Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 210505

Area Forecast Discussion
National Weather Service Mobile AL
1105 PM CST Mon Feb 20 2017

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


06Z issuance...Cloud coverage is gradually increasing and
ceilings lowering as ongoing shower activity enters the local
area from the west. Lowest cigs down to LIFR categories are
possible. Strongest band of precip progged to enter SW AL after
21.09z and western FL panhandle by 21.16z, with a few
thunderstorms also possible mainly during daytime hours. Expect
IFR ceilings, southwesterly to westerly winds, and perhaps a few
showers to linger into Tuesday night, especially across western
portions of the local area. Lower cigs/vis and gusty winds will be
possible in the heaviest showers and thunderstorms. /49


.PREV DISCUSSION... /issued 1007 PM CST Mon Feb 20 2017/

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

UPDATE...Based on latest model guidance and obs trends, have
updated POPs to more accurately reflect the timing of showers and
thunderstorms tonight through Tuesday night. At this time, the
main squall line and its attendant trailing stratiform region are
visible on neighboring radars advancing eastward across southern
MS and eastern LA, with a few isolated showers also forming ahead
of this line across coastal southeast MS. This squall line is
currently expected to reach the AL border by roughly 6 AM CST and
the northwestern FL panhandle around 9 AM CST.

Showers with brief, heavy downpours will be the primary threat,
though a few rumbles of thunder could potentially mix in Tuesday
afternoon as modest instability develops across the local area.
Still looking at rainfall amounts of 1 to 3 inches for most areas,
though a few locally higher amounts will be possible. The
heaviest precip is then progged to exit the local area to the
south and east late Tuesday night. /49

PREV DISCUSSION... /issued 359 PM CST Mon Feb 20 2017/

NEAR TERM /Now Through Tuesday/...Weather maps continue to
reflect a highly amplified representation in the upper level
height fields with a sharp ridge axis extending northward from the
western Caribbean to the Ohio River Valley. West of this feature,
trof axis was moving eastward over the Plains states. At the
surface, a narrow north to south high pressure ridge extends from
the eastern Great Lakes, south to Florida. An assessment of deep
layer moisture from 20.12Z weather balloon releases show Gulf
moisture opening up from the western Gulf, northward into the
eastern Plains ahead of frontal zone. From a climatological
perspective, moisture values in these areas range from 150% of
normal over the western gulf to 300-400% of normal from Kansas to

Moving into the near term forecast, clouds are forecast to begin
spreading in from the west and may begin to see a small chance of
showers breaking out over the western zones by midnight. Patchy
fog will also be mentioned as well. Anomalously high levels of
deep layer moisture shifts eastward over the central Gulf coast
where pwat values range between 1.5 to 1.7 inches by daybreak
Tuesday and continuing thru the course of the day. As deep layer
ascent, associated with the slow eastward movement of a large and
complex low pressure system over the deep south operates on this
moisture, categorical chances of showers will be in place. A few
storms possibly mixed in as well. Locally heavy rains possible.
Area rain gauges could see rain amounts totaling 1 to 3 inches. A
few areas could see locally higher amounts. There is a marginal
risk of excessive rain along the Gulf coast, which equates to a
2-5% probability of rainfall exceeding flash flood guidance. Risk
of severe weather is low.

Overnight lows remain well above seasonal, ranging from the upper
50s interior to mid 60s coast. With clouds and rains, daytime
highs Tuesday more tempered as seen past couple days, ranging
from 69 to 73. /10

SHORT TERM /Tuesday night Through Thursday night/...The ECMWF,
GFS, and NAM models are in excellent agreement in the placement of
the upper low pressure area over southern Mississippi/southeast
Louisiana by early Tuesday evening. All the models then move the
system southeast through midweek, reaching the east central Gulf
of Mexico by late Wednesday evening. The low then takes a jog to
the east across the Florida penisula, reaching the Caribbean
Islands by the end of the short term. A line of moderate to heavy
rain showers with embedded thunderstorms (60 to 90 percent chance)
will be ongoing Tuesday evening across south central Alabama and
the western Florida panhandle, with scattered lingering showers
further to the west. This line of heavy rain will move east of
the forecast area before midnight, however isolated to scattered
warp-around rain showers will persist across the entire forecast
area through Wednesday afternoon as the upper low tracks further
southeast over the north central Gulf, and an associated surface
low forms over the northeast Gulf. Drier air will follow in the
wake of the system Wednesday night through Thursday night, with
no precipitation in the forecast.

Additional widespread rainfall amounts Tuesday evening through
Wednesday afternoon are expected to range from one-quarter to
three-quarters of an inch east of the line from Camden, Alabama
to Pensacola, Florida, with less then one-quarter of an inch west
of this line.

Low temperatures Tuesday, Wednesday and Thursday nights will range
from 53 to 60 degrees. Cooler high temperatures are forecast for
Wednesday due to the rain and clouds, but still around 8 degrees
above normal, ranging from 70 to 75 degrees. Very warm high
temperatures will return on Thursday with highs ranging from 74
to 79 degrees, except for lower 70s along the beaches. /22

LONG TERM /Friday Through Monday/...The dry period will persist
through Friday afternoon as weak upper and surface ridges remain
over the region. A large broad upper level trough stretching from
the Pacific Northwest to the northern Great Plains will moves east
and carve out a longwave trough over the eastern conus by late in
the week, as a very strong surface low pressure area across
northern Missouri lifts northeast into southeast Canada by the
start of the weekend. An associated cold front quickly approaching
the region from the northwest on Friday will pass through the
area late Friday night into early Saturday morning. Only isolated
light rain showers are expected across the forecast area Friday
night with the passage of the front, followed by another dry
period over the weekend. Temperatures will continue to trend much
above normal on Friday prior to the passage of the cold front,
with highs over the weekend in the mid 60s to lower 70s. /22

MARINE...A broad area of surface low pressure moves into the gulf
coast Tuesday and brings a period of unsettled weather. Rains
could be heavy...lowering visibility to a mile or less at times
Tuesday. The low drops southeast over the eastern gulf and the
Florida peninsula Wednesday. a light to moderate onshore flow
early in the period becomes northeast Wednesday and Thursday. seas
generally 2 to 4 feet. over the weekend...the passage of a strong
cold front brings an increase in northerly flow and building seas
in its wake. /10




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