898 
FXUS63 KLSX 061218
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
618 AM CST Tue Nov 6 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 335 AM CST Tue Nov 6 2018

A pair of PV anomalies was noted upstream early this morning on 
both water vapor imagery and the RUC 1.5 PVU analysis. These 
features are forecast to rotate through a broad trough located over 
the northern half of the CONUS before phasing tonight near the Great 
Lakes. The passage of the trough which results from the phasing of 
these features will allow a secondary cold front to approach the 
region late tonight and then move through the region tomorrow (see 
discussion below). 

Models were depicting areas of H7 and H85 frontogenesis at 09z 
which were reasonably well correlated with light precipitation and
developing radar echoes observed over WY/NEB/CO early this 
morning. These areas of forcing are forecast to diminish over the 
next several hours followed by the new development of stronger 
areas of forcing over KS which then begin stretching eastward into
MO. By 00z, these areas of frontogenesis at H7 and H85 are 
forecast to stretch across northwestern into northern MO before 
decaying. Model forecasts show that new areas of frontogenesis 
then develop farther to the southeast and progress southeastward 
through late tonight. Taken together, this suggests that light 
rain should develop in KS this afternoon and then spread into 
central MO before precipitation begins to shift southeastward 
overnight. 

Highs today should be several degrees warmer than yesterday across 
most of the area due to lack of cloud cover for most of the day. The 
exception will be across the far northern CWA where persistent cloud 
cover will lead to a much more modest rise compared to yesterday. 

Breezy conditions and gusty winds are expected today, especially 
across the northeast half of the LSX CWA. BUFKIT soundings show that 
20-30kt wind gusts should mix down to the surface after 14z. 

Kanofsky

.LONG TERM...  (Wednesday through Monday)
Issued at 335 AM CST Tue Nov 6 2018

A broad-based longwave upper TROF will prevail over the central 
CONUS at the beginning of the period and should continue into the 
upcoming weekend.  Most models at this point then shift the TROF 
axis slightly eastward heading into early next week, resulting in a 
more NW flow setup.  A couple of storm systems will affect our 
region during this time:  one during Thursday-Friday and another on 
Monday.  This pattern will certainly favor below normal temperatures 
for a prolonged period of time, and if the upper TROF axis indeed 
does shift more to the east, it will open the door for much below 
normal temperatures as chilly Canadian airmasses drop down. 

The first storm system on Thursday-Friday has been intriguing for a 
couple days now, potentially giving our region its first taste of 
snow with a 1-2 punch of upper level disturbances.  Both of these 
disturbances are far away at this time:  one over the north Pacific 
just off the coast from Alaska and the other over northern Canada. 
A good model consensus then has the Pacific disturbance tracking 
into our region first, on Thursday night, with the second dropping 
down from the north late on Friday.  The first disturbance will have 
access to more moisture and overall have a more favorable track to 
affect the most area and currently is depicted to create a fairly 
broad swath of light precipitation across the northwest half of the 
forecast area--namely north of I-44 in MO and I-70 in IL.  If this 
were to be all rain throughout the event, then I have probably spent 
too much time talking about this already.  However, this system will 
be affecting our forecast area during a time when noticeably colder 
air will be settling in for the long haul and during a time of day 
that is more favorable for non-liquid precipitation during the 
fringes of the snowy season.  Most of the precipitation at this time 
looks set to move in during Thursday evening and move out after 
midnight.  There is good consensus on there being a strong presence 
of cold (sub-freezing) air above 1000ft AGL with decent potential of 
evaporational cooling at the boundary layer.  While precipitation-
types will likely be rain when the precipitation first moves in, 
with saturation of the column, there should be a rather quick change 
to snow, within a couple hours of onset in most cases.  However, any 
snow accums on the ground will be limited by warm ground and what 
will likely be slightly above freezing surface temps for much of the 
event.  As it stands now, snow accumulations of less than an inch 
are forecast primarily for areas north and west of STL metro, mostly 
on grassy surfaces, with an anticipated minimal impact on roadways. 
Confidence in this system looks medium at this time, with decent run-
to-run consistency the past couple of days on track and QPF, but 
changeover to snow will depend heavily on generating enough 
precipitation to saturate the column and this is dependent on an 
accurate assessment of the track of a shortwave currently over the 
Pacific that will be moving along the broad base of an upper TROF: a 
scenario that doesn't lend to much confidence this far out.  The 
track of the second half of this 1-2 punch mainly favors areas north 
and northeast of the forecast area, with perhaps another shot of 
mixed rain-snow late Friday afternoon or evening for areas near 
Quincy. 

Another NW flow disturbance may impact our region on Monday, with 
the air likely cold enough to support another round of light mixed 
rain-snow precipitation, but initial impact assessment looks low, 
plus it is far out in time and lots can change here.

The main takeaway for the weather after Thursday's system is below 
normal temperatures, at times much below, will continue and look to 
persist well into next week.

TES

&&

.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 508 AM CST Tue Nov 6 2018

SPECIFICS FOR KUIN: An extensive MVFR cloud deck was located over
the region at TAF issuance. Conditions are likely to remain MVFR 
at KUIN for most of the first 0-6 hours before diurnal mixing 
leads to rising cloud bases and an improvement to VFR. Gusty west
winds are expected to diminish around sunset. Mid/high clouds will
increase after 21z ahead of a weak disturbance.

SPECIFICS FOR KCOU: An extensive MVFR stratus deck was located 
upstream from KCOU at TAF issuance. Based on satellite imagery, 
the southwestern edge may graze KCOU during the first 0-6 hours, 
but VFR conditions are expected to prevail for the majority of 
the valid TAF period. Gusty west winds will diminish around 
sunset, and mid/high clouds will also begin to increase after 
07/03z ahead of an approaching disturbance. There is a slight 
chance of rain after 21z.

SPECIFICS FOR KSTL, KSUS, KCPS: An area of MVFR clouds was
rotating through the region in the wake of last night's low
pressure system. The southern edge of this cloud deck should graze
the St. Louis metropolitan area terminals during the first 0-6
hours before diurnal mixing leads to rising cloud bases. However,
given the erosion noted on the southern edge of the cloud deck on
satellite imagery, it is also possible that the BKN/OVC portion of
the MVFR cloud deck could remain just north of the terminals.
Given the latest satellite trends, the most likely outcome 
appears to be 2-4 hour window of MVFR ceilings in the St. Louis 
metropolitan area. VFR conditions with gusty west winds are then
expected for most of the daylight hours. There is a slight chance
of rain after 07/03z.

Kanofsky

&&

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

$$

WFO LSX