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FXUS63 KLSX 052314
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
614 PM CDT Fri Jun 5 2020

.SHORT TERM... (Through Late Saturday Night)
Issued at 235 PM CDT Fri Jun 5 2020

Early this afternoon, a MCV was located over southwestern Illinois
tracking east-southeastward and was agitating the cloud cover around
it. An earlier MCS had tracked into our forecast area this morning
and since mostly dissipated with the most active portion having
tracked well south of the area into northern Mississippi. The
atmosphere was less unstable in our region as a result of the cooler
air in place, with temperatures just now pushing into the 80s. A
surface cold front extended from near the Quad Cities area into
extreme northern Missouri, but was quiet along it, with little in
the way of convective cloud development and plenty of debris cloud
from the MCS over our region. Otherwise, an upper level RIDGE axis
has moved into the southern Plains and was beginning to amplify over
the past several hours, while "Cristobal" has unmoored from the
Yucatan Peninsula and drifted back into the southern Gulf of Mexico
as it strengthened back into a Tropical Storm.

The MCV will exit our forecast area during the late afternoon hours
and may kick up a brief shower before exiting. Otherwise, a dry
evening is expected as the upper level RIDGE will continue to
amplify through Saturday as the axis slides east, eventually
building overhead.

For tonight, much of the night looks quiet weather-wise, and the
signal for another nocturnal MCS developing well northwest and
racing into our area late has weakened but not entirely disappeared.
The overall pattern supports this scenario with the baroclinic zone
weaker and also sliding further northeast. A cold front also
sliding into our area from the north tonight and eventually
orienting more "backdoor" style on Saturday may also help sustain a
low PoP threat late tonight. This window of opportunity for rain,
if realized, would then continue into Saturday morning. Kept low
mentionable PoPs of 20-30%, which matches CLIMO for this time of
year. Otherwise, a dry period is expected to begin no later than
Saturday afternoon and continue into the beginning of the LONG TERM
section.

The cooler airmass that does build in for Saturday will not be a
huge impact, but will cut into what temperatures could otherwise
warm into given the very warm pattern in place. The easterly low
level flow should keep 90s at bay for most of the forecast area, but
temps will still remain above normal. Afternoon heat index values
should remain below 100F, with the highest in central and southeast
MO where the highest ambient temps are expected to exist.

TES

.LONG TERM... (Sunday through Next Friday)
Issued at 235 PM CDT Fri Jun 5 2020

The strong consensus continues to exist between the GEFS and EC ENS
H500 mean on the main features through next week. The amplified
upper RIDGE axis will be overhead to begin this period on Sunday
with this feature translating east to the eastern CONUS by Tuesday.
This represents a speed up of the departure of the RIDGE from what
was advertised 24hrs ago. The main takeaways from this through the
middle of next week are: (1) very little change on the time window
of influence from a cooler airmass from the Great Lakes, with
maintenance of a weak easterly flow at the low levels through Monday
to keep rain chances and hot temperatures at bay; (2) the
remnants of "Cristobal" are still expected to be steered through our
region but the preferred window of time for high rainfall probs
moves to late Monday night and Tuesday with anticipated impacts
towards the backend of that window possibly continuing into Tuesday
night.

Kept rain chances close to nil Sunday and Monday while undercutting
max temps below 90F in most places.

Late Monday night and Tuesday is now the preferred time window for
rainfall associated with "Cristobal" with 2-4" expected with locally
higher amounts. The tropical nature of the atmosphere during this
time will severely limit thunder probs while keeping a very
efficient rate of rainfall for convective showers. Again, if this
event is realized, it will be the earliest impact by a tropical
storm in this area in recorded history. Max temps should be below
persistence on Tuesday with what could be a pair of very mild nights
Monday night and Tuesday night.

In the wake of "Cristobal", ensemble means show the edging of the
base of an upper TROF followed by a surface cold front late next
week. This would favor a temperature rebound with an overall
unfavorable pattern for rain (below CLIMO--23-25%) before at or
above CLIMO PoPs on rain associated with the cold front late week.

TES

&&

.AVIATION... (For the 00z TAFs through 00z Saturday Evening)
Issued at 557 PM CDT Fri Jun 5 2020

A cold front extending across the MO and IA border east through
northern IL, just north of UIN will move southward through the taf
sites tonight and Saturday morning. Light surface winds tonight
will become east-northeasterly Saturday morning. Mainly just high
level cloudiness is expected through the forecast period with a
few diurnal cumulus clouds Saturday afternoon.

SPECIFICS FOR KSTL: A cold front extending across the MO and IA
border east through northern IL, just north of UIN will move
southward through STL late tonight and Saturday morning. Light
surface winds tonight will become east-northeasterly Saturday
morning. Mainly just high level cloudiness is expected through the
forecast period with a few diurnal cumulus clouds Saturday
afternoon.

GKS

&&

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

$$

WFO LSX
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