Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 231217
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
717 AM CDT Thu Oct 23 2014

.SHORT TERM:  (Through This Afternoon)
Issued at 313 AM CDT Thu Oct 23 2014

Broad N-S band of showers and isolated thunderstorms ahead of plains
shortwave currently stretch from western IA, through nw MO, into se
KS, and the primary forecast question for today is how far east this
rain will get.   Strong ridging with very dry low level air remains
locked in place from the Great Lakes to the Ohio Valley by storm
system working its way up the east coast, and this will certainly
inhibit any substantial increase in lower tropospheric moisture.
Believe that the combination of shortwave energy, moisture
advection, and isentropic ascent will cause upstream showers to work
work into our far nw counties this morning, with this precip then
struggling eastward during the day and "withering on the vine" from
central MO into west central IL during the afternoon hours as
shortwave outruns the limited low level moisture return and
encounters much drier air.  Still working on fine-tuning the
details, but at this time planning to limit any mention of precip to
areas northwest of a Cuba-St Louis-Hillsboro line, with highest PoPs
just east of Kirksville.

Truett

.LONG TERM:  (Tonight through Wednesday)
Issued at 313 AM CDT Thu Oct 23 2014

(Tonight through Saturday)

With no precip anticipated, focus thru this period will be temps.

Mdls are in good agreement with upper trof moving E of the region
overnight and swly flow at the sfc returning ahead of the next
cdfnt. This cdfnt is progd to push thru Sat morning, which may
require adjustments to temps on Sat with future updates.

With clouds expected to clear late tonight, have trended twd the
cooler guidance for tonight. With the thermal ridge advecting into
the region for Fri, have trended twd the warmer guidance. Similar
trends for Fri night with swly flow continuing overnight. Sat has
some forecast challenge with fropa expected in the morning. With
full insolation expected, have trended twd the warmer guidance even
with a nwly flow.

(Sunday through Wednesday)

Focus turns to precip chances on Tues.

23/00z mdls are in generally good agreement thru Tues morning, tho
the GEM is faster with the upper trof ejecting into the Plains. Have
focused PoPs on Mon night and Tues when the ECMWF/GFS have come into
better agreement. However, the 00z solns are somewhat different from
the prev 12z solns. Therefore, have still kept PoPs on the lower
side for now until a persistent signal is given.


Tilly

&&

.AVIATION:  (For the 12z TAFs through 12z Friday Morning)
Issued at 649 AM CDT Thu Oct 23 2014

Although broad area of WAA has produced some lowering of VFR
ceilings over western and northern MO, low level flow originating
from surface ridge over the Ohio Valley will continue to pump very
dry low level air (aob 3kft) into the FA throughout the day. While
increasing mid and high level moisture will lead to a general
increase in clouds from west to east during the day, this lower
level dry air should prevent ceilings from dropping below the VFR
category, and also cause the area of showers currently west of the
FA to struggle to spread east during the day. Showers have made
very little eastward progression so far this morning, and based on
this trend have opted to leave out mention of showers at all
locations at this time, and will let oncoming shift nowcast this
low-impact threat. Passage of shortwave and breakdown of the warm
advection should cause moisture to stratify and clouds to thin
heading into the evening, but subtle increase in surface dewpoints
may also lead to some predawn fog. All of the available MOS output
suggests some vsby restrictions after 06z, and have introduced
some MVFR vsbys at all location during the predawn hours with the
thinking that these trends will be fine-tuned by later shifts.

Specifics for KSTL: Clouds aoa 5kft will impact the area today and
into the evening. A few afternoon showers cannot totally be ruled
out, but at this point coverage is expected to be so spotty in the
metro area that the mention has been omitted from the forecast at
this time. However, did follow MOS trends of bringing in some MVFR
fog late tonight due to subtle increase in surface dewpoints.

Truett

&&

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

$$

WFO LSX






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