Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 221135

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
635 AM CDT Thu Mar 22 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 242 AM CDT Thu Mar 22 2018

The synoptic pattern early this morning features an amplified
flow regime across much of the CONUS, with a trough off the west
coast and another off the east coast. This will place the Midwest
within northwest flow through the period as upper-level ridging
continues to amplify over the west-central United States.

Main focus of the period will be on a chance of rain this morning
into this afternoon. Large scale ascent associated with the left-
ext region of a passing jet streak will help induce a low-level
jet response across portions of the southern and central Plains.
The advection associated with the ascending portion of the low-
level jet towards the upper-level jet streak will help foster mid-
level frontogenesis in the 700-850mb layer over portions of
southern IA southeastward into east/northeast MO and western IL.
The main question, as is typical in these situation, will be does
the low-level dry air beneath 850mb win out, or will the forcing
overcome the dry near-surface airmass. It appears that the
ARW/SREF and even the GFS have a decent handle on the light band
of showers over IA currently. Therefore, have leaned on these
solutions for the forecast today, which also jives fairly well
with the previous forecast. We do expect the forcing to be enough
to overcome the drier low-level airmass across portions of
northeast MO and western IL, thus low-end likely pops were
maintained there. Otherwise, have chances of some light rain
elsewhere over northeast MO/eastern MO/western IL. Amounts are
expected to be very light (GFS amounts likely too high). We can`t
completely rule out some light sleet or light snow mixing in with
the precipitation at onset this morning as the column wet bulbs,
but think rates will be too light and surface temps too marginal
to support any extended period of light snow/sleet.

The frontogenetical forcing should shift generally east of the
area this evening into tonight. However, the region will then be
within a broad zone of isentropic ascent which could continue to
support some shower activity across portions of Illinois into
southeast MO. Temperatures today will range from near 60 over
central MO to the mid 40s across northeast MO and western IL. Lows
tonight will be warmer, in the upper 30s to mid 40s.


.LONG TERM...  (Friday through Wednesday)
Issued at 242 AM CDT Thu Mar 22 2018

An approaching low pressure system will bring periods of
SHRA/TSRA to the area from Fri afternoon through Sat evening.
Models continue to depict a PV anomaly sailing headfirst into an
upper ridge axis and surviving the passage, which produces an
unusual track for the system`s surface low. The surface low
initially lifts northeastward (as would be expected within
southwest flow aloft) before beginning to move southeastward (as
would be expected within northwest flow aloft) on the downstream
side of the upper ridge. The baroclinic zone likewise initially
lifts northeastward into MO on Fri as a warm front before stalling
as the surface low begins to move first southeastward and later
eastward along it.

Highs on Friday will be warmest along the extreme southwestern
part of the CWA (parts of central and southeast MO) which should
be closer to the surface warm front. A noticeable temperature
gradient is then expected across the CWA on Saturday, although
the specific details may change slightly based on the boundary`s
position. The northern CWA (northeast MO and west central IL) may
struggle to rise out of the 40s due to clouds, precipitation, and
its position on the cool side of the boundary. The southern CWA
(southeast MO and southern east central MO) is expected to reach
the mid/upper 60s during the early afternoon. Once the surface low
has shifted eastward into the Ohio Valley on Sat night, the
trailing end of the baroclinic zone sinks southward as a cold
front, bringing a cool night to the entire area with lows in the

The same boundary is then lifted back northeastward as a warm
front on Sunday night in response to a developing surface low over
the plains, which in turn develops in response to an approaching
upper trough. Although the models disagree on the details, there
is general agreement that a PV anomaly within the base of the
trough splits from the prevailing flow and may even become a cut-
off low centered slightly south of the Four Corners region. This
scenario would maintain broad southwesterly flow across our area
until the main system either rejoins the flow and lifts into the
central CONUS or meanders slowly across the southern plains
towards the southeastern CONUS. Although there is some uncertainty
regarding how the upper air pattern will evolve, broad
southwesterly flow aloft downstream of a slow-moving low over the
southwestern CONUS would support several periods of SHRA/TSRA for
our area from late Sunday night through the middle of next week.



.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 635 AM CDT Thu Mar 22 2018

VFR conditions are expected through the period. A system will pass
through today bringing some light showers to the sites, but
anything that falls will be very light and should not cause any
cig/vsby restrictions. Otherwise, cigs may lower closer to MVFR
towards the end of the period as low-level moisture increases but
will leave any mention out for now given the uncertainty.
Otherwise, light and variable winds this morning will transition
to more ESE winds today into tonight.





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