Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 270235
Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
835 PM CST Sat Nov 26 2016
Issued at 835 PM CST Sat Nov 26 2016
Looks like another November evening filled with fog/stratus
challenges. Fog/stratus from this morning never did mix out and
continues to affect portions of southern MO. Latest satellite
imagery shows this area of fog/stratus expanding over the past
couple of hours. This expansion to the north/northeast should
continue through the overnight hours as low-level wind fields turn
more southerly. The main forecast challenge will be deciphering
whether this low-level moisture manifests itself more as fog or
more as stratus as it lifts/develops northeast.
Latest obs and webcams down near Springfield, MO show pretty
dense fog, but they remain a bit closer to the surface ridge axis.
Further to the north, the pressure gradient increases a bit which
would tend to favor a bit more mixed boundary layer resulting more
in stratus (a few hundred feet off the ground) than fog. The
latest hi-res guidance, however, is pretty aggressive with fog
development across much of the CWA overnight and many of these
guidance members develop fog well into IL. Will continue to
monitor obs as this moisture spreads into the region as a Dense
Fog Advisory could be needed should fog become more prevalent than
the more favored stratus scenario.
.SHORT TERM... (Through Late Sunday Afternoon)
Issued at 249 PM CST Sat Nov 26 2016
Primary concern for tonight is the potential for fog and stratus
again. There is still one area of persistent stratus over south
central Missouri this afternoon. It`s possible that this stratus
will expand northward after sunset. Forecast soundings across the
area are hinting at this with boundary layer dewpoint depressions
decreasing to zero or near zero at various times through the night
depending on which model is viewed. Boundary layer wind is higher
tonight than last night...so think fog is less likely than stratus.
Aforementioned boundary layer wind may also mix in drier air from
aloft to keep the stratus from redeveloping...so do not have a whole
lot of confidence in either occurring. Will tell the evening shift
to keep an eye out and keep their fingers on the "amendment" button.
Wind will be increasing on Sunday as low pressure strengthens over
the Great Plains. Strong low level moisture convergence on a 40-
50kt 850mb jet should spread showers into northern and central
Missouri by the afternoon. Normally, would lean toward warmest
guidance for temperatures with strong warm advection pattern, but
with increasing clouds and afternoon showers...expect temperatures
to top out close to today`s readings in the mid 50s.
.LONG TERM... (Sunday Night through Next Saturday)
Issued at 249 PM CST Sat Nov 26 2016
Moisture convergence is maximized across eastern Missouri and
Illinois between 00Z and 06Z tomorrow evening. A minimal amount
of instability is forecast and therefore only a slight
chance/isolated mention of thunder is retained with predominant
rain showers in the forecast for Sunday evening.
The storm system is expected to deepen and occlude across the
northern Plains and upper Midwest through Tuesday. This will result
in deep southwest flow across the lower Missouri and middle
Mississippi River Valleys and temperatures well above normal. Local
conditional climatology suggesting high temperatures in the upper
50s to lower 60s areawide through Monday.
Appears that the heaviest precipitation with the approaching trough
Monday and Monday night will be across the southeastern half of the
CWA. It is here that the best signal for moisture convergence and
upper level divergence is shown by model guidance. Have once again
retained slight chance/chance of thunder for these time periods, but
believe the predominant weather will be rain showers.
Given the occluding nature of the system, the actual cold front
really doesn`t arrive until Tuesday afternoon. This should result in
in one more day of above normal temperatures and it actually could
be the warmest since the low clouds should have cleared to the east.
Cold advection takes hold the remainder of the work week as the deep
trough of low pressure slowly slides across the center of the CONUS
and the surface ridge remains anchored from the northern Plains to
the lower Mississippi River Valley. This will result in
temperatures returning to normal and a dry forecast.
.AVIATION... (For the 00z TAFs through 00z Sunday Evening)
Issued at 526 PM CST Sat Nov 26 2016
Challenging TAF forecast for tonight as fog/stratus will be
possible at all sites. Fog/stratus from earlier this morning
across southern MO never mixed out this afternoon. The guidance
members that are handling this scenario the best are expanding the
stratus/fog northeastward tonight as the pressure gradient over
the region tightens. As a result, have gone much more pessimistic
at all sites with this TAF package, introducing IFR cigs. Could
also see some fog, but think that winds will stay up enough to
keep dense fog at bay. Debated going with prevailing LIFR in some
sites, but will wait and see how things play out over the next few
hours as confidence for tonight is fairly low.
Low clouds should slowly mix out Sunday morning, with all sites
going VFR by the late morning/early afternoon hours. However,
mid/high clouds will thicken up through the day as a disturbance
approaches from the west. Winds will become gusty during the
afternoon, with southerly gusts in excess of 20 knots likely. A
few rain showers will be possible late in the period.
SPECIFICS FOR KSTL:
Stratus to the southwest over southern MO is expected to expand
and lift northeastward tonight. Therefore, have trended towards a
much more pessimistic forecast with this package, introducing IFR
cigs. Debated including a TEMPO group for LIFR cigs, but will
watch trends over the next few hours as confidence in tonight`s
forecast is admittedly low.
If low stratus does indeed develop, then it will persist into the
mid/late morning hours on Sunday. Conditions should then go VFR
Sunday afternoon, with southerly winds gusting in excess of 20
knots. A few rain showers will be possible late in the period and
into the planning period as a disturbance approaches from the