Area Forecast Discussion
Issued by NWS St. Louis, MO

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

294
FXUS63 KLSX 171755
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1255 PM CDT Sun May 17 2015

.SHORT TERM:  (Through Late This Afternoon)
Issued at 400 AM CDT Sun May 17 2015

Don`t believe any one piece of guidance has a very good handle on
the specifics of today`s convective trends.

There is good agreement in the hi-res guidance that current N-S
band of convection will be weakening as the morning progresses and
northern section of the line north of I70 has certainly weakened
over the last 1-2 hours. Part of the line over s MO is remaining
fairly robust attm as it is still encountering MLCAPES of nearly
1000 J/kg, but also expect this area to become less intense with
time as it outruns this area of instability.  So, general idea of
the morning forecast is for current activity to weaken and
diminish as it attempts to propagate eastward, and forecast
attempts to reflect the convection along the outflow boundary
diminishing as it propagates eastward, while larger area of more
stratiform rain in the wake of the line advects more north than
east and also slowly diminishes.

Lots of uncertainly surrounds afternoon thunderstorm potential.
Obviously the rain-cooled air from this mornings convection will
temporarily stabilize AMS over the area, but models do indicate a
fairly rapid destabilization occurring early this afternoon over
the northwest half of our CWA where mid-level dry slow is forecast
to push into the area, with little if any CIN by mid afternoon.
Although NAM instability forecasts are likely too high, even the
more conservative GFS is forecastiong MUCAPES of 2000-2500 J/kg. Explicit
hi-res models do not redevelop any convection this afternoon and
while low level convergence/forcing appears to be almost non-
existent, there is good agreement that tail end of shortwave
rotating around the upper low to our north will be pushing through
this area during max heating. Based on these trends will certainly
continue PoPs throughout the area during the afternoon, with max
PoPs (in the high chance category) in a narrow corridor from
central IL to approximately KVIH.  If convection can get going,
deep layer shear of around 40kts should support organized severe
potential. It would appear that primary threats would be wind and
hail, but a few tornadoes cannot totally be ruled out due to some
low level turning between the southerly surface winds and
unidirectional SW flow aloft.

Truett

.LONG TERM:  (Tonight through Saturday)
Issued at 400 AM CDT Sun May 17 2015

Certainly, the exact location of this afternoon`s convection will
likely cause some adjustment in tonight`s PoPs, but overall
forecast trends will continue to reflect the precip threat
gradually diminishing from NW to SE tonight into Monday evening.

Despite very low MOS PoPs from central MO into central IL for
tonight and Monday have fought the urge to drop the mention of
showers and thunderstorms in these areas until the actual fropa
occurs later in the day on Monday. Have also tried to give PoPs a
bit of a diurnal flavor over southern areas, with higher values
during the afternoon during max instability.

Still appears some cool May weather is in the offing for the
middle of the upcoming week. True arrival of the cooler air
on Tuesday should send max temps into the 60s and lower 70s. Even
cooler temps are forecast by Wednesday as sprawling surface ridge
builds into the mid-Mississippi Valley while overrunning produces
a large area of overrunning rainfall in this dome of Canadian air,
and some locations over our NW counties may struggle to hit the 60
degree mark.

Temperatures will then moderate heading into the latter part of
the week, with another threat of showers and thunderstorms by next
weekend as return flow spreads warm, moist, and unstable air back
into the region.

Truett
&&

.AVIATION:  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1255 PM CDT Sun May 17 2015

Morning activity has diminished with redevelopment possible by
later this afternoon if skies can clear out enough and allow
atmosphere to become unstable once again. For now have VFR
conditions with gusty south to southwest winds near 25 kts at
times diminishing towards sunset. Timing and coverage hard to pin
down, so just kept VCTS mention through sunset, with more storms
possible just ahead of cold front late tonight. Front to move
through KUIN by 12z Monday, KCOU by 14z Monday and by 17z Monday
for STL metro area.

Specifics for KSTL:
Morning activity has diminished with redevelopment possible by
later this afternoon if skies can clear out enough and allow
atmosphere to become unstable once again. For now have VFR
conditions with gusty south to southwest winds near 25 kts at
times diminishing towards sunset. Timing and coverage hard to pin down,
so just kept VCTS mention through 01z Monday, with more storms
possible just ahead of cold front after 09z Monday. Front to move
through metro area by 17z Monday with winds veering to the west to
northwest.

Byrd
&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX






USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.