Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Jackson MS
1030 am CDT Wednesday Apr 23 2014

Update...earlier update removed mention of fog as it dissipated
rapidly after sunrise. Surface analysis has a 1023mb high centered
over northern Illinois ridging south over the lower Mississippi
River. Water vapor imagery shows a ridge aloft just to our west.
Both ridges surface and aloft will shift east this afternoon and
help maintain dry conditions over our County Warning Area under mostly sunny skies.
This will allow for warmer than normal temperatures. Hourly
observation temperatures continued to out pace forecast temperature
curves this morning so afternoon highs were increased a degree our
two. /22/

&&

Aviation...VFR conds prevail this morning and will continue through
the evening with nearly sky clear. Winds NE/east today at 5-10 knots. Models not
in good agreement with possibility of fog/status building in from
south 08z-14z but will hedge this direction. This could bring MVFR
to IFR conds to khbg/kmcb before VFR return by 14-15z Thursday and
winds pick up from the south. /Allen/

&&

Previous discussion... /issued 808 am CDT Wednesday Apr 23 2014/

Update...the dense fog advisory was canceled and weather grids were
updated to reflect latest fog distribution and latest temperature
trends.

Some fog remains across the Mississippi River valley and NE
Louisiana parishes this morning that at times has been dense but the
temporal scope of dense fog observations will not necessitate an
advisory in this area. Conversely...the dense fog advisory that was
in effect for southeast MS has been canceled as recent observations shown
the once dense fog dissipating quickly. Hourly temperature and dewpoint
trends were updated slightly with going trends. All updates are out.
/Allen/

Previous discussion... /issued 430 am CDT Wednesday Apr 23 2014/

Short term...today through Thursday night...it will be a foggy
start to the day in many places across the region...especially
across southeastern portions of MS. The weak front which moved
through yesterday failed to make a big push toward the coast and the
stalling and washing out of the feature in southern zones is
producing an environment of light winds and subtle moisture
convergence conducive for fog. We have hoisted a dense fog advisory
in effect through 9 am for areas roughly along and southeast of a
Brookhaven to Meridian line where fog should be thickest and less
patchy. In other areas where patchy dense fog will still be possible
we will allow the severe weather potential statement and graphics to cover risks.

After morning fog and low clouds burn off in the southern half of
zones anticipate a nice and warm day with winds generally 10 miles per hour or
less around highs around 80. Weak high pressure partly responsible
for the good weather today will be transitioning off to the east
tonight which will turn flow back to the south...although these
winds should remain rather light as well. Decent moisture not too
far to our south will be imported back north into southern zones
late tonight into early Thursday morning in the form of low stratus
clouds and perhaps a little fog. Low temperatures will be slightly
above average.

Tomorrow the bottom end of an fast-moving disturbance will be headed
at the region...increasing cloud cover and southerly winds. Wind
speeds and gusts will definitely be highest across the arklamiss
Delta although not to the level where a Wind Advisory would be
necessary. Despite scattered clouds temperatures should be able to top out
in the very warm middle 80s in some spots. A Pacific frontal system
coincident with the incoming trough axis should be kicking up
showers and thunderstorms to our west in the afternoon as moisture
tries to quickly recover up the axis of the MS river ahead of the
system. The best forcing with the system should be well to our north
and thermal middle level capping pushing north from the coast likely
prohibitive of storms south of I-20. Models have slowed down a
little with bringing the convection into the region...which is a
positive development if we are trying to minimize the severe weather
risk. But consensus still suggests 1000-1500 ml cape and sufficient
shear for a broken squall line or maybe even a supercell or two in
areas mainly along and northwest of a Winnsboro la...to Yazoo
City...to Eupora MS line in the late afternoon through middle evening
hours. The main risks should be hail and damaging wind gusts.
Activity should wane in intensity further east you go in the
forecast area with precipitation exiting well before daybreak Friday. /Bb/

Long term...Friday through Wednesday...broad ridging will be in
place by Friday morning following the passage of a cold front to the
north Thursday. As the end of the week progresses...this ridging
will become a little greater over the area which should result in
highs in the 80s...lows in the 60s and generally dry weather.

Our next big system will affect the region for the end of the
weekend into early next week. Early Sunday morning...a large upper
level low developing across the southwestern United States will
eject northeastward and take on a bit of a negative tilt. As this
occurs...a surface low will also develop on the leeside of The Rockies
and track with this upper low across the south Central Plains and
into Missouri by Monday. The European model (ecmwf) is a little more intense with
this surface low than the GFS on the 00z run...but both keep it
sub-1000mb. Ahead of this...moisture will increase across the region
with model precipitable water values around 1.6-1.8 inches depicted through Monday.
This will fall around the 99th percentile with regards to
climatology for this time of year...meaning this should lead to
efficient heavy rainfall. It appears that some locations could see
anywhere from 1-3 inches of rain(locally higher amounts) during the
Sunday through Tuesday time period. In addition...models show the
potential for multiple rounds of storms from about Sunday
afternoon/night through Monday before tapering off late Monday night
into early Tuesday as the front passes through. While the details
will certainly change over the course of the next several
days...parameters look good for all types of severe weather during
this time. Vertical totals increase to around 29-30c by Sunday
afternoon and instability wont be an issue as models bring middle 60s
dewpoints into the region...with even some 70 degree dewpoints. It
looks like the better instability/in terms of most unstable and
SBCAPE/ will come on Monday across the entire area with values
around 2500-3000j/kg. Plenty of shear will be available as low level
values will be around 30-40kts and deeper layer shear will be around
40-50kts. Will keep monitoring this over the next several days and
will remain mentioning this in the hwo/graphics.

The rain will exit the region during the morning hours Tuesday but
the large upper low and surface low will continue to spin across the
middle-Mississippi River valley region through at least the end of the
period. Both the GFS and European model (ecmwf) move it very slowly to the east
through the end of the week...leaving the southeastern United States
under large scale troughing. This will lead to cooler...below
normal...temperatures into the early days of may. /28/

&&

Preliminary point temps/pops...
Jackson 82 55 84 63 / 0 1 8 23
Meridian 82 51 84 60 / 0 1 8 26
Vicksburg 82 57 84 61 / 0 3 12 23
Hattiesburg 84 56 84 63 / 0 3 8 10
Natchez 83 59 82 63 / 0 5 8 11
Greenville 80 57 84 59 / 0 1 26 47
Greenwood 80 54 84 59 / 0 1 16 51

&&

Jan watches/warnings/advisories...
MS...none.
La...none.
Arkansas...none.
&&

$$

22/Allen/28