Areal Flood Advisory 66, NWS St. Louis, MO

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1225 AM CDT Sun Jul 18 2021

.SHORT TERM...  (Through Late Sunday Night)
Issued at 251 PM CDT Sat Jul 17 2021

The cold front responsible for the showers and storms experienced
the last couple of days has made its way just south of the CWA, and
high pressure is centered over the Midwest. Residual boundary layer
moisture just behind the front and 1,500-2,000 J/kg of SBCAPE per
SPC mesoanalysis has allowed for convection to form in portions of
central and southeastern Missouri. RAP soundings from these areas
show poor lapse rates and a warm nose at roughly 700-600 mb,
limiting convective growth. Therefore, I expect convection that
forms in these areas to remain weak and short lived.

Our focus for this evening and overnight is the upper-level
shortwave and MCV seen over eastern Kansas presently. It is expected
that the MCV will continue its southeastward trajectory, skirting
the southwestern and southern portion of the CWA this evening and
overnight. The 12z HREF kept convection well confined to portions
of southeastern Missouri and southwestern Illinois, but recent
HRRR runs have edged convection a little further north toward the
I-70 corridor as the MCV passes by. Given the aforementioned
lapse rates over the CWA and generally unfavorable environment for
convection, I'm not sure I buy in to this recent, more northerly
solution just yet. That being said, I've taken somewhat of a
middle ground and edged the chance of scattered showers a little
further north than the previous forecast and kept them going
longer tonight. With limited instability and 0-6 km shear around
15 kts, convection will remain weak and thunder will be limited.
However, precipitable water over southeastern Missouri and
southwestern Illinois is around 1.5-1.75", and if heavier showers
move over areas that saw high rainfall totals recently, an
isolated flash flood threat may be realized tonight, but I have
low confidence in that at this time.

Additionally, with the relatively same airmass in place over the
CWA tonight as was last night, I can't rule out another round of
patchy fog. Current surface obs show dry air pushing into the CWA
from the northeast, but I'm uncertain with how far this will make
it into the CWA. I currently have patchy fog in the forecast
mainly for portions of southeastern Missouri and southwestern
Illinois where surface moisture is higher thanks to recent
rainfall.

Tomorrow's convective trends largely depend on what happens tonight,
but confidence remains high that the best chance for isolated to
scattered showers and maybe a brief thunderstorm to occur is over
portions of southeastern Missouri where boundary layer moisture
will remain sufficient for convection. Temperatures will remain
below normal thanks to the post-frontal high pressure remaining
centered over the Midwest.

Elmore

.LONG TERM...  (Monday through Next Saturday)
Issued at 251 PM CDT Sat Jul 17 2021

Through the first half of the upcoming work week, the upper-
level pattern will hold, with a trough over the eastern U.S. and
an amplified ridge over the western half of the country. This will
keep the Middle Mississippi Valley in northwest flow aloft, and
at the surface, an area of high pressure will keep winds
northeasterly at the surface. This setup will help keep
temperatures at or below normal through Wednesday. For Thursday
through the end of the extended period, ensemble guidance shows
the western ridge building eastward. In response, the surface high
will drift eastward and return southerly low-level flow to the
CWA. This flow will advect warm, moist air into the region,
leading to an increase in temperatures and humidity by the week's
end. Ensemble solutions diverge near the end of the period with
the expansion of the ridge relative to the Middle Mississippi
Valley, leading to uncertainty with weather at the surface -
particularly rain chances. With the increase in boundary layer
moisture and uncertainty with the upper- level flow, I can't rule
out a low chance of convection over portions of the CWA, but my
confidence is higher in a mostly dry forecast for the week's end.

Elmore

&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1224 AM CDT Sun Jul 18 2021

VFR conditions are currently forecast through the valid TAF period,
but the lingering threat for fog at a few terminals and MVFR CIGs
this morning remain the dominant concerns for the forecast.

A surface cold front is still draped across the southern portion of
Missouri, which is aiding showers and thunderstorms in this area.
The terminals, however, have been dry for hours and should remain
that way through the period. While relative humidity is high at a
few terminals, no fog has yet to develop thanks to the slow approach
of drier air and cloud cover at all sites except KUIN. While no fog
is mentioned in this TAF issuance, if sites in the river valleys
(KSUS and KJEF) are able to cool rapidly and clear out, radiation
fog could develop.

Guidance is beginning to back off the concept of MVFR CIGs tomorrow
morning, but discrepancies still exist. While the TAFs never
introduced lower categories into the terminals, the SCT MVFR wording
was kept to account for the possibility of lower clouds tomorrow. If
MVFR CIGs do form, they should only last a few hours before lifting
to VFR. Winds will remain light and generally out of the northeast
through the period.

MRB

&&

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

WFO LSX

Hazardous Weather Outlook
National Weather Service Saint Louis MO
641 PM CDT Sat Jul 17 2021

ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027-
034>036-041-042-047>052-059>065-072>075-084-085-099-182345-
Adams IL-Audrain MO-Bond IL-Boone MO-Brown IL-Calhoun IL-Callaway MO-
Clinton IL-Cole MO-Crawford MO-Fayette IL-Franklin MO-Gasconade MO-
Greene IL-Iron MO-Jefferson MO-Jersey IL-Knox MO-Lewis MO-Lincoln MO-
Macoupin IL-Madison IL-Madison MO-Marion IL-Marion MO-Moniteau MO-
Monroe IL-Monroe MO-Montgomery IL-Montgomery MO-Osage MO-Pike IL-
Pike MO-Ralls MO-Randolph IL-Reynolds MO-Saint Charles MO-
Saint Clair-Saint Francois MO-Saint Louis City MO-Saint Louis MO-
Sainte Genevieve MO-Shelby MO-Warren MO-Washington IL-Washington MO-
641 PM CDT Sat Jul 17 2021

This Hazardous Weather Outlook is for portions of eastern and
central Missouri as well as west central and southwest Illinois.

.DAY ONE...Tonight.

Isolated thunderstorms are possible tonight across portions of
southeast Missouri and southwest Illinois. Severe thunderstorms are
not expected.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

Widely-scattered thunderstorms are possible Sunday in southeast
Missouri, southwest Illinois and west-central Illinois.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not anticipated tonight.

MRB