Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 201029

Area Forecast Discussion
National Weather Service Huntsville AL
529 AM CDT Thu Sep 20 2018

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 219 AM CDT Thu Sep 20 2018

RGB nighttime imagery indicates little to no valley fog development
in our forecast area thus far. It is developing in valleys along the
Cumberland and Appalachian regions to our northeast. Temp/Dew Point
spreads are over 5 degrees at many locations this morning after deep
mixing Wednesday afternoon. After the previous few days of valley fog,
will still keep patchy fog in our eastern valleys later this morning
through around 13Z.

For today, the 5h and 8h high positions were over middle TN and will
shift east into the Carolinas tonight. Will go with persistence
forecasting again today with regard to temperatures, with lower to
middle 90s for most valley locations.

.SHORT TERM...(Tonight through Saturday)
Issued at 219 AM CDT Thu Sep 20 2018

Will hold off on fog potential tonight based on the more limited fog
this morning and another day of heating and deep mixing and drying of
the boundary layer. On Friday, as deeper moist southerly flow
develops, surface dew points will climb a bit more and heating may
not be as intense. Also, there may be isolated to scattered showers
and thunderstorms, although pinpointing location with a lack of a
well defined focusing mechanism is difficult. The upper level
impulse/shortwave moving through the southern Plains becomes ingested
into the deeper westerlies across the lower MO Valley on Saturday.
Several MCS clusters will likely form along the frontal boundary from
Oklahoma through lower Ohio valley. The upper ridge axis will still
be anchored over the southeast including central and north Alabama
on Saturday. Thus, my confidence in thunderstorm coverage is lower
than what the suggested blends gives (i.e. likely/numerous). The
larger scale MCS activity will likely remain well to our west and
northwest through Saturday night, while we may see activity
precipitated by either differential heating southeast of these
complexes or weak streamline convergence noted in low to mid levels
between the ridge to our east and trough axis to our west. Will go
just above suggested blends for high temperatures on Saturday.

.LONG TERM...(Saturday night through Wednesday)
Issued at 219 AM CDT Thu Sep 20 2018

Upper ridging will slowly push eastward at the start of the long
term period, with southwest flow extending through the lower and mid
levels. This will continue an influx of moisture across the region,
with PW values potentially exceeding 2 inches on Sunday. Meanwhile, a
weak shortwave and associated vorticity max will develop and move
into the Mid Mississippi and Ohio Valleys. Models differ on the
strength of this feature, as well as just how far south it will reach
given the ridge set up over the extreme southeastern CONUS. This
will play a large role in just how much coverage of precip the TN
Valley will see, with the GFS bringing the feature and precip through
the area and the ECMWF keeping much of the precip to our north,
along a stalled frontal boundary. While we may certainly see a higher
coverage of showers and thunderstorms, confidence is not high enough
to include likely PoPs in for Sunday.

By Monday, an upper trough initially over the northern Rockies will
push eastward and amplify over the Plains. Surface cyclogenesis will
occur, with a cold front trailing south from the parent low. Well
ahead of this system across the TN Valley, southwest flow will
maintain abnormally high moisture across the region, with several
weaknesses moving across the area. Given the abundance of moisture
in place, it won`t take much for showers and thunderstorms to
develop, so isolated to scattered storms will remain possible both
Monday and Tuesday. The aforementioned trough and surface cold front
will continue to push eastward through the first part of the new work
week, reaching the Mississippi Valley late on Tuesday. Forecast
models are starting to come into better agreement with the timing of
this system, however discrepancies remain with the GFS roughly 6 to
12 hours faster than the ECMWF. Still several days away, so these
discrepancies are expected, but the good news is that models agree
that most of the upper level dynamics will remain well north of the
TN Valley so at this point, the severe potential is low. Of course,
this could change over the next 7 days. Given the uncertainty
regarding timing, have maintained the blended guidance, with the bulk
of the precip falling on Wednesday.

Near to slightly above normal temperatures are expected through at
least the first part of the long term period, with cloud cover and
scattered precip keeping temps from warming above the lower 80s on
Sunday. We`ll see a brief warming trend on Monday and Tuesday, as
precip coverage decreases. The real story, and what most people are
waiting on, is the potential for the aforementioned cold front to
move through and bring more fall-like temperatures to the area. The
front will likely still be in the area by the end of the long term
period, but there is fairly high consistency that by the end of the
week, our high temperatures will generally be in the 70s, with
overnight lows in the 50s.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 529 AM CDT Thu Sep 20 2018

VFR flight weather conditions are expected during the next 24 hours.





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