Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 250007

Area Forecast Discussion
National Weather Service Birmingham AL
707 PM CDT Tue Apr 24 2018

For 00Z Aviation.


Through Tonight.

A broad upper low over Tennessee was producing a large area of
clouds over the eastern United States. Cloud cover and northwest
winds have maintained cooler than normal temperatures across
central Alabama this afternoon with readings in the 60s. Scattered
showers will continue to track southeast through the late afternoon,
and dissipate towards sunset. There may be a few lingering showers
across the eastern counties this evening, and kept slight chance
of showers. Models keep high levels of low level moisture through
the overnight hours, and kept skies mostly cloudy for most areas,
with the southwest counties more likely to see some clearing.


Wednesday through Monday.

Upper-level flow will becomes westerly on Wednesday as the upper
low finally reaches the Mid-Atlantic coastline, and a potent
shortwave in the northwest flow digs into Oklahoma. Low-level flow
will remain northerly behind the departing low pressure system,
preventing a significant warm-up. Morning low clouds will
gradually lift, lingering the longest in the northeast counties.
Highs will be in the 70s, except in the northeast where more
cloudcover will keep highs in the 60s. Mid and high level clouds
will begin to increase from the west. Couldn`t rule out a few rain
drops reaching the ground late in the afternoon in Marion/Winston
Counties ahead of a mid-level speed max, but most should remain
dry during the day.

An initially diffuse cold front will sink into the area Wednesday
night and stall out. Low-level moisture is initially very limited
along the front, due to offshore/northwest winds ahead of it. But
models are coming into agreement that a wave of low pressure will
develop along the front over the ArkLaMiss due to strong synoptic
forcing via the potent shortwave. Strengthening moist isentropic
lift will cause rain to move in from the west late Wednesday
night. The GFS is often too slow to saturate during these
situations, so the wetter ECMWF/European ensemble mean/Canadian
was favored during this period. Widespread precipitation is
expected across the area Thursday due to strong lift from the
shortwave and the surface low moving through and sufficient
moisture. Raised PoPs to categorical across much of the area
given a wet signal in the European ensembles. Southerly winds
developing ahead of the surface low may allow dew points to reach
60 degrees across our southern counties. Cold temperatures aloft
will help compensate for lower dew points to allow some
instability to be present, and added in a slight chance of thunder
areawide. It`s not out of the question that some stronger storms
could develop across our southern counties or even an isolated
severe storm, as 500mb temperatures drop to -18 to -20C, with a
main threat of gusty winds/hail. Severe potential is still much
too low to mention in the HWO. Rainfall amounts look to generally
be a half inch to an inch, preventing much in the way of flooding
concerns. Added in some small PoPs in our eastern counties
Thursday night given uncertainty regarding the speed of the

Dry air moves in behind this system, but another dynamic shortwave
follows closely on its heels Friday with another cold front.
Moisture return in the wake of Thursday`s system is very
questionable, and latest deterministic models are dry for most of
the area. But some of the European ensemble members are wetter, so
will keep in some slight chance PoPs for Friday/Friday night. A
pattern change is still expected to begin Saturday through the
rest of the extended forecast period, with the current western
ridge/eastern trough upper-level pattern switching to a western
trough/eastern ridge. At low-levels a Bermuda high looks to set up
early next week. Rain-free conditions are expected for the weekend
and the first part of next week. Highs will be in the low to mid
70s behind the front Saturday, warming to the upper 70s to low 80s
Sunday and Monday.



00Z TAF Discussion.

Conditions will be slowly changing the next 24 hours as the pesky
and persistent upper low finally exit stage right. Overall, went
with a terminal forecast very similar to last night. We start off
with a large area of cumulus/stratocumulus and even a few breaks
at times. Ceilings will be VFR at 040-080. Expect some variability
in the cloud cover this evening before the clouds fill back in.
Have MVFR ceilings becoming widespread after 06z north and closer
to 10z south. Low level moisture was still hanging around and have
mentioned a tempo for IFR ceilings north and 4-6sm br around
daybreak. Ceilings will rise to VFR by 18z at all locations and
may even have no ceilings by the end of the period.

As the upper low finally exits, surface winds start of westerly
and will swing around to northerly around 10kts on Wednesday.




A moist air mass will remain in place the next few days. Scattered
light showers and patchy drizzle are expected through this evening
due to a slow moving low pressure system. A break in rain chances
will occur during the day on Wednesday. A cold front moving
through late Wednesday night and Thursday will then bring wetting
rains. Afternoon minimum relative humidity values will remain
above 50 percent through Thursday. Critical fire weather
conditions are not expected to be met.


Gadsden     52  68  51  64  48 /  20  10  40  80  20
Anniston    53  70  51  66  49 /  20  10  30  80  20
Birmingham  54  72  54  66  50 /  20  10  50  80  10
Tuscaloosa  54  74  55  67  50 /  10  10  60  80  10
Calera      54  71  53  66  49 /  20  10  50  80  10
Auburn      53  71  53  70  51 /  10   0  10  80  30
Montgomery  54  75  54  70  51 /  10   0  30  80  10
Troy        54  74  54  73  53 /  10   0  10  60  20




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