Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 191759

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1259 PM CDT Wed Jul 19 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 355 AM CDT Wed Jul 19 2017

Dangerous heat will remain the main story today with no changes to
the current suite of excessive heat warnings and heat advisories.
The expansive upper high currently centered in the vicinity of NW OK
will shift eastward today and tonight moving to near the KS/MO
border by 12Z Thursday. Meanwhile surface winds will veer to
southwesterly this morning. The combination of the shifting upper
level high/ridge eastward, low level southwesterly flow, and warming
lower tropospheric temperatures should translate to hotter temps
today in most areas. However, can`t disregard 2 areas of ongoing
precipitation early this morning and potential impacts. The first is
an area of predominately showers in central IL skirting the
northeast part of the CWA, associated with a southeast moving MCV.
This MCV and associated cloudiness will exit east of the area this
morning but may see a few spot showers in south central IL through
daybreak. The second region of precipitation is a growing region of
showers and thunderstorms across southwest IA/northwest MO/northeast
KS associated with a southwesterly LLJ. Guidance suggests this
activity will ooze a bit east-southeast with the steering flow aloft
and weaken after daybreak and present indications are it will remain
west of the CWA. The residual clouds from this convection could
hamper the temp climb this morning in the far western CWA, but
thinking is they will also thin with time allowing the heat-up this
afternoon. I have notched temps back a few degrees in north central
and central MO given the anticipated clouds with highs still
expected in the lower to middle 90s. The greatest readings today
will be focused in the St. Louis metro area where all indications
are that highs will range from the upper 90s to low 100s with peak
heat index values of 105-110 degrees. Elsewhere afternoon heat
indices will be in the 103-107 range. Lastly a number of the CAMS
suggest potential for isolated diurnally driven thunderstorms across
eastern parts of the CWA this afternoon. This certainly seems
reasonable and have introduced some slight chance pops. Mild to warm
conditions expected tonight. Focus for any organized showers and
thunderstorms should be well north of the CWA along the east-west
baroclinic zone within northern portion of the ring of fire.


.LONG TERM...  (Thursday through Tuesday)
Issued at 355 AM CDT Wed Jul 19 2017

Dangerous heat wave will continue to be the story through at least
Saturday. High temperatures Thursday/Friday/Saturday are expected
to be mostly in the upper 90s to low 100s. Urban areas of St.
Louis will likely approach 105F, particularly on Friday and
Saturday. Maximum heat index values each day within the excessive
heat warning are expected to be in the 105-110F range, with
portions of the St. Louis urban area slightly above 110F. Further
south, areas such southeast Missouri and southwest Illinois should
experience dewpoints low enough to prevent maximum heat index
readings from reaching warning criteria so no plans to upgrade the
advisory to a warning at this time. Those counties would need a
4th day of 105+ heat index values to justify a warning and Sunday
remains a highly uncertain forecast at this juncture in time.

As alluded to above, lots of uncertainty with respect to Sunday`s
forecast. Timing of synoptic cold front is obviously important, but
both the GFS/ECMWF suggest potential of convection Saturday night
into Sunday well ahead of the actual cold front. Leaned on a
warmer solution for now and have highs in the low to mid 90s with
max heat index values in the mid 90s to near 100 degrees. Actual
highs could be at least a few degrees warmer if there is less
convection and associated cloud cover however, and would make it
more likely heat headlines of some kind would need to be extended
for areas currently residing in the ongoing excessive heat

Cold front should make it through the CWA by late Sunday night and
finally put an end to this heat event. Early next week looks quite a
bit cooler, though how much cooler is certainly an open ended
question. Modeling has wavered on the amount of cool air behind this
front so leaned close to climatological values for now.
Precipitation chances look very low on Monday with front south of
the CWA, though could see some isolated-scattered activity
potentially on Tuesday as boundary retreats northward as a warm



.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1222 PM CDT Wed Jul 19 2017

VFR through the period with diurnal cumulus clouds and occasional
cirrus. Winds will remain generally westerly to southwesterly
during the daytime hours and then diminish overnight.



MO...Heat Advisory until 8 PM CDT Sunday for Crawford MO-Iron MO-
     Madison MO-Reynolds MO-Saint Francois MO-Sainte Genevieve
     MO-Washington MO.

     Excessive Heat Warning until 8 PM CDT Saturday for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Franklin MO-Gasconade MO-
     Jefferson MO-Knox MO-Lewis 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.

IL...Heat Advisory until 8 PM CDT Sunday for Randolph IL.

     Excessive Heat Warning until 8 PM CDT Saturday for Adams IL-Bond
     IL-Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-
     Jersey IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-
     Montgomery IL-Pike IL-Saint Clair IL-Washington IL.



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