Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 210918
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
418 AM CDT Sat Apr 21 2018

.SHORT TERM...
Today and Tonight.

Currently this AM, a surface high pressure ridge is centered over
the Great Lakes with a generally east/east-southeast flow across
Central Alabama expected through tonight. We should have one more
dry day today as our airmass continues to moderate with milder high
temperatures. Slightly lower readings will be in the east closer to
the ridge, but overall warmer than Friday. Moisture return will
occur slowly today ahead of our next rain maker as a surface low
pressure system develops in the lee of the Rockies moving into the
Texas Panhandle today and into Arkansas tonight. This in turn will
bring a warm front to our doorstep tonight. While dew points will
increase tonight, the warm front and better moisture should remain
closer to the coast tonight. However, with increasing low level
winds developing tonight we should see enough increased moisture and
lift for some showers to spread across the area, especially after
midnight.

08

.LONG TERM...
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
totals.

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-inforcing 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.

32/Davis

&&

.AVIATION...
06Z TAF Discussion.

No changes in the terminal forecast trends, but did mention some
MVFR ceilings moving in generally after 00z south to north.

BMX 00z roab indicated a dry atmosphere across Central Alabama.
This was due to a large dome of high pressure covering much of the
eastern Conus. Winds overnight will range from calm to east
northeast around 5kts. Winds just above the surface will range
from 20-25kts, but will be below LLWS criteria.

The pressure gradient increases on Saturday and expect winds to
increase after 14-16z to between 10-15kts. Winds will veer
slightly to east and southeast.

Moisture will increase as the winds become southeast above the
surface. Expect some clouds to inch in from the southeast after
21z and move west to northwest. Added the mention of some MVFR
ceilings into TOI/MGM by 03z.

Beyond the forecast period, moisture continues to increase ahead
of the next frontal system. Rain chances and flight restrictions
look likely into Sunday.

75


&&

.FIRE WEATHER...

Afternoon RH values will fall to or below 35 percent this
afternoon, for portions of the area. Northeast to east 20ft winds
remain below 10mph. 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.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     73  53  64  55  66 /   0  50  90 100  70
Anniston    73  54  66  57  68 /   0  50  90 100  70
Birmingham  75  57  66  58  69 /   0  50 100 100  70
Tuscaloosa  76  58  69  59  71 /   0  60 100  90  50
Calera      74  57  67  58  69 /   0  50 100  90  70
Auburn      72  56  66  58  71 /   0  50  90 100  70
Montgomery  76  59  71  60  74 /   0  50 100  90  50
Troy        75  58  71  61  75 /   0  50 100 100  50

&&

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

&&

$$



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