Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 170225 AAA

Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
925 PM CDT Fri Mar 16 2018

.DISCUSSION...See updated information for land areas below.


.UPDATE...Deeper moisture and ascent is on the decrease across
our area this evening as a shortwave impulse embedded within zonal
mid level flow is weakening and moving east of the region. Radar
shows a few residual showers over the forecast area, but coverage
has decreased over the past 1-2 hours. We recently sent an update
to adjust POPs through the rest of the evening, and will monitor
trends to see if additional updates are necessary. Short term and
high resolution model trends support additional development of
rain showers over our interior southeast MS and southwest AL
zones, especially in the 09Z-12Z time frame ahead of another
approaching mid level shortwave impulse. We will keep POPs in the
chance range over the interior after midnight/toward early
Saturday morning, with lower POPs over our southeastern zones.
Patchy fog development still looks possible overnight into early
Saturday morning, primarily over southeast MS and portions of
southwest AL. /21


.PREV DISCUSSION... /issued 622 PM CDT Fri Mar 16 2018/

DISCUSSION...Updated for latest aviation discussion below.

00Z issuance...MVFR to occasional IFR ceilings and visibility will
continue this evening as areas of showers move eastward across the
area. An area of heavier rain and associated reductions in cigs
and visibility may impact our TAF sites this evening before
17.06Z. Low ceilings and some patchy fog are expected for the
overnight period. Cloud bases will begin to lift tomorrow morning
but are expected to remain at MVFR levels. Winds, at or below 10
knots, will continue from the south and southeast through the
period. 05/RR

PREV DISCUSSION... /issued 401 PM CDT Fri Mar 16 2018/

NEAR TERM /Now Through Saturday/...Southerly flow will continue to
transport deep layer moisture northward across the region in advance
of a slow moving cold front well to our northwest. Continued warm
and moist air advection will maintain a chance for isolated to
scattered showers overnight. The combination of increasing moisture
and instability along with weak shortwaves aloft will lead to
scattered showers and isolated thunderstorms on Saturday. Heavy rain
will be the main threat with any thunderstorm and no severe storms
are expected through Saturday. Clouds and increasing moisture will
keep lows much warmer, in the upper 50s and low 60s inland to mid
60s along the coast. Much warmer and more humid on Saturday with
highs in the upper 70s to near 80 inland. Coastal locations will be
in the low to mid 70s. /13

SHORT TERM /Saturday night Through Monday night/...An upper
ridge over the central states weakens considerably while shifting
eastward into much of the eastern states Sunday night. This is in
response to a deep upper low which meanwhile develops over the
central Plains, along with an associated deep surface low. Ahead
of this developing system, another surface low over the east
central states moves off into the western Atlantic through Sunday
and brings a trailing cold front southward to near the northern
portion of the forecast area Saturday night, possibly moving into
the central portion of the area. The frontal boundary lingers near
this position Sunday morning then moves into the interior Gulf
coast states as a warm front Sunday afternoon as the Plains
surface low begins to slowly advance eastward. The upper low
continues eastward and across the mid Mississippi river valley by
Monday evening then begins to fill/shear out later Monday night
while moving to near the Ohio river valley. The associated deep
surface low moves to near the mid Mississippi river valley by late
Monday afternoon, then likewise moves to near the Ohio river
valley Monday night and meanwhile brings a strong cold front
through the forecast area.

While there is good confidence regarding these larger scale
features, uncertainty with the timing and strength of a series of
shortwaves set to move across the forecast area during the period
makes for a difficult pop forecast. In addition, the environment
is anticipated to become conducive for potential severe storm
development, but the timing issues make it rather difficult to
pin down the best potential periods for possible severe weather.
It appears that a Mesoscale Convective System (MCS) will develop
in response to a strong series of shortwaves, and looks to affect
the forecast area at some point between Sunday afternoon into
Monday. The MCS could be quicker and move through as early as
Sunday afternoon, or later from Sunday night into Monday. Even
outside of the MCS feature, additional convective development will
be possible due to a continuing series of shortwaves moving
across the area.

Will have slight chance pops for most of the area Saturday night
except for chance pops over the westernmost portion where some
better forcing associated with shortwaves will be possible. Likely
pops prevail Sunday into Sunday night with likely pops over the
eastern portion of the area on Monday (and good chance pops further
to the west), due to the MCS along with the otherwise continuing
series of shortwaves. Chance to likely pops follow for Monday
night as the front is expected to move through the area. Regarding
the potential for strong to severe storms, for Saturday night
while CAPE values near 500 J/kg are indicated, model soundings
show that this instability will be elevated with unfavorable lapse
rates at low levels. There is a good deal of dry air aloft, so it
is possible that a strong storm or two could develop. Model
soundings show convection becoming surface based on Sunday but
also with the elevated dry air giving way to a moist profile as
deep layer moisture increases over the region. MLCAPE values of
500-1000 J/kg develop over at least the western half of the area,
but could be lower further to the east due to subsidence effects
associated with the weakening upper ridge shifting across the
area. The amount of shear depends on the timing of shortwaves/MCS
development, but may be sufficient by Sunday afternoon to support
possible severe storms. Severe weather looks possible over the
forecast area Sunday night through at least Monday as instability
increases fairly significantly and shear is expected to increase,
but the above discussed uncertainties make it difficult to
determine if this potential encompasses both Sunday night and
Monday or one or the other. For now, have issued a graphic
depicting a slight risk for severe storms for Monday with the
caveat that this may begin as early as Sunday night. /29

LONG TERM /Tuesday Through Friday/...Other than slight chance
pops over the interior portion of the area on Tuesday due to some
lingering patches of rain, dry conditions follow for the long term
period. Cooler air flows into the region in the wake of the cold
front, but at this time overnight lows look to remain above
freezing. /29

MARINE...A predominately light to occasionally moderate south to
southwest flow is expected to continue over the coastal waters
through Monday as Atlantic high pressure continues to ridge west
across the eastern Gulf and a frontal boundary remains nearly
stationary well to the north of the marine area. A stronger cold
front approaches the marine area Monday night and is expected to
move east across the coastal waters Tuesday. /13




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