Area Forecast Discussion
Issued by NWS Jackson, MS

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FXUS64 KJAN 251711 AAB
AFDJAN

Area Forecast Discussion...UPDATED
National Weather Service Jackson MS
1211 PM CDT Sat Mar 25 2017

.UPDATE...
Updated for 18Z aviation discussion

&&

.AVIATION...
18Z TAF discussion:
Ceilings will rise to VFR during the afternoon, but isolated
thunderstorms will still pose a risk of lower ceilings and
visibilities at impacted TAF sites. While the majority of the rain
will move east of the area tonight, low clouds and fog will
develop by late evening and be responsible for MVFR/IFR
conditions, especially after midnight. Conditions will improve
back to VFR by 26/15Z./26/


&&

.DISCUSSION...
Squall line about to exit CWA at the moment, but other convection
beginning to fire west of the river. This should be the trend for
the rest of the day as areas behind the convective line clear and
begin destablizing where the sun can get on the ground. Latest
HRRR showing convection refiring around the I55 corridor around
noon and intensifying the further east it gets. Will update the
HWO to reflect best severe threat in the east for later this
afternoon./26/

Prior discussion below:

Sunrise update:

Did some tweaks to the forecast to speed up timing of main north
to south oriented band of convection progressing from the I-55
corridor (where it is currently) to mainly east of the CWA by
early afternoon. Still expecting at least some showers and storms
to form in the wake of the main line as we go through the late
morning and into the afternoon hours. Severe thunderstorm watch
#84 was canceled an hour or two ago and the current main band of
convection is sub-severe, although it certainly has potential to
re-intensify over eastern portions of MS later this morning. Of
course any storms later today in the wake of the main band of
convection will have access to more instability and could also be
severe. /BB/

Previous Main Discussion:

The squall line of thunderstorms we have been monitoring all night
currently extends roughly from GWO...to TVR...to HEZ and continues
to press east (albeit a little more slowly than before).
Noticeable lack of bowing segments (and associated severe
warnings) in our region at present is due to instability
decreasing toward the heart of the region, as was anticipated.
Latest consensus of hi resolution model guidance suggests after
this squall line pushes past the I-55 corridor after sunrise some
heating (and thus boosted instability) over eastern MS could re-
invigorate the line before largely clearing east of our region in
the 2 to 4 pm time range window. The main risk with any of these
severe storms in this time range will be damaging wind gusts and
hail, although there should still be enough low level shear to not
totally discount the prospect of a tornado.

Of course there is still uncertainty on whether a surface trough
lying back west of the current squall line will have enough
atmospheric fuel to trigger an additional severe storm threat
midday through early evening along and east of the MS River in
the wake of the exiting squall line. Latest consensus is now
suggesting odds of more than a few potent cells in the wake of
main squall line is not overly high, but if any isolated cells do
gain considerable strength and organization they may contain a
greater hail threat due to steeper lapse rates aloft moving in
from the west. The current severe weather threat advertised by the
storm prediction center kind of hinges on potential for more cells
developing in the wake of the main line and, considering trends,
the main risks may be focused further east with the next update
from them. For now we will try to relay subtleties of timing and
uncertainty in the HWO/graphics.

By this evening we expect isolated to scattered showers and at
least a few storms to be streaming from southwest to northeast
over central and southeastern areas as main upper lift departs but
a lingering boundary drifts down into these areas and provides a
triggering mechanism. Confidence is high any remaining strong
storms after sunset should be decreasing in intensity before
midnight. This stream of a bit of convection could continue into
Sunday morning in these areas before stopping as short wave
ridging moves back into the Lower Mississippi Valley. Otherwise,
Sunday should feature scattered clouds and sun with much warmer
temperatures. On Sunday night a few showers and a stray storms
could again form, mainly late, in northern to far western zones as
the next disturbance in the active pattern approaches from the
west.

This next disturbance looks to arrive during the day Monday with
associated surface low pressure and frontal features mainly
focused north of our region. Despite the latter, we do expect a
good deal of thunderstorm activity to develop across our region
midday through early evening owing to at least moderate
instability (MLCAPE values of 1500+ j/kg) manifesting from surface
warmth and steep atmospheric lapse rates. Wind shear will be
moderate at best, but should be sufficient for supercell
structures and perhaps convective line segments with damaging wind
gusts. It is hard to completely rule out a tornado but convective
threats on Monday should mainly be hail and high winds (and there
are some signals of hail to golf ball size or greater could be in
the cards with some storms, at least in the northern half of
zones). The Storm Prediction Center currently is advertising a
slight risk of severe thunderstorms for central and northern zones
and there is decent chance this risk could be upgraded for some
spots in later outlooks.

Only lingering and sub-severe convection is anticipated after
Monday evening and, once again, no big cold front will be coming
through to stabilize the atmosphere. A good deal of Tuesday into
Wednesday will likely pass with only insignificant precipitation
as we will be in between systems. However, the break from the
active train of southern stream systems will not last long and a
big chunk of energy should be pushing into the region again by
late Wednesday or early Thursday. The associated frontal system
with this system will probably pass closer to our region this time
and impart more lift and wind shear on the atmosphere. Instability
could reach the moderate range (depending on timing) and the
potential juxtaposition of this instability and decent wind shear
is concerning. Currently we are advertising good chances for
thunderstorms late Wednesday through Thursday night, however due
to timing uncertainties and being in the long range of the
forecast we are still holding off on mentioning a risk of severe
weather at this juncture. Expect the HWO/graphics to probably
begin including a risk of severe storms in this time range once
the mentioned timing uncertainties can be ironed out. Otherwise,
in the wake of this system there could be one, maybe two, days of
relatively quite weather before the next big weather maker arrives
on our doorstep during next weekend. /BB/

&&

.PRELIMINARY POINT TEMPS/POPS...
Jackson       71  60  82  63 /  56  40  10   9
Meridian      70  61  83  63 /  75  51  21   9
Vicksburg     74  57  84  63 /  66   8   5  10
Hattiesburg   71  63  84  64 /  75  35  18  11
Natchez       75  60  84  64 /  57  21   6  10
Greenville    71  52  80  62 /  23   3   4  17
Greenwood     69  58  79  61 /  76  16   5  13

&&

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

&&


$$



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