Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 241154
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
654 AM CDT Thu Apr 24 2014
.SHORT TERM: (Today through Friday
Issued at 329 AM CDT Thu Apr 24 2014
Strong upper level trof is moving east through the Great Plains
this morning. The surface reflection of this trof extends from
eastern Nebraska/Kansas southwest into western Texas. Looks like
good moisture surging ahead of the trof with upper 50s to mid 60s
dewpoints all the way up into eastern Kansas. All short-range
guidance has slowed the eastward progression of this system today,
and thereby also slowing down the precipitation associated with
it. This should allow for a warmer day than previously forecast.
Have bumped up temperatures toward warmer MAV guidance for today.
Should see little but spotty showers across most of the area this
morning, but with increasing chances of precipitation by 18-20Z.
Additional heating of course mean more instability than previously
forecast as well, and the NAM is forecasting a narrow corridor of
1200+ J/Kg MUCAPE ahead of the front this afternoon along and
south of the I-70 corridor. With deep layer shear in excess of 40
kts it does look like some severe storms will be possible.
Convective mode is somewhat indeterminate with shear strong enough
for supercells, but instability probably lacking a bit. I kind of
like the latest run of the HRRR which as a broken line of storms
moving across the area this afternoon, tho with a bit more
instability the convection could easily be more discrete
initially. Front now looks to be into central Missouri by around
00Z, along the Mississippi River by 03-04Z, and through Salem
Illinois by about 06Z. Have adjusted PoPs for this slower
progression through tonight.
Temperatures on Friday are still somewhat in question. A relatively
mild Pacific airmass will build in behind the front on Friday.
850mb temps don`t really fall off that much and are 10-12C by 00Z
Saturday. This would suggest mid to upper 70s which makes MOS
numbers a bit too cool across most of the area. Bumped up a bit
over MOS to compensate.
.LONG TERM: (Saturday through Wednesday)
Issued at 403 AM CDT Thu Apr 24 2014
Guidance continues to come into better agreement for the weekend.
The secondary front which was forecast to sweep all the way
through Missouri into Arkansas earlier in the week is now only
going to get to southern Iowa according to the model collective.
Still showing a good low level baroclinic zone stretching from
southwest Missouri up into Iowa Saturday through Sunday with a
southwest low level jet ramping up to 40-50kts flowing
perpendicular to the thermal gradient. All models print out some
light precip in response to this on Saturday, primarily over
central and western Missouri but I wouldn`t be surprised to see it
a bit further north/northeast as well if this verifies.
Temperatures will be highly dependent on how much sunshine we end
up getting, but I leaned heavily on the warmer MAV numbers.
Strong warm advection continues ahead of the ginormous trof which
will be moving over the Rockies Saturday night, and it looks like
there could be some nocturnal convection as a a result...mostly
across northern sections of the CWFA. The trof moves into the Great
Plains on Sunday and closes off while also becoming negatively
tilted. The resulting wound up, slow moving storm system will draw
abundant moisture into the Mid-Mississippi Valley from the Gulf
producing at least a couple of rounds of showers and thunderstorms
Sunday into Monday night before the associated cold front comes
through and drier air works into the area. Temperatures should stay
seasonably mild through the Monday with highs near normal.
Tuesday and Wednesday the storm system moves slowly overhead and
then on into the Ohio Valley. Models print out widespread precip
over the area as cold air aloft moves in with the cut off low.
Heaviest amounts look to be across northern Missouri and southern
Iowa associated with the deformation zone. While this makes sense
conceptually, still not sure I totally believe the medium range
models as this is a highly anomalous pattern for late April/early.
Stuck relatively close to guidance though as the mass fields on the
GFS and ECMWF are in very good agreement considering strength of
this system and its anomalous nature.
.AVIATION: (For the 12z TAFs through 12z Friday Morning)
Issued at 633 AM CDT Thu Apr 24 2014
Needless to say, a complex forecast for this period for area TAF
sites. Showers/thunderstorms currently ongoing over western
Missouri will continue to move ENE this morning, largely fueled by
warm advection and a strong southwesterly low level jet. The HRRR
has maintained a good handle on the evolution of the precipitation
overnight, thus have taken it into consideration regarding future
trends today. Thinking is for SHRA/TSRA from western Missouri to
impact KCOU by around 15Z-16Z and KUIN by 16Z-17Z. This
precipitation should remain northwest of metro TAF sites, thus
have not included precipitation mention for metro sites until late
this afternoon when SHRA/TSRA associated with the cold front are
expected to impact the area. Expect KCOU and KUIN to see a few
rounds of SHRA/TSRA this afternoon, prior to the passage of a cold
front through the terminals by around 00Z (KCOU) and 02Z (KUIN).
For metro TAF sites, expect SHRA/TSRA activity to increase by
ahead of the front, and continue until the front passes between
03Z-04Z, with lingering showers for perhaps another 2 hours
affecting area TAF sites.
Prior to the onset of precipitation, expect continued
gusty southerly/southeasterly winds and VFR flight conditions
along with increasing midlevel clouds. Flight conditions will
decrease to MVFR during SHRA/TSRA, and slowly improve late tonight
post- FROPA. Winds will quickly veer to the west in the wake of
the front, and remain between 10-15KT overnight.
Specifics for KSTL:
VFR flight conditions expected to prevail through the first part
of the forecast period. An area of showers and thunderstorms over
western Missouri continues to move ENE this morning, and is
expected to remain just northwest of the terminal given latest
HRRR guidance. Expect SHRA/TSRA activity to ramp up late this
afternoon ahead of an advancing cold front, and persist until the
front passes through the terminal around 03Z-04Z. Flight
conditions will decrease to MVFR in areas of precipitation, with
ceilings slowly improving overnight after SHRA has ended and
drier air begins to filter into the region in the wake of the cold
front. Winds will remain southerly/southeasterly today, gusting to
20-25KT, and veer quickly to the west behind the front tonight,
remaining between 12-15KT sustained. Clouds are expected to
scatter out by daybreak, providing VFR conditions through the end
of the period on Friday.
.PRELIMINARY POINT TEMPS/POPS:
Saint Louis 73 50 78 55 / 80 80 0 10
Quincy 68 47 76 51 / 80 80 0 10
Columbia 71 46 78 51 / 80 70 0 10
Jefferson City 72 48 79 50 / 80 80 0 10
Salem 74 49 76 51 / 70 80 0 10
Farmington 75 49 77 49 / 80 80 0 10