Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 171720

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1220 PM CDT Mon Jul 17 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 325 AM CDT Mon Jul 17 2017

Backdoor cold front in large part responsible for yesterday`s
shower and thunderstorm activity is currently approaching a
KCDJ>>KBLV>>KMVN arc. This boundary`s south/southwest progress
will slow a bit during the day today, as well as largely wash out
by the evening hours. Isolated to widely scattered showers and
storms are expected to fire along this boundary early this
afternoon, with the main focus being across southeast Missouri. Am
not expecting quite as much coverage as that of yesterday due to
weaker convergence along front in addition to some subsidence at
mid/upper levels of the troposphere.

Highs today will be several degrees cooler than that of yesterday
behind the backdoor cold front, though still near to slightly above
normal for the middle of July. Leaned slightly above MAV/MET
guidance generally for high temperatures today, especially in the
St. Louis metro area where a distinct cool bias has been noted since
the beginning of July.

Any remaining isolated shower and thunderstorm activity should
quickly wane this evening. Rest of the night is expected to be dry
with a mostly clear sky and a light east/southeast wind. MOS low
temperatures looked reasonable tonight, though did lower a couple of
degrees from MOS numbers across the eastern Ozarks where lightest
winds should reside.


.LONG TERM...  (Tuesday through Sunday)
Issued at 325 AM CDT Mon Jul 17 2017

The main story continues to be the major heat wave forecast to take
a grip on the Nation`s midsection this week.

Tuesday will mark the onset of dangerous heat as an expansive upper
level high becomes established over the central U.S. and warm front
retreats across the CWA. As discussed yesterday, the retreat of the
warm front is a bit slower than was advertised several days ago.
Present indications are the front will be draped through
northwest- central-southeast MO into southern IL at daybreak
Tuesday and become more north-south oriented and be roughly near
the MS River by early Tuesday evening. Temperatures will still
rise into the 90s on Tuesday and moisture/Td pooling along the
front will aid in producing heat indices over 100+. The highest
readings should be in metro St. Louis where it seems you can never
forecast warm enough temperatures these days.

The massive upper high will continue to strengthen and expand
eastward on Wednesday-Thursday with the high center in the vicinity
of eastern KS by early Wednesday evening, and in the mid MS Valley
on Thursday. West-southwesterly lower tropospheric flow and a
pronounced thermal ridge are ideal for big time heat. Despite some
weakening of the upper high and east-southeast shift into the TN
Valley region Friday-Saturday, the lower tropospheric west-southwest
flow and thermal ridge remain well intact and the intensity of the
thermal ridge actually increases. Present thinking is the worst
conditions and most oppressive/deadly heat will be in the Thursday-
Saturday time frame when highs will be in the 97-103 range and
afternoon and early evening heat index values will range from 105-
113. The worst conditions within the CWA should be within the metro
St. Louis area where the urban heat island exists and highs will be
routinely in the low 100s.

The current excessive heat watch looks on target with the main
impacted area where we are expecting at least 4 days of heat index
values of 105+. We will keep the watch in place with this forecast,
and expand it across the remainder of northeast MO and west central
IL where these conditions are expected to evolve. The watch will
likely be converted to an excessive heat warning within the next 24
hours. Adjacent areas of southern MO and southern IL where dew
points and heat index values are expected to be lower will
ultimately require a headline later this week, probably a heat

The guidance still remains varied on when the heat wave will break
and when we will finally see a cold fropa. The ECMWF suggests this
will occur Sunday/Sunday night with the GFS lagging by about 24
hours. Confidence is low at this point on favoring any particular



.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1210 PM CDT Mon Jul 17 2017

VFR flight conditions are expected to prevail across most of the
area today and into tonight. An area of MVFR and IFR stratus/fog
over northeast Missouri is dissipating, and should be up to VFR
within the next couple of hours. Additionally, there is a weak and
diffuse front stalled over eastern Missouri. This front will
provide a focus for thunderstorm development today...and a few
storms have already popped up over southeast Missouri and
southwest Illinois. Expect this activity to continue this
afternoon...possibly spreading northwest into parts of central and
northeast Missouri. Expect any storms that form to weaken and
dissipate during the evening.

Guidance is hinting at fog late tonight across a good portion of
the area. Current indications are that the fog will be mostly
MVFR...but some pockets of IFR are possible. Any fog that forms
should dissipate within a few hours of sunrise on Tuesday morning.


VFR flight conditions are expected to prevail at Lambert through
Tuesday afternoon. Isolated showers and thunderstorms are possible
primarily south of the terminal this afternoon in the vicinity of
a weak front that is stalled over the region. Any storms that form
will likely dissipate during the evening. Fog in the vicinity is
also a possibility late tonight...though think the fog will stay
out of the terminal.



MO...Excessive Heat Watch from Tuesday afternoon through Saturday
     evening for Audrain MO-Boone MO-Callaway MO-Cole MO-
     Franklin MO-Gasconade MO-Jefferson MO-Lincoln MO-Marion MO-
     Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike MO-Ralls
     MO-Saint Charles MO-Saint Louis City MO-Saint Louis MO-
     Shelby MO-Warren MO.

     Excessive Heat Watch from Wednesday afternoon through Saturday
     evening for Knox MO-Lewis MO.

IL...Excessive Heat Watch from Tuesday afternoon through Saturday
     evening for Madison IL-Monroe IL-Saint Clair IL.

     Excessive Heat Watch from Wednesday afternoon through Saturday
     evening for Adams IL-Bond IL-Brown IL-Calhoun IL-Clinton IL-
     Fayette IL-Greene IL-Jersey IL-Macoupin IL-Marion IL-
     Montgomery IL-Pike IL-Washington IL.



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