheltered/valley areas
of NE AL earlier this morning has now diminished, with some cloud
cover beginning to work its way ewd into NW AL. A few showers have
also developed across portions of extreme NW AL, in response to an
oncoming shrtwv aloft out of the srn/mid Plains states. Convection
should continue to gradually develop/spread ewd thru the day, as the
trough axis moves into the region. Primary lifting mech will be
provided by weak dynamic forcing along the advancing trough and
buoyant energy increasing thru the day. Even so, this should mainly
result in gen tstms with perhaps a strong storm or two capable of
gusty winds and brief heavy rainfall. Afternoon conditions will again
be warm/muggy, with highs climbing well into the mid/upper 80s range.

.SHORT TERM...(Tonight through Thursday night)
Issued at 531 AM CDT Tue Sep 19 2017

Guidance is in reasonably good agreement in predicting the mid-level
trough to cross the TN Valley tonight. Although most of the afternoon
convection should dissipate by sunset, some showers and storms may
linger in the region through Midnight aided by synoptic scale ascent
associated with the trough. The trough axis is expected to shift
southeastward across the southern Appalachians on Wednesday, as a
500-mb ridge builds northward into the eastern Great Lakes downstream
from an amplifying longwave trough over the western CONUS.
Developing northerly flow aloft will advect a drier mid- level
airmass into the region as this occurs, and this along with lack of
any source of ascent should lead to a lower coverage of showers and
storms on Wednesday. Mid-level winds will strengthen and veer to the
northeast from Wednesday night-Thursday night as the 500-mb ridge
strengthens to our northwest and the trough to our southeast evolves
into a cutoff low over the Carolinas. This pattern will maintain the
southwestward advection of dry air aloft, with only isolated
showers/storms possible on Thursday afternoon when a weak wave is
predicted to cross the region in northeast flow aloft.

Regarding temperatures, highs should warm from todays values back
into the u80s/l90s on Wednesday and Thursday based on the assumption
of less cloud cover and precipitation each day. However, despite the
presence of drier air aloft, little change is expected in the quality
of low-level moisture and this should result in afternoon heat index
values possibly as high as 95-100 degrees for portions of the
forecast area by Thursday. Based on favorable conditions for
radiational cooling, lows will range from the u60s in the west to the
l60s in the east -- with patchy fog possible each morning.

.LONG TERM...(Friday through Monday)
Issued at 531 AM CDT Tue Sep 19 2017

A fairly strong upper ridge will encompass much of the central CONUS
Friday morning, while a fairly strong upper trough digs southward
along the west coast. At the surface, high pressure will build over
the eastern half of the CONUS. The TN Valley will remain on the
southern fringe of the upper high, with an east-southeasterly flow
through the lower levels helping bring an influx of moisture across
the area. This will aid in diurnally driven thunderstorms on Friday,
though at this point, the highest coverage looks to be across the
east. This pattern will continue into Saturday, though guidance hints
at developing a cut off low along the northern Gulf and brings that
northward and into the area late Saturday afternoon and into Sunday.
This may enhance the coverage of diurnally driven thunderstorms,
however the upper high will remain to our north and may limit some of
this coverage.

High pressure will maintain its hold across the TN Valley heading
into the next work week. Meanwhile, TC Maria will lift northward
along the western Atlantic, though with the latest guidance looks to
remain over the open waters. Moisture will generally be cut off from
the local area as winds shift to a more northeasterly direction,
limiting the chance for showers and thunderstorms on Monday and
Tuesday. Given the strength of the upper high, the aforementioned
trough won`t make much of a progression as it moves east of the
Rockies, and looks to stall over the Plains. Thus, a dry forecast is
expected across the TN Valley at least for the start of next week.

Temperatures through the period will remain at or slightly above
normal for late September. Highs will generally warm into the mid to
upper 80s, with overnight lows falling into the mid to upper 60s.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1248 PM CDT Tue Sep 19 2017

Clusters of shra/tsra are moving ewd between the KMSL terminal and
the KHSV airport this early Tue afternoon. This activity is reducing
cigs/vis at the airports into the 1-2K ft/1-2SM range at times with
vrb winds gusting near 30KT with the heavier storms. This activity
will continue thru 20Z and then much of the convection should be
along/E of the I-65 corridor. Overall conds will be improving heading
into the evening hrs with VFR conds developing across the area. Some
patchy -br is possible at both airports late tonight given the
rainfall today coupled with light winds.





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