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Area forecast discussion 
National Weather Service Huntsville Alabama
955 PM CDT Sat Aug 29 2015

Update...
evening update to public forecast.

&&

Discussion...
current water vapor imagery still indicating presence of middle-level
cutoff low just south of the Alabama/MS Gulf Coast this evening. An
initial vorticity maximum rotating cyclonically around this feature -- which
brought widespread showers/thunderstorms to the region earlier today
-- continues to weaken...with only isolated pockets of light rain and
sprinkles noted across the forecast area this evening. However...infrared
satellite data does suggest that a gradual increase in synoptic scale
ascent is currently underway to the south-southeast of the forecast area this
evening -- in conjunction with another weak vorticity maximum prognosticated to shift
north-northwestward into the region by 12z. A rather large area of moderate/heavy
rain and embedded thunderstorms associated with this feature --
currently across SW Georgia/southeast Alabama -- should develop northeastward into
the region between 09-12z...and will increase probability of precipitation for roughly the
southeastern half of the forecast area to account for this. Although
bulk of the precipitation will likely occur without thunder...a few
embedded storms will be possible and will include a low probability for
deeper convection in updated weather grids. Otherwise...hourly
temperatures/dewpoints and derived elements have been adjusted to
indicated warmer and more humid conditions anticipated for remainder
of the night. Patchy fog will be possible this evening and during the
early morning hours on Sunday...prior to arrival of next round of
rainfall from the southeast.

70/dd

&&

Aviation...
/issued 658 PM CDT Sat Aug 29 2015/
for 00z tafs...short term threat for convection has ended at both
terminals...and although a few light sprinkles of -ra may occur at
either terminal between 30/00-01z...we will not include in taf due to
low nature of impact. Given widespread coverage of moderate/heavy rafl
earlier today...br/fg will become a concern as surface winds diminish and
back to east-southeast later this evening. However...overcast cs ceilings currently around
20 kft will begin to fall to around 9 kft as the next weak disturbance
approaches from the southeast...and this may reduce threat for visibilities lower
than 3-5 sm. Models are a bit unclear regarding timing of arrival of
this next disturbance...but threat for thunderstorms and rain should begin to increase
around 12z at both terminals. Scattered/numerous storms will be possible through the
morning and into the early afternoon hours Sunday...before primary Gulf
Coast trough begins to lift northeastward and away from the region --resulting
in an improvement in conds late in the taf period.

70/dd

&&

Previous discussion...
/issued 228 PM CDT Sat Aug 29 2015/
quite a bit of scattered rain showers/thunderstorms and rain was occurring mainly west of
Interstate 65 as of 19z. Based upon trending...will continue with
scattered rain showers/thunderstorms and rain through the evening hours...with the activity
expected to linger to between 02z-03z time frame. This precipitation is in
response to the southeast low level flow associated with the slow
northeast movement of the weak upper low at 500 millibars in the
northern Gulf of Mexico.

Short wave energy rotating around the northeast periphery of the
upper low on Sunday will result in a higher chance pop for the
forecast area. Will keep probability of precipitation toward higher end to account also for
the northeast movement of the upper low as it begins to open up and
shift northeast into central Alabama by 18z Sunday and into northeast
Alabama by 00z Monday. Will keep in a lingering 20 pop for the
forecast area on Monday in the wake of the upper low moving away from
the area. Weak ridging in the low-middle levels will shift eastward into
the region by Tuesday with continued low afternoon rain showers/thunderstorms and rain chances
due to afternoon heating/instability. Will continue with low afternoon
probability of precipitation for Wednesday and Thursday. The forecast area is expected to
remain on the northern fringe of the low level southeast flow and
moisture from the remnants of Erika during this time period.

By Friday...another upper low will take better shape in the northern
Gulf of Mexico. This feature...combined with enhanced low level
moisture...increasing precipitable water values and afternoon heating/instability...
will lead to a better chance of rain showers/thunderstorms and rain across portions of the
forecast area. Will follow closer with the slightly cooler GFS temperature
guidance through the extended forecast periods.

Tt

&&

Hun watches/warnings/advisories...
Alabama...none.
Tennessee...none.

&&

$$

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