Area Forecast Discussion
Issued by NWS Mobile, AL

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903
FXUS64 KMOB 201755
AFDMOB

Area Forecast Discussion
National Weather Service Mobile AL
1155 AM CST Wed Feb 20 2019

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

&&

.AVIATION...
20/18Z issuance...LIFR ceilings prevail across most of the region
this afternoon, and will continue into tonight. Some areas of
fog continues across much of the area, especially over the
western Florida panhandle where it is patchy dense. Expect
potential for more widespread dense fog across much of the
southern half of the forecast area again tonight. Scattered
showers and isolated thunderstorms will move into areas to the
west of I-65 this afternoon and continue into the evening hours.
Some strong storms possible, producing strong wind gusts. South
winds 10-15 knots with occasional gusts around 25 knots expected
this afternoon, but winds expected to decrease tonight as a weak
front stalls across southeast MS and southwest AL. Even with the
occasional surface winds gusting up to around 25 knots at times,
kept mention of LLWS for early this afternoon as south winds up to
around 40 knots persist at around 2000 ft AGL. 12/DS

&&

.PREV DISCUSSION... /issued 935 AM CST Wed Feb 20 2019/

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

UPDATE...We have allowed the Dense Fog Advisory for southern
portions of Mobile and Baldwin (and adjacent Marine zones).
Better deep layer mixing has mixed out the dense fog, but light
fog continues across the region. Visibilities in most locations
where the fog remains are in the 2 to 4 mile range. There are a
few spotty areas over the western Florida panhandle where visibilities
remain around 1 mile, even 1/4 mile at PNS, but these lower visibilities
are somewhat limited at this point. Winds are still rather gusty,
occasionally up to 20 to 25 mph. Updated products will be out
shortly.

Still expect showers and thunderstorms to develop to the west and move
into portions of the forecast area later this morning and this afternoon.
Continues to look like that there is a marginal/slight severe risk today,
particularly over our northwestern counties (especially northwest
of a Camden, AL to Wiggins, MS line). 12/DS

PREV DISCUSSION... /issued 627 AM CST Wed Feb 20 2019/

DISCUSSION...See updated information for land areas and aviation
discussion below.

UPDATE...Visibility has remained remained 1-2 miles or slightly
better across interior portions of Mobile and Baldwin counties
this morning, and suspect that localized dense fog remains
confined near coastal portions of these counties. We have trimmed
the Dense Fog Advisory to only include the coastal portion of
Mobile/Baldwin counties through 9 AM. Localized dense fog will
also continue to impact portions of Mobile Bay, the Mississippi
Sound east of Pascagoula and adjacent coastal waters out 20 nm
through 9 AM.

A High Risk of rip currents will continue along area beaches
through tonight as moderate onshore flow combines with long
period swell and high tidal ranges. We also extended the high rip
current risk into Thursday afternoon as there is indication of
continued long period southerly swell through the day. /21

AVIATION...
12Z issuance...IFR to LIFR ceilings prevail, along with MVFR to
IFR visibility reductions due to fog early this morning. Ceilings
remain IFR for the most part today, though could briefly rise to
MVFR this afternoon before deteriorating to IFR to LIFR again late
tonight into early Thursday morning. Areas of fog could also
reduce visibility to IFR or lower again tonight. Southeast winds
10-15 knots with occasional gusts around 20 knots expected today.
Winds decrease tonight as a weak front stalls across southeast MS
and southwest AL. Kept mention of LLWS early this morning as
south-southeast winds around 35 knots persist around 2000 ft AGL.
Numerous SHRA and possibly a few TSRA will spread across the area
later this morning and into this afternoon. A few storms could be
strong to severe west of a Camden to Mobile line today. /21

PREV DISCUSSION... /issued 508 AM CST Wed Feb 20 2019/

NEAR TERM /Now Through Wednesday night/...Early morning surface
analysis places an area of low pressure near the MS/LA border, with
a cold front extending south from this feature across central and
southwest LA. A warm front also extends ahead of the area of low
pressure and is draped across interior southeast MS and far
southwest AL, where temperatures are in the mid 60s to around 70
degrees with dewpoints close to the same. Temperatures are cooler in
the 50s across much of the western FL panhandle and interior south
central AL north of the warm front. A brisk east to southeast flow
prevails across our region early this morning, with gusty winds at
times near the coast. Patchy fog has mainly remained confined to
southern portions of our forecast area early this morning. Dense
fog, if any, is probably relegated to immediate coastal portions of
Mobile and Baldwin counties. We will continue to monitor
observations over the next couple of hours, and may trim the Dense
Fog Advisory to only include coastal portions of Mobile/Baldwin
counties by 6 AM if observations at KMOB/KBFM/KCQF remain above 1/2
mile.

Another complicated forecast today as potential for locally heavy
rainfall and localized severe weather are on tap for portions of
southeast MS and southwest AL late this morning into this afternoon.
An increasingly moist airmass through the deep layer will spread
across our area today and tonight underneath southwest flow aloft
and also as a strong mid to upper level jet lifts northeast from the
southern Plains to the Great Lakes region. The surface cold front
will subsequently push eastward across MS this morning, before
gradually stalling across far southeast MS and interior portions of
southwest AL this evening through late tonight. Short range and high
resolution model guidance is in agreement with bringing a linear
complex of convection eastward across MS and southeast LA ahead of
the front this morning, before this convection pushes into southeast
MS and interior portions of southwest AL late this morning into this
afternoon. POPs today will be highest (80-100%) roughly northwest of
a Mobile to Camden line where moisture and ascent will most favor
convection, but scattered to numerous showers ahead of the line will
support chance to likely POPs farther east over the remainder of the
area. A moderately unstable airmass will be in place across
southeast MS and interior southwest AL ahead of the approaching line
today, with MLCAPE values ranging from around 500 to as high as 1200
J/KG per RAP/NAM/GFS forecast soundings. Strong deep layer and low
level shear will also be in place, with 850 mb flow of 40-50 knots
oriented along the line favoring potential for damaging wind gusts.
Curved hodographs will also bring a potential for a couple of
tornadoes today. A Marginal Severe risk will be in place roughly
west of a Camden to Mobile line, with a Slight severe risk extending
across interior southeast MS and potentially into portions of
interior southwest AL where instability and shear will be a little
more favorable for severe potential. The severe threat should
primarily occur from late this morning into this afternoon, but
upper dynamics will tend to weaken by mid to late afternoon.

Shortwave impulses in southwest flow aloft will allow for additional
moistening and ascent tonight, and expect additional development of
showers and potentially a few storms again this evening into late
tonight, with coverage most enhanced near the frontal boundary that
will extend over the northwestern part of the forecast area.
Although as strong storm or two will be possible early in the
evening, instability and shear tends to decrease tonight, so the
severe threat currently looks quite low tonight. Areas of fog may
redevelop tonight, and we may have to monitor for dense fog at times
late. /21

SHORT TERM /Thursday Through Friday night/...
Scattered showers continue Thursday as the southwest to northeast
surface boundary is expected to stall and elongate near I-65. The
two major synopic drivers the Bermuda High and the long wave trough
over the Southwest CONUS remain firmly established in the shortterm.
This pattern maintains the southwesterly upper level flow, but down
at the surface the low level winds are southerly. Convergence
between the southerly low level winds and the stalled surface
boundary could result in additional showers or even a few isolated
thunderstorms Thursday afternoon. As such have increasing south to
north Pops throughout the forecast area with the highest Pops near
the boundary. By Thursday evening and into the overnight hours the
surface boundary should begin to lift as a weak warm front
bringing it out of our forecast area and into central Alabama.

Friday does see mid-level dry air from the Gulf begin work its way
over the central Gulf Coast. This is likely as a result of air being
advected in from the subsidence rich region over the Bahamas, but
the low levels should remain quite moist. Model soundings show
precipitable water values at or above 1.60 inches. Moreover with
surface temperatures forecast to be 8-10 degrees above normal
reaching the upper 70s, there is a fair possibility of isolated
thunderstorms in the afternoon mainly over the interior counties.
The window of opportunity however is small and restricted to the
afternoon and early evening as additional dry mid level air is
anticipated to be advected in from the Gulf. This effectively
nullifies rain chances in the late evening and overnight hours.
DJ/17

LONG TERM /Saturday Through Tuesday/...
Saturday sees a shortwave trough over northern Texas embedded within
the aforementioned longwave trough move quickly through the southern
plains. This shortwave spurred on by a 100kt mid-level jet takes on
negative tilt throughout the day and rapidly deepens a 995mb surface
low over eastern Kansas. It is this low pressure system and trailing
cold front that shifts the moisture axis eastward from western
Louisiana over to southern Alabama. Convection along the cold front
should reach the southeastern Mississippi by late Saturday. Storms
will have access to approximately 1500 J/kg CAPE and modest 0-6km
wind shear around 40 kts. This should be sufficient for isolated
strong storms within the line as it moves east through our forecast
area Saturday evening into Sunday morning. As the cold front pushes
through Sunday and Monday, skies clear and temperatures fall back
into seasonal normals in the upper 60s with rain chances near zero.

North winds give way to northeasterly winds late Monday as the
high behind the previous cold front shifts east. Winds continue to
shift southeast Tuesday morning owing to a weakly deepening low
near the Texas coastline. This low develops beneath the divergence
region of and upper level jet and gradually travels eastward
during the day Tuesday. It will be the eventual formation of this
feature that would bring increased rain chances for Tuesday
leading into the middle part of next week. DJ/17

MARINE...Areas of dense marine fog will remain possible this
morning, primarily over Lower Mobile Bay, the Mississippi Sound east
of Pascagoula and adjacent waters out to 20 nm, where a Dense Fog
Advisory continues until 9 AM CST. Additional areas of fog may form
again this evening through early Thursday morning over these same
areas, as well as near bays/sounds/near shore waters from Pensacola
to the Okaloosa/Walton county line. The pattern will continue to
support areas of marine fog at times through the remainder of the
week.

Otherwise, a moderate to occasionally strong southeasterly wind flow
early this morning will gradually diminish later today. Exercise
caution headlines have been included through the morning hours. A
generally light onshore flow will then prevail Wednesday night
through Friday, increasing to moderate onshore flow late Friday
night through late Saturday. The next cold front is expected to push
across the marine area by Sunday. /21

&&

.MOB WATCHES/WARNINGS/ADVISORIES...
AL...High Rip Current Risk through Thursday afternoon for ALZ265-266.

FL...High Rip Current Risk through Thursday afternoon for FLZ202-204-
     206.

MS...None.
GM...None.
&&

$$

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