Area Forecast Discussion
Issued by NWS Jackson, MS

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343
FXUS64 KJAN 200458
AFDJAN

Area Forecast Discussion
National Weather Service Jackson MS
1158 PM CDT Thu Apr 19 2018

.UPDATE...
Updated for 06Z aviation discussion

&&

.AVIATION...
06Z TAF discussion:
VFR conditions will prevail through the period. Winds may be gusty
to near 15kts today before subsiding tonight. /28/

&&

.DISCUSSION...
Evening surface analysis had a 1033mb high centered over the upper
Mississippi River Valley that was ridging south across our western
zones. This high will shift slowly east through the night
maintaining a cool dry north to northeast wind. This will allow
temperatures to bottom out cooler than normal. Temperatures were
on track and no changes to the forecast were needed. /22/

Prior discussion below:

Tonight through Friday afternoon: High pressure at the surface,
and ridging aloft, will continue to build into the region over the
next 24 hours. Although clear skies are expected tonight,
northerly winds will still remain up around 5 mph. This will keep
lows from achieving what they potentially could if they were calm,
as well as keep chances for any frost development at bay. Still,
a chilly night is on tap as lows range from the upper 30s to lower
40s.

Then on Friday, a pleasant early-spring day is expected across the
forecast area.  Winds will be more northeasterly during the day.
However, they won`t be as gusty as previous days.  Under sunny skies
yet again, highs will range from the middle 60s to lower 70s. /19/

Friday night through Thursday: The period will start off with a
closed low and associated trough over the Four Corners region.
Ahead of this feature, shortwave mid- level ridging and associated
ridge axis is progged to be located from the Gulf Coast through
the Mississippi River Valley. The ridge is expected to deamplify
as the aforementioned closed low and trough ejects into the
Southern Plains by Saturday morning. Southeasterly low-level winds
ahead of the approaching system will help advect better moisture
into the area with dewpoints increasing into the mid to upper 60s
mainly along and south of I-20 by Sunday morning. Strong forcing
via 60m/12hr height falls, diffluent upper level flow, and
positive differential vorticity advection combined with the
increasing moisture will allow precip to overspread the area ahead
and along of an approaching cold front Saturday night through
Sunday afternoon. Once again, the GFS continues to remain a faster
outlier solution and will thus lean more towards the slower ECMWF
timing with rain and thunderstorms moving through the Delta late
Saturday night into early Sunday morning, the I-55 corridor around
late morning to early afternoon, and east of I-55 early afternoon
to early evening before exiting into Alabama Sunday night.

Uncertainty still exists with respect to convective evolution and
severe potential with this event. While strong forcing and
instability will be in place (upwards of 500-1000 J/kg of SBCAPE
during the late morning to afternoon), the strongest shear,
especially at the low-levels, will be tied to a small 50-60kt low-
level jet that will traverse the area right on the leading edge of
the precipitation. As such, there isn`t a great deal of
juxtaposition between the instability and shear. Nonetheless, 30-
40kts of deep layer shear should still be realized which would be
enough to warrant a low end severe threat primarily for damaging
winds along and south of the I-20 corridor where the best moisture
and instability will be located. Will continue to hold off on
mention in the HWO/graphics, but a marginal risk area could
eventually be needed. Despite the progressive nature of this system,
strong forcing and 1.5-1.75" PWATs will result in locally heavy
rainfall. Latest 12z guidance has continued to indicate the
potential for a swath of 3-4" of rain in a short amount of time,
however exact placement of these amounts remains highly uncertain as
indicated by the large spatial spread in both GEFS and EPS ensemble
members. The flooding potential will need to be closely monitored
and a limited risk area could also be needed in the HWO/graphics.

Following the frontal passage, dry northwest flow will persist
across the ArkLaMiss as the weakening closed low opens into a
shortwave trough and phases with a stronger northern stream low over
the Great Lakes region. A few northwest flow showers could linger
through Monday and Tuesday before a reinforcing cold front and
associated showers clear out the lingering moisture Wednesday into
Thursday morning. Weak ridging is expected to gradually build into
the area towards the end of next work week as another upper low
begins to encroach on the West Coast. /TW/

&&

.PRELIMINARY POINT TEMPS/POPS...
Jackson       41  69  49  75 /   0   0   0   8
Meridian      40  71  47  76 /   0   0   0   4
Vicksburg     42  69  49  75 /   0   0   0  12
Hattiesburg   43  73  50  75 /   0   0   0   5
Natchez       42  70  49  76 /   0   0   0  17
Greenville    41  66  48  73 /   0   0   0   9
Greenwood     40  68  48  74 /   0   0   0   7

&&

.JAN WATCHES/WARNINGS/ADVISORIES...
MS...None.
LA...None.
AR...None.

&&


$$

28



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