Area Forecast Discussion
Issued by NWS St. Louis, MO

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241
FXUS63 KLSX 220258
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
858 PM CST Wed Feb 21 2018

.UPDATE...
Issued at 858 PM CST Wed Feb 21 2018

At 8pm, the 32F isotherm extended from near Springfield, Illinois
southward to Metro East Saint Louis and to near Chester, Illinois.
This places much of STL metro and Missouri in the sub-freezing air
at the surface, and thus, vulnerable to freezing rain/drizzle. This
line has only moved slightly west over the past 3-4 hours.

Solid steady precipitation has developed over southeastern MO and
southern IL since 6pm and is starting to make a move northward
towards STL metro. This will represent a steadier, slightly
heavier, precipitation from the drizzle much of the STL metro area
has already received. This precipitation is expected to maximize
its extent towards midnight into much of the STL metro area before
retreating and exiting to the east by 3am.

This does NOT mean the event is over. The latest HRRR/RAP/NAM
continue to indicate development of new pcpn around central MO
shortly after midnight and extending eastward into the STL metro
area and Quincy by 3am and continuing thru the morning rush before
retreating/exiting to the northeast by mid-late morning.

Temperature trends overnight all indicate an ever so slight
warming because, in part, to the moisture push as winds become
more easterly, and this should also be as a result of latent heat
release from the freezing pcpn as ice develops. Look for temps to
rise to or above freezing in the STL metro area from east to west
very slowly thru mid-morning Thursday with relief only occurring
to central MO and the Quincy area when pcpn ends by mid-late
morning with a very slow temp rise anticipated and a cooler start
at those locations.

Headlines still look good, but see the need for an early
cancellation overnight for a sliver of the current area in the
east and areas westward during the morning Thursday but everything
is just so close right now as to justify continuing current
expiration at this time.

TES

&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 331 PM CST Wed Feb 21 2018

Freezing rain continues this afternoon across parts of southeast MO.
Precipitation will spread through southeast MO and southwest IL this
evening as another surface wave moves northeastward along the
stationary front that extends from LA northeast through eastern KY.
The precipitation should move through the St Louis metro area late
this evening.  This precipitation should shift east of our forecast
area by 12Z Thursday.  The precipitation type should be initially
freezing rain, but may change to liquid rain late tonight as the
surface temperature gradually warms above the freezing mark. Another
area of precipitation will move northeastward through mainly
northeast MO and west central IL late tonight and Thursday morning
in a region of low-mid level warm air advection ahead of a southwest
flow shortwave.  With an elevated warm layer over the subfreezing
surface layer the precipitation type may be initially a mix of snow
and sleet, but will change over to freezing rain.  Surface
temperatures in this area will not warm above the freezing mark
until late Thursday morning. Will continue the winter weather
advisory this evening for central and southeast MO into parts of
southwest IL, then for most of our forecast area for late tonight
and Thursday morning. Did not see much precipitation left for late
Thursday morning and afternoon, plus by this time the temperatures
across the entire area should rise above 32 degrees. Much of the
forecast area will receive a glazing of ice accumulation, possibly
up to one tenth inch in isolated spots. Northeast MO may pick up a
light dusting of snow and sleet late tonight.

GKS

.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 331 PM CST Wed Feb 21 2018

(Thursday Night - Saturday Night)

Very active 48 hours still on tap from Thursday night through
Saturday night with multiple rounds of rainfall and increasing
chances of thunderstorms Friday night and especially on Saturday.
First wave of rain still looks to be Thursday night as an upper-
level disturbance interacts with quasi-stationary front to the south
and east of the area. Still appears to be a rather quick hit of
rainfall across much of the area with heaviest amounts focused
across portions of southeast Missouri and southwest Illinois close
to the frontal boundary. Fairly broad/weak 850-hPa low-level jet and
a lack of elevated instability suggest rainfall rates should stay
on the light to moderate side. Continue to believe this particular
round of rainfall will likely not cause too much in the way of
problems, but will likely continue to saturate soils and further
elevate some area rivers and streams. Temperatures Thursday night
are expected to stay above freezing across the entire area, so not
expecting any freezing rain.

Some good news on Friday into Friday evening as bi-state area should
be dry behind departing shortwave trough. Surface frontal boundary
will also slide back southeastward into the lower Mississippi and
Ohio River Valleys with surface high pressure across the northern
Plains exerting its influence into the area. Highs on Friday will be
on the mild side for February in the 40s and 40s.

Low-level warm/moist advection intensifies across the area Friday
night as a stronger shortwave trough begins to eject out of the
Rocky Mountains. Widespread rainfall is expected across the bi-state
region north of the developing warm front. Some elevated instability
so continue to mention thunder chances across southern portions of
the forecast area. Stronger low-level forcing, presence of weak
instability aloft, deep warm cloud depths, and precipitable water
values 200-300 percent of normal for late February suggest the
potential for heavy rainfall rates. Area of biggest concern for
Friday night and into Saturday will likely be on portions of
southeast Missouri and southwest Illinois where 2-3 inches of rain
are forecast Thursday night through Saturday evening.

Attention then on Saturday afternoon and early evening will be how
far north warm front reaches and track of surface low across the
Missouri Valley. Models have trended a bit slower and further
northwestward with track of aforementioned surface low. Increased
probability of thunderstorms across much of CWA as appears warm
front will lift to near central portions of the area by mid
afternoon on Saturday. Very strong deep-layer shear strong dynamics
aloft suggest an increasing threat for severe weather within the
warm sector. Major unknown is how far north warm front gets and how
quickly as well as widespread rainfall Friday night and early
Saturday morning may slow its northward progress. In addition,
degree of destabilization within the northern portion of the warm
sector is unknown due to forecast low cloud cover and potential
shower/thunderstorm activity continuing through the day on Saturday.
Will have to keep a close eye on this period moving forward given
the very strong dynamics. Probably will not take too much
instability to potentially yield severe thunderstorms.


(Sunday - Next Wednesday)

Broad area of surface high pressure will filter into the region on
Sunday through Monday. Expecting a welcome break from the rain along
with temperatures remaining slightly above normal day and night.

The next chance of rain appears to be on Tuesday night and next
Wednesday as another upper-level disturbance ejects out of the
desert southwest.

Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Thursday Evening)
Issued at 550 PM CST Wed Feb 21 2018

Deteriorating conditions will spread back into the TAF sites this
evening. Beginning with FZDZ and IFR CIGs at STL metro sites and
this will expand north and west overnight and Thursday morning.
Look for IFR conditions to prevail for much of the day on Thursday
and into Thursday night. Freezing pcpn will transition to all rain
late tonight or Thursday morning, as warmer temps build back in.
NE surface winds will veer E and SE by Thursday night.

SPECIFICS FOR KSTL: Could toggle some between MVFR and IFR CIGs
but believe IFR will eventually win out and prevail for much of
the valid period. Pcpn already in place will become -RA overnight
as warmer air builds in and will lessen or be more intermittent
after sunrise.

TES

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Winter Weather Advisory until noon CST Thursday for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
     Gasconade MO-Iron MO-Jefferson MO-Lincoln MO-Madison MO-
     Moniteau MO-Montgomery 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 midnight tonight to noon CST
     Thursday for Knox MO-Lewis MO-Marion MO-Monroe MO-Pike MO-
     Ralls MO-Shelby MO.

IL...Winter Weather Advisory until noon CST Thursday for Calhoun IL-
     Greene IL-Jersey IL-Macoupin IL-Madison IL-Monroe IL-
     Montgomery IL-Randolph IL-Saint Clair IL.

     Winter Weather Advisory from midnight tonight to noon CST
     Thursday for Adams IL-Brown IL-Pike IL.

&&

$$

WFO LSX



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