Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 240547
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1147 PM CST Fri Feb 23 2018

.UPDATE...
Issued at 1014 PM CST Fri Feb 23 2018

New 00z runs of NAM and GFS as well as hi-res models have all
shifted the heavy rainfall axis northward from what the day shift
saw into the heart of the STL metro area. Have expanded the FFA to
cover this new forecast and upped the rainfall amounts to around 2
inches thru Saturday evening.

Edged temps down a bit for most areas on Saturday with east flow,
thick clouds, and presence of pcpn, will be tough to rise much,
especially above MOS values.

TES

&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 307 PM CST Fri Feb 23 2018

Forecast issues continue involve the potential for freezing rain,
flooding, and severe weather over the next 24 hours.  Will be
issuing a winter weather advisory for tonight for a small part of
northeast Missouri.  Will also be expanding the flood watch to
include more of east central Missouri as well as southwest and south
central Illinois.

Large area of rain entering southern Missouri from Arkansas and
Oklahoma will continue to move northward late this afternoon and
this evening.  This will be in response to strong moisture
convergence on the nose of strengthening low level jet.  The rain
will become widespread from south to north across Missouri and
Illinois as ascent increases in the right entrance region of a jet
streak over the Great Lakes.  Forecast soundings over far northeast
Missouri late tonight into early Saturday favor freezing rain and
some sleet which could produce some light glazing as surface
temperatures fall to freezing or just below.  Will put a winter
weather advisory out for Knox and Lewis counties where I expect this
threat.

There will also be a chance of thunderstorms late this evening into
the overnight hours as elevated instability moves into areas along
and south of I-70.  The combination of abnormal high PWATS and
strong forcing will cause persistent moderate to occasionally heavy
rain to fall over the same areas, particularly over southeast
Missouri into south central Illinois.  The chance for thunderstorms
will increase on Saturday as an upper trough lifts out of the
central Rockies into the Midwest further increasing ascent over the
area.  An associated surface low will move from Kansas into the
western Great Lakes from Saturday into Saturday night. A surface
warm front will move north into the area which will act as a focus
for thunderstorms to develop on. These thunderstorms will likely
help enhance rainfall rates across the watch area on Saturday
afternoon and Saturday evening.

In addition to the flood threat, there is the potential for a few
strong to severe thunderstorms on Saturday afternoon and evening
across the southeast half of the CWA along and south of the warm
front.  The limiting factor will be the amount of rain and clouds
that will limit the surface based instability ahead of the cold
front that will move across eastern Missouri and southwest Illinois
during the late afternoon and early evening hours.  With this said,
the models are showing enough instability for thunderstorms to
develop as the cold front moves through, with ample low and deep
layer shear for a few organized severe storms.

Britt

.LONG TERM...  (Saturday Night through Next Friday)
Issued at 307 PM CST Fri Feb 23 2018

Sunday into Tuesday still looks dry as the upper trough moves off to
the east and a large surface anticyclone move across the area.
Temperatures will be above normal.  Then the GFS/ECMWF are showing
an impressive system moving into the central CONUS mid-late next
week.  This system will draw moisture northward from the Gulf
bringing a chance of showers by Wednesday.

Britt

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1147 PM CST Fri Feb 23 2018

Precipitation has nearly overspread all TAF sites and will reach
UIN imminently. Pcpn should be all rain except could mix with some
snow in UIN initially. IFR conditions have enveloped all TAF sites
except UIN and will likely reach there by 09z. After a break in
the rain for most of the morning and early afternoon, round two of
the rain will move back in during Saturday afternoon and exit in
the early evening. Round two of the rain will be more convective,
and have added in some VCTS to account for it for now. Light
NE surface winds will gradually veer E and strengthen by Saturday,
and further veer to SW and W during Saturday evening. Winds
notably will strengthen significantly with advent of SW to W
direction as the dry line/cold front passes and will see gusts to
25-30kts.

SPECIFICS FOR KSTL: IFR conditions will prevail until passage of
dry line/cold front Saturday evening. Steady rain will exit by
mid-morning with round two returning by late afternoon. Could see
some thunder nearby with the second round. After winds veer
gradually from NE to SE to SW by Saturday evening, look for
substantial strengthening with dry line/cold front passage with
gusts to 30kts.

TES

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch through Saturday evening for Crawford MO-Franklin MO-
     Gasconade MO-Iron MO-Jefferson MO-Madison MO-Osage MO-
     Reynolds MO-Saint Charles MO-Saint Francois MO-Saint Louis
     City MO-Saint Louis MO-Sainte Genevieve MO-Warren MO-
     Washington MO.

     Winter Weather Advisory from 3 AM to 9 AM CST Saturday for Knox
     MO-Lewis MO.

IL...Flood Watch through Saturday evening for Bond IL-Clinton IL-
     Fayette IL-Madison IL-Marion IL-Monroe IL-Randolph IL-Saint
     Clair IL-Washington IL.

&&

$$

WFO LSX



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