Area Forecast Discussion
Issued by NWS St. Louis, MO

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44
000
FXUS63 KLSX 181945
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
245 PM CDT Mon Mar 18 2024

.KEY MESSAGES...

- A hard freeze (low temperatures below 29 degrees) is forecast
  again for almost the entire area, threatening tender vegetation.

- Gusty winds and warm, dry air will make for elevated fire
  danger and erratic fire behavior along and west of the
  Mississippi River tomorrow.

- We will remain dry into the later part of the week, though
  temperature uncertainty increases markedly by Thursday.


&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 244 PM CDT Mon Mar 18 2024

Stratocumulus clouds that developed this morning amidst strong
cyclonic flow aloft will begin to dissipate as the wave`s axis
passes through the region this evening. The loss of diurnal
instability will also erode the cloud deck, leading to a clear and
cold night across the region. Temperatures overnight will fall
below freezing areawide, and it is very likely (90%+ chance) that
we see another hard freeze (colder than 29 degrees) tonight for
all but the immediate St. Louis metropolitan area. While the
growing season is not truly in effect despite our recent warm
stretch, vegetation that bloomed early will be threatened
overnight.

A weak mid-level wave drops south from Canada overnight along deep
northwest flow, forcing a surface low and cold front to approach the
Mid-Mississippi Valley tomorrow morning. Poor moisture return will
allow the front to pass through the region on Tuesday with no
precipitation, but the strengthened southwest wind at the surface
and aloft directly ahead of the front will help advect anomalously
warm and dry air into eastern Missouri. Temperatures tomorrow will
rise noticeably higher than today`s did, by 15-20 degrees in some
spots. Those along and south of the Missouri River will see the
warmest temperatures, reaching the mid-60s during the afternoon.
This warm, dry air alongside stronger winds will also lead to
erratic fire behavior and elevated fire danger in Missouri and far
western Illinois. Heed caution with any exposed flames and be
mindful of anything that can inadvertently spark a fire. The colder
air behind the front lags to our north, but by Wednesday
temperatures fall back to near/slightly-below mid-March normals.

MRB

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 244 PM CDT Mon Mar 18 2024

The ridge-trough upper-level pattern currently across North America,
with a cutoff low embedded in the ridge over the Desert Southwest,
will begin to erode starting Wednesday. The resultant near-zonal
flow aloft will introduce a more active pattern, higher uncertainty
in our temperature forecast, and increased potential for
precipitation later in the week. Our first threat for rain comes
Thursday, when the cutoff low to our west dislodges and drifts east.
At the same time, most guidance depicts an amplified shortwave
diving into the Great Lakes region amidst the cyclonic flow of a
larger trough in eastern Canada. These features will be the focus
for precipitation to our north and to our south, however we look to
be forcing-starved between them. As such, barring any significant
shift in these waves` characteristics, most of the forecast area
will be dry. Chances for rain are highest (around 30%) in far
northern and southern Missouri, closer to the forcing aloft.

This rain threat lingers in the forecast through into Friday and
Saturday and expands across the forecast area, but this is moreso a
reflection of how uncertain the forecast is. The track, speed, and
amplitude of the shortwave forcing any rain is more uncertain later
in the week, leading to broad, low rain chances (15-30%). That said,
there will be plenty of dry time and any rain that does fall likely
will be light. Low level moisture and humidity, as depicted in
ensemble guidance, looks anomalously dry. Also, with instability at
a premium (MUCAPE less than 100 J/kg) any convection that manages to
develop will be very shallow and weak. The differences mentioned
above also impact our temperature forecast, which becomes highly
uncertain by the weekend. While we will generally remain near to
below average into Friday, the temperature spread increases
dramatically by the weekend. Subtle differences in the ensemble
guidance regarding the upper level pattern (a weak ridge overhead vs
northwest flow) is the main culprit for the uncertainty.

By Sunday, however, ensemble guidance converges a bit more on a
large-scale upper-level trough digging into the western CONUS. The
exact evolution is far from certain, but would result in southwest
flow aloft and increased potential for more widespread and
beneficial precipitation Sunday into early next week. ML/AI-based
severe weather probability guidance is cluing in on the more
favorable synoptic pattern for at least some threat of severe
weather as well, and the CPC Precipitation Outlooks valid for this
period also highlight the region for increased potential for above-
normal precipitation. Temperature uncertainty remains relatively
high during this time as well, but generally speaking we should warm
to at least near-normal if not a bit warmer still.

MRB

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1150 AM CDT Mon Mar 18 2024

Stratocumulus clouds currently at the terminals will erode with
sunset, and dry VFR conditions will prevail through the forecast
period. There are some indications for LLWS for a few hours
overnight at KUIN, but conditions look more like turbulent mixing
than true wind shear. Winds pick up tomorrow corresponding with a
dry cold front passage, ending up out of the west by sunset
tomorrow.

MRB

&&

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

$$

WFO LSX


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.