National Weather Service Raw Text Product

467 
FXUS63 KLSX 162317
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
617 PM CDT Thu Jul 16 2020

.SHORT TERM...  (Through Late Friday Night)
Issued at 258 PM CDT Thu Jul 16 2020

Following today's modest reprieve from hot and humid conditions will 
be a return to the sultry summer air mass Friday. The certainty 
revolving around Friday afternoon heat indices is only somewhat 
stunted by the struggle to pinpoint details with regard to a stalled 
boundary and resulting thunderstorm potential. 

Issues are minor through late tonight. Surface high pressure will 
gradually move east through Friday morning. Surface flow turns 
southwest with increasing low to mid-level moisture behind the 
departing high. Where light winds intersect with better surface 
moisture (recent rainfall) over southeast Missouri and portions of 
southwest Illinois, we could see a brief period of patchy fog in the 
pre-dawn hours Friday morning. 

There are two features to watch overnight. One is another compact 
vort feature that tracks west to east along the Kansas/Oklahoma 
border late today and through southern Missouri through the first 
half of Friday. This will likely assist the stalled boundary in 
additional thunderstorm development over southern Missouri. Hi-res 
guidance continues to point to a slight northward shift in the 
boundary, which become quite difuse Friday afternoon. Isolated to 
perhaps scattered storms are possible over the southern sections of 
the forecast area, potentially reaching as far north as St. Louis by 
early afternoon. The second feature is another small vort that track 
from Nebraska into southern Iowa Friday afternoon. There are some 
hints that a weak area of convergence extends south of this and 
results in isolated storms over central Missouri. Therefore, 
maintained slight chances for isolated thundertorms at the height of 
diurnal instability.

Though moisture and temperatures increase Friday, apparent 
temperatures above 100 degrees look more probable from central 
Missouri through southeast Missouri. Given the lingering question 
with the position of the boundary and/or resulting impacts from 
thunderstorms, there still isn't enough confidence to say 
definitively that heat indices will achieve the one day criteria. 
It's more likely that duration criteria will be met, provided lack 
of thunderstorm development and persistence in Saturday-Monday 
temperatures trends.

Maples

.LONG TERM...  (Saturday through Next Thursday)
Issued at 258 PM CDT Thu Jul 16 2020

Heat remains the primary concern through the weekend and potentially 
into early next week. However, the struggles continue with regard to 
nailing down the magnitude and locations impacted by higher heat 
indices, largely due to thunderstorms potential and timing in a cold 
front next week. Consequently, there are some reservations and lower 
confidence in the duration of the upcoming warmth. 

It continues to look as though the upper ridge will extend from the 
southwest CONUS, east through the mid-Atlantic seaboard Saturday. 
However, the strength of the H5 ridge is somewhat offset east of the 
Mississippi Valley with surface high pressure and mid-level height 
rises from west to east through the southeastern CONUS.

An upper trough begins to enter the northern plains Saturday with 
several vort features ejecting east in the Upper Midwest from the 
base of the trough. An increasing pressure gradient begins to 
develop in the mid-levels with a strengthening jet stretching from 
the central plains into the northern Great Lakes. As the upper 
trough moves east through the weekend, a surface cold front begins 
to sink southward.

We lie squarely in between, where a remnant vort from the central 
plains continues to be relatively stagnant over a diffuse stationary 
boundary. Guidance continues be quite variable in the degree of 
diurnal activity along and north of the this boundary as is slowly 
drift north Saturday and Sunday. As this is all occurring, 
disturbances to the north provide some potential for MCS development 
around the northern periphery of the upper ridge. 

At this point, concerns for heat still exist with the caveat that 
isolated to scattered storms could develop either day. The greatest 
bust potential would stem from any southward placement in MCS 
development out of the north. Therefore, there is still far too much 
uncertainty for any headline call. Current forecasts in the mid to 
upper 90s are between the upper 50th and 75th percentile from Friday 
through Monday.

By early next week, trends are showing a southward push to the cold 
front and thunderstorm potential, which should effectively end the 
concern for higher heat indices. It's beginning to look a bit more 
active early next week.

Maples

&&

.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 617 PM CDT Thu Jul 16 2020

VFR conditions, dry weather, and light surface winds of less than
10kts will prevail at the TAF sites through the valid period.
There is a low chance (20%) of thunderstorms Friday afternoon but
too low to mention at this time. Otherwise, look for E-SE surface
winds to gradually veer more S-SE during the valid period. 

TES

&&

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

$$

WFO LSX