Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 251227
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
627 AM CST Wed Feb 25 2015

.SHORT TERM:  (Through Late This Afternoon)
Issued at 353 AM CST Wed Feb 25 2015

Split flow upper level pattern feature two systems. A nthrn wave
that will begin affecting the FA later this aftn into Thu mrng and a
stronger sthrn stream storm. The sthrn energy is expected to track
from the Red River Valley today into the sthrn Appalachians
tonight. This path will keep the effects from this feature well to
the south of the CWA...though will be a significant impact for
locations from NE TX east into the Carolinas. The nthrn system is
dropping SE out of the Dakotas this mrng with a SFC low in along the
NE/SD border as of 9Z with a bndry extending SE across sthrn
IA...nthrn IL/IN and arcing NE in Canada. The bndry is expected to
slowly sag south thru the day with the low tracking just to the SW
of the bndry...which takes it from ern NE this mrng to somewhere
across far ern KS or wrn MO by 00Z this evng. The SREF and ECMWF
appear to be wrn outliers. This system will be in a weakening mode
as it drops SE thru the region overnight. Overall speed of the system
seems to have slowed some over the past 24 hrs and as such have backed
off on increasing PoPs across the nthrn zones until late this
aftn. The slower system also means thicker cloud cover will be
delayed allowing temps to rise a bit more than previously thought.
Temps may be marginal for snow at the onset of the event as
well...allowing for a possible rain snow mix. Therefore...have
significantly lowered QPF and snow accums prior to 00z and have
them confined primarily along and north of US HWY 36 with snow
accums of less than an inch. Have also delayed the start time of
the ADVY til 21Z due to the above reasoning. Added Brown and Pike
Counties in IL to the ADVY and now have it ending at 15Z
Thu...instead of 12Z.

2%

.LONG TERM:  (Tonight through Tuesday)
Issued at 353 AM CST Wed Feb 25 2015

Clipper system centered over west central Missouri will continue
to slide to the southeast tonight...with colder air filtering in
from the north. Because of slightly warmer temps over central MO
could still see some mixed rain and snow over this region for a
few hours after 00z Thursday. Otherwise, heaviest snowfall will be
across northeast MO and west central IL. Even though system
weakens as it exits region, will see lingering light snow through
at least 18z Thursday before tapering off. Storm total numbers
will range from 3 to 4 inches far north to less than an inch
southeast of the St. Louis metro area.

Colder air to filter into the region on Thursday, so will have
non-diurnal temps, with mid morning highs most locations, then
either steady or falling temps through the afternoon hours. Thursday
night will be our coldest night as fresh new snowfall, light north
wind and clearing skies allow temps to fall below zero for areas
north of the I-70 corridor. Rest of forecast area will see lows in
the single digits above zero. As for wind chill indicies,
approaching advisory criteria for northeast MO and west central IL,
but it`s too early to issue an advisory at this time.

Beyond that, the extended models continue to have timing, placement
and strength differences with the prolonged period of precipitation
this weekend and into early next week. Best chances of precipitation
continues to be the Saturday night/Sunday timeframe and Monday
night/Tuesday timeframe. Continue to check back frequently for
updates to this event.

Byrd

&&

.AVIATION:  (For the 12z TAFs through 12z Thursday Morning)
Issued at 608 AM CST Wed Feb 25 2015

IFR conditions assoc with snow after 00Z are the primary concern.
An area of low pressure will weaken as it moves from ern NE to SW
MO today spreading snow across the terminals overnight. The next
12 hrs should be quiet with light erly flow and gradually
increasing mid/high clouds. Snow is expected to move into nthrn MO
and w cntrl IL around 00Z and CIGs/VSBYs should quickly drop with
the onset of precip. It is possible that there may be a mix of
ra/sn for a brief prd when precip begins...esp across NE MO but is
expected to transition to snow by evng. Snow will spread south
thru the evng...lasting thru 12Z...except at KUIN where the event
should be winding down. Since IFR conditions are 12-18 hrs
out...tried not to get too specific with CIGs/VSBYs. Am confident
both elements will be lower at some point drng the heaviest
snowfall. Nrly winds begin to ramp up towards the end of the prd.

Specifics for KSTL:

VFR for the next 18hrs with light erly flow and gradually
increasing mid/high clouds. Snow should dvlp shortly after 6Z with
CIGs/VSBYs dropping to IFR quickly once snow begins. Tried not to
get too specific with CIGs/VSBYs since the event is 18+ hrs out.
Am confident that both elements will be lower at some point drng
the heaviest snowfall. Winds increase after 12Z Thursday as the
SFC low pulls away strong high pressure builds into the region
from sthrn Canada.

2%

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...WINTER WEATHER ADVISORY from 3 PM this afternoon to 9 AM CST
     Thursday FOR Knox MO-Lewis MO-Marion MO-Monroe MO-Ralls MO-
     Shelby MO.

IL...WINTER WEATHER ADVISORY from 3 PM this afternoon to 9 AM CST
     Thursday FOR Adams IL-Brown IL-Pike IL.

&&

$$

WFO LSX






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