Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 230859
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
359 AM CDT Thu Apr 23 2015
.SHORT TERM: (Through Late This Afternoon)
Issued at 348 AM CDT Thu Apr 23 2015
Will continue the frost advisory early this morning for the entire
area, although the mid level cloudiness across parts of central
and southeast MO will hinder frost formation. There will still be
at least patchy frost across this area. The most widespread frost
will likely be across northeast MO and west central IL where the
sky has been crystal clear, the surface winds were light near the
surface ridge axis, and the surface dew points were quite low for
late April. Much of the mid level cloudiness across central and
southeast MO will shift southeast of the forecast area by early
afternoon. With plenty of sunshine for most of the area later this
morning and this afternoon, along with slowly rising 850 mb
temperatures, and rising upper level heights, the high
temperatures this afternoon should be a little warmer than
Wednesday, albeit still a little below normal.
.LONG TERM: (Tonight through Wednesday)
Issued at 348 AM CDT Thu Apr 23 2015
Weather conditions should be quiet for at least the first half of
tonight with an H5 ridge axis moving across the area. At the same
time, surface winds will increasingly become more southerly around
the back side of a surface high pressure center located over the
Ohio Valley. This will aid low-level moisture return across the
region ahead of the next low pressure system, which will be
located near the NM/TX/Mexico border by 12z Fri. Isentropic ascent
could support isolated to scattered SHRA/TSRA after midnight,
especially across the southwestern CWA.
Models show the vort max initially lifting northeastward into the
plains on Fri before the H5 pattern over the north central CONUS
and southern Canada steers this disturbance more towards the east
than towards the north, a path which takes it across Missouri on
Fri night into Sat. As the decaying LLJ shifts eastward on Friday
morning, some remnant convection could move into the western CWA.
A warm front will lift northeastward through the area on Friday
afternoon/night with the approach of the surface low. Convective
trends are less clear between 18z-00z, but convective coverage
should increase after 00z due to increasing large scale ascent
ahead of the vort max, a strengthening southwesterly LLJ, and
favorable dynamics in the LER dynamics of an approaching H5 jet.
This is especially true for the northern half of the CWA.
The more eastward motion of the low pressure system on Saturday
means that the tail end of the E-W boundary won`t sink south as a
cold front until Saturday night, which means that thunderstorm
chances remain in the forecast for most of Saturday and into early
Saturday night. Outflow boundaries from the previous night could
serve to focus convection on Saturday. How far south the cold
front then sinks will influence precipitation chances for early
next week when a new surface low forms somewhere along it.
While the Fri/Sat disturbance moves away from the area and is
eventually absorbed by a large and complex low centered near the
Maine coast, another low pressure system will be developing over
the desert southwest by early next week. Model solutions diverge
for Tuesday and beyond due to different handling of a vort max
near the ND/MN/Canada border and its subsequent evolution and
interaction with other features across the central US.
.AVIATION: (For the 06z TAFs through 06z Thursday Night)
Issued at 1130 PM CDT Wed Apr 22 2015
VFR conditions and dry wx will prevail at the TAF sites thru the
valid period. Otherwise, light NW surface winds will become
variable in direction from west to east during the course of
Thursday, and eventually settle from the east.
Specifics for KSTL: VFR and dry thru the period. Light NW surface
winds will become variable in direction late Thursday afternoon
for a few hours as a high pressure RIDGE moves in before becoming
easterly by late evening.
MO...FROST ADVISORY until 8 AM CDT this morning FOR Audrain MO-Boone
MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln MO-Madison MO-
Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike
MO-Ralls MO-Reynolds MO-Shelby MO-St. Charles MO-St.
Francois MO-St. Louis City MO-St. Louis MO-Ste. Genevieve
MO-Warren MO-Washington MO.
IL...FROST ADVISORY until 8 AM CDT this morning FOR Adams IL-Bond IL-
Brown 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-St. Clair IL-Washington IL.