Area Forecast Discussion

FXUS63 KLSX 172105
Area Forecast Discussion
National Weather Service Saint Louis MO
305 PM CST Sat Feb 17 2018
.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 305 PM CST Sat Feb 17 2018
A strong upper level disturbance was located over northwestern IL 
this afternoon.  This feature helped to drive pcpn development over 
northeast MO and west-central IL during the late morning and early 
afternoon at the same time WAA aloft was the dominant force for pcpn 
development and movement further to the south across much of the 
forecast area.  The forcing for all areas has since moved off to the 
east, leaving behind areas of drizzle that are exiting hastily to 
the east.  Widespread low cloudiness has kept temps down all day in 
the 30s, but some clearing is occurring over northeast and central 
MO, where temps are rising into the 40s.  
Skies are expected to clear for all areas by 9pm, and with high 
pressure settling in and moving thru, look for near calm winds for 
much of the night as well.  With a wet ground from the wet snow and 
rain that fell today and a late clearing, the calm winds and clear 
skies should provide good conditions for radiational fog.  The only 
item that will reduce this potential is drier air also advecting in 
with the high pressure build.  Have continued the mention of patchy 
to areas of fog for overnight with the evening shift monitoring any 
need for any further upgrades.  High pressure is expected to be 
progressive enough where southerly winds increasing late for areas 
north and west of STL metro should prevent much fog there. 
Otherwise, look for min temps in the 25 to 30 range. 
Flow aloft already begins to back SW again on Sunday but any 
disturbances to create lift will hold off until nightfall, leaving 
what should be a dry day with increasing and thickening clouds late 
in the day.  A tightening pressure gradient at the surface with 
southerly flow will result in wind speeds of 15 to 20mph with gusts 
to 35mph.  This strong south flow will propel temps well into the 
50s on Sunday afternoon with 60F possible for parts of central and 
east-central MO.
.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 305 PM CST Sat Feb 17 2018
(Sunday Night - Tuesday Night)
***Active weather pattern early next week...beneficial rainfall 
Warm and active period will begin on Sunday night as strong low-
level warm/moist advection intensifies across the area. Best chance 
for shower activity (and potentially a few thunderstorms) will be 
across portions of south-central Illinois where strongest forcing 
is forecast to reside. Temperatures on Sunday night will be on the 
mild side with a non-diurnal swing as a warm front quickly moves 
through the CWA.
Chances of showers and possibly a few storms continue through Monday 
and Monday night, but not expecting an all day or night rain. 
Expecting there will be breaks but weak vorticity maxima will 
continue to move rapidly northeastward within the deep southwest 
flow aloft. Probably will be one of those cases where most areas 
recieve some measurable rainfall both on Monday and Monday night, 
just with quite a bit of dry time between showers. Monday still 
appears to be the warmest day of the forecast period with 850-hPa 
temperatures near +10C and at least some breaks in the rainfall. 
Shouldn't be looking at anything as warm as observed this past 
Thursday due to the possibility of rain and high chances of overcast 
sky conditions. That being said, still expecting highs in the upper 
60s to low 70s across much of the area which is about 20-30 degrees 
above normal for mid/late February.
Main focus for heavier rainfall continues to look to be on Tuesday 
into early Tuesday night. Showers with embedded thunderstorms will 
focus along and slightly poleward of a cold front which will only 
slowly progress southeastward in time. Still some placement and 
timing differences with this cold front, but consensus is that a 
couple inches of rainfall will be possible across much of the 
southeastern half of the area Tuesday-Tuesday night with locally 
higher amounts possible. Main result of this expected rainfall would 
be very beneficial to area rivers/streams as they are very low at 
the moment due to the ongoing drought. Soil moisture should also 
help be recharged. Due to these antecedant conditions, not expecting 
much negative impact at this time (if any), but cannot rule out a 
small chance at some flash flooding or minor river flooding. 
There are some signals of the potential for heavy rainfall rates 
with very deep warm cloud depths (~10 kft) and precipitable water 
values of 1.25-1.50" which nears or excedes record values for KSGF 
and KILX within the month of February. If the cold front's progress 
slows down more or stalls out across southeastern sections of the 
area, the threat for river flooding and flash flooding would 
Another potential concern is how much post-frontal precipitation 
there will be and how quickly surface temperatures drop below 
freezing behind the front. Large disagreement between the GFS and 
ECMWF. While both have rapidly dropping temperatures as a shallow 
Canadian airmass builds into the area, the ECMWF has a narrow axis 
of moderate precipitation in the form of freezing rain potentially 
mixing with or changing over to sleet before ending. Meanwhile, the 
GFS looks to be a classic Schmocker rule scenario with precipitation 
ending before the subfreezing air advects into any one location. 
(Wednesday - Next Saturday)
Large model disagreement continues into the middle of next week. The 
ECMWF has another wave within the southwest flow aloft which brings 
another round of sleet and freezing rain to much of the area whereas 
the GFS and CMC are basically dry until Thursday night. Favored the 
agreement between the deterministic GFS/CMC which is also supported 
by nearly every GEFS ensemble member.
Broad model agreement in another moderate rainfall event for late 
week or early next weekend. Highest chances for rainfall at this 
early juncture appear to be Friday/Friday night.
.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1225 PM CST Sat Feb 17 2018
Pcpn has ended at COU and will end at STL metro sites and UIN by
20z. IFR CIGs currently in place at the TAF sites will also
gradually edge eastward this afternoon, with clearing expected at
all sites by nightfall or shortly after. With wet ground, light
winds from high pressure building in. and clear skies, look for
some VSBY reductions in fog, especially for STL metro sites
because of the delayed onset of SE winds on the backside of the
high pressure. These VSBY reductions should all rapidly improve by
mid-morning, but depending on how this all plays out, could see
some potential for IFR VSBYs overnight. For now bottomed out VSBYs
at 3-4SM. As the area of high pressure moves away on Sunday, look
for SE-S surface winds to become gusty. 
SPECIFICS FOR KSTL: IFR conditions to improve during the mid-late
afternoon to MVFR with clearing at or shortly after nightfall.
Some VSBY reductions in fog late tonight with potential for IFR.
Winds becoming gusty with fog dissipation on Sunday morning.