Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 290500

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1200 AM CDT Sat Apr 29 2017

Issued at 932 PM CDT Fri Apr 28 2017

Two surface frontal boundaries exist over our region this evening.
The northern one extends from near Chicago southwestward to
Quincy and to near Sedalia. This has not moved much over the past
few hours and if CAA and pressure rise trends are any indication,
is undergoing frontolysis and will dissolve soon. The second
frontal boundary is southeast of our CWA and extends from just
north of Evansville, IN to just south of Cape Girardeau to near
Poplar Bluff. It is this front that will soon become the main
front as its northern cousin dissipates.

The 00z NAM as well as the various iterations of the HRRR models
continue to support the instability axis across southern MO and
far southern IL. While thunderstorm coverage, currently growing in
southeast MO and southern IL, will continue to increase and
expand northward, it is over the nearly stationary instability
axis in the southern CWA where the greatest threat for excessive
rain will be along with an isolated severe storm which will
probably tend to favor hail as the primary severe threat.

The northward expansion of the pcpn later tonight will be
courtesy of the southern front (by this time...just "the front")
buckling back to the northwest in response to a strong low
pressure wave approaching from the southwest. It is how far this
front buckles back to the northwest which will decide how this
forecast evolves on Saturday, particularly in the afternoon and
evening. Current model depictions generally favor the front
getting thru STL metro, but not much more, but there remains an
unusually large spread despite this being within a 24hr forecast.
This should enable pcpn to expand into STL metro later tonight and
into much of central MO with little problem, but questionable on
whether it reaches UIN before 12z/Sat. Instability for thunder
will also tend to lag the northward expansion of pcpn.

All in all, the overall forecast remains in play, with increasing
rain chances and main initial flood threat in the south--expanding
northward after daybreak on Saturday.



.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 339 PM CDT Fri Apr 28 2017

Showers and thunderstorms that are ongoing over southwest and
south central IL are occurring in association with a region of
elevated CAPE, and lift via a south-southwesterly LLJ and
migratory impulse in the southwest flow aloft. Present indications
are this activity will continue to move east and exit the CWA
before 00Z in association with continued eastward motion of the
upper level disturbance. Thereafter there may be a rather tranquil
period until late this evening. The wavy warm front currently
extends from western KY through the bootheel curving southwest
through AR into eastern OK. This front is expected to lift slowly
northward and become better defined across the far southern CWA -
southeast MO and southern IL tonight. Late this evening, another
impulse currently located in KS will translate across the region
providing some large scale ascent. More importantly the broad and
stout southwesterly LLJ is expected to result in strong lift along
the broad frontal zone, with the ascent primarily focused along
the front and into the cool sector. This is expected to result in
widespread thunderstorm development from 03-06Z across southern MO
and southern IL. Attendant with these storms will be a threat of
large hail, as well as heavy rainfall/flash flooding giving there
west-east alignment and high rainfall rate and training potential.
The axis of storms and the northward and growing adjoining
stratiform rain region should then gradually spread northward
through the overnight hours with the strongest storms shifting to
the east into the OH Valley region.

A stormy and volatile period is expected on Saturday with potential
for both severe weather as well as heavy rain and flash flooding.
There remains uncertainty in the specifics that we probably won`t be
able to nail down until as we get much closer. The overall scenario
has the mid/upper flow continuing to back and become more south-
southwesterly as the upper low/trof shift across the
southern/central Rockies into the Plains. The next result is the
warm front will lift northward through central/eastern MO and
southern IL during the day as a surface low tracks into eastern OK.
The result will be an expanding unstable warm sector and therefore
risk of surface-based storms with attendant hail and damaging wind
threats. Complicating the whole scenario is an ongoing MCS will move
from western MO and into central MO during the late morning and then
across the remainder of the CWA during the afternoon hours. Current
thinking is that while WAA generated showers and storms will be
possible at just about anytime, this anticipated MCS will re-
energize by afternoon and pose a greater severe weather threat as it
taps the instability of the expanding warm sector. A very moist air
mass with PWs greater than 200% of normal will promote high rainfall
rates and any locations seeing longevity of storms and training will
have a heightened flash flood threat.


.LONG TERM...  (Saturday Night through Next Friday)
Issued at 339 PM CDT Fri Apr 28 2017

Saturday night, upper low to continue lifting northeast out of
southern Plains into region, which will begin to lift warm front
back to the north inconjunction with deepening surface low. Strong
low level jet will pull in plenty of moisture, so continue to see
PWs in excess of 1.5 inches, with some models indicating close to
PWs around 2 inches. Will see several rounds of showers and storms
Saturday night through Sunday. Model differences on location of main
axis of highest qpf still a problem with NAM the furthest north.
Will keep superblend solution, which is a bit further north than
yesterday and a bit less than the WPC qpf. As for chances of severe
weather, strong to severe storms possible, especially over east
central/southeast MO and southwestern IL Saturday night.

By Sunday as surface low moves due north over western MO, warm front
to slide north of forecast area, while surface low drags cold front
through region during the afternoon and early evening hours. So dry
slot to move in behind cold front with precipitation tapering off
towards Midnight Sunday. Strong to severe chances look best over
southeast MO and far southern IL.

Storm total qpf still looks to be between 3 and 5 inches with the
highest amounts extending along the I-44 corridor in MO and I-55
corridor in IL. This could lead to a significant hydrologic event
with moderate to major flooding occurring on a number of river
basins. Flash flood watch to remain in effect through 06z Monday,
though have added a few more counties along the northern side of the

Will see wrap around showers on backside of system late Sunday night
through Monday. Beyond that, another brief break in the
precipitation before next system moves Tuesday night through
Thursday night. Extended models having differences in timing and
placement with this system, so went with a blend for now. Otherwise,
coolest temps will be on Monday with highs in the 50 to 60 degree
range, then moderate back into the 60s for the remainder of the
forecast period.



.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1157 PM CDT Fri Apr 28 2017

IFR conditions are expected to prevail for the TAF sites for much
of the valid period, especially as a frontal boundary to our south
will be slow to pull northward. Rain will continue to increase
from the south and impact most TAF sites thru much of the valid
period once it moves in. Looks like the best opportunity for a
break from the IFR conditions will be with the front moving to the
north late in the valid period, but it may not move thru any TAF
sites except for STL metro, and did lift CIGs into MVFR-low end
VFR as a result. Surface winds will be NE-E north of the front
and will begin to become oriented this way this evening and will
veer southerly when the front pushes north thru STL metro later on
Saturday. Winds will also become gusty later tonight and Saturday
as the storm system to the west winds up.



MO...Flash Flood Watch through late Sunday night 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...Flash Flood Watch through late Sunday night 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.



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