Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 171602

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
1102 AM CDT Thu Aug 17 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 253 AM CDT Thu Aug 17 2017

The large elongated MCS that existed late yesterday evening from
central MO into central OK has lost considerable organization over
the last 4+ hours with the concentrated deep convection/remnant
MCS sinking ssewd through SE OK and NW AR. Across eastern MO and
western IL, the cloud tops have warmed considerably, the overall
extent of showers/rain has been decreasing, and tstorms were
confined to a small area in SE MO. This remaining activity was
located well ahead of an advancing cold front currently extending
through NW MO, and was occurring in association with low level
MCON via a southwesterly LLJ and positively tilted short wave
moving into western MO. Present indications are the remaining
showers and isolated tstorms will move to the east of the CWA by
dawn in response to veering of the LLJ and migration of the short
wave. This is largely supported by the CAMS. A threat of showers
and thunderstorms will persist across far eastern MO and southwest
IL this morning in the wake of the current activity and ahead of
the advancing cold front, albeit low. Even this low threat will
shrink/diminish with time as the cold front continues to advance
east across the CWA this morning and afternoon. The most favored
area to see some new development would probably be southeast MO
into southwest-south central IL late morning/early afternoon. In
the wake of the cold front, skies will clear with westerly winds
advecting drier air into the region. Highs today should be
seasonable and within a few degrees of normal for mid August.

Weak high pressure, lowering dew points, and light west winds will
prevail tonight. This combination and clear skies, should result in
pleasant nighttime temps and below average overnight lows.


.LONG TERM...  (Friday through Wednesday)
Issued at 253 AM CDT Thu Aug 17 2017

By 12z Friday, another shortwave will be located upstream over the
northern plains. This feature will take a more southeasterly track
compared to the previous shortwave, and it will bring a chance of
SHRA/TSRA to most of the area on Friday night and early Saturday
morning. Precipitation will likely be southeast of the CWA shortly
after daybreak because the upper trough axis will have shifted
east of the CWA by 12z. An broad and flat upper ridge will build
into the central CONUS behind the departing shortwave for the rest
of the weekend and early next week. There are a few weak
disturbances in the flow noted for early next week, but some of
them appear to be the result of convective contamination. Please
see the section below for information about the weather pattern on
the day of the total solar eclipse.

By the middle of next week, a deep low pressure system moving out
of the Gulf of Alaska will induce downstream height rises over
the intermountain west, allowing a cutoff low off the coast of
California to rejoin the flow as an open wave. The building ridge
over the west will yield a northwest flow pattern aloft over
MO/IL. One or more features within this flow pattern may bring a
chance of rain to the area. The shift to northwest flow aloft will
allow a backdoor cold front to move into the area on Wednesday
night and Thursday, bringing slightly cooler temperatures to MO/IL
during the middle and end of the week.


At this point, it looks like our area might squeak by with at
least a chance of good viewing conditions somewhere along the path
of totality on early Monday afternoon before clouds and
precipitation begin to increase across the area on Monday night.

High temperatures on Monday will likely be in the lower 90s across
the area, although temperatures will locally fall by as much as
20 degrees during totality. NWS forecast grids at hourly
resolution will not capture the full temperature drop during the
brief 1-2 minute duration of totality.

The primary forecast issue for Monday is the potential for
mid/high clouds. As noted in an earlier forecast discussion,
diurnal cumulus clouds should dissipate to some extent prior to
totality during the partial phase of the eclipse. This effect is
caused by reduced diurnal heating within the moon`s shadow. In
terms of mid/high clouds, the latest model depictions of
condensation pressure deficits are simultaneously both high (dry)
enough to offer hope and low (moist) enough to be discouraging,
giving credence to the adage about the misery of uncertainty. The
GFS appears to be affected by convective contamination and its
CPD solution was discarded at this time.

The ECMWF, GFS, and GEM still show a broad ridge stretching across
the eastern two-thirds of the CONUS at 12z Monday...although if
the ridge gets much flatter, it won`t be much of a ridge.

Compared to 24hrs ago, the ECMWF shows good continuity with its
depiction of a weak shortwave over MN at 12z Mon. The GFS and GEM
appear to have convective contamination in the H5 vorticity
field, which makes it difficult to discern whether these models
might also depict a shortwave over IA or MN which could increase
cloud cover over our area on Monday.

The good news is that Sunday night`s H85 LLJ will be focused
across NEB/SD, and the even better news is that the magnitude of
the forecast Corfidi vectors is quite small across the region.
This means that it is unlikely for a rapidly propagating MCS to
race southeastward into our area during the pre-dawn and early
morning hours, leaving convective debris clouds to block the view
of the eclipse.

Please consider the above forecast with the same caveats as any
other Day-5 forecast; there is normally a fair amount of
variability this far in advance.



.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 630 AM CDT Thu Aug 17 2017

A cold front was located across western MO early this morning. A
band of stratus resulting in IFR flight conditions prevailed ahead
of the front from west central Il across northeast and central MO.
IFR conditions will prevail through around 14-15z along this
corridor and then the movement of the cold front and westerly
wind shift will resulting in diminishing clouds/VFR conditions
and eventually clearing. Further east including metro St. Louis,
heating of the moist air mass ahead of the front is expected to
result in the development of stratocu producing MVFR flight
conditions from mid-late morning, with the cold fropa and westerly
winds also resulting in improvement to VFR by midday. The westerly
winds will be a bit gusty from midday into the afternoon. Weak
high pressure and light winds are expected tonight with VFR flight
conditions continuing.


Heating of the moist air mass ahead of an approaching cold front
is expected to result in the development of stratocu producing
MVFR flight conditions from mid-late morning. The cold fropa and
westerly winds should result in improvement to VFR by midday. The
westerly winds will be a bit gusty from midday into the afternoon.
Weak high pressure and light winds are expected tonight with VFR
flight conditions continuing.





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