336 
FXUS63 KLSX 292341
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
541 PM CST Tue Jan 29 2019

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 315 PM CST Tue Jan 29 2019

Bottom line up front: Latest high resolution and ensemble guidance 
is showing slightly colder temperatures tonight than the previous 
forecasts, so we've adjusted accordingly. With that, the Wind Chill 
Warning has been extended south to include much of the St. Louis 
metro area. Additionally, we've expanded the portion of the warning 
that lasts into Thursday. While the area warned through Thursday 
should easily reach into warning criteria tomorrow morning, lighter 
winds on Thursday morning will likely keep the wind chill between 15 
and 25 below in many locations. Rather than having both a warning 
and advisory in effect for the same areas, we instead have gone with 
only a warning, which for many areas may be adjusted to an advisory 
tomorrow.

Onto the discussion: A deep upper level low is currently centered 
over northern MN. The associated surface low is over the Great 
Lakes, with a cold front trailing southwest through roughly St. 
Louis as of 2pm. Widespread clouds and light radar returns have 
blossomed along the front in the last few hours in response to 
slightly better moisture along and south of the Missouri River. 
Thanks to very dry air at the surface, much of this snow is not 
reaching the surface, but we have had a few reports of light snow 
across the region.

The front will continue to push south through the afternoon, and 
snow will likely become more widespread as it moves into even better 
moisture. However, snow will remain light, and fairly short 
residence times will limit snow accumulations to under and inch. The 
bigger impact of this frontal passage will be a reinforcing shot of 
very cold air, bringing sub-zero temps to much of the area tonight. 
Furthermore, winds will remain elevated through the night, which 
coupled with the very cold temperatures will result in dangerously 
cold wind chills. 

Very cold temperatures will remain in place through tomorrow as 
arctic high pressure settles over the area. Fortunately with the 
high overhead, winds will diminish and boost wind chill readings 
somewhat. Meanwhile, a shortwave embedded in the NW upper level flow 
will drop through the Great Plains, spurring weak lee cyclogenesis. 
A warm front will extend eastward from the weak low, with the upper 
reaches of the front pushing into central MO by tomorrow afternoon. 
This may bring on yet another bout of light snow for areas west of 
the Mississippi through the evening. Despite warmer air arriving in 
the mid levels, surface temperatures will remain quite cold into 
Thursday morning.

BSH

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 315 PM CST Tue Jan 29 2019

The upper pattern that has yielded our current outbreak of very 
cold air will continue for the first part of this period:  a 
longwave upper TROF over the eastern CONUS and a longwave upper 
RIDGE over the western CONUS.  

The Arctic airmass will be slow to pull away on Thursday and also we 
are expecting a lot of cloud cover as easterly surface winds begin 
to pick up some.  Factoring in everything, it appears dangerous 
level wind chills of 15 below or lower will hang on through the 
morning hours of Thursday for much of northeast MO and central IL, 
still constituting the single wind chill "event".  Thus, the wind 
chill headlines for these areas have been extended until noon 
Thursday. 

One final clipper system is progged to drop down in the resultant NW 
flow aloft with most of the energy associated with it missing our 
region to the north.  The portion of this system that will impact 
the forecast area will be the surface frontal boundary being dragged 
southward through, not unlike the event this afternoon.  The 
precipitation for most of this event is expected to fall as 
snow, but again, with most of the energy passing to the north of our 
region, this should also keep any accumulations to a minimum 
(dusting or less).  The frontal boundary will settle and stall just 
to our south late Thursday night as a weak wave of low pressure 
slides along it, maintaining the threat for light precipitation for 
areas south and east of STL metro until close to sunrise Friday 
morning.  However, as cloud ice is expected to be lost by this 
point, the light precipitation will attempt to briefly transition 
from light snow to freezing drizzle before ending. 

At this point, all of the models forecast a major pattern change 
heading into the weekend and early next week.  This pattern will 
overall feature progressive flatter flow, bottling up the cold 
Canadian air for a time, and instead, steering milder Pacific 
airmasses into our part of the country. 

Weak upper ridging will first build in for the weekend and Monday, 
and coupled with southerly flow, should result in much milder 
temperatures.  The MEX MOS does have max temps well into the 60s for 
Sunday and Monday, but concerns at this time revolve around stratus 
cloud development, limiting any major warmup.  If the GFS is right, 
a great opportunity may avail itself on Monday afternoon as the dry 
slot of a system pushes through, but that is also 6 days out, so it 
is difficult to zero in on those little details right now.  Did 
introduce low 60s into some areas for Sunday and Monday for now. 
Some light rain chances are possible during the weekend, but a 
better chance should exist on Monday with a stronger system and 
front affecting the area.  Precipitation-types from Friday 
night through early Tuesday should be all rain.

The mild pattern may already be breaking down by next Tuesday as a 
deep upper TROF attempts to build into the central CONUS on the 
heels of another storm system rolling through.  The initial look at 
this system favors mostly rain, but it may end as snow for many 
areas as colder seasonable air returns to the region.

TES

&&

.AVIATION...  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 523 PM CST Tue Jan 29 2019

A small band of flurries and light snow has developed impacting
KSUS/KCPS. This snow may linger for up to an hour at KCPS, but is
expected to have pushed south of KSUS by 00z. Clouds will also
continue to push south this evening, leaving much of tonight clear
at terminals. Guidance is suggesting a small band of low VFR cigs
will push south late tonight into Wed morning. Confidence is
somewhat low regarding coverage of these clouds and have kept at
SCT for now. Winds will diminish Wed morning as the surface ridge
builds into the region. Another round of light snow is possible
late Wed afternoon into the evening and may impact KCOU. Currently
expect the bulk of this snow to remain south of the terminal, but
small shifts in the location are possible and will continue to
monitor. 

SPECIFICS FOR KSTL: Expect ongoing light snow to push south of the
terminal by 00z. Strong NW winds will slowly diminish Wed morning.
Remainder of TAF period expected to be VFR.

Tilly

&&

.CLIMATE...  

Record lows and record "low" high temperatures

               Wednesday (Jan. 30)         Thursday (Jan. 31)

St. Louis     -9 (1936)/ 10 (2004)        -9 (1936)/ 10 (1951) 

Columbia      -9 (1936)/ 8 (1965)        -12 (1936)/ 5 (1985)

Quincy       -13 (1965)/ 1 (2004)        -14 (1996)/ 3 (1985)


&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Wind Chill Advisory until noon CST Wednesday for Cole MO-
     Crawford MO-Franklin MO-Gasconade MO-Iron MO-Jefferson MO-
     Madison MO-Moniteau MO-Osage MO-Reynolds MO-Saint Francois 
     MO-Sainte Genevieve MO-Washington MO.

     Wind Chill Warning until 3 PM CST Wednesday for Audrain MO-Boone 
     MO-Callaway MO-Lincoln MO-Montgomery MO-Saint Charles MO-
     Saint Louis City MO-Saint Louis MO-Warren MO.

     Wind Chill Warning until noon CST Thursday for Knox MO-Lewis MO-
     Marion MO-Monroe MO-Pike MO-Ralls MO-Shelby MO.

IL...Wind Chill Advisory until noon CST Wednesday for Monroe IL-
     Randolph IL.

     Wind Chill Warning until 3 PM CST Wednesday for Bond IL-Calhoun 
     IL-Clinton IL-Fayette IL-Jersey IL-Madison IL-Marion IL-
     Saint Clair IL-Washington IL.

     Wind Chill Warning until noon CST Thursday for Adams IL-Brown IL-
     Greene IL-Macoupin IL-Montgomery IL-Pike IL.

&&

$$

WFO LSX