872 
FXUS63 KLSX 070528
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1128 PM CST Sun Jan 6 2019

.UPDATE...
Issued at 850 PM CST Sun Jan 6 2019

Adjustments to rain timing and increasing concerns on high winds
for some areas on Monday. 

New model runs continue to confirm what they were trending with
earlier in the day, in delaying the timing of the rain chances for
tonight and Monday. Outside of any random sprinkles falling from a
mid cloud deck (higher than 8000ft), onset of the best rain
chances look to be delayed until after midnight for areas north
and west of STL metro, and until after sunrise Monday for STL
metro and areas to the south and east. Rainfall amounts look
light, so despite the high PoPs, QPF is expected to be less than a
tenth of an inch for most areas.  

Rain should exit by midday Monday for most areas save southwest
IL. Clouds will also clear quickly after rain exit, and deepening
mixing layer thanks to SW surface winds should be able to tap into
the western edge of the leftover low level jet as it slowly drifts
eastward. The strength of this low level jet will be near 40kts
when it begins to be tapped into during the late morning and
midday hours for northeast and central MO and west-central IL, and
offers a brief window for gusts to 40mph for many of these areas,
which is just below Wind Advisory criteria. The oncoming shift
overnight will need to evaluate for Advisory potential. The 
timing, strength, and placement of the low level jet for later in 
the day does not favor these high wind gusts as much for STL metro
and areas to the S and E. Either way, clearing and a stout W-SW
surface flow will result in decent warming potential for many
areas, well into the 60s--still well below record values (see
CLIMATE section below). 

TES

&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 340 PM CST Sun Jan 6 2019

Mid/high level clouds have been increasing in coverage and thickness 
today ahead of an approaching low pressure system. Despite the 
increasing cloud cover, the influence of a ridge axis aloft as well 
as steady warm air advection and the passage of a warm front have 
allowed temperatures to reach the mid-50s to lower 60s by early 
afternoon across all but the northernmost part of the CWA. These 
temperatures are 20-25+ degrees above normal for this time of year.

The low pressure system will continue moving through the region 
tonight and tomorrow, placing the LSX CWA beneath a broad region of 
DCVA located downstream of the upper trough axis. Ahead of the 
approaching upper low, an increasing H85 jet will transport moisture 
northeastward through the region tonight and tomorrow. At the 
surface, a cold front will sharpen across the central CONUS tonight 
before slowly moving through MO/IL tomorrow. BUFKIT soundings show 
that an initially dry air mass below 500-600 hPa gradually moistens 
with time. Finally, models depict steady isentropic ascent along the 
300-315 K surfaces in the presence of ample moisture, and the 
overall area of ascent moves across the region tonight and tomorrow.

Taken together, this suggests that mid/high clouds will continue to 
thicken late this afternoon and tonight (well ahead of the cold 
front) due to isentropic ascent, which is consistent with satellite 
imagery as well as the increasing mid-level echo noted across MO/IL 
on composite radar imagery over the last few hours. Once the column 
reaches saturation, further lift will result in a broad corridor of 
rain which moves across the region from late tonight through 
tomorrow afternoon. Rain will end during the day tomorrow as the H85 
jet shifts eastward and strong isentropic downglide occurs near and 
behind the cold front. Wind speeds and gusts will increase as the 
pressure gradient tightens, but the strongest gusts will probably 
occur near and shortly after the cold frontal passage, especially 
after 18z. Immediately behind the front, model guidance depicts 
nearly unidirectional winds from the surface to the frontal 
inversion aloft, which is conducive for mixing those stronger wind 
gusts down to the surface. At this time, it does not appear that 
the winds will be strong enough to require any headlines. 

Tonight's overnight lows will be noticeably warmer than last night 
due to thick cloud cover, steady warm air advection, and periods of 
precipitation. With a warm start to the day, highs should easily 
reach the 60s across most of the area tomorrow, which would be 20-30 
degrees warmer than average for this time of year. The area of 
greatest uncertainty for tomorrow's high temperatures is across 
central MO. Depending on how quickly the clouds clear out behind the 
departing low pressure system, the current forecast may not be warm 
enough for the western part of the CWA. 

Kanofsky

.LONG TERM...  (Monday Night through Next Sunday)
Issued at 340 PM CST Sun Jan 6 2019

Conditions will be rather benign through much of the period. Most 
sensible weather steers around the immediate forecast area with 
limited opportunities for measurable precipitation.

Temperatures will remain above normal Tuesday with forecast highs 
ranging from the upper 40s to lower 50s. However, a cold front will 
extend south of a shortwave, which pushes west to east through the 
southern Great Lakes Tuesday. The front has no moisture to work with 
and passes through dry, but will be effective in advecting colder 
air into the region late Tuesday into Wednesday. 

Synoptic troughing over the eastern CONUS will then couple with an 
amplified ridge over the Western U.S., setting up west to northwest 
flow aloft through the Midwest. Surface high pressure builds in from 
the northern plains through the midweek period, resulting in dry but 
cool conditions through Thursday. Given the trajectory of the colder 
H8 air, expect slightly below normal temperatures over Illinois to 
near normal temperatures over central Missouri. There will be plenty 
of sunshine Wednesday, possibly into early Thursday. Highs will 
generally range from the lower 30s to around 40 degrees. 

The only mention to precipitation comes late in the week. 
deterministic spread lends less confidence in any one solution. 
Despite this, chances are low for precip with general trends showing 
surface high pressure weakening over the Midwest. An upper shortwave 
pushes southeast through northwest flow aloft, while low pressure 
pushes east to west through the gulf states Friday into Saturday. 
Each feature stays just far enough to the north and south 
(respectively) of the area with limited moisture depth and dry air 
to overcome.

Clouds continue to linger into Saturday with another upper level 
wave dropping down from the north during the afternoon. This looks 
to pass through dry as well with the main impact being a 
reinforcement in north and northwest flow. Temperatures are near or 
slightly above normal Saturday just ahead of this feature.

Maples

&&

.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1128 PM CST Sun Jan 6 2019

Rain chances will be on the increase overnight from west to east,
beginning with UIN and COU over the next few hours, and then at 
STL metro sites after sunrise Monday morning. In the meantime, a 
SW low level jet 0f 45-50kts has developed and resulting in LLWS 
at all TAF sites, ending from west to east overnight. Rain and 
lower ceilings in the MVFR category should occur nearly concurrently
later tonight and Monday morning before clearing occurs. SE surface
winds 10-15kts with modest gusts at times will gradually veer SW 
by Monday. Gusts look to increase substantially at UIN and COU, 
where 30-35kts possible by midday and early afternoon. Considered 
IFR category at UIN but for now left as MVFR. 

SPECIFICS FOR KSTL: Timing for rain still looks set for mid to
late Monday morning and LLWS in the meantime. Can't rule out a 
stray sprinkle before 14z but impact looks nil. The strong wind 
gusts affecting UIN and COU later on Monday, for now, still appear
to remain to the west but will keep an eye on trends with the new
models later tonight.

TES

&&

.CLIMATE...  

Record high temperatures and year

     Monday 01/07

STL   73 (2008)

COU   75 (1965)

UIN   70 (1907)


&&

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

$$

WFO LSX