Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 231759
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1159 AM CST Tue Jan 23 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 307 AM CST Tue Jan 23 2018

The region remains under the influence of a deep layer cyclone
early this morning with the surface low currently centered in the
Great Lakes and upper low in northwest IL. Stratus blankets the
mid-upper MS Valley into the Plains. Weak impulses rotating within
the cyclonic upper flow continue to bring some light
snow/flurries to the CWA. The most organized area of snow
occurringin response to large scale forcing and mid level
frontogenesis associated with one of the strongest vort maxes has
shifted east of the CWA as the upper low pulls northeast. Lastly
the tight pressure gradient continues to result in strong and
gusty northwest winds.

The deep layered cyclone/upper low will continue to move
northeast today with the upper low in the eastern Great Lakes by
early evening. Today looks rather blustery with gusty northwest
winds continuing this morning and then decreasing during the
afternoon as the gradient weakens. Given the westward extent of
stratus, I think it will be slow to clear and have slowed the
clearing trends of the previous forecast. Even this less
optimistic forecast may be too fast, with the new forecast
showing clearing in central MO beginning this afternoon and over
eastern MO and southwest IL early-mid evening. Scattered flurries
and light snow showers will continue to impact parts of eastern MO
into western IL this morning with no accumulation expected. In
light of the aforementioned cloud cover and continued weak CAA
early this morning, I expect the temperature rebound today will be
small...on the order of 3-6 degrees.

Tonight looks rather tranquil with relatively light winds and
seasonably cold temperatures. A northwest flow short wave trof
will move from the northern-central Plains into the MS Valley by
daybreak Wednesday. Weak large scale forcing will accompany this
trof, however moisture is limited and is currently forecast to be
confined to the mid-upper levels. As such, present indications are
this wave will result in increasing clouds late evening into
overnight and will be largely void of precipitation within the
CWA.

Glass

.LONG TERM...  (Wednesday through Monday)
Issued at 307 AM CST Tue Jan 23 2018

The aforementioned northwest flow short wave/PV anomaly was noted
over western MT on early morning water vapor imagery and RAP 1.5
PVU analysis fields. As noted above, this feature will continue
sliding southeastward towards MO/IL tonight, but the lack of
moisture will preclude widespread precipitation when this feature
moves through the region on Wed. A surface high pressure center
then shifts southeastward across the region on Wed night, setting
the stage for return flow on Thu.

The combination of building heights aloft and favorable southwest
surface winds will yield warm temps on Thu and Fri. The current
forecast temperatures may be 5-10 degrees too cool based on how
quickly temperatures rose over the last couple of days. Breezy
conditions are also expected due to the modest pressure gradient
between the surface high over the southeastern CONUS and surface
cyclogenesis occurring over the northern plains and US/Canada
border induced by a vigorous upper disturbance as it crosses the
Rockies.

A previously stalled weak feature over Baja CA is forecast to
lift into MO/IL on Fri ahead of the developing low pressure system
over the northern plains. This feature could produce light rain
over parts of the area on Fri aftn, but the best opportunity for
widespread precipitation will be from Fri night through Sat night.
During this time, the cold front associated with the
aforementioned surface low will stretch across the Midwest and
central plains while a couple of PV anomalies move across the area
and produce large-scale ascent. The cold front is expected to
move through the region on Fri night or Sat. Model disagreement
regarding the location, evolution, and strength of the surface and
upper air features becomes apparent by 12z Sat and is highly
noticeable by 12z Sun, therefore there is currently low confidence
in temps and precip timing for the upcoming weekend and beyond.

Kanofsky

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1137 AM CST Tue Jan 23 2018

Winds are expected to weaken over the area by late this
afternoon. MVFR ceilings will persist the rest of the day into the
evening hours. Low pressure in the Ohio Valley will continue to
push east into the Northeast US. The wrap around moisture from the
departing system will be resposible for the lower ceilings
through the evening hours into early Wednesday morning. Ceilings
will slowly lift to VFR from west to east over the region late
tonight into early Wednesday morning.

SPECIFICS FOR KSTL:
Winds will begin to relax after 21z at the terminal. MVFR ceilings
will persist through the rest of the day and into the late evening
hours. Ceilings are expected to lift to VFR after 05z Wednesday.

Kelly

&&

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

$$

WFO LSX



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