Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 201211
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
711 AM CDT Wed May 20 2015

.SHORT TERM:  (Through Tonight)
Issued at 316 AM CDT Wed May 20 2015

A broad area of rain progresses across the forecast area today in
association with a wave of low pressure riding along a
quasistationary frontal boundary. The precipitation is forced and
maintained by a strong southerly H85 LLJ interacting with a
quasistationary boundary. Isolated embedded thunderstorms are
possible. The upper vort center behind the surface wave moves into
weakly confluent flow and merges with a separate vort center which
originates over Manitoba. The resulting trough forms over Iowa
tonight and moves to the east or slightly south of east. Surface
high pressure will have already spread into the forecast area by
this time and no additional precipitation is expected with the
trough overnight, although the slight increase in lift could
prolong any ongoing showers across the eastern forecast area
between 00-06z.

The forecast area remains on the cool side of a quasistationary
boundary today and extensive clouds/precip will limit insolation.
These unusually cool conditions might set or tie some climate
records. Please refer to the climate section below.

42

.LONG TERM:  (Thursday through Tuesday)
Issued at 316 AM CDT Wed May 20 2015

High pressure remains in place on Thursday and Friday. An upper
ridge also builds over the central US on Friday then amplifies on
Saturday ahead of a deepening trough over the western half of the
US. Several disturbances lift through the trough between Saturday
and Monday. These disturbances should bring a period of unsettled
weather to the forecast area, especially between Sunday and
Tuesday. Similar to the last two weekends, day to day
precipitation chances will depend both on synoptic scale
shortwaves and on any MCVs which form upstream. Refinements are
likely. Temperatures begin to warm up again on Saturday when winds
become southerly around the retreating surface high. A more
noticeable jump in warmth is expected on Sunday after a warm
frontal passage.

42

&&

.AVIATION:  (For the 12z TAFs through 12z Thursday Morning)
Issued at 705 AM CDT Wed May 20 2015

Have used extrapolation of current radar trends for onset of the
rain this morning, and the latest (10z) HRRR data to time the end
of the rain later this morning and early this afternoon. For
cig/vsby trends, it looks like a very atypical May day with MVFR
vsbys in rain/fog, and IFR to MVFR cigs developing in the steady
rainfall. I`ve gone with prevailing MVFR vsbys/IFR cigs in COU by midday
where low level saturation should be greatest due to the rain this
morning, but elsewhere I`ve kept cigs MVFR with the thinking that
rainfall will be somewhat less, and because NE winds will be
attempting to advect relatively drier low level air in from the
ridge to our north. This thinking dovetails fairly well with 06z
MAV guidance. Heading into tonight there is lots of
uncertainty...will the southward oozing of the surface ridge scour
out the residual low level moisture or will the moisture become
trapped beneath an inversion? Because of this, forecast specifics
trail off early this evening, and will let day shift take a crack
at this time period as we get a better feel for trends later this
afternoon.

Specifics for KSTL: Steady rain is expected to develop by mid
morning, with cigs/vsbys heading into MVFR category by midday.
Latest HRRR guidance suggests rain ending by mid afternoon, but
have maintained MVFR cigs into the evening due to saturation
produced by the rainfall. Some lingering MVFR vsbys in fog is also
anticipated for several hours after the rain ends.

Truett

&&

.CLIMATE:
Issued at 316 AM CDT Wed May 20 2015

Record cool high temperatures for May 20th are as follows.

St. Louis, MO....55/1924 (KSTL)
Columbia, MO.....57/1924 (KCOU)
Quincy, IL.......57/1967 (KUIN)
Farmington, MO...66/1954 (co-op station)


&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX





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