Area Forecast Discussion
Issued by NWS Birmingham, AL

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959
FXUS64 KBMX 211742
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
1142 AM CST Wed Feb 21 2018

.UPDATE...
For 18Z Aviation.

&&

.SHORT TERM...
Today and tonight.

Record high temperatures are expected at all four climate sites
today as a very anomalous upper-level ridge remains parked just
off the coast of the Carolinas. Deep southerly flow through the
column should veer to south-southwest with time, which may result
in temperatures a degree or two higher than observed on Tuesday.
500mb heights in our southeast counties are expected to reach a
whopping 590dm today, and most locations should see temperatures
crack the 80F mark. Dewpoints in the mid 60s will make it feel
more like the end of spring or early summer, and this enhanced
moisture should lead to isolated or scattered shower activity
primarily driven by daytime heating. However, an approaching front
should provide enough of a focus for enhanced rain chances late
this afternoon into the evening across the Northwest. Due to the
strength of the ridge, the front should come to a halt near the
northwestern tip of our forecast area this evening and then
retreat northward overnight.

87/Grantham

.LONG TERM...
Thursday through Wednesday.

The stationary front over North MS and into West TN will slowly
begin to lift northward as a warm front on Thursday. There could
still be some showers and isolated thunderstorms in the far
northwestern counties early Thursday, but this should start to
clear out as we get into Thursday night. The rest of Central AL
will again see unseasonably warm temperatures Thursday and Friday
as the high pressure to our east continues to usher in warm/moist
air off the Gulf of Mexico.

On Friday, an upper trough swings through the Great Basin and a
surface low develops on the lee side of the Rockies late Friday
night into Saturday morning. This low quickly moves northeastward
through the day on Saturday and stretches a cold front down
through the Mississippi River Valley. Models have come into
slightly better agreement, but the GFS still deepens the low more
than the ECMWF does and moves the cold front through Central AL
quicker during the overnight hours Saturday night into Sunday
morning. This system features a fairly strong low level jet
(50+kts at 850mb) and a 100-115kt upper level jet begins to nose
into Northern AL.

Here in Central AL, dewpoints are running high, and daytime
heating builds up quite a bit of instability. However, timing of
this front brings it in during the overnight hours, so instability
might be on the lower end (300-500 J/kg), but still sufficient to
support thunderstorms. With the strong low level jet across the
area along and ahead of this front, there will be quite a bit of
vertical shear in place to support organized storm development.
Therefore, expect a line of storms to develop along the cold front
and push into Central AL. The severity of these storms is still
uncertain with several limiting factors including marginal lapse
rates and lower instability. Will continue to monitor for model
trends in the next few days. For now, I don`t have enough
confidence to include mention of severe in the HWO.

Meanwhile, another shortwave trough swings through the larger
upper level trough and will bring an impulse through the area
just behind the departing front. This will lead to another round
of rain to move through during the day on Monday, mainly south of
the I-59 corridor.

Ridging builds in across the area Tuesday under more zonal flow
aloft, leading to drier conditions.

25/Owen

&&

.AVIATION...
18Z TAF Discussion.

A mix of VFR and MVFR cigs at mid day as a sct to bkn cu field
streams northward across the area. Southerly winds will be gusty in
the 15-20kt range through the afternoon, due to enhanced daytime
mixing. Winds will subside this evening, remaining out of the south
at 4-7kts overnight. Expect cigs to lower, with widespread MVFR
conditions, and localized IFR and LIFR, most likely at MGM and TOI
early Thursday morning.

Isolated to scattered showers are possible this afternoon, mainly
north and west. Confidence of activity at any terminal is only great
enough to mention VCSH at TCL.

14

&&

.FIRE WEATHER...

Unseasonably high RH values continue for the next several days.
With the higher relative humidity values and good overnight
recovery, do not expect there to be any fire weather concerns.

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
None.

&&

$$



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