Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service La Crosse WI
1132 PM CST Sat Nov 28 2015

Short term...(tonight through Sunday night)
issued at 225 PM CST Sat Nov 28 2015

Nothing much to see here...please keep moving to the good stuff
below. Really no change in thinking from recent days...with
shortwave ridging continuing to build in aloft to wrap up the
weekend...all while a low level ridge axis remains centered pretty
much overhead. Another mainly clear and chilly night is in the works
tonight...with an increase in middle level clouds expected from the
southwest late. Those will thicken up into Sunday as warm advection
ascent aloft Cranks into gear ahead of the closed low lifting out
onto the central/northern plains...with temperatures gaining a couple
degrees over today's readings via simple airmass moderation...though
thickening clouds won't help matters. Much warmer Sunday night with
a blanket of thicker 6-10kft clouds... though with no precipitation concern
given plentiful dry air below that layer (as evidenced by 900mb dew
points pushing -30c!).

Long term...(monday through saturday)
issued at 225 PM CST Sat Nov 28 2015

By far the most challenging portion of the forecast will come later
Monday into Tuesday...with all eyes focused on the potential for
accumulating snowfall across the area. 12z guidance suite continues
the overall colder thermal profile theme as noted in the 00z and 06z
runs...tied to a slight southward shift in the middle level circulation
passing through northern Iowa and pretty much squarely across the
County Warning Area. That general idea still supports a warm advection Wing of
precipitation rolling northeastward through the area roughly 21z-
06z...with solid moisture transport pointed toward the lower lakes
but wrapped back into a notable trowal feature out ahead of the
approaching upper low. However...the biggest change with the
slightly more southerly track is for less warm air to be yanked
northwestward into the trowal feature...with forecast profiles
supporting more of a snowy scenario farther east into parts
of western Wisconsin.

With all of that said...still have some concerns models are
underdoing the amount of warm they seem to do quite often
in these types of scenarios...with higher confidence that eastern/
southeastern counties are likely to see just a cold rain during the
initial precipitation band...maybe mixed with a little sleet at times given
enough of a cold dome below 800mb for a time. The freezing rain
threat looks pretty small at the moment with stronger southeasterly
flow likely to hold surface temperatures in the middle 30s through Monday
night...but something to watch in the near term.

Otherwise...the thing to really watch remains snow accumulation
potential...with increasing confidence that parts of the area may
get some pretty good snow amounts. Don't foresee snow ratios being
anything too impressive with only a thin dgz and a relatively "warm"
thermal profile throughout the column. But...having the partial
remnants of Sandra wrapping into the system combined with a period
of rather strong upper difluence/forcing ahead of the closed low are
all pluses for what could be some pretty good quantitative precipitation forecast amounts in excess
of a half to perhaps three quarters of an inch...taking into account
the entire storm and some additional deformation-driven precipitation into
Tuesday. Sref plumes seem to be clustering in toward this wetter
solution as well...only increasing confidence that this event may
ultimately require some headlines.

For the moment...the most likely areas to see significant snow of
4-6" or more (early estimate) would be much of southeast Minnesota
into parts of northeast Iowa...especially along and west of the
Highway 63 corridor. Not to be forgotten...could see some higher
totals into Clark/Taylor counties as long as enough warm air doesn't
wrap north to mix those areas with more sleet. Based on remaining
questions about amounts and placement...prefer to forego a Winter
Storm Watch but will hit the idea hard on social media...web

Wrap around showers should persist into Tuesday and Tuesday night
within a lingering deformation axis on the back side of the
departing upper low. By this point...precipitation should pretty much be
all snow with the arrival of colder air tucked beneath the upper
low...with perhaps another inch or two across western/northern areas
where surface temperatures should hold below freezing.

Sharp drying looks to take hold Wednesday morning as the system
opens and tracks into southern Ontario...with increasing sunshine by
midday at the latest. Thereafter...just not a whole lot to talk
about into late week and the first half of the weekend as upper
ridging builds through much of the central Continental U.S. And milder Pacific
air takes hold. Given the setup with loads of dry air prognosticated to lay
out across the region...have a hard time seeing much of any cloud for perhaps some passing cirrus. Temperatures should also
respond upward nicely...potentially warming well into the 40s by
Friday/Saturday with stronger southerly flow in place. Of course...
any deeper snow pack will heavily modulate that warm well as
potentially deliver some rather chilly nights Wednesday/Thursday night with
light surface winds and clear skies.


Aviation...(for the 06z tafs through 06z Sunday night)
issued at 1132 PM CST Sat Nov 28 2015

Leading edge of middle-upper level clouds just beginning to move into
northeast Iowa and southeast Minnesota. These clouds will continue to move
northeastward overnight...resulting in ceilings from 9-10 kft
eventually lowering to between 5-6 kft by middle to late Sunday
morning and continuing through the rest of the period. Winds will
be light and variable at klse. At krst...light westerly winds
tonight will become light and variable Sunday morning and then
pick up a bit from the east-southeast by Sunday afternoon.


Arx watches/warnings/advisories...


Short term...Lawrence
long term...Lawrence

National Weather Service Glossary of Abbreviations