Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 250003

Area Forecast Discussion
National Weather Service Mobile AL
603 PM CST Fri Feb 24 2017

.DISCUSSION...Updated for latest aviation discussion below.


00Z issuance...VFR conditions start off this forecast period at
25.00Z, but expect cigs and visibility to gradually reduce to MVFR
to IFR levels this evening ahead of an approaching cold front.
Confidence in LIFR or lower visibility thresholds being met is
lower tonight, but cannot be completely ruled out at KMOB/KBFM/KPNS
prior to frontal passage. The front is expected to move from
northwest to southeast across the region generally between
25.09-13Z. A return to VFR conditions with increasing northwest to
north winds is expected following passage of the front. /21


.PREV DISCUSSION... /issued 320 PM CST Fri Feb 24 2017/

NEAR TERM /Now Through Saturday/...Surface and upper level ridging
will continue to weaken and shift eastward through tonight as a cold
front approaches from the west. Cloud cover and precip chances
increase as the front approaches, moving into our Mississippi
counties around midnight and then east across the remainder of the
area through Saturday morning. Isolated to scattered showers are
expected with the front mainly after midnight. Some patchy dense fog
is expected to develop during the evening hours ahead of the front,
but will quickly dissipate after midnight as mixing increases with
frontal passage. An isolated thunderstorm or two is possible but
weak instability and lack of upper level dynamics will preclude any
strong storms. Lows will range from the low to mid 50s inland to low
60s along the coast.

Saturday will feature clearing skies along with cooler and drier
conditions. Northerly flow will continue to advect cool and dry air
into the region in the wake of the front through the day as high
pressure builds eastward out of the southern plains. Highs on
Saturday will be in the low to mid 60s inland to upper 60s and low
70s to the south. /13

SHORT TERM /Saturday night Through Monday night/...An upper trof
over the interior eastern states moves off into the Atlantic
through Sunday while another trof over the western states weakens
while advancing into the Plains.  The Plains trof gradually
shears out across the northeast states and into the western
Atlantic through Monday night while another well defined upper
trof advances across the western states, with an associated
surface low developing over the central Plains.  A surface high
over the southeast states Saturday night moves off into the
western Atlantic through Sunday night and sets up a return flow over
the forecast area on Sunday which continues through Monday night.
Very dry deep layer air over the area early in the period, with
precipitable water values around 0.25 inch, increase somewhat on
Sunday, then more significantly on Monday with values rising to
around 1.5 inches, about 200% of normal. MLCape values increase to
500-750 J/kg Monday afternoon mainly over southeast Mississippi
and extreme southwest Alabama followed by values around 750 J/kg
across the entire area Monday night. Wet bulb zero values will be
fairly low, around 10 kft, with 0-1 km Helicity values that may
approach 150 m2/s2. Despite these indications, model soundings
show lapse rates near moist adiabatic from approximately 900-700
mb showing that much of the indicated CAPE values are well
elevated, so will continue to monitor at this point. Will continue
with dry conditions through Sunday night, except for small pops
over a small portion of southeast Mississippi and interior
southwest Alabama late Sunday night. Chance to good chance pops
follow for Monday and Monday night due to moderate isentropic lift
along with the significantly improved deep layer moisture. Lows
Saturday night range from the lower/mid 30s inland to near 40
closer to the coast, then trend much warmer by Monday night to the
lower 60s inland with mid 60s closer to the coast, values which
are just a bit below daytime highs typically observed this time of
year. Highs on Sunday will be in the mid 60s to near 70, with
lower to mid 70s for Monday. /29

LONG TERM /Tuesday Through Friday/...An upper trof advances
across the Plains and into the eastern states through mid week,
with an associated surface low well to the north bringing a cold
front through the forecast area late Wednesday afternoon into
Wednesday night. A surface high builds into the region through
Friday in the wake of the front. Deep layer moisture remains high
ahead of the approaching front with MLCAPE values of 500-750 J/kg.
0-1 km Helicity values of 100-150 m2/s2 are expected on Tuesday
with much lower values on Wednesday. Will need to monitor the
potential for strong storm development on Tuesday, but otherwise
general thunderstorms are expected until the front moves through.
Will have chance to good chance pops through Wednesday night, and
other than small pops over the easternmost portion on Thursday,
dry conditions follow in the wake of the front. Highs on Tuesday
will be near 80 ranging to mid 70s at the coast followed by
slightly cooler values on Wednesday before trending much cooler
for Thursday and Friday with highs in the lower/mid 60s. Lows
Tuesday night will be rather mild and in the mid 60s then trend
much cooler through Thursday night to the lower 40s inland with
upper 40s at the coast. /29

MARINE...A cold front will move across the marine area late tonight
into Saturday morning. Offshore flow increases in the wake of the
front late tonight into Saturday. Onshore flow weakens on Sunday and
becomes southerly again by Monday ahead of another approaching
system. /13




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