Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 200602

Area Forecast Discussion
National Weather Service Huntsville AL
102 AM CDT Fri Apr 20 2018

For 06Z TAFS.


.NEAR TERM...(Tonight)
Issued at 928 PM CDT Thu Apr 19 2018

High pressure will continue to build into the Tennessee Valley
region overnight, promoting tranquil conditions. As of 02z,
temperatures have dropped off into the mid 40s to around 50 degrees
thanks to the clear sky. With the cool, dry air mass in place,
temperatures will continue to drop overnight, with lows bottoming out
in the mid to upper 30s by early Friday morning. Given these cool
temperatures, there will be a localized patchy frost threat,
especially across Southern Middle Tennessee where temperatures will
be slightly lower. However, the pressure gradient will remain tight
enough to allow for a consistent 5-6kt NNE wind overnight. This light
breeze will help to keep the boundary layer mixed enough to prevent
any widespread frost development and issues. The forecast remains on
track so have only made minor adjustments based on current

.SHORT TERM...(Friday through Saturday night)
Issued at 241 PM CDT Thu Apr 19 2018

On Friday, as the surface high shifts east and upper ridging shifts
into the Ozarks and central Gulf Coast, dry east-southeast flow will
develop over the TN valley. Warm advection will be rather neutral at
best Friday, but as the column/thicknesses enlarge due to ridging and
stronger solar heating, temps should warm into the mid to upper 60s.

On Saturday, the ridge will shift into the OH valley to the southeast
Coast with broad southwesterly flow aloft developing. This will
enable moisture advection to increase and modest warm advection.
Mid and high clouds will likely increase by Saturday evening.
Increasing diffluence aloft develops quickly Saturday night as a
series of shortwaves ejects from the main upper trough moving out of
the Plains. The resulting combination of theta-e advection in low to
mid levels and good UVVs should enable showers to become scattered
to numerous rather quickly late Saturday night, especially in
northwest AL into southern middle TN.

.LONG TERM...(Saturday night through Wednesday)
Issued at 241 PM CDT Thu Apr 19 2018

Looks like another weekend will end on a soggy note across the TN
Valley. At least temps won`t be too bad, highs in the lower 60s (~10
degrees below normal), and lows in the lower 50s (normal values).

A cutoff upper-level low will weaken and slow down as it moves out
of the Southern Plains. An upper-level jet streak on the back side of
the low will put us in a diffluent pattern and the left exit region
of the jet. With the sfc low remaining to our south, this will be the
primary source of lift. Light rain will start Saturday night, but
soundings will saturate pretty quick by Sunday morning as a
southeasterly LLJ ~40-50kts moves in with PWATS reaching ~1.2". Lift
will be further enhanced on Sunday with WAA. All of this will lead to
rainfall will becoming widespread and heavier Sunday and Sunday
night. The upper-level low will make slow eastward progression and
continue to weaken on Monday due to an upper-level ridge along the
southeast Atlantic coast. Temps will be a near reflection of Sunday,
but POP chances will slowly decrease through the day leaving most
with scattered showers by Monday night.

By Tuesday, a strong shortwave moving into the Northern Plains will
give this upper-level low the push it needed to lift northeast.
However, it doesn`t get far before it becomes embedded within the
flow around the shortwave approaching from the northwest. It will not
be as breezy on Tuesday as it will be on Sunday and Monday since the
pressure gradient will be more relaxed, however scattered showers
remain in the forecast. There is still some model discrepancy about
the evolution of the shortwave, if it will be an open or closed low,
but none the less it looks like the system will bring a cold front
into the area on Wed, keeping rain chances lingering through mid-


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 102 AM CDT Fri Apr 20 2018

VFR conditions will prevail at both HSV/MSL, with periods of sct ci
clouds anticipated in northwest flow aloft. Although north-
northeasterly sfc flow has diminished into the 5-10 knot range, a
strong east-northeasterly low-level jet has warranted inclusion of
LLWS through 12Z. Otherwise, we expect sfc winds to gradually veer to
east-northeast and diminish over the course of the valid TAF period.





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