718 
FXUS63 KLSX 071943
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
243 PM CDT Fri Oct 7 2022

.SHORT TERM...  (Through Late Saturday Night)
Issued at 236 PM CDT Fri Oct 7 2022

The primary impact will be the frost/freeze conditions late tonight 
into early Saturday. By Sunday, the cooler-than-normal airmass will 
moderate back to normal. 

Latest surface analysis shows a cold front has pushed well to the 
southeast, stretching from the Tennessee Valley westward through 
Arkansas. Northerly surface flow has set up behind the boundary 
around the eastern periphery of high pressure, which will 
continue to build southeast through late tonight.

Short term guidance depicts the center of the high elongating 
from the mid-Mississippi Valley westward into central Kansas. The
source region for the airmass that is being funneled into the 
region incorporates dewpoints in the mid to upper 20s, which is 
consistent across multiple pieces of short range guidance. The 
southward extent is the question with the I-70 corridor being an 
average delineation where air moderates slightly to the south and
east. While the DESI grand ensemble 50th percentile shows low 
temperatures in the low to mid-30s in the freeze warning and mid 
to upper 30s south and east, dry air and 850 mb temps are within a
couple degrees of upstream air in the Dakotas this morning. Very
dry conditions already precede this cold push, which begs to lean
toward the lower end of DESI percentile data. This is especially
true given the microclimate behavior in parts of the CWA with the
potential for localized minimums. I would not be surprising to see
notoriously cooler outlying locations fall into the upper 20s, 
especially through sections of northeast Missouri. Temperatures 
moderate slightly to the southeast, but only to the low/mid-30s. 
Therefore, the freeze warning and frost advisory look solid. 

The core of the cooler 850 mb airmass moves east Saturday. Along 
with dry air and sunshine, mid-level moderation will allow us to 
tack on 25-30 degrees for highs in the low-60s. This still runs 
about 10 degrees or so below normal. The surface high moves slightly 
east mitigating the potential for sub-freezing lows Saturday night. 
Nonetheless, clear skies and light wind will drop temperatures into 
the mid-30s to near 40 degrees with frost being a concern in some 
areas. 

By Sunday, the southern shift in the surface high bring 
southwesterly return flow with a slight warming of mid-level 
temperatures over Saturday. Temperatures will return near normal 
with highs in the low-70s being representative of much of the area.


Maples

.LONG TERM...  (Sunday through Next Friday)
Issued at 236 PM CDT Fri Oct 7 2022

Near and above normal temperatures are expected the first half of 
the week before a midweek cold front brings the potential for 
rainfall. Below normal temperature return behind the front later in 
the week.

The base to the upper level trough will begin to transition to the 
east from Sunday into Monday. As this occurs, surface ridging will 
extend from the mid-Atlantic across the gulf states with west and 
southwesterly flow setting up from the central plains into the mid-
Mississippi Valley. Initially, weaker warm air advection results in 
near normal temperatures with highs in the low to mid-70s Sunday. By 
Monday, southwesterly mid-level flow begins to strengthen underneath 
the axis of a modest upper level ridge. 10-15C air at 850 mb is 
poised to move northeast through the day Monday pushing temperatures 
near to above normal with highs in the mid to upper 70s. 

Two areas are worth watching late Monday into Tuesday with the first 
chance at some much needed rainfall. A northern stream shortwave 
tracks southeast out of the northern plains, while a southwestern 
cutoff upper low is kicked northeast ahead of an amplifying, 
positively tilted trough. Both pieces act in tandem to provide at 
least some upper level lift ahead of the western trough in the 
vicinity of a strengthen LLJ late Monday night into Tuesday. 
However, moisture return remains unimpressive with the southeastern 
half of the CWA holding onto dry air in the lower 10k feet. This 
keeps the potential for measurable rainfall on the order of a few 
hundredths of inch from Columbia to Quincy.

Though several fragments of upper level vorticity are embedded 
withing the southwesterly flow ahead of the western trough, moisture 
remains limited at least until midweek. Unfortunately, that is also 
where guidance diverges and ensembles provide little confidence for 
beneficial rainfall. Little change in probabilistic guidance would 
lean in favor of less than one-quarter of an inch with measurable 
(0.01) rainfall likely (60 percent), but quickly decreasing below 30 
percent as thresholds are raised above one-quarter of an inch. Both 
the deterministic ECM/GFS are indicating a broad, deep upper low 
taking shape over southern Canada by late Wednesday into Thursday. 
The main difference seems to be the progressive nature of the GFS as 
the upper trough remains positively tilted over the central U.S. On 
the other hand, the ECM deepens the system slightly quicker with a 
slower arrival of the surface front, giving rise to better moisture 
return and higher yield in rainfall late Wednesday. So, while 
guidance has been trending lower on precipitation, there is still 
some hope for beneficial rainfall. 

Northwesterly flow dominates the region behind the frontal boundary 
as the strong upper low traverses the U.S./Canadian border. Cooler 
than normal temperatures are favored late next week.


Maples

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1243 PM CDT Fri Oct 7 2022

VFR conditions are expected through a commanding majority of the
TAF period with few, minor caveats. 

A secondary cold front is reinforcing cold air southward with
scattered/broken MVFR level cloud bases along the boundary. 
Observations indicate that broken MVFR of 2500-3500 feet has been
hit/miss and momentary. It is not enough to justify a prevailing
group and TEMPO may even be a far stretch with these instances
being isolated and only lasting a few minutes.

Predominant wind direction will remain out of the north, becoming
light and variable overnight, potentially calm around sunrise.
Much colder temperatures will allow some steam fog in immediate
vicinity of north/south oriented rivers. While steam fog could
develop on west/east rivers, air may be too dry to promote spread
too far from streams, limiting confidence of any direct impact.
However, it may be something to watch through early Saturday.
Expect winds to gradually pick up to around 5 knots Saturday
morning, shifting out of the west as high pressure sinks south.

Maples

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Frost Advisory from 1 AM to 9 AM CDT Saturday for Boone MO-
     Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Jefferson MO-Lincoln MO-Madison MO-Moniteau MO-
     Montgomery MO-Osage MO-Reynolds MO-Saint Charles MO-Saint 
     Francois MO-Saint Louis City MO-Saint Louis MO-Sainte 
     Genevieve MO-Warren MO-Washington MO.

     Freeze Warning from 1 AM to 9 AM CDT Saturday for Audrain MO-
     Knox MO-Lewis MO-Marion MO-Monroe MO-Pike MO-Ralls MO-
     Shelby MO.

IL...Frost Advisory from 1 AM to 9 AM CDT Saturday for Bond IL-
     Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-
     Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL-
     Randolph IL-Saint Clair-Washington IL.

     Freeze Warning from 1 AM to 9 AM CDT Saturday for Adams IL-Brown 
     IL-Pike IL.

&&

$$

WFO LSX