-- Discussion containing changed information from previous version are highlighted. --
196
FXUS64 KHUN 041059
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
559 AM CDT Sat May 4 2024

...New AVIATION...

.NEAR TERM...
(Today)
Issued at 418 AM CDT Sat May 4 2024

Nocturnal convection will continue to spread eastward to the east
of the I-65 corridor early this morning, with this activity
occurring within a moist southerly flow regime in the lower
troposphere between a ridge of high pressure along the Atlantic
Seaboard and an area of low pressure lifting northeastward across
KS. Although deeper lift associated with a shortwave trough
(currently crossing the TN Valley in prevailing WSW flow aloft)
has contributed to a bit more thunderstorm activity than
originally anticipated, weak mid-level WSW winds (10-20 knots)
will hinder any risk for storm organization, with lightning and
brief heavy rainfall the main impacts. Patchy/locally dense fog
will also gradually develop through sunrise given the very moist
nature of the boundary layer, but observational data does not
warrant an SPS at this time.

Present indications are that the remnants of the overnight
precipitation regime will shift slowly northeastward and out of
the region between 12-15Z, warranting a sharp gradient in POPs
from SW-to-NE across the CWFA. Beyond this timeframe, weak
subsidence is anticipated as the shortwave trough advances further
northeastward into the central Appalachians, with only isolated-
widely scattered showers and storms possible during the afternoon
hours. Partly cloudy-mostly sunny skies will allow temperatures to
reach the l-m 80s this afternoon.

&&

.SHORT TERM...
(Tonight through Monday night)
Issued at 418 AM CDT Sat May 4 2024

The general synoptic pattern across the TN Valley will remain
largely unchanged tonight and Sunday, with light/moist southerly
flow expected to persist in the low-levels beneath a zonal flow
regime aloft. There are some indications that another weak mid-
level vort max (perhaps convectively-induced) may track eastward
across our region late this evening, and with some agreement from
the 00Z CAMs on an increase in convection, we have included a low-
medium chance POP between 3-9Z. This activity will in all
likelihood exit the region prior to sunrise on Sunday, with a
warm/humid but dry start to the day followed by development of
isolated afternoon showers and storms as temps rise into the m-u
80s.

Over the course of the day tomorrow, another shortwave trough is
predicted to lift northeastward from the Red River Valley of OK/TX
within the flow around a strong upper low tracking east-
northeastward from the Great Basin into the central Rockies. Due
to the motion of the upper low, a well-defined lee cyclone will
evolve across eastern MT throughout the day, forcing a warm front
to spread northeastward across the TN Valley late Sunday
night/early Monday morning. Deep lift generated by the warm front
and approaching shortwave trough will produce a fairly widespread
coverage of rain and storms that may begin across our western
zones as early as 00Z Monday before spreading further
northeastward. Although mid-level winds will back to SW and
increase to 15-25 knots preceding the arrival of the trough, shear
does not appear strong enough at this point to warrant concern
for organized storm structures.

Most NWP guidance suggests that the effective warm front will lie
across the OH Valley by 12Z Monday, with remnants of nocturnal
precipitation expected to spread northeastward and out of the
region during the late morning. A 500-mb shortwave ridge to the
east of the upper low (discussed in the paragraph above) will
translate across the region, yielding a lower but non-zero
coverage of showers and storms Monday afternoon and night. Low-
level SW flow will strengthen in the wake of the warm front, with
dewpoints expected to rise into the u60s-l70s.

&&

.LONG TERM...
(Tuesday through Friday)
Issued at 418 AM CDT Sat May 4 2024

Latest long range guidance from the global models suggests that
the presence of a mid-level ridge will continue to limit the
coverage of warm sector showers and thunderstorms across the TN
Valley on Tuesday, with a breezy, warm and humid day expected as
highs rise into the m-u 80s. However, by Tuesday night, we will
need to keep a close eye on thunderstorm activity to our northwest
that should develop along a Pacific cold front/dryline that will
surge eastward ahead of an occluding cyclone across eastern
MT/western ND. There are some indications that frontal convection
across southern portions of MO/IL may evolve into a broken MCS
Tuesday evening that could potentially track southeastward into
the local area early Wednesday morning and may feature a risk for
strong-severe storms as mid-level westerly flow will likely
increase to 35-40 knots prior to its arrival in our region.

However, a more reasonable scenario is that the initial MCS will
not reach our region due to the development of another weak area
of low pressure across the southern High Plains that should eject
northeastward into MO by Wednesday evening. In this case,
redevelopment of thunderstorms would occur along the path of the
surface low (across MO) early Wednesday afternoon that could
potentially grow upscale into a larger and more significant MCS
that would reach our region late Wednesday night/early Thursday
morning. This convective system would be entering an environment
of even stronger deep-layer shear (with WSW flow aloft of 45-50
knots) and moderate-high levels of instability driven by surface
dewpoints in the lower 70s beneath a plume of steep lapse rates
aloft. Thus, a notable risk of severe thunderstorms producing
large hail and damaging winds may materialize during this
timeframe. A third and final round of storms may occur on Thursday
night/Friday morning prior to the eventual passage of a cold
front that should bring a drier/cooler airmass into the region by
next weekend.

&&

.AVIATION...
-- Changed Discussion --
(12Z TAFS)
Issued at 559 AM CDT Sat May 4 2024

The remnants of nocturnal rain/storms continue to spread eastward
and out of northern AL early this morning, with clearing mid-
level clouds across the western half of the area supporting
development of patchy, locally dense fog in a few locations. We
will indicate IFR conditions in a 1-hr long TEMPO group for MSL
and MVFR conditions at HSV, where brief development of BR/FG could
occur thru 13Z. Diurnal warming of the moist boundary layer will
encourage development of a sct-bkn Cu field by 16Z, but coverage
of afternoon showers/storms appears too low to include in the TAFs
attm. However, a slightly greater coverage of convection may
occur btwn 3-9Z, which is when a weak mid-level disturbance will
track eastward across the TN Valley. Outside of convection, light
SSW winds and mostly clear skies may allow for the development of
patchy BR/FG once again early Sunday morning.
-- End Changed Discussion --

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...None.
TN...None.

&&

$$

NEAR TERM...70/DD
SHORT TERM...70/DD
LONG TERM...70/DD
AVIATION...70/DD