Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 141729

Area Forecast Discussion
National Weather Service Huntsville AL
1129 AM CST Thu Dec 14 2017

For 18Z TAFS.


.NEAR TERM...(Rest of Today)
Issued at 948 AM CST Thu Dec 14 2017

A cold front continues to push southward across the the region this
morning, with high pressure slowly building southward through the
day. Despite northerly surface flow, abundant sunshine has allowed
temperatures to warm up nicely so far, with temps currently in the
lower to mid 40s. The weak cold air advection will temper some of
the diurnal heating today, though plentiful sunshine will still warm
temperatures into the upper 40s to lower 50s by this afternoon.
Previous forecast package was on track so no major changes were made.

.SHORT TERM...(Tonight through Saturday night)
Issued at 202 AM CST Thu Dec 14 2017

It will be a chilly night areawide with lows in the 20s
except over the larger water bodies where readings stay near 30-32F.
Flow should be just enough to prevent any fog development. A
positively tilted trough/vorticity lobe will drop southeast from the
eastern TN valley into TX on Friday. A relatively dry atmosphere
should keep the sky mostly cloud free. A surface ridge will be
building east across the southern U.S. into Friday night. After highs
in the 40s, lows Friday night should once again drop into the 20s.

Saturday should be a nice day as southerly flow advects 850 mb
temperatures back into the 6-7C range. Clouds are unlikely to return
before dusk. Highs should reach the lower 50s at most valley

As has been advertised in recent days, a quick moving shortwave in
mid to upper levels will be ejecting northeastward out of TX Saturday
night, reaching the MS river around 12Z Sunday. Good upper level
divergence at 250mb across the mid South and TN valley will support
good UVVs and induce a strong south-southwesterly low level jet of
50-60kt. Resulting isentropic ascent will be strong with good
pressure advection indicated on the 295-305K levels. PWs will
increase above 1.0 inches with very good MTVs also progged. So,
despite low levels beginning rather dry with dew points in the 30s,
will bring rainfall into most of the area after midnight Sunday
morning. An elevated MCS (mostly showers) will develop and expand
out of MS into north AL and southern TN at the nose of the strong
low level jet and best isentropic ascent during the early morning
hours of Sunday. Sounding profiles may support a few ice pellets
mixing in over southern middle TN or northeast AL, but confidence is
not high enough to include just yet. Have also left thunderstorms out
for now due to positive Showalter Indices (SI), but we can`t rule
out some higher based thunderstorms rooted at or above 750 mb based
on the steepening lapse rate profile. Will monitor future model runs
as we move closer to the event.

.LONG TERM...(Sunday through Wednesday)
Issued at 202 AM CST Thu Dec 14 2017

In the beginning of this forecast period widespread rainfall
associated with a shortwave trough axis moving within the
subtropical jet. This rainfall will be occurring over the MS River
Valley and gradually moving east into the western portions of the TN
River Valley. In addition, a warm front should be moving north from
the Gulf of Mexico towards the TN Valley on Sunday. This will result
in temperatures rising after midnight and during the day on Sunday.
Lows may actually have occurred around midnight on Sunday morning.

Meanwhile, the kinematics accompanying the trough axis include a
substantial increase in wind shear with low-level SRH values between
200-300 m2/s2 and curved hodographs/veering profile from the sfc to
850 mb. What this means is that despite a generally stable moist
adiabatic environment, broad rotating and bowing segments will be
possible especially as the warm front stalls over the TN Valley. In
addition, daytime highs will be in the low 50s but there may be a
north-south gradient in later forecasts as greater confidence is
realized on warm front movement.

As this trough moves off to the NE, the TN Valley (and much of the
Southeast) will be under the influence of subtropical ridge centered
just southeast of Florida over the Bahamas with strong SW flow aloft
and southerly surface flow. The resulting warm air advection will
bring some relief to the cold temperatures with daytime highs near
60 on Monday and overnight lows in the 40s. However, as an elongated
trough moves within the subtropical jet over S Plains/MS River
Valley, will likely see additional shower development within the SW
flow (weak undulations within the SW flow).  This may occur before
the main trough and accompanying cold front arrives on
Tuesday/Wednesday. There is typical model timing differences but
given the trajectory of this front, temperatures should be only
modestly affected and mostly the overnight lows once the front
passes. Furthermore, will keep showers in the forecast for both
Monday and Tuesday given the timing differences and potential
pattern change. Speaking of the pattern change, it does appear that
mean troughing/blocking pattern that had been in place for a while
over the eastern half of the country will be replaced with ridge and
trough. Left Wednesday dry and slightly cooler after the front
passes sometime on Tuesday.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1129 AM CST Thu Dec 14 2017

VFR conditions will prevail through forecast period at the KMSL and
KHSV terminals. Only a few passing high clouds are anticipated. Winds
should generally remain from the northwest to north and below 5 kts
through much of the period.





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