Area Forecast Discussion
Issued by NWS St. Louis, MO

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816
FXUS63 KLSX 220202
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
902 PM CDT Mon May 21 2018

.UPDATE...
Issued at 902 PM CDT Mon May 21 2018

At 8pm, a weak surface cold front has nearly stalled, sitting from
near Mount Sterling, IL thru far western Saint Charles County
Missouri into central Crawford County Missouri. Light southerly
flow existed ahead of the front and westerly flow behind with a
narrow band of cumulus clouds right along the front. Our office
has had a nice front row seat to these cumulus clouds whose towers
have been steadily collapsing over the past couple hours. Dry
weather is expected for the forecast area overnight with what
should be patches and areas of fog developing, especially ahead of
the weak front where higher dewpoints, low depressions and
clearing skies and light winds will make for good conditions for
radaiational fog. Models not very bullish on dense fog
development, however. This weak front should make very little
progress eastward overnight, with any anticipated drying or
lowering of dewpoints behind it minimal due to very weak flow
behind it.

Otherwise, look for temps to fall into the 60s overnight. Another
warm day is expected on Tuesday with max temps in the low to mid
80s, adding to our already record warm start to May.

TES

&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 339 PM CDT Mon May 21 2018

(Tonight)

Slow-moving cold front is entering central Missouri early this
afternoon, with scattered showers and storms developing along a weak
convergence zone across southeast Missouri into southwest Illinois.
Latest CAMs suggest this ongoing activity will be the focus through
the early evening hours before activity weakens and eventually
dissipates with loss of instability. Remainder of the night should
be dry with decreasing cloudiness. Some fog concern overnight as
lower dewpoints will be slow to filter into the bi-state area. This
would be moreso true in favored valley locations where more clearing
occurs.

Lows tonight are expected to be in the 60s across the region,
remaining on the mild side for mid/late May.

(Tuesday)

Surface ridge of high pressure looks to be in control beginning on
Tuesday with a partly to mostly sunny sky. High temperatures on
Tuesday are forecast to reach the lower to mid 80s, or about 5-10
degrees above normal.

Gosselin

.LONG TERM...  (Tuesday Night through Next Monday)
Issued at 339 PM CDT Mon May 21 2018

(Tuesday Night - Thursday Night)

Dry/mild weather pattern will continue through the middle of the
week as surface high pressure center strengthens and remains
anchored across the Great Lakes. Look for lows in the 60s and highs
in the mid to upper 80s under a partly to mostly clear sky.

(Friday - Memorial Day)

Weather pattern looks like early summer this upcoming Memorial Day
weekend. A weak disturbance aloft is forecast to come out of the mid-
Missouri Valley and becomes nearly stationary. End result is to have
some type of a shear axis near the area which should help initiate
convection, which likely will focus in the afternoon/evening hours
when instability is maximized. While widespread thunderstorm
activity does not look likely through the holiday weekend, isolated
to scattered storms look possible each day, and this type of
activity certainly could impact any outdoor activity plans.

Temperatures likely will remain above normal through the holiday
weekend with continued highs in the mid to upper 80s and lows in the
60s. It appears the end of the month of May will likely finish how
it started with well above normal temperatures.

Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 645 PM CDT Mon May 21 2018

Predominantly VFR conditions, dry weather and light winds will
prevail at the TAF sites thru the valid period. Some high-end MVFR
CIGs will affect some STL metro sites early this evening where
some additional clouds have developed near weak surface cold front
until some dissipation occurs after sunset. Another item of
concern is fog potential. STL metro sites will not benefit much of
drier air behind the front with it still upstream at 23z/Mon and
very weak wind field behind it and crossover temps all in the
upper 60s. This points to potential of a period of IFR VSBYs at
most STL metro sites, possibly all. Have introduced fog at all STL
metro sites, indeed taking CPS and SUS into IFR category late
tonight, but held off on STL for now and will re-evaluate this
evening. Lower crossover temps and better mixing and drying at COU
and UIN should lead to a more brief and less of an issue with fog
at those sites.

SPECIFICS FOR KSTL: Introduced fog with crossover temps in the
upper 60s and versus forecast min temps is indicative of IFR VSBY
potential. Did not forecast IFR just yet, but may be upgrading to
add in this evening after further evaluation.

TES

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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