Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion...updated 
National Weather Service St Louis MO
911 PM CST sun Nov 29 2015

issued at 905 PM CST sun Nov 29 2015

Weather features across the region remain fairly nebulous this
evening, but it appears that weak and slowly increasing warm air advection
around 850mb is becoming the main driver of precip across the
region this evening. Although the increase in upward vertical velocity with the warm air advection is
certainly subtle, in the moisture laden air mass it appears to be just
enough to lead to the development of several areas of light rain.

Pops have been tweaked several times during the evening primarily
on radar trends, and this will likely be the case for the rest of
the evening and into the predawn hours given the fact that the
forcing is so weak it's difficult to use model output to pin down
exact trends. Mass fields as well as hi-res precip output
certainly suggests that with time the main focus of the precip
should be over the northwest half of the County Warning Area where the warm air advection remains
the most persistent; however, given the extent of the moisture i'm
not certain if threat will ever totally end in southeast parts of
the forecast area.

Temps will likely hold nearly steady in most areas.



Short term: (through late tonight)
issued at 255 PM CST sun Nov 29 2015

The cloudy and rainy weather pattern will continue tonight as
southwest flow shortwaves continue to move through our area ahead of
the upper level low currently located over Wyoming and Utah. The majority
of the rain should be on the light side with just drizzle at times.
The best chance of rain will likely occur in the evening with the
latest radar depicting plenty of light rain upstream of our forecast
area across western MO. Temperatures will be warm enough that the
precipitation will be all liquid rain with no freezing or frozen
precipitation expected. Lows tonight should be similar to the
previous night. Went just above MOS guidance due to the overcast
cloud cover. The MOS temperature guidance has been a little too
cold on nighttime lows the past couple of days.


Long term: (monday through next sunday)
issued at 255 PM CST sun Nov 29 2015

The influence of the main storm system center itself will be felt on
Monday and Monday night as an upper level disturbance rotates around
the southern periphery of the closed low system as it eventually
tracks to our north. This will combine with what will initially be
an occluded front passing thru late in the day, with a cold front
shortly after.

Another rain event will thus result on Monday and perhaps into Monday
evening, with the best rainfall amounts over areas north and west of
stl Metro where the better upper level support will reside. Up to a
third of an inch of liquid equivalent will be possible from this

P-types will be entirely rain with vertical temp profiles strongly
supportive of liquid falling, and surface temps remaining above
freezing when any pcpn is falling. Cold surfaces not a factor
with it being early in the season and lack, thus far, of any
significant recent cold air outbreaks.

With extensive cloud cover anticipated, favored daytime Max temps
Monday at or slightly below the coldest MOS.

Models remain in good agreement from Tuesday thru early
Thursday, with a synoptically unfavorable position for pcpn on
Tuesday with a wound-up storm due north, and northwest flow aloft
Wednesday and Thursday with surface high pressure dominating. Temp
recovery on Tuesday looks intriguing, with the edge of a cooler
airmass trying to build in, but otherwise, a very favorable setup
for a nice warmup, with multiple sightings of the sun anticipated for
the first time in days and deep mixing for late November with breezy
west-southwest winds.

Look for slightly below average temps Wednesday as the colder air has
a better chance to deepen and already a recovery to slightly above
average by Thursday.

There remains model disagreement later on Thursday on how to handle
the evolution of a shortwave in this northwest upper flow but with moisture
expected to be at a premium by this time, its impacts are expected
to be minimal to nil.

Otherwise, building heights and ridging aloft then takes hold for
Friday and Saturday with what could be well above average temps
as southerly surface flow kicks in. Current forecast reflects
Max temps in the 50s, but could see some 60s from this setup.

Yet another cutoff system could impact our region for next Sunday,
but timing tends to favor the slower solutions here so went on the
low-end with pops for liquid pcpn, plus plenty of time to watch the



Aviation: (for the 00z tafs through 00z Monday evening)
issued at 544 PM CST sun Nov 29 2015

Little change in conditions from previous forecast. IFR across
southern and western Missouri should gradually spread further
north and east overnight into Monday with areas of light to
moderate rain. Ceiling heights have been pretty difficult to
predict over the past couple of days, and I expect tonight into
Monday to continue to be tough. Generally speaking though,
ceilings should fall through low MVFR from southwest to northeast
through tonight with IFR becoming widespread on Monday. Areas of
rain will continue through Monday afternoon.

Specifics for kstl:

Ceilings between 1000-2000 ft will continue to prevail with
periods of light rain tonight. Eventually I expect the ceiling to
fall below 1000 feet...tho timing is uncertain. The 00z taf may be a
bit pessimistic on timing and the IFR could hold off until after
12z, but some model guidance actually brings the IFR in more
quickly than my taf around 06-07z. This weather pattern is giving
US very little to really latch on to to help US with these confidence remains low. Should see periods of rain
continuing on Monday...but finally ending Monday evening after a
cold front moves through.



Lsx watches/warnings/advisories:


Weather forecast office lsx

National Weather Service Glossary of Abbreviations