900 
FXUS63 KLSX 012153
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
353 PM CST Fri Mar 1 2019

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 347 PM CST Fri Mar 1 2019

A weak ridge of high pressure centered over eastern Arkansas and 
extending up the Mississippi Valley will drift southeast tonight. 
Another, much stronger high centered over southwest Canada will 
begin nosing into Missouri and Illinois tonight, bringing the 
leading edge of the cold air down into southern Illinois and 
southeast Missouri by early Saturday morning.  Upstream temperatures 
in Iowa, Nebraska, and South Dakota are generally in the 20s at this 
time on the leading edge of the high, and this agrees well with MOS 
forecast lows for Saturday morning across our area.  There is light 
precipitation showing up on regional RADARs over northern Missouri 
and Iowa up into Minnesota and Wisconsin associated with a shortwave 
that's moving across the Midwest.  There haven't been many reports 
of the precipitation reaching the ground in Iowa and none in 
Missouri, but I can't rule out a few sprinkles or flurries this 
evening across northeast Missouri and west central Illinois.

The cold advection weakens a bit on Saturday as the low level flow 
veers a bit from the northwest to the north-northeast.  This will 
allow for a least a modest diurnal rise in temperatures into the 
afternoon.  However the guidance looks too warm (especially the MAV) 
given the northerly flow and likely overcast sky.  I leaned toward 
the colder guidance for highs Saturday which yielded highs around 30 
in northeast Missouri to the low and mid 40s from St. Louis and 
further south.  There may be some sprinkles or flurries in the 
afternoon due to increasing low and mid level frontogenesis ahead of 
Saturday night's storm system.  I do not expect any of this 
precipitation to be heavy enough to accumulate as forecast soundings 
show a dry layer around 850mb as well as a dry boundary layer.

Carney

.LONG TERM...  (Saturday Night through Next Friday)
Issued at 347 PM CST Fri Mar 1 2019

Our main focus this forecast remains centered on the winter storm 
for Saturday night and Sunday, followed by very cold weather and 
well below normal temperatures through the first part of next week.

Overall the new guidance suite is a tad further south and not quite 
as robust with QPF with the upcoming winter storm system for Saturday
night into Sunday. It also looks like the onset of precipitation 
may be a bit slower. There is really a lack of much organized forcing
on Saturday evening and thus for most areas I think it will remain
dry. The possible exception is for perhaps the extreme southern 
portion of the CWA - southeast MO and far southwest IL. Across that
area both the ECWMF and NAM are showing some light QPF for a portion
of the evening, and that appears to be primarily in response to 
low level WAA. Thermal profiles suggest it could be a wintry mix 
on the NAM while the GFS and ECWMF would suggest a rain/snow mix. 

The real thrust of the winter storm still looks on track for overnight
Saturday into Sunday morning or 06-18z Sunday. Forcing/ascent 
associated with the fast-moving migratory upper trof, increasing 
low-mid level frontogenetic forcing, and divergence with the upper
level jet exit region should result in a large west-east oriented
band of snow overspreading spreading the area from the west, entering
central MO between 06-09z and reaching south central IL just before
daybreak. While there could be a period of accumulating sleet 
across southeast MO and southwest IL where there are suggestions
of an eroding elevated warm layer, diabatic cooling should change
any precipitation over to snow by 12z. The snow should come to an
end as the fast-moving upper trof zips by with measurable snowfall
ending by early afternoon along the MS River and in our eastern 
CWA by 21z.

A climatological snow band study by Gravelle indicates the median
width of the 4+ inch snow band in these types of events is on the
order of 85 nm. Meanwhile applying techniques of Browne and 
Younkin, the composite track of the 850 mb low would place the 
axis of heaviest snowfall from roughly the Lake of the Ozarks to 
around Mount Vernon, IL. This region is also where omega is 
briefly maximized within the dendritic growth zone on Sunday 
morning. Overall liquid to snow ratios will increase during the 
event, primarily due to CAA. This currently looks like a widespread
winter weather advisory level event with snowfall amounts ranging
from 2-3 inches across northeast MO/west central IL to 4-5+ 
inches along and south of Interstate 70 with isolated 6+ inch 
amounts possible - likely within the aforementioned KAIZ-KMVN 
corridor. We are leaving the winter storm watch in place for now 
after collaboration, and to help with the uncertainty in the 
forecast for neighboring offices to the west. Hazardous travel is 
expected after the onset of snowfall/wintry precipitation late 
Saturday night and especially on Sunday morning with snowfall 
rates maximized at 1/2 to 1 inch per hour and due to the cold 
ground temperatures. North-northeast surface winds will also be 
increasing and become gusty during the morning and into the 
afternoon associated with the tightening pressure gradient and 
deepening surface low to the south-southeast of the region. These 
winds could produce some blowing and minor drifting of snow, 
adding to the hazardous travel.

The period from Sunday night into Monday still looks exceptionally 
cold for early March with temperatures around 30 degrees below 
normal. While the winds will be diminishing late Sunday evening 
into Monday morning, they will still be high enough when combined 
with temperatures falling to near zero to produce dangerous wind 
chills across parts of northeast MO and west central IL.

A deep long wave trof centered through eastern NOAM will keep it 
cold with below average temperatures into midweek. The GFS and ECMWF 
suggest there is potential for more wintry precipitation Thursday 
into Thursday night although the pathways getting there are a bit
different.
  
Glass

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1130 AM CST Fri Mar 1 2019

A broad area of MVFR ceilings stretches from northeast Missouri
into central and southeast Missouri and southern Illinois at this
time. Additionally, IFR ceilings and some fog is being reported
across the eastern Ozarks. The ceilings rise to VFR levels from
the St. Louis Metro area eastward along the I-70 corridor and to
the north up I-55 and Illinois Highway 67 into central Illinois.
The lower ceilings are expected to improve slowly with time up to
low-end VFR for most of, if not all the area...but this may take 
all night in some spots. The eastern Ozarks in southeast Missouri
may be one of those places that stays below 3,000 feet for the
entire night. Otherwise, light and variable wind will prevail
until Saturday morning as a weak ridge of high pressure drifts
across the area. The wind will shift to the northwest as a cold
front moves through the Mississippi Valley on Saturday morning.

SPECIFICS FOR KSTL: 

Expect low-end VFR ceilings to continue to prevail at Lambert this
afternoon and into tonight. Some of the model guidance is hinting
that the ceiling could drop below 3,000 feet for a short period
this afternoon, but this seems unlikely given the warming
temperatures this afternoon. Light and variable wind under the 
influence of weak high pressure will turn to the northwest 
Saturday morning as a cold front moves through the area.

Carney

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Winter Storm Watch from Saturday evening through Sunday 
     afternoon for Audrain MO-Boone MO-Callaway MO-Cole MO-
     Crawford MO-Franklin MO-Gasconade MO-Iron MO-Jefferson MO-
     Lincoln MO-Madison MO-Moniteau MO-Monroe MO-Montgomery MO-
     Osage MO-Pike MO-Ralls MO-Reynolds MO-Saint Charles MO-
     Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte 
     Genevieve MO-Warren MO-Washington MO.

IL...Winter Storm Watch from Saturday evening through Sunday 
     afternoon for Bond IL-Calhoun IL-Clinton IL-Fayette IL-
     Greene IL-Jersey IL-Macoupin IL-Madison IL-Marion IL-Monroe 
     IL-Montgomery IL-Pike IL-Randolph IL-Saint Clair IL-
     Washington IL.

&&

$$

WFO LSX