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fxus64 kjan 271759 aaa 
afdjan

Area forecast discussion...updated
National Weather Service Jackson MS
1259 PM CDT Thu Jul 27 2017

..dangerous heat today before strong cold front moves in Friday &
Saturday...

Update...
updated for 18z aviation discussion

&&

Aviation...
18z taf discussion:
overall quiet afternoon & VFR conditions are expected across the
region today with winds mainly from the west-northwest around
5-8kts or so. A few rogue showers may be possible along the
Highway 82 corridor but not enough confidence to put into the
tafs. Expect a cold front to move towards the area early tomorrow
morning through tomorrow, which will bring some increasing chances
of -shra & thunderstorms in the vicinity near glh, gwo, gtr & cbm around 28/10-12z. These
showers & thunderstorms in the vicinity may move south and gradually dissipate but not
before bringing some chances for thunderstorms in the vicinity near hks, Jan, mei & nmm
around 28/17z or so. Any showers or storms could bring brief
reductions in ceilings &/or visibilities but conditions will
mostly remain VFR tomorrow. More storms may be possible to
redevelop along the cold front late tomorrow night in the north,
moving south, after the end of the taf period. /DC/

&&

Discussion...
main story in this forecast update continues to dangerous heat &
heat stress issues today. Anomalous deep mid-upper level ridging
(595-596dm @ 500mb) extends over the Southern Plains and eastward
into our northwestern areas with nearly 594-595dm 500mb heights
analyzed in the morning upper air analysis. Pws are dry across the
area with only around or less than 1.5 inches areawide so not
expecting any convection today. In addition, some isentropic
subsidence continues as drier air is filtering in down in the
lower levels of the thermal profiles in the local 12z soundings,
especially down to 850mb. We have also continued to warm around
the 20-21 deg c @ 850mb & 23-25 deg c @ 925mb. This will support
easily in the mid-upper 90s, just like yesterday. With dewpoints
holding up in the low-mid 70s, we should easily reach around the
95-98 degree range today, a degree or two warmer than yesterday.
The cold front analyzed to the north over the Central Plains &
into the mid-MS valley is having some significant high level
clouds but the 250mb upper ridging and isentropic subsidence
should keep only a few of these high clouds making it into the
Delta & Highway 82 corridor before dissipating, so this shouldn't
have any impact on high temperature forecasts today. Overall the
only potential to limit heat indices could be some mixing,
especially in the south & east, other than areas in the Pine belt
that local effects from extreme rainfall the other day could hold
up dewpoints some. Kept dewpoints only mixing to around 70 degrees
but wouldn't be surprised if some areas reach into the upper 60s,
which could limit heat indices & heat stress some. Overall
previous thinking looks good and kept the heat advisory going for
the same area & the severe weather potential statement/graphics the same. /DC/

Prior discussion below:

Today through tonight...

Dangerous heat and accompanying humidity will pose a threat to the
arklamiss today. A broad, steep, and stagnant upper ridge centered
over the Western Plains will continue to slowly build eastward into
the arklamiss as the day continues resulting in 500 mb heights
increasing to around 593dm. 850 temps will once again reach levels
just above 20 deg c throughout the region. High temperatures on
yesterday ranged from 94-97 degrees areawide and all indications
support a 1-2 degree increase areawide today. Models were aggressive
with dewpoints mixing down today, especially in the eastern portions
of the region where temperatures will near triple digits. Dewpoints
were adjusted to dampen the aggressive mixing scenario. This
resulted in increased forecasted Max heat indices over eastern
portions of Mississippi, thus the heat advisory was expanded
eastward. Expect heat index values to range from 105-109 throughout
most of the arklamiss except in the Pinebelt region where heat
indices will range from 100-104. Moisture will begin to increase
overnight ahead of an approaching cold front. The increasing
moisture and cloud cover will lead to a warm, muggy night.
Temperatures will only fall to the upper to mid 70's in the Delta
and the mid to lower 70's elsewhere. There is still some uncertainty
with the timing of the passage and precipitation associated with it,
however precip is expected to hold off until Friday morning. /Jpm3/

Friday through next Wednesday night,

The greatest weather impacts are expected during early portions of
the long range period, particularly from Friday into Saturday when
hot and humid conditions are expected to combine with a strong cold
front to bring the potential for a few rounds of storms capable of
producing marginal severe weather and localized flooding. The first
round of storms may approach northern portions of the arklamiss and
the Highway 82 corridor by daybreak Friday as a decaying convective
system moves into the forecast area. The effects of the morning
convection on subsequent convective development are unclear with
guidance showing a multitude of possibilities, but it does appear
at least that temperatures and heat stress will be reduced over
northern portions of the forecast area and have adjusted the
severe weather potential statement/graphic to account for this.

For now, have followed along with the idea that the early Friday
morning convection will dissipate in time to allow for renewed
convective development during peak heating over northern portions
of the area, but the greatest potential for very moist and
unstable air and severe weather/heavy rain potential may be
along/south of the I-20 corridor from late afternoon into Friday
evening. The threat for any intense convection should become more
confined Saturday to southern portions of the area where several
cams and model ensembles suggest another convective system may
move through, but this is highly conditional on prior convective
evolution, so confidence is quite low concerning the severe
weather/heavy rain threats for Saturday. Overall the primary
change to the previous severe weather potential statement/graphics was to distinguish the
threats for Friday and Saturday more.

Looking ahead, the strong front will push through the arklamiss
by Saturday night as an amplified upper level pattern and deep
trough become established over the eastern Continental U.S. Resulting in dry
northerly flow and unseasonably low relative humidity values from
Sunday through early in the week. As we go into the middle of next
week, the airmass should modify with greater moisture resulting
in increased diurnal convective precipitation coverage. /Ec/

&&

Preliminary point temps/pops...
Jackson 96 74 93 74 / 4 4 28 47
Meridian 97 73 92 73 / 6 3 32 55
Vicksburg 95 74 92 73 / 3 4 28 46
Hattiesburg 95 73 94 75 / 9 3 20 48
Natchez 94 73 90 74 / 4 3 23 46
Greenville 95 77 90 72 / 4 15 49 43
Greenwood 95 76 89 72 / 4 12 48 44

&&

Jan watches/warnings/advisories...
MS...heat advisory until 8 PM CDT this evening for msz018-019-025>063.

La...heat advisory until 8 PM CDT this evening for laz007>009-015-016-
023>026.

Arkansas...heat advisory until 8 PM CDT this evening for arz074-075.



&&

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