Chris's Corner - The Winter Break is About to End

Area Forecast Discussion
National Weather Service Saint Louis MO
319 PM CST Wed Jan 9 2019

.LONG TERM... (Friday through Next Wednesday)
Issued at 316 PM CST Wed Jan 9 2019

(Friday - Saturday Night)

The potential for significant accumulations of snowfall continues to
be the main forecast concern with the upcoming winter storm which is
expected to affect the bi-state region beginning Friday afternoon.
Latest model guidance has been fairly consistent over the past 24
hours, suggesting a period of primarily warm-air advection driven
snow from Friday afternoon through late Friday night. This is where
the brunt of the accumulating snowfall is likely to occur with this
upcoming storm, and where the highest forecast confidence resides.
There remains far more uncertainty with the Saturday afternoon and
night timeframe as models diverge with a northern stream shortwave
trough and the potential for a closed midlevel low developing. This
helps to effectively lengthen the duration of additional snowfall
accumulations through Saturday night like the ECMWF model continues
to suggest. However, most other guidance moves out the accumulating
snowfall essentially by late Saturday morning. While light snowfall
is shown to continue into the afternoon hours across the a majority
of the area, the belief is that the light intensity of the falling
snow and temperatures just above freezing will greatly reduce
additional snowfall amounts.

Precipitation is still expected to begin across the area from the
west/southwest to the east/northeast Friday afternoon into early
evening. Very dry air at low levels will likely delay the onset from
happening a few hours earlier, but persistent forcing for ascent
throughout the atmospheric column will eventually achieve top-down
saturation. At the moment, we are looking at a start time for
central and southeastern Missouri around mid afternoon, northeast
and east-central Missouri toward dusk, and west-central and
southwest Illinois by early evening. This predicted onset time could
lead to aggravated travel problems as it may start to accumulate
during or just before the Friday evening commute.

Precipitation type initially could very briefly start off as rain
and/or sleet, but evaporative cooling/wetbulbing should lead to a
rapid changeover to plain snow. Nearly all model guidance continues
to suggest this is an all snow event thereafter, with the exception
the 1800 UTC NAM which has sleet across parts of southeast Missouri
early Friday night. This solution was discounted for now because the
NAM has a well-known bias with being too strong with the low-level
jet.

Light to moderate snow is likely to continue through much of Friday
night, with the most intense snow likely between the late evening
and early overnight hours when a lead shortwave trough moves through
the region. Model guidance continues to suggest a wide range of
total liquid equivalent precipitation possibilities, with the GFS on
the drier side and the ECMWF/NAM much wetter. Due to the antecedent
dry air and lack of very strong forcing to more rapidly overcome the
low-level dry air, did weigh the drier GFS a bit more compared to
the wetter model solutions.

Regarding snowfall efficiency (snow-to-liquid ratio/SLR), model
soundings show the maximum lift (associated with the aforementioned
midlevel shortwave trough passage) to be centered within the
dendritic growth zone. This factor is combined with high moisture
content (PWAT values 80-90% of normal) and a near isothermal layer
between the surface and 650 hPa. This isothermal layer helps
dendrites to aggregate into larger snowflakes which accumulate
faster. Finally, at the surface, air temperatures should be in the
upper 20s to near 30 degrees Friday night. Because of the reasons
outlined above, SLR values Friday night probably will be near or
just above climatology (12:1). Toward the end of the event, any snow
that does fall from late Saturday morning into the afternoon will
likely have a difficult time accumulating due to surface
temperatures just above freezing and the light intensity of the
falling snow. Therefore, an event average SLR will likely be near
10:1. Using that ratio and with a blend of the GFS (more heavily
weighted) and the wetter ECMWF/NAM for QPF, widespread snowfall
totals of 4 to 6 inches are generally expected. Higher amounts are
possible, particularly if steadier snow does continue longer on
Saturday/Saturday evening like the ECMWF depicts.


(Sunday - Next Wednesday)

The deterministic ECMWF model remains an outlier through Sunday
night with a closed midlevel low developing across the mid-
Mississippi Valley. This solution was discounted for now as other
deterministic and ensemble guidance suggests this is an unlikely
scenario. However, if the ECMWF were to be correct than light snow
may continue across parts of the area off and on into Sunday evening.

Moderating temperatures are likely early to mid next week as
mid/upper level heights rise and 850-hPa temperatures climb back
above 0C. Highs in the 40s are expected on Tuesday and Wednesday
with lows in the 20s and 30s.

The next chance of precipitation is Tuesday night into Wednesday as
a cold front moves through the region. At this time, most of the
precipitation should be plain liquid rain, but rain may briefly mix
with or changeover to snow before ending as the colder air returns
behind the boundary.


Gosselin