Areal Flood Advisory 20, NWS Mobile, AL

Area Forecast Discussion
National Weather Service Mobile AL
107 PM CDT Mon May 3 2021

.NEAR TERM /Now Through Tuesday/...A shortwave trof over the
Ohio and Tennessee River valleys weakens while ejecting off into
the western Atlantic tonight. A large upper trof located from the
northern Plains to the 4 Corners region meanwhile advances
eastward, and becomes oriented along the Mississippi River valley
by late Tuesday afternoon. This upper trof is actually a phased
southern and northern stream system, and a surface low associated
with the northern stream slowly progresses from Wisconsin to
across the Great Lakes while a surface low associated with the
southern stream moves along the Red River valley then across the
mid Mississippi River valley Tuesday afternoon. These two surface
lows will bring a trailing cold front to a northern Louisiana to
western Tennessee line by late Tuesday afternoon. The forecast
area remains in a rather moist southerly deep layer flow with
precipitable water values increasing from 1.5-1.7 today to 1.5-2.0
inches by Tuesday afternoon. MLCAPE values this afternoon range
from 1500-3000 J/kg with mid level lapse rates around 6.5 C/km,
and model soundings also show abundant dry mid level air to
enhance downdraft production. With the modest shortwaves moving
across the area and daytime heating, this environment will support
likely pops north of a MOB-EGI line this afternoon with coverage
gradually diminishing tonight.  0-6 km bulk shear will be
generally low around 25 knots but higher values of 30-35 knots are
possible. The fairly high instability values and limited shear
values suggest slow moving storms with precipitation loading/mid
level dry air entrainment as the main mechanisms leading to
strong, potentially severe storms with damaging wind gusts and
large hail. Locally heavy rainfall is also possible. A severe
thunderstorm watch is in effect for interior areas until 8 pm. The
Storm Prediction Center (SPC) has a slight risk over interior
areas with a marginal risk over most of the remainder through
tonight.

For Tuesday, a series of vigorous shortwaves will advance eastward
across the forecast area during the day while the upper trof
advances toward the region.  Bulk shear values will be low Tuesday
morning, 20-25 knots, but then increase to 35-45 knots in the
afternoon with the highest values over the northwestern portion of
the area.  MLCAPE values of 1500-3000 J/kg are expected, and mid
level lapse rates look a bit steeper when compared to today and
around 7C/km.  It is not clear at this time as to how convection
will evolve in response to the pattern/environment, with scenarios
in Convection Allowing Models (CAMS) ranging from a cold pool driven
line of convection advancing into the forecast area to near the I-65
corridor Tuesday afternoon or a line of convection that develops
further to the west and doesn't move into the area until the
evening. The cold pool driven line of convection is certainly a
possible scenario given that model soundings show abundant dry
air above 650 mb to contribute to downdraft production. Larger
scale guidance such as the GFS and ECMWF strike a compromise with
a line of convection advancing into the northwesternmost portion
of the area late Tuesday afternoon. SPC has an enhanced risk over
southeast Mississippi and interior southwest Alabama with a slight
risk over much of the remainder of the area. Damaging winds,
large hail, and a few tornadoes are possible.  Lows tonight will
be mild and mostly in the lower 70s, and highs on Tuesday range
from the lower 80s near the coast to the mid to upper 80s further
inland. A high risk of rip currents continues through the period.
/29

&&

.AVIATION...
18Z issuance...MVFR ceilings prevail over the area through the
period, with localized IFR conditions possible in showers and
storms moving across the area during the afternoon and evening.
Southerly winds around 15 knots decrease to around 10 knots
tonight then increase back to around 15 knots Tuesday morning.
/29

&&

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

FL...High Rip Current Risk through Friday morning for FLZ202-204-206.

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

This product is also available on the web at:
http://weather.gov/mob

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