Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 182332
AFDMOB

Area Forecast Discussion
National Weather Service Mobile AL
532 PM CST Wed Jan 18 2017

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

&&

.AVIATION...
00Z issuance...With near coastal airports already reporting VISBY
drops as the fog that has hung in offshore moves inland, am expecting
drops to low end IFR or LIFR levels overnight over airports over the
coastal counties. If tonight is anything like last night, there will
be temp increases in wind speeds, causing temp rises in VISBYS, but
leaving ceilings at or below IFR levels. Again with sunrise, VISBYS
and CIGS are expected to rise into VFR levels, with afternoon SHRA
and TSRA causing local drops.

/16

&&

.PREV DISCUSSION... /issued 312 PM CST Wed Jan 18 2017/

NEAR TERM /Now Through Thursday/...Dense fog is expected to
rapidly redevelop this evening in the nearshore waters/coastal areas
and spread inland through the late evening and overnight hours as a
warm and moist airmass continues to overspread the area. A Dense Fog
Advisory has been issued for the marine area and for our southern
row of counties in southeast Mississippi...southwest Alabama and
Northwest Florida through Thursday morning. The advisory may have to
be expanded northward later this evening.

Showers and thunderstorms will increase in coverage from west to
east during the day on Thursday as a shortwave ejects out of the
southern plains and sends a weakening cold front toward the area.
The Storm Prediction Center has placed the Gulf Coast in a Marginal
Risk for severe storm development on Thursday. However, widespread
convection is expected to keep instability levels low and the lack
of significant deep layer shear will also act to limit the severe
weather risk. The primary threats will be locally heavy rain and
gusty winds in the stronger storms during the afternoon hours. Lows
tonight will generally be in the low to mid 60s. Highs on Thursday
will be in the low to mid 70s. /13

SHORT TERM /Thursday night Through Saturday night/...This portion of
the forecast will be comprised of two main periods of potentially
severe weather, the second of which may be significant.  The first
of these two periods is associated with an upper trof oriented from
the central Plains to Louisiana which takes on an increasing negative
tilt while sweeping across the southeast states Thursday night. Deep
layer lift associated with this feature is expected to have created
an active line of convection which will have advanced eastward across
much of the forecast area Thursday afternoon, then continues across
the western Florida panhandle and south central Alabama Thursday
evening before exiting to the east. Have included areas of fog
developing across the area late Thursday night which dissipate by
mid Friday morning.

The 850 mb jet associated will be about 30-35 knots, which results
in 0-1 km helicity values up to around 125 m2/s2. MLCape values in
this environment are expected to be around 500 J/kg and considering
the shear am expecting primarily strong storms to affect the eastern
portion of the area Thursday evening with a few severe storms
possible, with the main threat being damaging wind gusts. Per
coordination with SPC, a Marginal Risk for Severe Storms will be in
place for the entire eastern portion of the area Thursday evening as
the line moves through. Precipitable water values will be near 1.5
inches which are around 190% of normal for this time of year. While
QPF amounts Tuesday evening will be typically be 0.5 to 0.75 inches,
given the abundant deep layer moisture in place, there is the
potential for heavier amounts to fall in a short amount of time (less
than an hour), and will maintain a Limited Threat of Flooding through
Thursday evening.

The second period of severe weather - which could be significant - begins
Friday night as another upper trof advances into the Plains and
strengthens. This next upper trof will actually be comprised of two
systems, the first of which sends a 100-120 knot upper jet across the
forecast area Friday night, followed by a second rather vigorous
system which amplifies the trof pattern over the Plains and brings
another 100-120 knot upper jet over the forecast area Saturday night.
One notable attribute of the second system is that it will bring a
deep upper low across the Red River valley Saturday night that is at
least 3 standard deviations below normal. The surface pattern
responds to the first system with the strengthening of a surface low
over the central Plains on Friday along with a warm front which is
expected to advance from the northern Gulf into the forecast area
Friday night. The Plains surface low moves northward on Saturday as
the warm front continues northward through the remainder of the
forecast area on Saturday, likely during the morning hours. Another
surface low meanwhile develops and deepens while advancing across the
southern Plains through Saturday night.

MLCape values will be around 500 J/kg Friday evening then increase
to roughly around 1000 J/kg late Friday night, then increase further
to 1500-2500 J/kg on Saturday. The presence of the warm front in the
forecast area Friday night through Saturday morning (possibly delayed
exiting north of the area until Saturday afternoon) in this
increasingly unstable environment along with a vigorous series of
shortwaves (ejected downstream across our area from the strengthening
Plains upper trof) is certainly cause for concern. The 850 mb jet
increases to 40-50 knots late Friday night then diminishes to around
30 knots by midday Saturday, with similar values through Saturday
night although there is the potential to increase to 40-50 knots.

Based on the timing of the best deep layer lift, which includes a bit
of a respite around midday Saturday, expect a round of severe
weather late Friday night through Saturday morning as the warm front
lifts northward through the area, followed by a second round of
severe weather developing Saturday afternoon and continuing through
Saturday night.  With the warm front in the forecast area with a
strong 850 mb jet, suspect that 0-1 km helicity values will be much
higher than the 200 m2/s2 currently indicated by guidance, and
considering the indicated MLCape values, the potential for tornadoes,
large hail and damaging winds all look possible.

With the forecast area in the warm sector for the remainder of the
period and some question about the strength of the low level jet, see
a continuing risk of damaging winds, along with some potential of
large hail and tornadoes depending on the amount of shear.  One
change that occurs during the period involves the amount of deep
layer moisture. Precipitable water values will be around 1.5 inches
late Friday night through Saturday, but look to decrease to around
1.0 inch Saturday night. Considering the strength of the forcing and
abundant deep layer moisture but that QPF amounts for late Friday
night through Saturday will be typically 1.0-1.5 inches, will issue
a Limited Risk for Flooding graphic for this time period. /29

LONG TERM /Sunday Through Wednesday/...A deep upper low embedded
within an upper trof over the southern Plains continues eastward
across the southeast states through Sunday night followed by upper
ridging building into the region through Tuesday. An associated
surface low deepens while moving from the southern Plains to across
the southeast states, and brings a strong cold front through the area
late Sunday into Sunday night. Progressively drier deep layer air is
expected to move into the region on Sunday, with precipitable water
values dropping to around 0.5 inches, about 60% of normal. Despite
the drier air moving into the area, forcing will be fairly vigorous
initially and sufficient to support likely pops for much of the area
at least Sunday morning, after which pops decrease to chance. There
is some potential for strong to possibly severe storms Sunday morning
as MLCape values of 500-1000 J/kg will be present with the deep layer
forcing and 0-1 km helicity values of 100-150 m2/s2.  Have included
slight chance pops for Monday due to potential wrap around patches of
rain, but dry conditions may begin as early as this period and follow
through at least Tuesday. A chance for rain returns on Wednesday as
another cold front moves through the area. /29

MARINE...Showers and thunderstorms will increase in coverage along
with increasing winds and seas on Thursday ahead of an approaching
cold front. Rain and thunderstorm chances will continue to increase
through the weekend as multiple systems affect the Marine area. A
strong cold front will move through the Marine area late Sunday into
Monday with a strong West to Northwest flow in the wake of the
front. A Small Craft Advisory will be needed this weekend into early
next week. /13

&&

.MOB WATCHES/WARNINGS/ADVISORIES...
AL...Dense Fog Advisory from 9 PM this evening to 10 AM CST Thursday
     for ALZ261>266.

FL...Dense Fog Advisory from 9 PM this evening to 10 AM CST Thursday
     for FLZ201>206.

MS...Dense Fog Advisory from 9 PM this evening to 10 AM CST Thursday
     for MSZ078-079.

GM...Dense Fog Advisory until noon CST Thursday for GMZ630>635-650-
     655.

&&

$$

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