Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 232331

Area Forecast Discussion
National Weather Service Huntsville AL
531 PM CST Tue Jan 23 2018

For 00Z TAFS.


.NEAR TERM...(Tonight)
Issued at 233 PM CST Tue Jan 23 2018

Visible satellite imagery continues to show a broken-overcast
stratocumulus deck associated with a low-level thermal trough axis
impacting the northern half of the forecast area this afternoon. The
cloud cover has been most widespread north of the TN River, and has
been sufficient to have an impact on temperatures, with mid-afternoon
readings ranging from the l/m 40s along and north of the AL-TN
border to the lower 50s south of the river.

Although recent model guidance unanimously agrees that northwest
flow will diminish overnight as a surface ridge axis builds eastward
from a high over the southern Plains, it is not in very good
agreement regarding how quickly the stratocu deck will clear the
region. The GFS/ECMWF suggest a more rapid clearing trend as the
instigating 850-mb thermal trough axis shifts rapidly east-
northeastward into the central Appalachians this evening. However,
the NAM and UK Met Office models indicate that clouds could linger
across the northeastern half of the CWFA through sunrise tomorrow, if
not even a few hours longer. This makes the low temperature forecast
tonight quite uncertain, and we have attempted to blend guidance to
account for these factors. However, lows across the TN counties and
northeast AL could end up being a few degrees warmer than currently
forecast if clouds indeed persist through sunrise.

.SHORT TERM...(Wednesday through Thursday night)
Issued at 233 PM CST Tue Jan 23 2018

An amplifying mid-tropospheric shortwave trough is expected to drop
southeastward from western NE into MO overnight, and spread rapidly
southeastward across TN/KY tomorrow before reaching the southern
Appalachians by 00Z Thursday. Although guidance does suggest that
weak elevated vertical motions will develop as this system crosses
the region tomorrow morning, moisture in the same layer will be
insufficient to support more than an increase in cloud cover.
Stronger low-level subsidence in the wake of the departing trough
will allow any lingering stratocumulus clouds to dissipate shortly
after sunrise, leaving sunny skies during the afternoon, and this
should allow temps to warm into the lower 50s for most of the area.

Northwest flow aloft will increase in the wake of tomorrow morning`s
trough, providing mostly clear skies on Wednesday night/Thursday
morning. At the surface, a broad ridge will translate northeastward
across the TN Valley providing calm winds, and the combination of
these factors should allow for strong radiational cooling. Lows will
likely fall into the mid 20s for most outlying areas, and (although
not explicitly mentioned in the forecast) conditions will be
favorable for patchy freezing fog in river valleys and near large
bodies of water. Light/variable winds will continue on Thursday, with
abundant sunshine anticipated as a mid-level shortwave ridge
approaches the region from the southwest, and this should provide
even warmer max temps in the mid 50s. A slight warming trend will
continue Thursday night, as stronger southerly return flow begins to

.LONG TERM...(Friday through Tuesday night)
Issued at 233 PM CST Tue Jan 23 2018

During the day on Friday, southwest return flow around the surface
high (over the Carolinas and off the eastern seaboard) increases,
especially near and west of the I-65. Further east, winds may remain
a bit more southeasterly keeping highs a few degrees cooler (52 to 56
degrees). Not expecting much cloud cover, so couldn`t rule out a few
highs reaching 60 degrees, but think higher temperatures in the west
will mainly remain below 60 degrees. Models tighten up the pressure
gradient in response to the storm system pushing into Kansas and the
western Great Lakes region. Thus kept gusts up to around 20 mph in
the forecast in the afternoon.

Models remain consistent from last night keeping the best
convergence and moisture content west of Alabama through Friday
night, maybe into Saturday morning. Thus only have isolated to widely
scattered chances of light showers in the forecast after midnight on
Friday into the early morning hours on Saturday. There will be deep
enough moisture and ample convergence for mostly cloudy conditions to
develop around midnight across northern Alabama. Newest guidance
looks to be coming into a little better agreement on the evolution of
the next storm system moving through southeastern Canada during this
period and into the weekend. Models tend to develop stronger
convergence and deeper moisture ahead of the longwave trough axis
associated with this system Saturday afternoon into Saturday night. A
complicating factor is that models continue to show some upper level
forcing/energy pushing northeast from the central Gulf of Mexico at
the same time. However, they are very different with the speed and
movement of this feature. This may help to either increase rain
chances or keep them lower (ECWMF solution) or delay high
precipitation chances (GFS solution). At this point went a bit below
blended guidance given the uncertainty with 30 to 40 percent pop
Saturday afternoon. Highs in the lower 60s look reasonable given the
925 mb temperatures and mixing of boundary layer.

There is quite a spread in model precipitation guidance given the
differing solutions with the energy pushing northeast or east-
northeast from the Gulf of Mexico Saturday night through Sunday. Thus
keeping a bit below guidance, with high chance or likely pops
Saturday night into Sunday. Timing of heavier rainfall and if it will
hang around through Sunday is very uncertain. Due to limited
instability and the uncertainty with this forecast, leaving
thunderstorms out of the forecast with through Sunday.

Somewhat cooler air returns Sunday night into early next week, as
the storm system pushes east of the area. Highs return to the mid 40s
to lower 50s and lows drop back into the upper 20s to 30s.


.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 531 PM CST Tue Jan 23 2018

VFR conditions will prevail through the next 24 hours at the KMSL and
KHSV terminals. Mostly clear to partly cloudy skies will persist and
winds will generally remain from the north-northwest through the





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