258 
FXUS63 KLSX 180815
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
315 AM CDT Tue Jul 18 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 314 AM CDT Tue Jul 18 2017

Focus today continues to be temps, dewpoints and resultant HI 
values. 

Mdls are in general agreement for today, tho some important 
differences do exist. The upper ridge is progd to strengthen and 
build ewd into the region today, with the 850mb thermal ridge also 
building ewd slightly. The sfc ridge will retreat slightly allowing 
the weak sfc fnt to move nwd with winds becoming sly to perhaps even 
swly across wrn portions of the CWA. 

This presents the forecast problem for today. Dewpoints are expected 
to pool along the fnt similar to Mon. Have raised dewpoints across 
wrn and into nrn portions of the CWA where the fnt is expected to 
linger, tho may still be too low across more nrn portions of the 
CWA. Also raised temps across this region with mdls with warmer 
850mb temps and sfc winds becoming more sly. However, some 
uncertainty remains exactly where the warmer temps and warmer 
dewpoints will reside. Still, believe that areas where temps are 
lower will be offset by higher dewpoints and still result in HI 
values around 105. 

Have therefore converted the entire area previously in a watch for 
today to an Excessive Heat Warning and continued this warning 
through Sat. 

Otherwise, isod to widely sct TSRA will again be possible thru the 
central portions of the CWA. More than enuf CAPE will be available, 
but little to no shear and such warm temps shud prevent any storms 
from becoming severe. 

Tilly



.LONG TERM...  (Wednesday through Monday)
Issued at 314 AM CDT Tue Jul 18 2017

Main weather story and focus in the extended will continue to be the 
dangerous heat wave expected to last through at least Saturday. 
Mid/upper level anticyclone will slowly migrate eastward from the 
south-central Plains toward the Mississippi River by Friday 
afternoon. Some weakening of the anticyclone and associated ridge 
axis occurs this weekend as flow aloft becomes more northwesterly. 
Some hope for an end in at least the oppressive heat sometime either 
Sunday, or more likely, for Monday. Plenty of uncertainty in 
temperatures on Sunday and Monday however. This is due to timing 
differences in the cold frontal passage with the previous 12Z ECMWF 
and current 0Z CMC being on the faster end of the guidance envelope 
with the GFS and new ECMWF being quite a bit slower. A slower 
frontal passage would likely yield higher temperatures than 
currently forecast for Sunday and the necessity of heat headlines 
potentially being extended another 24 hours. 

Forecast high temperatures slowly warm Wednesday through Saturday as 
the 850-hPa thermal ridge slides southward into the CWA. Highs are 
expected to range from the mid 90s to near 100 degrees on Wednesday 
and warm to the upper 90s to mid 100s on Saturday (warmest 
conditions each day within the St. Louis urban area). Similar to 
previous shifts, leaned a few degrees above guidance for high 
temperatures through Saturday. Heat index values will generally be 
in the 100-110 range, with warmest values on Friday and Saturday. 
Will leave rest of watch remain for now, but most of rest of CWA 
will likely need to be expanded to warning. Exception looks to be 
across southeast Missouri counties where dewpoints each afternoon 
will be lower than remainder of CWA. Appears these counties may need 
an advisory vs. a warning with peak heat index values Wed-Sat in the 
upper 90s to low 100s. Also the possibility that forecast dewpoints 
each afternoon are a couple of degrees too high for portions of east-
central and southeast Missouri downwind of the Ozarks. Forecast 
southwest wind direction appears favorable for downslope off of the 
Ozark Plateau which may lead to deeper mixing and the aforementioned 
possibility of lower dewpoints. However, this in turn may lead to 
higher ambient temperatures so going heat index values in the 
current forecast look reasonable either way.  

Which should come as no surprise given the anomalous ridging 
across the mid-Mississippi Valley, dry weather is expected through
at least Saturday. Do have PoPs coming in Sunday-Monday 
associated with a frontal passage. Vegetation in many areas have 
already become stressed after the previous stretch of heat last 
week. With this upcoming heat wave likely more intense and a 
longer duration, area vegetation including crops will become 
further stressed. The bi- state area will definitely be in need 
of some widespread rainfall next week. If the warm/dry weather 
continues too much longer, the onset of drought will become an 
increasing concern.


Gosselin


&&

.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 618 PM CDT Mon Jul 17 2017

Isod TSRA possible at KCOU for the first few hours of TAF period.
Expect fog to develop late tonight across much of the region, 
impacting KCOU/KSUS/KCPS. Otherwise, winds will be light and
generally nely thru tonight. Winds will gradually veer to
eventually become sely to sly on Tues. 

Tilly

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Excessive Heat Watch from Wednesday afternoon through Sunday 
     evening for Crawford MO-Iron MO-Madison MO-Reynolds MO-
     Saint Francois MO-Sainte Genevieve MO-Washington MO.

     Excessive Heat Warning from noon today to 8 PM CDT Saturday 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 Wednesday afternoon through Sunday 
     evening for Randolph IL.

     Excessive Heat Warning from noon today to 8 PM CDT Saturday 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.

&&

$$

WFO LSX