Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 190506
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1106 PM CST Tue Feb 18 2020

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 240 PM CST Tue Feb 18 2020

Early this afternoon, an upper level longwave TROF dominated the
north-central CONUS with a longwave upper RIDGE just offshore the
Pacific Northwest.  A flatter zonal flow prevailed over much of the
southern CONUS.  A sheared upper level disturbance was embedded
within the flow over eastern Nebraska and the lift associated with
this feature was helping to induce precipitation development over
southern Missouri and southern Illinois.  At the surface, a cold
front was well southeast of our region with a cold area of high
pressure centered over the far northern Plains beginning to build
into our region.  Clouds continued to prevail over southeast
Missouri and southern Illinois where precipitation is trying to get
going while skies have mostly cleared of clouds for areas farther to
the northwest.  Temperatures ranged from the upper 30s and lower 40s
for most areas with northwest to north winds around 10-15mph.

The upper level pattern will largely persist through this period.
The upper level disturbance inducing precipitation development over
southern MO and IL will slide east by early this evening shifting
the small threat for precipitation out of our area by sunset, with
clouds clearing out from northwest to southeast.

This will leave the cold area of surface high pressure to dominate
our region during this period, with northerly surface winds and
below normal temperatures.

Another upper level shortwave disturbance will attempt to induce
precipitation on Wednesday night, but it will have to overcome a dry
column and model guidance have this feature trending weaker with
time as it shears out and breaks up.  We had a dry forecast going
and do not see any reason to change that at this time.

Min temperatures tonight and Wednesday night are expected to slip
into the teens north of I-70 and low-mid 20s for areas south of I-
70.  Max temps on Wednesday are forecast to be 5-10 degrees below
normal, with readings peaking from the low 30s in northern MO to the
low-mid 40s in southern MO/IL.

TES

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 240 PM CST Tue Feb 18 2020

The first portion of this period appears to be a dry period.  The
GEFS H500 mean advertises a dampening migratory RIDGE with the axis
sliding through our region on Saturday.  At the same time, the GEFS
surface pressure mean has the strong area of high pressure
continuing to build into our region on Thursday with the ridge axis
over much of our region early Friday morning before a return flow
begins later on Friday and continues into Saturday.

This all suggests weak to no lift on what should be a dry column
with very modest moisture return until Saturday.  Below normal temps
are expected to continue Thursday and Friday, with the coldest day
in this stretch being Thursday due to the combo of a building area
of cold high pressure with cloudy skies.

By Saturday, seasonable temperatures should make a return to the
region.

This is where the relatively high confidence portion of the forecast
comes to an end, especially with respect to temperatures and their
effect on precipitation types.

The GEFS H500 mean shows the upper RIDGE axis east of our region
on Saturday night, with SW flow aloft then prevailing until passage
of a Pacific storm system through our region early Monday.  At
the surface, an inverted TROF approaches a rapidly moistening
column ahead of it, with this TROF sliding through our area on
Sunday night with a wave of low pressure tracking to our south.

There is good correlation to a high probability precipitation event
in our region for much of the period from Saturday night through
early Monday with GEFS mean QPF showing a healthy rise and greater
than 90% of members supporting this.  Temperature variance does
increase substantially, however, with nighttime dips in temperature
frequently near thresholds where a switch in pcpn-type may occur,
especially for areas north and west of the STL metro area.  That
said, it looks like we are dealing with a predominantly rain event
for much of the forecast area, with the best chances for some snow
mixing in at the beginning and very end of the event.  Despite these
openings for the potential of snow, snow accum potential looks low
at this time.

Another system already looks set to affect our region late on
Tuesday with the GEFS H500 mean showing a deepening TROF over the
High Plains, maintaining the active weather pattern that begins
Saturday night.  Large temp variances once again with some potential
for another round of snow in portions of the forecast area, but
confidence is quite low on the specifics.

TES

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1037 PM CST Tue Feb 18 2020

A large, strong ridge of high pressure over the northern Plains
will build southeastward into our area. A northerly surface wind
will veer around to a northeast direction Wednesday morning.
Increasing mid-high level cloudiness is expected Wednesday
afternoon and evening.

SPECIFICS FOR KSTL: A large, strong ridge of high pressure over
the northern Plains will build southeastward into our area. A
northerly surface wind will veer around to a northeast direction
Wednesday morning. Increasing mid-high level cloudiness is
expected Wednesday afternoon and night.

GKS

&&

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

$$

WFO LSX



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