Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 281116
Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
616 AM CDT Tue Mar 28 2017
.SHORT TERM... (Through Late Tonight)
Issued at 336 AM CDT Tue Mar 28 2017
The synoptic pattern this morning features two upper-level
features of note, one PV anomaly pushing eastward across portions
of the Mid-Atlantic and a strong upper-level low digging across
the Four Corners region. This will place the region within
shortwave ridging aloft through the period, providing mainly dry
conditions today into much of tonight.
The region continues to see copious cloud cover early this
morning with even some light, patchy drizzle in a few locations.
While the patchy drizzle should taper off quickly after sunrise,
the clouds will likely hang tough through much of the day. A
surface ridge currently over the Northern Plains will drift to the
east today. This ridge position will provide a steady
northeasterly flow into the region through the day. With clouds
expected to linger through much of the day given limited mixing
beneath a building inversion and cool northeasterly surface flow,
see little reason why we will warm very much this afternoon.
Therefore, have lowered temps several degrees from the previous
forecast with highs likely to struggle in the mid/upper 50s to low
For tonight, the strong upper-level low will eject into the
Southern Plains, allowing mid/upper-level flow to back over the
Midwest. In response to the ejecting upper-level low, a low-level
jet will ramp up overnight across the Central Plains. While the
bulk of the activity associated with this increasing isentropic
ascent will remain just off to the west overnight, some
showers/isolated storms may creep into portions of central MO
towards 12Z Wednesday morning. See the long term section below for
additional details on this approaching storm system.
.LONG TERM... (Wednesday through Monday)
Issued at 336 AM CDT Tue Mar 28 2017
The aforementioned approaching low pressure system will be
located over the NM/TX border at 12z Wed and makes only slow
eastward progress on Wed and Thu, and it will continue to
influence wx conditions until it finally moves east of the area on
Thu night. The surface warm front associated with the surface low
will make quick northward progress on Wed morning, bringing
low-level moisture back into the region and setting the stage for
strong to severe thunderstorms on Wed night when a 40-50kt LLJ
develops ahead of the approaching mid/upper low. Although the
greatest instability is forecast farther south in AR, models
still depict ample large-scale ascent, H7-H5 lapse rates of
6.5-7.5 deg C/km, 40-60 kts of 0-6km shear, and at least a few
hundred J/kg of MUCAPE across the CWA. Taken together, these
factors will be sufficient for strong to severe thunderstorms
across the southern half of the LSX CWA on late Wed night into
early Thu morning. Given the slow motion of this low pressure
system and depending on overnight convective trends, a separate
period of strong to severe convection which develops on Thu
afternoon could briefly affect the eastern or southeastern CWA
before the threat shifts eastward.
Although precipitation may linger into Fri morning behind the
departing low pressure system, dry weather is expected for most
of the day on Fri due to shortwave ridging between systems. Dry
conditions should persist through at least the first half of Sat
before large-scale lift ahead of yet another closed low located
across the southwestern CONUS begins to affect the region. Model
solutions for the path and strength of this feature diverge
quickly after 00z Sun, but there is enough general agreement to
support a chance of rain through Mon across most of the area
associated with its passage.
Temperatures will remain mild through the period and no extremes
are currently anticipated. The coolest day will likely be on Fri
when highs only reach the mid-50s, but most other days should
experience highs in the 60s to perhaps near 70 degrees.
Overnight low temperatures may fall into the upper 30s across
northeastern MO and west central IL on Fri night/early Sat
morning, but they are not expected to fall below freezing.
.AVIATION... (For the 12z TAFs through 12z Wednesday Morning)
Issued at 616 AM CDT Tue Mar 28 2017
Low stratus will be the main focus of this TAF period. Low stratus
is currently affecting all sites, ranging from MVFR to high-end
IFR. Have highest confidence in IFR at COU this morning, with less
confidence in the St. Louis metro sites. Think IFR is likely to
stay out of UIN as drier air has worked in on northeasterly
winds, thus will go prevailing MVFR there. Conditions should
improve to at least MVFR at all sites by this afternoon, with
perhaps even a brief period of VFR cigs early this evening.
However, an approaching low pressure system will bring lowering
cigs once again towards the end of the period along with
increasing chances for showers.
SPECIFICS FOR KSTL:
Low stratus will persist through much of the day. Low-end MVFR to
high-end IFR will persist this morning, with cigs slowly rising to
high-end MVFR by late this afternoon into early this evening.
Could even see a brief period of VFR cigs this evening, but
confidence in this is not high. Otherwise, additional low cigs
will move in towards the end of the period along with a chance of
showers as an area of low pressure approaches the region.
North/northeast winds today around 5-10 knots will veer to the
east overnight into Wednesday morning.