Area Forecast Discussion
Issued by NWS Birmingham, AL

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426
FXUS64 KBMX 221746
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
1246 PM CDT Sun Oct 22 2017

.UPDATE...
MESOSCALE UPDATE.

&&

.SHORT TERM...

Skies ranged from partly cloudy east to overcast west across
central Alabama early this afternoon. Temperatures ranged from the
low 80`s far southeast to the lower 70`s in the southwest across
our forecast area. Surface winds were generally out of the
southeast between 10 and 20 mph with breezy conditions at times.

Aloft... water vapor imagery continues to depict a sharp trough
moving into southwest Missouri with satellite derived winds near
jet level continuing to indicate winds between 30 and 60 knots
across portions of Louisiana, Mississippi and Alabama.

Visible satellite imagery shows thickening cloud cover across our
west and southwest counties while radar imagery shows rain with
some thunderstorm activity affecting portions of Sumter, Marengo,
Greene and Hale Counties. This activity is expected to continue
and move further northeast this afternoon as a cold front
approaches from the west.

A synoptic-scale cold front extended from Southeast Missouri
through eastern Arkansas and into northwest Louisiana while a pre-
frontal trough extended from far eastern Arkansas nearly parallel
to the Interstate 55 corridor south into northeast Louisiana. A
maritime-influenced warm front was analyzed along the immediate
Mississippi coastline and extended northeast into southwest
Alabama, generally encompassing the coastal counties, and analyzed
further east into the western Florida panhandle.

Vigorous convection continues to persist across southeast
Mississippi and into southwest Alabama, particularly in the
vicinity and to the south of the warm front. The warm front is
expected to slowly lift northward this afternoon and evening ahead
of the approaching cold front. How far north this boundary
advances will determine the potential for severe thunderstorms.

Vertical wind profiles along with ample dynamical support aloft
favors organized thunderstorm activity along with severe storm
potential, however, weak lapse rates in the mid to upper levels
remains a limiting factor. Modified 12Z BMX sounding continues to
yield meager low level instability and various model-derived point
soundings across our central and southern counties continues to
depict maximum low-level instability values near 1000 J/kg roughly
near and south of the U.S. Highway 80 corridor with lower values
to the north.

Based upon latest trends, highest solar insolation along with best
thermal advection is occurring across our southeast counties
where surface temperatures are in the low 80`s and are expected to
reach into the mid 80`s this afternoon. Dew point values are
gradually increasing across our forecast area with higher values
in the upper 60`s generally along and south of the Interstate 85
and U.S. Highway 80 corridors. Expect dew points to rise into the
lower 70`s in these areas later this afternoon and into the early
evening hours.

Overall, potential for severe weather remains low across our
forecast area with the best potential for a strong to locally
severe storm being across our southern counties this afternoon and
evening with strong winds being the primary risk. If enough low
level instability materializes, updrafts could become strong
enough to support a conditional tornado risk.

JH/05

Previous short-term discussion:Today and Tonight.

Models have come into a better alignment for the frontal passage
through tonight, so confidence is quite high that the widespread
rainfall will occur, thus have gone with 100 PoP across the area.
Best chances during the daytime hours will be west of I-65, with
all areas seeing rain tonight. On a whole looking at 1.5 to 2.5
inches of rain with the higher totals in the east this evening as
the large swath of rain will increase in size this afternoon into
tonight as it swings east. Do not think that this would be a
widespread flash flooding event, as some of these totals will
occur over a 6 to 9 hour window. There could be some localized
rises in normal trouble spots, but feeling is that nuisance
flooding is the only concern. As for instability, it does not look
great, especially with the southeasterly flow ahead of the front,
barely swinging southerly just ahead. A few stronger storms
definitely could occur with wind being the main concern, but
widespread severe weather is not anticipated based on the models.
SPC only has Central Alabama in the general thunderstorm outlook
with a marginal concern just to our southwest. As we get through
the morning, there could be some adjustment based on mesoscale
features, but feel as though this is a good starting point at this
time. A mid morning meso analysis will be done based on the new
models and upper air support to see if any changes are needed for
this afternoon.

16

.LONG TERM...
Monday through Sunday.

Models are in better agreement with faster progression of the
upper trough on Monday. Rain should come to an end before noon,
and a mild afternoon is expected with some breaks in the clouds.
An upstream shortwave will reinforce the cold advection regime on
Tuesday into Wednesday with afternoon temperatures in the upper
50s to lower 60s in the North on Wednesday.

A transition to warmer conditions is expected on Thursday and
Friday as southerly flow develops in response to a large trough
forming in the western CONUS. Model spread is quite large on the
timing of the next front sometime in the Friday night through
Sunday timeframe. Our forecast leans more toward the slower and
more consistent ECMWF with the highest rain chances centered on
Saturday night.

87/Grantham

&&

.AVIATION...
18Z TAF Discussion.

Early morning MVFR cigs have become scattered as the air mass
warmed and mixing ensued. A large area of mostly light rain has
pushed into southwest Alabama ahead of a MCV over southeast
Mississippi. This rain shield will continue to track north and
eastward this afternoon, with rain likely reaching the I-65
corridor by 21z. TSRA has been confined to areas near the coast,
and for now do not anticipate any tstms at any TAF site until
after 03z. By 03z, strong forcing associated with an approaching
upper level trof will bring heavier rain and embedded tstms to all
areas, starting in the west and pushing eastward overnight. Even
though the forecast will call for prevailing 3 miles with TSRA,
vsbys will likely fall below 3 miles at times in the heavier rain
bands. The back edge of the rain will likely reach the I-65
corridor arnd 09z with near the AL/GA state line by 12z.

Cigs will be mostly above 3k feet agl thru 00z, with lcl cigs
near 2k feet agl, especially west of I-65. Cigs will transition to
IFR category by 03z across areas along and west of I-65, with IFR
cigs at all sites by 08z. Southeast sfc winds could gusts up to
20 knots until the rain shield arrives, then likely weaken as the
air mass stabilizes. The winds will gradually veer overnight and
become westerly between 09z and 12z as a cold front pushes
eastward.

58/rose


&&

.FIRE WEATHER...

A cold front will move through the area tonight, providing a
widespread soaking rainfall through Monday morning. Dry through the
rest of the week. Relative humidity values should remain above
critical thresholds. There are no fire weather concerns.

&&

.PREV DISCUSSION...
/Issued 1242 PM CDT Sun Oct 22 2017/
SHORT TERM...

The 12Z BMX sounding depicts a large dry layer from roughly 850
mb upward to around 500 mb. The low and mid levels are expected to
gradually moisten and become saturated by this afternoon and into
the evening hours.

Clouds are increasing across Alabama at this hour with the
thickest cloud cover present over our southwest counties where
rain was affecting portions of Sumter, Marengo and Greene
Counties. Thunderstorm activity was not far away from our
southwest counties and is expected to affect portions of our
southwest counties through the next few hours.

Aloft... a trough of low pressure was present along southern border
between Kansas and Missouri and was moving east. Areas to the
south and east of the trough base had satellite-derived wind
retrievals from 250-350 mb between 30 and 70 knots across northern
Louisiana, north-central Mississippi and into western Alabama.

A cold front was analyzed from near Chicago, Illinois that
extended south-southwest across eastern Arkansas and into far
eastern Texas with a surface low of 1012 mb analyzed near St.
Louis, Missouri. Ahead of the front was a squall line along a pre-
frontal trough that extended from far southwest Missouri south
through eastern Arkansas and into north-central Louisiana.

A maritime-influenced warm front was evident per the Mixing Ratio
field that extended from near Baton Rouge, Louisiana east across
southeast Louisiana and into far southeast Mississippi and into
the coastal sections of Mobile and Baldwin Counties in Alabama. A
mesoscale convective vortex (MCV) was associated with ongoing
convection across southeast Mississippi.

Despite generally favorable wind profiles and divergence aloft,
surface-based instability across much of our forecast area was
relatively meager with values currently less than 500 J/kg. Some
destabilization is expected through this afternoon but will be
hampered by further increasing cloud cover that will inhibit solar
insolation. The warm front near the Gulf Coast is expected to
only slowly advect northward, keeping higher instability values
confined to areas to the south of our forecast area.

JH/05

Previous short-term discussion:Today and Tonight.

Models have come into a better alignment for the frontal passage
through tonight, so confidence is quite high that the widespread
rainfall will occur, thus have gone with 100 PoP across the area.
Best chances during the daytime hours will be west of I-65, with
all areas seeing rain tonight. On a whole looking at 1.5 to 2.5
inches of rain with the higher totals in the east this evening as
the large swath of rain will increase in size this afternoon into
tonight as it swings east. Do not think that this would be a
widespread flash flooding event, as some of these totals will
occur over a 6 to 9 hour window. There could be some localized
rises in normal trouble spots, but feeling is that nuisance
flooding is the only concern. As for instability, it does not look
great, especially with the southeasterly flow ahead of the front,
barely swinging southerly just ahead. A few stronger storms
definitely could occur with wind being the main concern, but
widespread severe weather is not anticipated based on the models.
SPC only has Central Alabama in the general thunderstorm outlook
with a marginal concern just to our southwest. As we get through
the morning, there could be some adjustment based on mesoscale
features, but feel as though this is a good starting point at this
time. A mid morning meso analysis will be done based on the new
models and upper air support to see if any changes are needed for
this afternoon.

16

LONG TERM...
Monday through Sunday.

Models are in better agreement with faster progression of the
upper trough on Monday. Rain should come to an end before noon,
and a mild afternoon is expected with some breaks in the clouds.
An upstream shortwave will reinforce the cold advection regime on
Tuesday into Wednesday with afternoon temperatures in the upper
50s to lower 60s in the North on Wednesday.

A transition to warmer conditions is expected on Thursday and
Friday as southerly flow develops in response to a large trough
forming in the western CONUS. Model spread is quite large on the
timing of the next front sometime in the Friday night through
Sunday timeframe. Our forecast leans more toward the slower and
more consistent ECMWF with the highest rain chances centered on
Saturday night.

87/Grantham

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     66  69  48  66  39 / 100  80  10   0  10
Anniston    67  70  50  67  39 / 100  90  10  10  10
Birmingham  65  70  50  68  41 / 100  50  10   0  10
Tuscaloosa  64  71  52  69  41 / 100  20  10   0  10
Calera      65  71  51  67  41 / 100  50  10   0  10
Auburn      68  72  52  68  42 / 100 100  10  10  10
Montgomery  69  74  52  71  43 / 100  60  10   0  10
Troy        70  75  53  70  42 / 100  80  10   0  10

&&

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

&&

$$



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