Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 220454

Area Forecast Discussion
National Weather Service Birmingham AL
1154 PM CDT Sat Apr 21 2018

For 06Z Aviation.



Skies remain mostly clear for northern portions of Central AL as
scattered clouds have pushed into areas generally along and south
of I-20. Expect these to continue to lift northward this
afternoon and evening, increasing in coverage through tonight. No
major changes with the temperatures today and tonight with highs
expected in the mid 70s this afternoon and dropping to the upper
50s to low 60s tonight. The biggest change made in forecast is in
the early part of the extended. Models have slowed down on
bringing the rain in tonight and tomorrow, so have pushed back the
higher PoPs until after 12z. Best coverage is still expected
Sunday afternoon with 100% rain area-wide. Other aspects of this
system mentioned in the extended forecast below remain valid.


Sunday through Friday.

The upper low will be located over the AR/OK border region
starting off Sunday morning, within a compact neutrally tilted
southern stream trough. A surface trough will be located over
AR and LA with an embedded weak surface low. A warm front will
extend from AR southeastward to the AL Gulf Coast. A surface ridge
over the Mid-Atlantic will result in easterly to southeasterly
winds across the area. An expansive area of showers and embedded
thunderstorms will be ongoing over MS Sunday morning, associated
with a LLJ/warm conveyor belt. A dry layer will remain in place at
700 mb over Central Alabama, but moist southwest flow and strong
upper forcing aloft in the left exit region of an upper-level jet
streak, and low-level moist isentropic lift/warm air advection
should result in at least scattered light showers across the area
ahead of the main activity.

The area of widespread showers over MS will move eastward into
West Alabama Sunday morning. The expectation remains that the
surface warm front and associated surface-based instability will
not be able to lift northward ahead of this activity, due to the
easterly component to the winds across the area, as well as
considerable cloudiness. The WRF-ARW does try to bring in some 65
dewpoints and associated instability ahead of this activity, but
seems too aggressive with moisture return compared to the guidance
consensus. The 12km NAM also seems too aggressive with
instability, and does not even have the agreement of its 3km nest.
Some of this seems to be due to the more elongated nature of the
upper low in its solution which is out of step with the rest of
the guidance. So, while there could be some embedded
thunderstorms, this first wave of activity is not expected to be
severe. Models are suggesting that some heating will occur in the
wake of the morning activity over MS, with scattered convection
forming along the surface trough underneath the mid-level dry
slot, close to the cold core of the upper low, in an area of weak
low level shear but strong deep layer shear. Will have to monitor
our far western counties for any gusty winds/hail potential, but
current thinking is that these will weaken as they move into AL
during the early evening due to the loss of daytime heating and
lack of a strong LLJ, and as they enter a more stable air mass in
the wake of earlier precipitation. Convection is also expected to
develop along the Gulf Coast and northern Gulf which is another
limiting factor. Therefore, severe potential remains too low to
mention in the HWO at this time.

Models are trending a bit slower with the mid-level dry slot
Sunday night, so PoPs were raised across the area for this period.
The upper low will continue to drift slowly eastward towards
eastern MS. Widespread precipitation will likely hinder any
surface-based instability. Some guidance is hinting at a meso-low
and associated area of increased low-level shear tracking near our
far southeast counties, but current thinking is that it will
remain just southeast of our area. Latest QPF forecasts are now
indicating the best chance for area-average amounts greater than
3" has shifted from the east-central to our southeast counties,
which is where the highest PWATs and possible meso-low will be.
These areas are currently in D0 drought with high FFG values, and
did not see much rainfall last weekend. Chances of widespread
flash flooding across the area remain too low to mention in the
HWO at this time based on the placement of the heaviest rainfall
totals and antecedent conditions, and the possibility that
coastal convection could cut off parts of the area from the higher

The cutoff upper low will meander across the region Monday and
Tuesday. Deeper moisture will push off to our north and east, but
wraparound low-level moisture and the cold core aloft will result
in continued light showers/drizzle and associated cloudiness.
There could be enough instability for a couple embedded
thunderstorms in East Alabama on Monday given cool mid-level
temperatures but nothing severe is expected. High temperatures
across the northeast counties are uncertain given model
differences regarding the strength of a developing CAD wedge. The
upper low will open up into a wave and lift away from the area
Tuesday night in response to a couple kicker waves in the northern
stream. Models indicate two different strong shortwaves will move
through in the northwest flow aloft during the second half of the
week, re-enforcing the eastern trough and being accompanied by
cold fronts. Very little in the way of Gulf moisture return or
instability is expected, but there will be enough dynamics and
Pacific moisture to continue to mention rain chances, with timing
details still to be resolved.



06Z TAF Discussion.

Deteriorating conditions expected the next several hours and
these restrictions will last beyond the forecast period. Scattered
rain showers will develop just after daybreak and increase in
through early afternoon. Did back up the timing on some of the
rain. If showers develop before daybreak, there will be a chance
of some lower ceilings. Some thunder is possible and continued the
PROB30 for thunder at all locations except ANB/ASN after 18z.

Northeast to southeast winds around 10kts are anticipated
overnight, with Central Alabama positioned between a low west and
a high east. Ceilings will lower to MVFR between now and 11z and
reaching 012 by 14z. The terminals will be located north of a
warm front much of the day Sunday and as the rain falls, ceilings
may lower to IFR in spots, but not confident on the coverage or
timing. Will also have to monitor the surface temps and dew point
depression as a VIS restriction may develop too in precipitation.
Winds will increase as a surface low approaches with some gusts
around 20kts possible. Widespread rainfall will begin tapering off
west to east Sunday night and Monday with continued ceiling

Winds just above the surface increase overnight into the 32kt
range and continue into Sunday. If surface winds materialize,
these values should remain just below any LLWS criteria.




Afternoon relative humidity values will fall to near or slightly
below 35 percent this afternoon, for portions of the area.
Northeast to east 20 ft winds remain below 10 mph. Critical fire
weather conditions are not expected to be met. Rain chances return
tonight, with widespread rainfall Sunday into Sunday night. A
moist air mass will linger over the area for the first half of the
upcoming week.


Gadsden     55  63  55  66  55 /  50 100 100  70  50
Anniston    56  64  57  68  55 /  50 100 100  70  50
Birmingham  58  66  58  69  56 /  50 100  90  60  30
Tuscaloosa  60  68  59  71  56 /  50 100  90  50  20
Calera      58  66  58  69  55 /  50 100  90  50  30
Auburn      56  66  58  71  56 /  50 100 100  50  40
Montgomery  59  70  60  74  56 /  50 100  90  40  20
Troy        59  71  61  75  57 /  50 100  90  40  20




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