Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 290258
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
958 PM CDT Tue Mar 28 2017

.UPDATE...
Issued at 913 PM CDT Tue Mar 28 2017

Latest NAM and HRRR model guidance is a little slower bringing
showers into our forecast area late tonight/early Wednesday
morning. It appears that showers will move into central MO around
12Z Wednesday morning, then move into northeast MO and west
central IL later in the morning, mainly north and west of STL as a
southerly low level jet brings increasing temperature and
moisture advection into this area. Lows tonight will be slightly
above normal for late March with a low level cloud cover along
with gradually increasing east-northeasterly surface winds as the
surface pressure gradient tightens.|

GKS

&&

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 327 PM CDT Tue Mar 28 2017

Clouds have stayed locked in over the area as the clearing over east
central Illinois that was working into the eastern CWA filled back
in with stratocumulus this afternoon.  Forecast soundings do show
that the low levels will stay saturated through the night, with an
increase in mid-high clouds as upper low currently over New Mexico
moves northeast into the Plains.  Showers and isolated thunderstorms
will move into the area late tonight per the GFS and NAM as the
upper ridge moves east and a band of strong moisture convergence
sets up on the nose of a 40kt low level jet.  This low level forcing
combined with increased mid-upper ascent from approaching low will
be enough to set up one round of showers and scattered thunderstorms
late tonight and tomorrow morning.  Then there may be a break during
the late morning and early afternoon before both models show another
round of showers and thunderstorms moving into central Missouri by
late afternoon.

Went closer to the GFS MOS temperatures over the next 24 hours.

Britt

.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 327 PM CDT Tue Mar 28 2017

Primary concern heading into Wednesday night is the potential for
severe weather. Guidance is in good agreement that there will be
thunderstorms over western Missouri at 00Z moving toward our area.
Forecast instability falls of pretty quickly in the evening with
MUCAPE values not exceeding 500 J/Kg on the GFS until after 09Z.
NAM has more energy in the evening pushing 1000 J/Kg, but it tends
to be a bit too high, and even with that high bias MUCAPE values
fall off overnight. Regardless...with 50-60kts of deep layer
shear, can`t say there won`t be severe storms; but the threat
should stay over our eastern Ozark counties where instability will
be greatest. Should be a better chance for severe storms on
Thursday ahead of the low. SBCAPE increases to around 2000 J/Kg
during the afternoon. Deep layer shear isn`t as impressive
initially at 18Z but increases to 40+ kts along and southeast of
I-44 and south of I-70. 0-1KM helicity is less than 100 m2/s2 for
the afternoon hours on Thursday so think the primary threats will
be hail and damaging wind rather than tornadoes.

Thunderstorm threat will be coming to an end Thursday night although
rain will likely continue on the west side of the low as it passes
for a good portion of the night.  Cooler and drier air will move
into the area on Friday and Saturday.  Overnight lows will dip into
the upper 30s and 40s with daytime highs mainly in the upper 50s to
mid 60s.  There is a chance there could be some rain on Sunday ahead
of the next system as the upper level shortwave digs into the
southern Plains. However the GFS and ECMWF disagree on how much
QPF to print out so have reduced PoPs for Sunday until we see a
little better agreement. The system lifts out of the Southern
Plains into the Mississippi Valley on Monday so rain chances for
early in the week look good. The GFS and ECMWF can`t agree on a
track for the surface system just yet with the GFS being further
north and therefore warmer than the EC. Think the current hedge
between the two with highs mainly in the 60s and lows in the mid
40s to low 50s looks good.

Carney

&&

.AVIATION...  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 536 PM CDT Tue Mar 28 2017

Latest satellite loops and observations depict some breaks in the
MVFR cloud deck along with a gradual rise in the cloud ceiling
height which may lead to temporary improvement to VFR conditions
later this evening, but this will be short lived as guidance
indicates prevailing MVFR conditions will return late tonight and
eventually down to IFR early Wednesday morning. It appears that
showers will move into the COU area by 12Z Wednesday and into the
rest of the taf sites later in the morning as a southerly low
level jet brings increasing low level moisture into the area ahead
of an approaching low pressure system over the southern Plains.
East-northeast surface winds will strengthen late tonight and
Wednesday morning as the surface pressure gradient tightens.

SPECIFICS FOR KSTL: Latest satellite loops and observations depict
some breaks in the MVFR cloud deck along with a gradual rise in
the cloud ceiling height which may lead to temporary improvement
to VFR conditions later this evening, but this will be short lived
as guidance indicates prevailing MVFR conditions will return late
tonight and eventually down to IFR Wednesday morning. It appears
that showers will move into the STL area Wednesday morning as a
southerly low level jet brings increasing low level moisture into
the area ahead of an approaching low pressure system over the
southern Plains. Northeast surface winds will strengthen late
tonight and Wednesday as the surface pressure gradient tightens.

GKS

&&

.PRELIMINARY POINT TEMPS/POPS...
Saint Louis     47  63  56  71 /   5  60  90  80
Quincy          42  55  50  61 /   5  70  90  80
Columbia        47  60  54  67 /  40  70  90  70
Jefferson City  49  62  55  68 /  30  60  90  70
Salem           48  64  56  72 /   0  30  70  90
Farmington      50  64  58  71 /   5  50  90  80

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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