Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 300454
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1154 PM CDT Mon Jun 29 2015
.SHORT TERM: (Through Late Tonight)
Issued at 347 PM CDT Mon Jun 29 2015
Isolated convection has formed over the last hour in a narrow
north-south band along the Mississippi River from near Quincy to
near St Louis. Could be a bit more development over the next
several hours, and then anticipate that this activity will quickly
dissipate with loss of heating.
However, upstream shortwave currently fueling convection over
eastern MN and nw WI will be dropping south overnight, and this
may return a low shower/thunderstorm threat to areas along and
north of I-70 in the midnight-daybreak time frame.
.LONG TERM: (Tuesday through Next Monday)
Issued at 347 PM CDT Mon Jun 29 2015
Weak ripples in the upper level NW flow will continue a threat of
showers and a few thunderstorms into the day on Tuesday. Have
attempted to reflect a bit of a diurnal trend to the PoPs with
afternoon heating and instability leading to a bit more of a
threat/coverage, but this trend may certainly be altered by the
arrival of the difficult to time shortwaves. At this point I`ve
only made modest tweaks to going forecasts, and will likely have to
save any major adjustments to the nowcast time frame.
Unfortunately, it appears that several rounds of convection...and
more heavy rain...are in the offing as we head into midweek. 12z
runs are maintaining trends of previous solutions in forecasting a
slight eastward shift of the upper level trof during this time,
which will allow baroclinicity over the plains to work back to
the east. Deep UVV is progged to develop across the FA on Tuesday
night due to strong shortwave in the NW flow sweeping into the
mid-Mississippi Valley, with resultant low level WAA producing
strong isentropic ascent over eastward advancing baroclinic zone.
Resultant strong lift and rapid eastward advection of lower
tropospheric moisture should cause elevated convection to develop
during the late evening over western MO and then grow into an MCS
which will then work its way east-southeast late Tuesday night and
early Wednesday morning. This round of convection should then be
followed by more storms late Wednesday and Wednesday night as yet
another shortwave in the NW flow propagates into the region, with
both the GFS and NAM suggesting nearly persistent WAA/theta-e
advection over the southern half of the CWA in this time frame.
Both the GFS and the NAM are progging PWATS of over 2 inches in
our area heading into midweek, so storms should be efficient rain-
makers...again... It does appear that the heaviest rainfall may be
located over the southwest or southern half of the CWA (avoiding
the critically water-logged northeast half of the FA), but in
reality no spot in our CWA needs any more wet weather. Have
mentioned this heavy rain potential in a hydrologic outlook
issued earlier this afternoon.
A chance of thunderstorms will certain carry into Thursday, but it
does appear that the barrage of shortwaves in the Tuesday night-
Wednesday night time frame...along with the convection they
produce...will cause a southward and westward displacement of the
low level baroclinicity, lessening the heavy rain threat by that time.
Medium range solutions maintain ridging over the western CONUS and
a mean trof over the northeast U.S.,which places mid-Mississippi
Valley in the heart of the NW flow regime throughout the period.
With shortwaves rippling through this NW flow it appears that
there will be a chance of showers and thunderstorms throughout the
Friday-Monday time frame. There should be lots of dry time and
certainly don`t see any all-day rains at this point, but for now
no solution really offers any significant rain-free period.
.AVIATION: (For the 06z TAFs through 06z Tuesday Night)
Issued at 1146 PM CDT Mon Jun 29 2015
Specifics for KCOU, KUIN: Radiational fog is possible at KUIN
because late afternoon rainfall has locally increased the low
level moisture, winds will be light, and skies will be mostly
clear. No fog is currently forecast at KCOU because T-Td spreads
were higher and no rain had occurred at KCOU earlier in the
period. Expect winds to back overnight and then veer again after
sunrise. Winds then become light/variable late in the TAF period.
Specifics for KSTL, KSUS, KCPS: Isolated convection continues to
percolate near and along outflow boundaries from earlier
convection. Given the light winds and nearly clear skies,
radiational fog should develop overnight in the St. Louis metro
area, especially at KSUS and KCPS where increased moisture from
rainfall during the late afternoon and mid-evening hours will
reduce the T-Td spread. Otherwise, expect winds to back overnight
and then veer again after sunrise. Rain is possible at KSTL late
in the 30-hour TAF period, but there is low confidence in the
timing or duration attm.